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Full text of "Mineralogy"

EARTH 

SCIENCES 
LIBRARY 



Bf RKELEY\ 

LIBRARY 

UNIVERSITY OF I 
CAtlFORNIA/ 




*w- 



EARTH 

SCIENCES 

LIBRARY 




MINERALOGY 



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WHITTAKER & CO,, LONDON, E.C 



MINERALOGY 



BY 

F. H. HATCH 

PH.D., F.G.S., M.lNST.C.E., M.I.M.M. 

AUTHOR OF "A TEXTBOOK OF PETROLOGY," AND JOINT-AUTHOR OF "MINING 

TABLES," "THE GEOLOGY OF SOUTH AFRICA," AND "THE GOLD- 

MINES OF THE RAND " 



FOURTH EDITION 

ENTIRELY REWRITTEN AND ENLARGED 



WITH 124 ILLUSTRATIONS 



WHITTAKER & CO. 
2, WHITE HART STREET, PATERNOSTER SQUARE 

LONDON, E.G. 

AND 64 & 66, FIFTH AVENUE, NEW YORK 
1912 



' ' ' v 



. . . 



EARTH 

SCIENCE* 

LIBRARY 




O^deot. 



CONTENTS 



CHAPTER 

PREFACE - 



PART I 

THE PROPERTIES OF MINERALS 
INTRODUCTION - i 

I. MORPHOLOGICAL CHARACTERS - 3 

Habit of crystallized minerals, 4 ; symmetry of 
crystals, 5 ; law of symmetry and of the constant 
angle, 8 ; zones, 9 ; classification of crystals, 9 ; 
regular or cubic system, n ; hexagonal system, 
15 ; tetragonal system, 17 ; orthorhombic or 
rhombic system, 19 ; law of rational intercepts, 
20 ; monoclinic system, 22 ; triclinic or anorthic 
system, 23 ; symbols, 24 ; hemihedrism, 29 ; 
hemimorphism, 30 ; twinning, 31 

II. PHYSICAL PROPERTIES - - 34 

Cleavage, 35 ; tenacity, 38 ; hardness, 39 ; colour, 
41 ; pellucidity and lustre, 42 ; refraction, 43 ; 
double refraction and polarization, 47 ; uniaxial 
crystals, 47 ; wave surface, 49 ; biaxial crystals, 
51 ; absorption of light pleochroism, 54; thermal 
properties, 55 ; electrical properties, 56 ; magnetic 
properties, 57 ; taste and odour, 58 ; surface 
energy, 58 ; density, 60 

III. CHEMICAL COMPOSITION - - 64 

Polymorphism, 66 ; isomorphism, 67 ; pseudo- 
morphism, 67 ; classification by chemical com- 
position, 68 



819493 



vi MINERALOGY 

PART II 
DESCRIPTIVE MINERALOGY 

CHAPTER PAGE 

I. THE ROCK-FORMING MINERALS - 83 

Quartz group, 84 ; felspar group, 86 ; felspathoid 
group, 95 ; scapolite group, 98 ; mica group, 98 ; 
amphibole and pyroxene group, 101 ; olivine 
group, 106 ; chlorite group, 109 ; talc, kaolinite, 
etc., in; zeolite group, 112; contact minerals, 
H3 

II. THE ORES 118 

Ores of platinum, 120 ; ores of gold, 122 ; ores of 
mercury, 127 ; ores of copper, 129 ; ores of silver, 
142 ; ores of lead, 149 ; ores of zinc, 154 ; ores 
of nickel, 161 ; ores of cobalt, 165 ; ores of iron, 
168 ; ores of manganese, 183 ; ores of bismuth, 
antimony, and arsenic, 188 ; ores of vanadium, 
193 ; ores of tin, 193 ; ores of molybdenum, 199 ; 
ores of tungsten, 200 ; ores of uranium, 201 ; 
ores of other rare metals, 203 ; ores of aluminium, 
204 ; appendix veinstones or gangue minerals, 
206 

III. THE SALTS AND USEFUL MINERALS OTHER THAN ORES - 208 

Carbonates, 208; sulphates, 214; nitrates, 219; 
chlorides and fluorides, 220 ; phosphates, 224 ; 
berates, 225 ; appendix other useful minerals, 227 

IV. GEMS - 229 

Diamond, 230 ; corundum, 231 ; spinel, 232 ; beryl, 
233 ; garnet, 234 ; topaz, 235 ; tourmaline, 236 ; 
zircon, 237 ; sphene, 238 ; turquoise, 238 ; chryso- 
beryl, 239 ; peridot, 240 ; opal, 240 ; chalcedony, 
240 ; quartz, 240 ; felspar, 241 

INDEX ........ 242 



PREFACE TO THE FOURTH EDITION 

THE first edition of this introduction to the study 
of Mineralogy was published in 1892. Since then 
the book has been reprinted three times, but till 
now without revision. Starting with the idea of a 
simple revision, I have found it necessary to rewrite 
and to enlarge it ; but in doing this, I have been 
careful to retain the essential features of its original 
arrangement. 

As before, there are two parts, the first of which 
deals with the properties of minerals, and the second 
with the description of the more important species 
that either make up rocks, or occur as ores, as 
salts, or as gems. This subdivision, which is iden- 
tical with the one originally adopted, has been found 
convenient by students ; but, as in all attempts 
at the classification of natural products, it is not 
free from inconsistencies and overlaps. Thus, for 
example, calcite, which, it cannot be denied, is a rock- 
forming mineral, has been relegated to the Salts in 
company with the other carbonate of lime and those 

vii 



viii MINERALOGY 

of magnesia, strontia, and baryta namely, aragonite, 
dolomite, magnesite, strontianite, and witherite. Simi- 
larly apatite, which is an accessory constituent of many 
rocks, is placed in the phosphate division of the Salts ; 
and tourmaline, zircon, and garnet, which are also 
frequent accessory constituents of rocks, appear among 
the Gems. Again, pyrites, which is almost ubiquitous 
enough to be regarded as a rock-forming mineral, is 
placed with the Ores ; and even there it is difficult 
to find its best position, for, although it is chiefly mined 
for its sulphur content, its natural position is with the 
ores of iron. 

Such classificatory inconsistencies need not, however, 
occasion any real difficulty to the reader, since the 
description of any given mineral can be found by refer- 
ence to the index, where the number of the page, 
containing its descriptive paragraph, is distinguished 
by heavy type. 

In selecting mineral types for description, I have 
endeavoured to include only those that either play an 
important role in the economy of Nature, or are sought 
after by man for industrial purposes. Rare minerals, 
which have found no industrial application, although 
they may be of the greatest interest to the Crystal- 
lographer or to the Chemist, are, from the standpoint 
of the present work, mere lusus natures, and as such 
have been rigorously excluded. 

In the choice of a name, where there are several 



PREFACE ix 

synonyms for a mineral, I have been guided by the 
British Museum Index. For physical constants I have 
chiefly relied on Hintze's " Handbuch der Mineralogie," 
but have also consulted Rosenbusch and Wulfing's 
" Mikroskopische Physiographic," Weinschenk's " Ges- 
teinsbiidenden Mineralien," Brush and Penfield's " De- 
terminative Mineralogy," and Miers' " Mineralogy." 
The compilation of the paragraphs dealing with the 
distribution of the ores and other useful minerals has 
involved some research in the publications of the 
various mining institutions, in order to carry out the 
principle, adopted by me, of restricting the list to locali- 
ties of industrial importance. 

For kindly consenting to read the proofs I have to 
thank my friends W. Campbell Smith, of the British 
Museum, and R. H. Rastall, Fellow of Christ's College, 

Cambridge. 

F. H. H. 

SEDGWICK MUSEUM, 
CAMBRIDGE. 
1911. 



MINERALOGY 

PART I 

THE PROPERTIES OF MINERALS 

INTRODUCTION 

THE geological study of the visible portion of the 
earth's solid crust has established the fact that it is 
chiefly made up of material which is either of igneous 
origin, having consolidated from a molten state, or con- 
sists of sediments that have accumulated at the bottom 
of former seas, and now lie piled up one above the other 
in strata many thousands of feet thick. These rock 
masses, whether igneous or sedimentary, are made up 
of the homogeneous inorganic substances known as 
Minerals. Besides those of which rocks are composed, 
there is a great variety of other minerals, filling chinks 
and fissures in the earth's crust, and comprising, inter 
alia, the valuable ore deposits which are the source of 
our metals. 

It is the business of the mineralogist to study the 
form, characters, and physical and chemical properties 
of these different kinds of mineral matter ; and the 






PROPERTIES OF MINERALS 



facts thus elicited afford a means by which the different 
species of minerals may be classified and distinguished. 
In accordance with this principle, Part I. of this book 
is divided into three chapters, of which the first deals 
with the morphological characters of minerals, the 
second with their physical properties, and the third 
with their chemical composition. 



CHAPTER I 

MORPHOLOGICAL CHARACTERS 

IN studying the properties of minerals, one of the first 
things that strikes the observer is their frequent occur- 
rence in geometrical forms bounded by plane surfaces. 
In these bodies, which are termed crystals, the com- 
ponent particles of matter are arranged in accordance 
with certain fixed laws of symmetry ; and that this is so, 
is evidenced by the fact that the physical properties of 
a crystal are found to bear a definite relation to its 
external geometrical boundaries. 

Mineral matter is said to be crystalline when it exists 
as part of a crystal or as an aggregation of crystals. In 
some minerals, however, the aggregation of the mole- 
cules is subservient to no law of symmetry; and in 
these there is consequently no interdependence between 
physical structure and external form. This is known 
as the amorphous state of matter. 

It must be borne in mind, however, that a crystalline 
mineral need not always present a definite geometrical 
form. The environment of the crystal during its forma- 
tion may have militated against the assumption of 
geometrical contours; but the crystalline or non-crys- 

3 



4 ' PROPERTIES OF MINERALS 

talline nature of a mineral will nevertheless be always 
indicated by its physical structure. 

The form or habit of crystallized minerals is deter- 
mined by the extent to which certain of the bounding 
planes (faces) of the crystals are developed at the 
expense of others. Such habits are the following : 

Tabular (or platy, etc.) : barytes. 
Prismatic (or columnar) : stibnite, epidote. 
A cicular (or needle-shaped) : cerussite. 
Capillary (or hair-like) : millerite. 

When the mineral is amorphous or minutely crystalline, 
it assumes an external shape which is bounded by other 
than plane surfaces. Such shapes are designated by the 
following terms : 

Nodular : blende, malachite. 
Globular : blende, calcite, marcasite. 
Botryoidal (like a bunch of grapes) : dolomite, chal- 
cedony. 

Mammillated : psilomelane. 
Reniform (or kidnev -shaped) : haematite. 
Stalactitic (pendent) : calcite, aragonite, limonite. 
Dendritic (branched) : copper. 
Wiry : silver. 
Mossy : copper. 
Leafy : gold. 

The internal structure of minerals having the external 
forms enumerated above is designated by the following 
terms : 

Granular. When composed of grains or small irregu- 



MORPHOLOGICAL 5 

larly-shaped crystals in close juxtaposition : calcite (in 
marble), magnetite. 

Massive or Compact. When the outlines of the con- 
stituent grains are invisible : haematite. 

In addition, some minerals show an internal fibrous 
or concentric laminated arrangement; thus, a mammil- 
lated or globular external form may be associated with 
an internal fibrous structure, as in wavellite, pyrites, 
and haematite, or with a concentric laminated structure, 
as in malachite and haematite. 

The faces of crystals may be smooth, drusy, striated, 
or curved. When smooth they reflect clear images of 
distant objects, a property which is utilized in the 
measurement, by the reflecting goniometer, of the angle 
formed by two faces of a crystal.* Drusy faces are 
those which are roughened by the presence of minute 
projecting crystals. Striated faces are marked by a 
series of parallel lines which may be due to repeated 
(polysynthetic) twinning, as in albite (see p. 92), or to 
a rapid alternation (oscillation) of two sets of faces, as 
in pyrites. Curved faces are produced by rapid succes- 
sion of small, so-called vicinal faces, the angle between 
each pair being a little less than 180. Diamond, 
dolomite, and gypsum, are minerals which frequently 
have curved faces. 

Symmetry of Crystals. If a sufficiently large number 
of crystals be examined, it will be found that they 

* Note that the angles measured by means of the reflecting 
goniometer, and quoted in most books of mineralogy, are not the 
face-angles, but their supplements. 



6 PROPERTIES OF MINERALS 

show differences in the symmetrical arrangement of 
their faces. It will also be found that they exhibit 
various degrees of symmetry. Thus, a crystal may 
possess what is termed centrosymmetry i.e., it has 
a centre of symmetry, through which every line will 
meet the crystal at similar points at its two ends.* It 
may also possess one or more planes of symmetry. A 
plane of symmetry is a plane dividing a body into two 
parts, each of which is the exact but inverse counter- 
part of the other ; that is to say, the two parts bear to 
one another the same relation that an image 
bears to its object, the mirror being equiva- 
lent to the plane of symmetry (see Fig. i). 

Every face of a crystal possessing centro- 
symmetry must have a corresponding face 
parallel to it on the opposite side of the 
FlG i._ A centre of symmetry, and there must also be 
PLANE OF f aces corresponding to these on the opposite 

oYMMETRY. 

sides of planes of symmetry, and making 
equal angles with them respectively. A group of faces 
which are thus mutually connected by symmetry is 
technically described as &form. 

Crystals possess other kinds of symmetry besides 
those depending on centrosymmetry and planes of 
symmetry. Thus, the faces of a crystal may be sym- 
metrically distributed about an axis of symmetry i.e., 
each of its faces can be brought into the position of 
another similar face by rotation about such an axis. 

* All crystals except certain hemihedral and hemimorphic forms 
see p. 29) possess centrosymmetry. 




MORPHOLOGICAL 



According as the angle through which the crystal is 
rotated, to produce this result, is J, J, J, or J, of the 





a, 

a b 

FIG. 2. FORM WITH NINE PLANES OF SYMMETRY. 

Three are shown in a, and six in b. 

total rotation to the original position, so the axis of 
symmetry is said to be binary, ternary, quaternary, or 
senary (or, according to some authors, digonal, trigonal, 




FIG. 3. HEXAGONAL FORM, WITH SEVEN PLANES OF SYMMETRY. 
VIEW (a) AND PLAN (&). 

Six are shown in &, and the seventh is the plane of projection. 

tetragonal, hexagonal). A plane of symmetry is always 
parallel to a possible face of the crystal ; an axis of 
symmetry is always parallel to a possible edge. 



8 PROPERTIES OF MINERALS 

For convenience in mathematical treatment, crystal 
forms are referred to fixed lines which are chosen, 
when possible, from the intersections of their planes of 
symmetry, and are known as crystal axes. Axes of 
symmetry are not necessarily crystal axes, but an axis 
of symmetry either coincides with a crystal axis, or 
bisects the angle between two equal crystal axes, or is 
equally inclined to all the crystal axes. 

The Law of Symmetry and of the Constant Angle. 
The fundamental principle which underlies crystal- 
lography may be formulated as follows : Every crystal is 
enclosed by plane faces, and is subservient to the law of 
symmetry, which requires that similar parts of a crystal 
shall be similarly modified. For example, if one corner 
of a cube is replaced by a face, the law of symmetry 
requires that all eight corners shall be so replaced. 
The resulting eight faces of the octahedron comprise a 
simple form, as do also the original six faces of the 
cube ; the compound form in which both sets of faces 
are present is termed a combination. 

It is also to be observed that the angle between similar 
pairs of faces of a crystal of a given substance is con- 
stant. Thus, the angle between two adjacent faces of the 
octahedron measures always 109 28' 16" ; while the cor- 
responding angle between two adjacent rhombic pyra- 
mid faces (in : ill) of sulphur, for instance, measures 
always 106 26'.* This is the law of the constant angle. 

* See footnote on p. 5. The angle (in : 111) for sulphur, 
as quoted in the textbooks, is 73 34'. The face - angle is the 
supplement of this viz., 180 73 34' =106 26'. 



MORPHOLOGICAL 9 

Zones. However complicated a crystal may appear 
to be, it will be found that the combination may 
generally be reduced to a few sets of faces, which, 
if produced, would intersect in edges parallel to the 
same straight line. A set of faces, therefore, which 
are parallel to the same line (the zone-axis) is termed 
a zone. The zone -axis is supposed to be drawn 
through the centre of the crystal at the intersection of 
the crystal axes. In the measurement of crystals by 
the reflecting goniometer, zones are of the greatest 
service. Once the crystal is set up so that a zone-axis 
is parallel with the axis of the instrument, the angles 
between all the faces belonging to that zone can be 
measured during a single rotation of the graduated 
circle. The reading of the scale is noted when the 
reflected image (the " signal ") from each face is suc- 
cessively in adjustment, and the angle between any two 
faces is the difference in their readings. 

Classification of Crystals. By considering crystals 
in the light of the three elements of symmetry described 
above, it is found that there are thirty-two possible 
cases, to which thirty-two classes of crystals correspond. 
They may, however, be conveniently grouped, by means 
of their planes of symmetry, into six systems. 

The greatest number of planes of symmetry possible 
in a crystal is nine; all forms possessing these belong 
to the REGULAR or CUBIC system.* Three of these 
planes are perpendicular to one another, and six bisect 
the angles between each pair of the first (see Fig. 2). 

* For the less symmetrical forms of the cubic system see p. 29. 



10 



PROPERTIES OF MINERALS 



The remaining systems are the following: The 
HEXAGONAL, with seven planes of symmetry, six of 




FIG. 4. TETRAGONAL FORM, WITH FIVE PLANES OF SYMMETRY. 
VIEW (a) AND PLAN (6). 

Four are shown in b, and the fifth is the plane of projection. 

which intersect in one straight line, and are inclined to 
one another at an angle of 30, the seventh being per- 
pendicular to the other six (see Fig. 3). 






FIG. 5 RHOMBIC FORMS, WITH THREE PLANES OF SYMMETRY. 

a and b are the view and plan respectively of the same crystal, and c is 
another rhombic combination showing three planes of symmetry. 

The TETRAGONAL, with five planes, four of which 
intersect in one straight line, and are inclined to one 



MORPHOLOGICAL 



11 



another at an angle of 45, the fifth being perpendicular 
to the other four (see Fig. 4). 

The RHOMBIC, with three planes of symmetry perpen- 
dicular to one another (see Fig. 5). 

The MONOCLINIC, with one plane of symmetry (see 
Fig. 6). 

The TRICLINIC, ANORTHIC, or ASYMMETRIC, with no 
planes of symmetry (see Fig. 7). 

The rhombohedral forms are considered by some to 
constitute a distinct system (the trigonal or rhombo- 





FIG. 6. MONOCLINIC FORM, FIG. 7. TRICLINIC CRYSTAL WITH NO 



WITH ONE PLANE 
OF SYMMETRY. 



PLANE OF SYMMETRY, BUT POS- 
SESSING CENTROSYMMETRY. 



hedral system), with three planes of symmetry, intersect- 
ing in one straight line, and inclined to one another at 
an angle of 60. They are often treated, however, as 
hemihedral derivatives of the hexagonal pyramids (see 
P. 30). 

The Regular or Cubic System of Crystals. This 
system comprises the following holohedral forms, which 
may occur alone or in combination : octahedron, cube, 
rhombic dodecahedron, icosi-tetrahedron, triakis-octa- 
hedron, tetrakis-hexahedron, and hexakis-octahedron. 
(For hemihedral forms see p. 29.) 






12 PROPERTIES OF MINERALS 

For the purpose of comparison those forms are re- 
ferred to three equal crystal axes standing at right 
angles to one another (and at right angles to the three 
principal planes of symmetry). Each crystal axis is a 
quaternary axis of symmetry. There are further four 
a ternary axes of symmetry, 

j equally inclined to them, and 

; six binary axes of symmetry, 

bisecting the angles between 

a them. 

I The octahedron is the simplest 

form of the regular system. It 
is composed of eight faces, each 
of which is an equilateral tri- 
angle, and cuts the three axes at unit distance. The face- 
angle between any two adjacent faces measured across 
an edge is 109 28' 16" (see Fig. 9). 

The cube is another simple and very common form of 
this system. It is contained by six 
square faces, each of which cuts one 
axis at right angles, and runs parallel 
to the other two, thus coinciding in 
direction with the principal planes 

of symmetry. The angle between 

c . FIG. 9. OCTAHEDRON. 

two faces measures 90 (see Fig. 10). 

The rhombic dodecahedron is contained by twelve equal 
rhombs (the diagonals of which bear to one another 
the ratio of i : V 2 )- Each face cuts two axes at unit 
distance, and is parallel to the third. The remaining 
six planes of symmetry of the regular system coincide 




MORPHOLOGICAL 18 

in direction with the faces of this form. The angle be- 
tween two faces which meet in an edge measures 120. 
The icosi-tetrahedron is a form contained by twenty- 
four deltoids (a plane quadrilateral figure bounded by 
pairs of adjacent equal straight lines). Each face cuts 




FIG. io. CUBE. FIG. n. DODECAHEDRON. 

two axes at unit distance, the third at a distance 
measured by a rational quantity, m, less than unity. 
The series is limited on the one hand by the cube, on 
the other by the octahedron (see Fig. 12). 

The triakis-octahedron, or three-faced octahedron, is a 





FIG. 12. ICOSI-TETRAHEDRON. FIG. 13. TRIAKIS-OCTAHEDRON. 

form contained by twenty-four isosceles triangles. Each 
face cuts two axes at unit distance, the third at a 
distance equal to a rational quantity, m, greater than 
unity. The series is limited on the one hand by the 
octahedron, and on the other by the rhombic dodecahe- 



14 



PROPERTIES OF MINERALS 



dron. The form is also known as the pyramidal 
octahedron ; this designation has reference to the shape, 
which is that of an octahedron with a three-faced 
pyramid on each face (see Fig. 13). 

The tetrakis-hexahedron, or four-faced cube, is a form 
contained by twenty-four isosceles triangles. Each face 
cuts one axis at unit distance, one at a distance equal 
to a rational quantity, m, and is parallel to the third. 
The series is limited by the cube and the rhombic 
dodecahedron. The form is also termed the pyramidal 
cube, from the fact that it can be described as a cube 





FIG. 14. TETRAKIS-HEXAHEDRON. FIG. 15. HEXAKIS-OCTAHEDRON. 

each face of which is surmounted by a four-faced 
pyramid (see Fig. 14). 

The hexakis-octahedron, or six-faced octahedron, is a 
form contained by forty-eight scalene triangles. Each 
face cuts one axis at unit distance, another at a distance 
equal to a rational quantity, m, and the third at a 
distance equal to a rational quantity, n. By the varia- 
tion of the values for m and n* the hexakis-octahedron 

* These letters, m and n, are used to denote the value of the 
intercept, or distance cut off on the axis by the face, reckoning 
from the point of origin. They represent any rational number, 
such as i, 2, 3, or , , , etc. 



MORPHOLOGICAL 



15 



may be made to approximate successively to all the 
simpler forms of the system. It is therefore the most 
general of all the forms of the regular system (see 




FIG. 16. COMBINATIONS. 
a, Cube ; o, octahedron ; d, dodecahedron ; e, icosi-tetrahedron. 

Fig. 15). For examples of the various combinations of 
simple forms, see Fig. 16. 

The Hexagonal System.* The forms belonging 
to the system are referred to four axes, three of which 
(a) are equal and similar, and cross c 

one another at an angle of 60 ; 
while the fourth (the principal 
axis, c) is unequal and dissimilar 
to the other three, and stands at <f.; 
right angles to the plane formed 
by them (see Fig. 18). The ratio 
of a to c is an irrational number, 
to be determined for each par- 
ticular mineral. It represents the 
relative lengths of the intercepts of the pyramid 
chosen for the fundamental form. Thus, in quartz the 
ratio a : c = 1*0999. The principal axis coincides with 
* So called on account of the horizontal section being a hexagon. 



FIG. 17. 



16 



PROPERTIES OF MINERALS 



a senary axis of symmetry, and there are six binary 
axes of symmetry perpendicular to it. 

The most important forms are the pyramids, the 
prisms, and the basal plane. The pyramids are obtuse 
or acute, being limited on the one hand by the basal 
plane, on the other by the prism. The proto -pyramids, 
or pyramids of the first order, are a series of forms con- 
tained by twelve isosceles triangles, each of which cuts 
two of the lateral axes at the unit distance, is parallel 
to the third, and cuts the vertical axis at a distance 
equal to c multiplied by a rational quantity, m (see 
Fig. 18). When m is infinitely small (i.e., equals o), we 




FIG. 18. PROTO-PYRAMID. 



FIG. 19. PROTO-PRISM. 



have the basal plane; when m becomes infinitely large 
(i.e., equals oc), we get the proto-prism. The basal plane 
is thus parallel to the lateral axes, and at right angles to 
the vertical. The proto-prism is a form composed of 
six faces parallel to one lateral axis, and to the vertical. 
Both forms can only occur in combination, and are 
often associated with one another (see Fig. 19). 

The deutero-pyramids, or pyramids of the second order, 
are a series of forms each of which is contained by 
twelve isosceles triangles, cutting one of the lateral axes 
at the unit distance, the remaining two at a distance 



MORPHOLOGICAL 



17 



equal to twice the unit length, and the vertical axis at a 
distance equal to c multiplied by a rational quantity, m. 
This series is also limited by the basal plane and the 
deutero-prism. 

The dihexagonal pyramids are a series of pyramids 
contained by twenty-four scalene triangles. Each face 
cuts one lateral axis at the unit distance, and another at 
a distance equal to a multiplied by a rational quantity, n, 
which must be less than 2 ; while the vertical axis is 
intercepted at a distance represented by me. There is 





FIG. 20. COMPOUND FORMS. 
c, Basal plane ; o, pyramid ; a, prism ; r, rhombohedron (see p. 29). 

a corresponding dihexagonal prism, in which the inter- 
cept on the vertical axis is infinitely great i.e., its faces 
are parallel to this axis. For examples of the combina- 
tions of simple forms, see Fig. 20. 

The Tetragonal System.* The forms of this 
system are referred to three axes at right angles to one 
another, two of which (a) are equal, the third (the 
vertical or principal axis, c) unequal (see Fig. 21). 
The principal axis (c) coincides with a quaternary 

* So called on account of the horizontal section being a tetragon 
or square. 

2 



18 



PROPERTIES OF MINERALS 



axis of symmetry, and the two remaining axes with 
binary axes of symmetry. Each pair of crystal axes 
lies in a plane of symmetry. The two remaining 
c planes of symmetry bisect the angle 

I between the two equal crystal axes 

(a). As in the Hexagonal system, 
we have pyramids and prisms of two 
orders (a proto and a deutero series) ; 
but in this system the pyramids 
consist of eight faces, and the prisms 
of four, instead of twelve and six 
respectively. There is also a ditetra- 
gonal pyramid, consisting of sixteen 
faces. The basal plane is parallel 
to the lateral axes, as in the Hexa- 
The proto-pyramids cut the lateral 
axes at the unit distance, and the vertical at a distance 
equal to me. When m equals infinity, the form becomes 



-^ a 

ft'' 



FIG. 21. 



gonal system, 




FIG. 22. PROTO- 
PYRAMID. 




FIG. 23. DEUTERO- 
PYRAMID. 



FIG. 24. DEUTERO- 
PRISM. 



the proto-prism. The deutero-pyramid cuts one lateral 
axis at unit distance, and is parallel to the other ; while 
the vertical is cut at a distance equal to me : by in- 



MORPHOLOGICAL 19 

creasing the value of m we approximate to the deutero- 
prism. 

For examples of the combinations of simple forms, 
see Fig. 25. 





FIG. 25. TETRAGONAL COMBINATIONS. 

s and p, Proto-pyramids ; e, deutero-pyramid ; m, proto-prism ; 
a, deutero-prism. 

The Orthorhombic or Rhombic System. The 

forms of this system are referred to three unequal 
and dissimilar axes, which stand at right angles to 
one another. One of these (c) having been chosen 
for the vertical, the shorter of the two laterals (the 
brachy-axis or br achy -diagonal, a) is directed towards 
the observer, while the larger (the macro-axis or macro- 
diagonal, b) is then transverse. Each crystal axis 
coincides with a binary axis of symmetry, and each pair 
lies in a plane of symmetry. 

The forms comprise pyramids, prisms, domes, and 
pinacoids, of which the last three can only be seen in 
combination. 

Starting from a fundamental pyramid which cuts the 
three axes at the unit distance, we derive various pyra- 
mids, according as we increase or diminish the distance 
cut off on the vertical, the macro- and the brachy-axes. 
Thus, besides the fundamental series, we get two series 



20 PROPERTIES OF MINERALS 

of pyramids, a brachy series and a macro series, each 
series being limited by the corresponding prism, brachy- 
or macro-prism, as the case may be. 

It must be borne in mind that the unit of measure- 
ment is different for each axis in the rhombic and suc- 
ceeding systems. The ratio of the intercepts of the 
fundamental form, which is chosen for each particular 
mineral, is a : b : c, or, if b be taken as unit, a : i : c ; a 
and c being irrational. Thus the ratio of the intercepts of 
the pyramid, chosen as the fundamental form for sulphur, 
is 0-8130 : I : 1*9037; or a : b : c = 0*8130 : I : 1*9037. 

c 

I 




\ 

i 
FIG. 26. FIG. 27. A RHOMBIC PYRAMID. 

These axial lengths are known as parameters, and the 
form used to determine them is termed the parametral 
form. The intercepts made on the crystal axes by any 
face of a crystal must be such that they can be ex- 
pressed as rational multiples of the parameters. This 
is known as the law of rational intercepts. 

The domes* are forms comparable to the prisms, but 
differing from them in being parallel to a lateral instead 
of to a vertical axis. Like the prisms, they may be 

* So called on account of their giving a roof-like termination to 
crystals. 



MORPHOLOGICAL 



21 



regarded as special cases of the pyramids i.e., as 
pyramidal faces that cut one of the lateral axes at an 
infinite distance ; they are, in fact, the lateral limiting 
forms of the brachy and macro series of pyramids 
respectively, just as the prisms are the limiting 
forms of the pyramids in a vertical direction. The 
brachy- dome cuts the macro-axis and the vertical axis, 
and runs parallel to the brachy-axis ; the macro-dome, 





FIG. 28. 
COMBINATION. 

P, basal plane ; 
z, pyramid ; 
k, macro-pinacoid ; 
M, prism ; 
d, brachy-dome ; 
o, macro-dome. 



FIG. 29. 
COMBINATION. 

m, macro-dome ; 
b, brachy-pinacoid. 



FIG. 30. 
COMBINATION. 

c, basal plane ; 

b, brachy-pina- 
coid; 

a, macro-pina- 
coid. 



on the other hand, cuts the brachy-axis and the vertical 
axis, and runs parallel to the macro-axis. 

The pinacoids are faces which cut one axis perpen- 
dicularly, and are parallel to the other two. In this 
sense the basal plane may be regarded as a pinacoid. 
The true pinacoids are two in number namely, the 
brachy-pinacoid, parallel to the vertical and the brachy- 
axis, and the macro-pinacoid, parallel to the vertical and 
the macro-axis. For compound forms, see Figs. 28, 29, 
and 30. 



22 PROPERTIES OF MINERALS 

The Monoclinic System. The forms of this 
system are referred to three unequal and dissimilar 
axes; of these, one is at right angles to the other 
two, which cut one another obliquely. The crystals 
are conventionally so placed that the observer looks 
into the obtuse angle (fi) formed by the inclined axis 
with the vertical. The inclined axis is termed the 
clino-axis (or clino -diagonal, a) ; while the perpendicular 
axis is known as the ortho-axis (or ortho-diagonal, b). 




FIG. 32. MONOCLINIC COMBINATION 
OF 

c, Basal plane; b, clino-pinacoid ; a, ortho- 
pinacoid ; /3 is the angle between the 
FIG. 31. axes a and c ; R, R, are right angles. 

The one plane of symmetry of this system contains the 
vertical axis and the inclined axis. It is represented as 
a crystal face by the clino-pinacoid (see Fig. 32). The 
crystal axis b is a binary axis of symmetry. In conse- 
quence of there being only one plane of symmetry, a 
pyramid, or more correctly hemi-pyramid, in this system 
consists of four faces instead of eight : two at the top, 
front ( ) or back ( + ), and two at the bottom, back ( ) 
or front ( + ) 

As in the rhombic system, the forms comprise 



MORPHOLOGICAL 



pyramids, prisms, domes, and pinacoids. These are 
classified according to their relation to the axes. Thus, 
besides the fundamental pyramid with unit values for 
the lateral axes, and those derived from it by the 





FIG. 33. MONOCLINIC COM- 
BINATION OF 

, Pyramid ; m, prism ; b, clino- 
pinacoid. 



FIG. 34. COMBINATION OF 

P, Basal plane ; n, clino-dome ; 
I, prism ; M, clino-pinacoid ; 
x andjv, ortho-domes. 



variation of the intercept on the vertical axis (the lateral 
axes being in this series cut at unit distance), there 
is an ortho- and a clino-series of pyramids, an ortho- 
and a clino-series of prisms and domes, and the two 
pinacoids the ortho-pinacoid and the clino-pinacoid. 
The terminal or basal plane is c 

parallel to the two lateral axes | 

(see Fig. 34). j / 

The Triclinic or Anorthic 
System. The forms of this sys- 
tem are referred to three un- 
equal and dissimilar axes which 
intersect obliquely. These axes 
are placed so that one axis is 
vertical (c), and the lateral axes 

are inclined from back to front (a), and from left to 
right (c}. 



? 



FIG. 35. 



24 



PROPERTIES OF MINERALS 



There is no plane nor axis of symmetry, and the 
simple forms consist only of the two parallel faces 
required by the law of centro-symmetry. Being open 
forms, they can only occur in combination. 

Here again the forms include pyramids, prisms, 
domes, and pinacoids, constituting macro- and brachy- 
series, as in the Rhombic system. Fig. 37 represents a 





FIG. 36. TRICLINIC COMBINATION 
OF 

c , Basal plane ; b, brachy-pinacoid ; 
a, macro-pinacoid. 

The angles between the common 
edges of these faces are equal to those 
between the axes, namely, a, p, and y. 



FIG. 37. TRICLINIC 
COMBINATION. 

P, Basal plane ; o, pyramid 
T and I, prisms ; M, brachy- 
pinacoid ; x, macro-dome. 



combination, consisting of basal plane (P), brachy-pina- 
coid (M), two prisms (T and /), a macro-dome (x), and a 
pyramid (o). 

SYMBOLS. 

The use of symbols to designate the forms and faces 
of crystals is so general, that a few words on the subject 
will not be inappropriate here. There are two principal 
methods in vogue namely, those of Naumann and 
Miller. Naumann's system was once popular on account 
of simplicity; but the Millerian system is better adapted 
for mathematical treatment, and has been adopted 
almost universally. In the former system (Naumann's) 



MORPHOLOGICAL 25 

the fundamental form is designated by an initial letter ; 
thus, O stands for the octahedron of the regular system, 
and P for the fundamental pyramid of the remaining 
systems. Other forms are represented by the addition 
to these letters of coefficients, including the sign for 
infinity ( oo ). When placed before the initial letter 
(e.g. } wP), these coefficients indicate what multiple of 
the intercept of the fundamental form is cut off by the 
faces of the form in question on the vertical axis ; but 
when they follow the letter, they refer to one of the 
lateral axes (e.g., Pm). The length of the intercept on 
one of the axes being always reduced to the unit value, 
it is only necessary to give the intercepts on two axes. 
Thus, mPn indicates a form that cuts off intercepts 
equal to me on the vertical and na or nb on one lateral, 
the intercept on the third axis being the unit distance. 

In the rhombic system, where it is necessary to 
distinguish between the long and short lateral axes, 
the signs - and ^ are used for this purpose by placing 
them above the initial letter. Thus Pm indicates a 
form that cuts off mb on the macro-axis, the intercepts 
on the two other axes being unity ; while Pm is a 
form with an intercept of ma on the brachy-axis. 

In the monoclinic system, again, we have to dis- 
tinguish between a perpendicular and an inclined axis. 
This is done by placing a bar straight or obliquely 
through the initial letter. Thus Pm indicates that m 
refers to the ortho-axis, while in Pm it refers to the 
clino-axis. 

In the Millerian system the symbol is composed of 



26 



PROPERTIES OF MINERALS 



the " indices " of the face. These are obtained by con- 
verting the multiples of the three fundamental inter- 
cepts (or parameters), for a given face, into fractions, of 
which the numerator is in each case I : the denominators 
of these fractions are then the "indices." Thus, a face 
of which the ratio for the intercepts is la : zb : 30, will 
have indices represented by the denominators of the 
fractions J, J, and J; and 632 will be its Millerian 
symbol. The symbol for the form of which (632) is one 
face is {632} . Naumann's symbol for this form would 
be 3?2 in the rhombic system. 

The symbols of some of the principal forms, according 
to both systems, are given in the following table. The 
first column shows the multiples of the fundamental 
intercepts on the three axes. 

REGULAR SYSTEM. 



Name of Form. 


Multiples 
of 


Naumann. 


Miller. 




a a a 






Octahedron ... 
Cube 
Rhombic dodecahe- 


III 

I oo oo 




00 O 00 


(Ill) 
(100) 


dron 
Icosi-tetrahedron 
Hexakis-octahedron 


I I 00 

i m m 

i n m 


ooO 
m O m 
m O n 


flip) 
(III)* 

(hkl) 



* The letters h, k, and /, are used in the Millerian symbols to 
represent any whole numbers, just as the coefficients m and n repre- 
sent any rational numbers in Naumann's system. The relation 

between hkl and m n may be expressed thus : ;// : n =- : -r. 

/ K 



MORPHOLOGICAL 
HEXAGONAL SYSTEM. 



27 





Multiples 








Name of Form. 


of P 


Naumann. 


Miller.* 


Bravais. 




a a a c 












\ 


Basal plane ... 
Pyramids 


oo oo oo I 
i oo i m 


oP 

Iff p 


(III) 
(hkk,eff) 


(oooi) 
(popq) 


Prism... 


i oo i co 


00 P 


(12!) 


(1010) 


Rhombohedron 




R 


(hi I) 





TETRAGONAL SYSTEM. 



Name of Form. 


Multiples of 

a a c 


Naumann. 


Miller. 


Basal plane 


oo oo I 


oP 


(ooi) 


Pyramids 
Deutero-pyramids 


i i m 

i oo m 


mP 

m P oo 


p? 


Prism 


I I CO 


oo P 


(no) 


Deutero-prism ... 


I 00 00 


oo P oo 


(100) 



RHOMBIC SYSTEM. 



Name of Form. 


Multiples 
a b 


of 

c 


Naumann. 


Miller. 


Basal plane ; . . 


00 00 


I 


oP 


(001) 


Pyramids 


I I 


m 


w P 


(hhl) 


Brachy-pyramids 


n i 


m 


m P n 


(hkl)h<k 


Macro-pyramids 


i n 


m 


m P n 


(h k I) h>k 


Prism 


i i 


CO 


oo^P 


(no) 


Brachy-domes . . . 


CO I 


m 


m P oo 


(ok I) 


Macro-domes ... i oo 


m 


m P co 


(hoi) 


Brachy-pinacoid oo i 


00 


CO P 00 


(oio) 


Macro-pinacoid ... 


I CO 


oo 


OO P CO 


(100) 



* In Miller's notation the three edges of a rhombohedron (p. 30) 
are taken as axes. Bravais' notation refers to four axes, three of 
which lie in one plane at 120 to each other, and have equal para- 
meters (<z), the fourth, with parameter c, being the vertical axis 
(see Fig. 17). 



28 



PROPERTIES OF MINERALS 
MONOCLINIC SYSTEM. 





Multiples 






Name of Form. 


of 


Naumann. 


Miller. 




a b c 






Basal plane ... 


oo oo i 


P 


(00 1) 


Pyramids 


i i m 


m P (h h ft 


Clino-pyramids 


nim 


m P n 


(hkl)k<k 


Ortho-pyramids 


i n m 


m P n 


(h k I) h > k 


Prism 


i i oo 


00 P 


(no) 


Clino-domes ... 


oo i m 


m P oo 


(0*5 


Ortho-domes 


i oo m 


m P oo 


(k o /) 


Clino-pinacoid 


oo I oo 


oo P oo 


(oio) 


Ortho-pinacoid 


I oo oo 


oo P oo 


(100) 



MODIFICATIONS IN THE SYMMETRY OF CRYSTALS. 

The symmetry of the forms of the different systems 
is subject to certain modifications, resulting in hemi- 





FIG. 38. OCTAHEDRON AND TETRA- 
HEDRON. 

The suppression of the shaded faces 
in the former gives rise to the 
latter form. 



FIG. 39. TETRA- 
HEDRON. 

With its corners trun- 
cated by a second 
tetrahedron. 



hedrism, hemimorphism, and twinning. In the first 
two cases the degree of symmetry is diminished ; in the 
last it is often increased. 



MORPHOLOGICAL 29 

Hemihedrism. Hemihedral forms are those which 
possess only half the full number of faces required by 
the symmetry of the system to which they belong. 
Such forms can best be understood by reference to the 





FIG. 40. PENTAGONAL DODECA- 
HEDRON. 

Derived from the tetrakis-hexahedron by 
the suppression of the shaded faces. 



FIG. 41. PENTAGONAL 
DODECAHEDRON (Pd). 

In combination with the 
cube c. 



full-faced (holohedral) form. The suppression of alter- 
nate faces, or groups of faces, in the holohedral form 
gives rise to a hemihedral derivative of that form. Thus, 
the hemihedral derivative of the octahedron is the 




FIG. 42. Two KINDS OF RHOMBOHEDRA. 

Derived by the suppression of alternate faces of the 
hexagonal pyramid. 

tetrahedron, composed of four equilateral triangles, as 
exemplified in an important silver -copper ore (tetra- 
hedrite). Another example in the regular system is 
the pentagonal dodecahedron, which may be derived from 



30 



PROPERTIES OF MINERALS 



the corresponding tetrakis-hexahedron, or six-faced 
cube, by the suppression of alternate faces. 

Fig. 41 shows a combination of the pentagonal 
dodecahedron with the cube, often found in iron 
pyrites. 

The hexagonal pyramid is also subject to hemi- 
hedrism, giving rise to the rhombohedron, a six-faced 
form characteristic of the minerals calcite, dolomite, 
spathic iron ore (see Fig. 42). The dihexagonal 
pyramid gives rise similarly to the scalenohedron (see 





FIG. 43. SCALENOHEDRON. 



FIG. 44. HEMIMORPHITE. 



Fig. 43), and the tetragonal pyramid to the sphenoid. 
The rhombohedral and scalenohedral forms are charac- 
terized by the ternary axis of symmetry coincident with 
the principal crystal axis and three binary axes of 
symmetry. 

Hemimorphism. Hemimorphism is a property 
possessed by certain crystals of presenting different 
forms at the opposite ends of an axis of symmetry 
(generally the vertical axis), instead of being terminated 
similarly at both ends as in normal crystals. The phe- 
nomenon of hemimorphism is closely connected with 



MORPHOLOGICAL 31 

that of pyro-electricity, since those minerals which are 
hemimorphic acquire positive and negative electricity 
at the ends or poles of the hemimorphic axis when 
warmed or cooled e.g., tourmaline and hemimorphite 
(silicate of zinc). 

Twinning". A twin is, as its name implies, a double 
crystal. The examination of a twin shows that it con- 
sists of two individual crystals, or two parts of one and 
the same individual, united on a common plane, or pene- 
trating one another symmetrically. To explain this phe- 





FIG. 45. TWINNED CRYSTAL. 

Produced by the rotation of one-half through an angle 
of 1 80 round the axis, 1 1'. 

nomenon, it is necessary to refer the two individuals to 
a plane the twinning plane with regard to which both 
are symmetrically disposed.* Now, it is found that the 
two twinned individuals can be brought into a position 
of complete parallelism if one of them be rotated 
through an angle of 180 on an axis (the twin axis t t' 
in Fig. 45) perpendicular to the twinning plane. The 
plane which unites the two individuals (the plane of 

* Note that the twinning plane can never be a plane of symmetry 
of the individual crystal if it is holohedral; on the other hand, it is 
always a possible face of the crystal. 



32 



PROPERTIES OF MINERALS 



composition] is often, but not necessarily, coincident 
with the plane of twinning. In some cases the two 
individuals completely penetrate one another, as in 
Figs. 46 and 47. These types can also be explained 





FIG. 46. CUBES TWINNED BY 
INTERPENETRATION . 



FIG. 47. Two MONOCLINIC 

CRYSTALS TWINNED BY 

INTERPENETRATION. 



by a hypothetical rotation of one individual about a 
twin axis. 

One of the most characteristic features of twinned 
crystals is the presence of re-entrant angles, whereas 





FIG. 48. GENICULATED TWIN 
OF TINSTONE. 



FIG. 49. CROSS-SHAPED TWINS 
OF STAUROLITE. 



the angles of simple crystals are always salient. Twin- 
ning frequently results in the production of knee- 
shaped (geniculated), arrow-headed, cross- and heart- 
shaped forms, as in the minerals tinstone, staurolite, 



MORPHOLOGICAL 



33 



and gypsum (Figs. 48, 49, and 50). Sometimes the 
same type of twinning is repeated in one and the same 
crystal, producing what is termed poly synthetic or lamellar 
twinning, the lamellae occasionally being so frequently 
repeated as to produce the effect of a mere striation, as 



FIG. 50. ARROW-HEADED TWIN OF GYPSUM. 

The dotted part shows the position of the rotated hah 
in the untwinned crystal. 

in plagioclase felspar. In some cases the compound 
crystal has a higher degree of symmetry than the 
untwinned crystals ; thus, rhombic and monoclinic 
minerals when twinned occasionally acquire pseudo- 
hexagonal symmetry e.g., aragonite and tridymite. 



CHAPTER II 

THE PHYSICAL PROPERTIES OF MINERALS 

ON page 2 it was stated that the physical properties of 
crystals bear a definite relation to their geometrical 
form. What is this relation ? Experiment has shown 
that the physical properties of crystalline bodies vary 
in a manner dependent on their direction in space, and 
that these directions are connected with the symmetry 
of the crystal, or, in other words, with the symmetrical 
arrangement of its molecules. Thus, while a sphere of 
glass when warmed remains a sphere, a sphere of sulphur 
becomes an ellipsoid, the axes of which are related to 
the axes of the crystal from which the sphere was cut. 
The optical properties of crystals are similarly related 
to their symmetry. These physical properties of a 
crystal are termed vector in contradistinction to the 
scalar properties, which are independent of direction. 
The vector properties are related to an internal structure 
which may be graphically represented by an ellipsoid. 
For crystals belonging to the hexagonal and tetragonal 
systems the ellipsoid is a spheroid i.e., it is one pro- 
duced by the rotation of an ellipse about one of its 
axes. In the regular or cubic system, the spheroid 

34 



PHYSICAL 35 

is a sphere, the generating ellipse being in this 
case a circle. In crystals belonging to the rhombic, 
monoclinic, and triclinic systems, the ellipsoid has 
three unequal axes. Such an ellipsoid is produced by 
the rotation of an ellipse about one of its axes, while 
the other axis is being lengthened or shortened in such 
a manner that its ends describe ellipses instead of 
circles. The triaxial ellipsoid has three planes of sym- 
metry, which in the rhombic system are coincident 
with the three planes of crystal symmetry. In the 
monoclinic system only one of these planes is coinci- 
dent with a plane of crystal symmetry, while in the 
triclinic system, since there is no plane of crystal 
symmetry, the ellipsoid is independent of the crystal 
form. 

Amorphous bodies, on the other hand, only possess 
scalar properties. 

In the present chapter the physical properties of 
minerals are considered under the following heads : 
MOLECULAR COHESION, AS EXEMPLIFIED BY CLEAVAGE, 
FRACTURE, TENACITY AND HARDNESS ; PHENOMENA 
RELATING TO LlGHT ; CHARACTERS DEPENDING ON 
THE ACTION OF HEAT, ELECTRICITY, AND MAGNETISM ; 
TASTE AND ODOUR; SURFACE ENERGY; DENSITY. 

MOLECULAR COHESION. 

Cleavage. A piece of Iceland spar, when tapped 
with a light hammer, splits into a number of rhombo- 
hedral bodies of variable size but of similar form. A 



36 PROPERTIES OF MINERALS 

piece of rock-salt, similarly treated, splits into cubes. 
Mica, on the other hand, splits into thin laminae. This 
property of crystals of separating along certain planes, 
related to the symmetry of the system to which the par- 
ticular crystal belongs, is termed cleavage. It is one of 
the most striking of the physical properties of crystals, 
and one that gives important evidence as to the mode of 
aggregation of the crystal particles : for the separation 
takes place in the direction of least cohesion i.e., per- 
pendicularly to that in which the particles exert their 
greatest attraction.* Planes of minimum cohesion are 
repeated over planes of symmetry and about axes of 
symmetry. 

Cleavage may be considered with regard to (i) quality 
or degree ; (2) direction. 

i. Quality or Degree. According to the character of 
the surface produced by splitting a crystallized mineral 
along its cleavage planes, we may distinguish between 
imperfect and perfect cleavage. A mineral possessing 
only an imperfect cleavage will, when broken, present 
a rough surface in which the planes of separation are 
frequently interrupted by cross-fractures and continued 
on a different level. The complete parallelism of the 
separated portions of the cleavage planes can be proved, 
however, by their being simultaneously illuminated 
when moved to and fro in a beam of light in such a 

* The cleavage of minerals is not to be confounded with that of 
rocks, which is a fissile structure due to the arrangement of the 
minerals constituting the rock, and in no sense a molecular 
structure. 



PHYSICAL 87 

manner as to reflect the rays towards the eye of the 
observer. A mineral with perfect cleavage will split 
along faces which in most cases are more smooth and 
even than the original faces of the crystal themselves. 

2. Direction. The planes of cleavage are always 
parallel to possible faces of the crystal ; and the number 
of intersecting planes varies with the number of faces 
which make up the form in question. Thus, a mineral 
with octahedral cleavage will split in four different 
directions ; with cubical, in three ; with prismatic, in 
two (except in the triclinic system) ; with pinacoidal, 
or basal, in one. Some minerals, however, possess 
cleavages parallel to the faces of more than one form, 
and these can usually be distinguished by their different 
quality, or degree of perfection. 

The following are examples of cleavage : 

Cubical: rock-salt, galena. 

Octahedral: fluorspar, diamond. 

Rhombic dodecahedral : zinc-blende. 

Pyramidal: sulphur. 

Prismatic : hornblende. 

Basal: mica, topaz. 

Pinacoidal: gypsum. 

Domatic : barytes. 

Rhombohedral : calcite. 

Two cleavages : felspar, barytes. 

In all these cases the cleavage may be regarded as 
perfect or fairly perfect. Examples of imperfect cleavage 
may be found in the minerals augite and olivine. 



38 PROPERTIES OF MINERALS 

Planes of parting are to be distinguished from true 
cleavage, as they only occur at intervals in the crystal, 
and are probably due to secondary twinning or incipient 
alteration. Planes of parting are seen, for instance, in 
corundum, diallage, and magnetite. 

The radiating lines of easy separation known as 
percussion figures, which are obtained when a crystal 
face is struck with a sharp point, are closely allied to 
cleavage. They are particularly well exhibited by 
mica (see page 101). 

Fracture. The nature of the surface produced by 
breaking a mineral is also a valuable diagnostic 
character. Fracture, as it is termed, must not be con- 
founded with cleavage. The former can only be pro- 
duced in a direction in which cleavage is not developed ; 
consequently, if the tendency to cleave is strongly 
developed in a mineral, it will be difficult to produce a 
true fracture. 

The fracture of minerals may be described in the 
following terms : 

Even: chalcedony. 
Uneven : tourmaline. 
Conchoidal: calcite, flint. 
Subconchoidal : quartz. 
Splintery : jade. 
Hackly: copper. 

Tenacity. Tenacity is manifested by the behaviour 
of a mineral when submitted to pressure, percussion, 
tension, torsion, or division. According to the results 



PHYSICAL 39 

obtained the mineral is said to be brittle, malleable, ductile, 
elastic, or sectile. Haidinger found that the ductility of 
the metals, as shown by the ease with which they could 
be drawn into wire, decreased in the following order : 
gold, silver, platinum, iron, copper, zinc, tin, lead ; and 
their malleability, as shown by the ease with which they 
could be flattened out under the hammer, as follows : 
gold, silver, copper, tin, platinum, lead, zinc, iron. 

A mineral is elastic when, after being bent, it returns 
to its original shape. Mica is elastic, chlorite and talc 
inelastic. When a mineral can be cut by a knife it is 
sectile ; horn-silver is a familiar example. 

Hardness. The term " hardness " signifies the 
resistance offered by a body to the separation of its 
particles. The relative hardness of minerals may be 
utilized as a means of identification. It is measured by 
the force required to scratch (i.e., to separate the super- 
ficial particles of) the mineral with a steel point, or the 
sharp-pointed fragment of some mineral harder than 
the one to be experimented upon. The results obtained 
from one and the same mineral are found to vary with 
the crystal face, and with the direction in one and the 
same face. 

This directional variation is a function of the sym- 
metry of the crystal, and if, on any face of a crystal, 
a curve of hardness be constructed by joining up the ends 
of lines drawn from a central point, whose lengths 
represent the hardness existing in their direction, it 
will be found to have the symmetry appropriate to 



40 



PROPERTIES OF MINERALS 



the crystal experimented upon. Thus, the curve of 
hardness on the face of the cube is intersected by the 
trace of four planes of symmetry (see Fig. 51), and that 
on a face of the octahedron by the trace of three planes 
of symmetry (Fig. 52). 

The relative average hardness of minerals may be 




FIG. 51. 



FIG. 52. 



expressed by reference to the following scale, devised 
by the Freiberg mineralogist Mohs : 



6. Felspar (adularia). 

7. Quartz. 

8. Topaz. 

9. Corundum (sapphire). 
10. Diamond. 



1. Talc. 

2. Gypsum. 

3. Calcite. 

4. Fluorspar. 

5. Apatite. 

Each of these minerals can be scratched by those 
that follow it, and will itself scratch those that precede 
it in the scale. 

Minerals having the hardness I 2 can be scratched 
by the finger-nail ; those lying between 3 and 5 by the 
point of the knife (with gradually increasing difficulty) ; 
6 can scarcely be marked by the knife, but is touched 
by a steel file ; 7 scratches glass ; 8 scratches the steel 
file ; while 9 is only to be scratched by the diamond. 



PHYSICAL 41 

These so-called " degrees of hardness" are only points 
arbitrarily fixed in a graduated scale. There is no con- 
stant ratio between the different numbers. Thus, the 
degrees of hardness of talc, gypsum, and calcite, lie 
within much narrower limits than those of topaz, 
corundum, and the diamond. Experiments made by 
Rosiwal, by grinding with a standard powder, showed 
that the relative hardness of the minerals used for the 
scale may be expressed by the following figures, 
corundum being taken at 1,000: diamond, 140,000; 
corundum, 1,000 ; topaz, 175 ; quartz, 120 ; felspar, 37 ; 
apatite, 6*5 ; fluorspar, 5 ; calcite, 4*5 ; gypsum, 1*25 ; 
talc, 0*33. 

PHENOMENA RELATING TO LIGHT. 

Colour. The colour of a mineral is either an intrinsic 
property as, for instance, that of gold, copper, cinna- 
bar, ruby-silver or it is accidental, and due to the inclu- 
sion or admixture of a small quantity of some colouring 
substance, as in rose-quartz, jasper, fluorspar, some 
varieties of felspar (e.g., the so-called amazon-stone), 
and most gem-stones. When powdered, most minerals 
show a different colour to that which characterizes them 
in the mass. Thus, haematite, which maybe dark brown 
or black in the mass, yields powder of a cherry-red 
colour. This property is utilized as a test namely, by 
filing off a little of the mineral, or by drawing it across 
the rough surface of a piece of unglazed porcelain ; the 
resulting mark is termed the streak of the mineral. 



42 PROPERTIES OF MINERALS 

The streak of a mineral is therefore the colour of its 
powder. 

Pellucidity. According to the amount of light trans- 
mitted by minerals, we may distinguish between their 
transparency, translucency, and opacity. The pellucidity 
of a mineral is to a certain degree dependent on its 
colour ; for a dark colour diminishes transparency. It 
is also influenced by the thickness of the specimen, 
some minerals which are opaque in thick pieces being 
translucent in thin plates or splinters. The opacity of 
minerals is not always an inherent quality, since in 
some cases it is the result of decomposition, hydration, 
etc. (e.g., the kaolinization of felspar and the serpen- 
tinization of olivine). 

Lustre. The lustre of a mineral is due, partly to the 
degree in which light is reflected, and partly to the nature 
of the reflected light. According to the degree or intensity 
of lustre, a mineral is said to be splendent (as blende), 
shining (as calcite), glistening (as magnetite), glimmering 
(as galena), or dull (as serpentine). According to the 
nature of the reflected light, the lustre is metallic (as 
in the native metals, pyrites, etc.), submetallic (as in 
pitchblende), adamantine (as in diamond and blende), 
vitreous (as in quartz), greasy (as in el&olite], pearly (as in 
diallage and bronzite), or silky (as in tremolite). 

There are a few other phenomena, produced either 
by peculiarities in the reflecting surface, or by the fact 
that reflection takes place from surfaces in the interior 
of the crystal. Such, for example, are the following : 



PHYSICAL 43 

opalescence a peculiar milky or cloudy appearance 
produced by the diffraction of light by minute fissures 
in the interior of a crystal (e.g., opal, moonstone) ; 
chatoyancy a changeable banded lustre like that of 
the eye of a cat. Chatoyant stones, like chrysoberyl 
(" cat's-eye "), when cut suitably, flash out bands of 
light which shift their position according to the way 
in which the stone is moved. This phenomenon is 
the result of a fibrous structure. When two or three 
systems of striations intersect, a star composed of four 
or six luminous rays is produced. This phenomenon 
is termed asterism, and is displayed by certain varieties 
of ruby, sapphire, garnet, and mica. Schiller is the 
name given to certain metallic and pearly lustres pro- 
duced by the reflection from the surfaces of minute 
enclosed plates, rods, or particles (as in diallage, bronzite, 
hypersthene, and avanturine). Change or play of colours 
is a phenomenon of diffraction produced at the surface 
of some minerals by a fine lineation (e.g., labradorite). 
Iridescence and irisation refer to the prismatic colours 
produced by the interference of light in the interior 
or at the surface of a mineral. In the former case 
the phenomenon is due to the presence of minute 
fissures, in the latter to the presence of a thin super- 
ficial film. 

Refraction. Isotropic media are those in which the 
velocity of transmission of light is independent of the 
direction in which the ray is transmitted. Anisotropic 
media are those in which the velocity of transmission 
changes with the direction. 



44 PROPERTIES OF MINERALS 

Gaseous, fluid, and amorphous substances and 
crystals of the regular system are isotropic ; substances 
crystallizing in the tetragonal, hexagonal, rhombic, 
monoclinic, and triclinic systems are anisotropic. 

In one and the same homogeneous isotropic substance 
light is transmitted without change of direction and 
without change in the velocity of transmission ; but in 
passing from one isotropic medium to another the ve- 
locity of light is changed, and there is consequently a 
change in direction, except in the particular case when 
the incident ray is normal to the plane separating the 
two media. This deflection of a ray of light in passing 
from one medium to another is termed refraction. The 
law of refraction may be formulated thus : In passing 
from an optically rare to an optically dense medium a ray of 
light is refracted or bent towards the normal to the bounding 
surface, and the angle of refraction bears a constant relation 
to the angle of incidence Jor the same two media. This 
constant ratio is the index of refraction, and may be 
expressed thus : 

__ sine of angle of incidence 
sine of angle of refraction 

As generally used, it refers to the index of refraction 
of an isotropic substance as compared to air. 

Since the velocity of light varies inversely with the 
density of the medium, the refractive index is inversely 
proportional to the velocity. 

The amount of deflection suffered by the incident 
ray when refracted depends also on its wave-length. 



PHYSICAL 45 

It is consequently different for rays of different colour. 
Refraction is therefore accompanied by dispersion, by 
which white light is resolved into the component 
colours of the spectrum. 

Now consider the case of light travelling from a 
denser to a rarer medium. Since in this case the re- 
fracted ray is deflected from the normal to the bounding 
surface, there must be a certain angle of incidence for 
which the deflected ray is parallel to the bounding 




N' 

itiiliti/l/fftn/f/Jia}i///JJfl//J/J////J/////M/f/ 
\ A, T 

IN' 

"C 

FIG. 53. DIAGRAM ILLUSTRATING THE PHENOMENON OF 
REFRACTION. 

PQ and ST, the parallel bounding surfaces of a plate of an isotropic 
substance in air; AB, the incident ray in air; BC, the re- 
fracted ray in a denser isotropic medium ; CE, the course of the 
ray in emerging from the denser medium again into air (CE is 
parallel to BA] ; NN', the normal to the bounding plane between 
the two media ; i, angle of incidence ; r, angle of refraction. 

surface. This angle, which varies in different sub- 
stances, but is constant for one and the same substance, 
is known as the critical angle. 

A ray of light, traversing a denser medium, and coming 
upon the bounding plane with a rarer medium at the 
critical angle, continues its course on emergence parallel 
to that plane (see Fig. 54). 



46 



PROPERTIES OF MINERALS 



If the ray in the denser medium strikes the bounding 
plane with a rarer medium at an angle greater than the 
critical angle, it undergoes total reflection (see Fig. 55). 

Since in the first case 



n = 



sin go 



sine of critical angle' 



and sin 90 = i, 



n = 



sine of critical angle" 

Consequently, the greater the index of refraction of a 
substance, the smaller its critical angle. 




30 




FIG. 54. DIAGRAM SHOWING THE 
PATH OF A RAY WHICH MEETS 
THE BOUNDING SURFACE WITH 
A RARER MEDIUM AT THE 
CRITICAL ANGLE (9). 



FIG. 55. DIAGRAM SHOWING THE 
PATH OF A RAY WHICH MEETS 
THE BOUNDING SURFACE WITH 
A RARER MEDIUM AT AN ANGLE 
OF INCIDENCE GREATER THAN 
THE CRITICAL ANGLE (0). 



The following are examples of this relation: 



Substance. 
Water 
Glass . . . 
Diamond 



Index of Refraction. 
I'33 

i'53 
2*42 



Critical Angle. 

48 35' 



40 45 
24 25' 



The small critical angle of the diamond explains 
why so much light is reflected from the interior facets 
of a brilliant; and the fact that this reflection is 



PHYSICAL 47 

accompanied by a large amount of dispersion of the 
coloured rays, determined by its high refractive index, 
accounts for the " fire " of this precious stone. 

Double Refraction and Polarization. When a ray 
of light passes from an isotropic medium into an 
anisotropic medium, the incident ray is separated into 
two rays, which traverse the anisotropic medium in 
different directions and at different velocities. Further, 
each ray is polarized i.e., its vibrations, instead of 
being in all azimuths at right angles to the direction 
in which the light travels, are now confined to one 
plane ; and the planes of polarization or vibration of 
the two rays are at right angles to one another. This 
is the phenomenon of double refraction. The passage of 
the two rays through the anisotropic medium is regu- 
lated in accordance with the elasticity of the particular 
medium ; and the distribution of the elasticity or, in 
other words, the nature of the figure of elasticity is a 
function of the symmetry of the system in which the 
substance crystallizes. We have to distinguish between 
crystals with a principal axis (tetragonal and hexagonal), 
and those without a principal axis (rhombic, monoclinic, 
and triclinic). 

Uniaxial Crystals. As explained on page 34, the figure 
of elasticity for crystals of the tetragonal and hexagonal 
systems, which have a principal axis, is a spheroid ; in 
other words, sections at right angles to the principal axis 
are circles, all other sections being ellipses. Both the 
velocity and the direction of vibration of the two rays 



48 PROPERTIES OF MINERALS 

produced by double refraction in this class of crystals 
are represented by the cross-section through the centre 
of the spheroid of elasticity at right angles to the incident 
ray. The vibrations of the rays take place respectively 
parallel to the greatest and least diameters of the section, 
and the lengths of these express the velocity of the rays. 
Since the section at right angles to the principal axis is a 
circle, and all diameters are equal, rays which enter the 
crystal parallel to this axis traverse it as through an 
isotropic medium i.e., without double refraction. This 
axis is distinguished as the optic axis, and crystals of the 
tetragonal and hexagonal systems are said to be uniaxial. 
Every other section is an ellipse, which is the more 
elongated the greater the angle made by the incident 
ray with the principal axis. In all the ellipses, how- 
ever, the one diameter remains equal to the diameter of 
the circular section. The refracted ray which vibrates 
parallel to this diameter travels with a constant velocity, 
and has a constant index of refraction (designated by o>). 
It is called the ordinary ray (O). 

Since the length of the second diameter* varies with 
the inclination of the incident ray to the principal axis, 
the ray vibrating parallel to it travels with a similarly 
varying velocity, and has, therefore, no constant index of 
refraction. The latter reaches its maximum or minimum 
value when the incidence is at right angles to the prin- 
cipal axis. The ray vibrating parallel to the second 
diameter is known as the extraordinary ray (E), and its 

* It will be either the major or the minor axis of the ellipse, 
according as the spheroid is prolate or oblate. 



PHYSICAL 49 

maximum or minimum index of refraction is represented 
by e. As with n, both co and e have a different value 
for differently coloured light. If the principal axis is 
the axis of greatest elasticity (in the case of a prolate 
spheroid), the crystal is said to be optically negative, 
and the refractive index of the ordinary ray is greater 
than that of the extraordinary ray (oT>e). If, on the 
contrary, the principal axis is the axis of least elas- 
ticity (in the case of an oblate spheroid), the crystal is 
optically positive, and e is greater than a* 

Wave Surface. The relation between the ordinary 
and the extraordinary ray in a uniaxial crystal can be 
best understood by a consideration of the nature of the 
wave surfaces produced by the propagation of the two 
sets of rays. 

Suppose a luminiferous wave motion initiated within 
a uniaxial crystal. Two sets of rays (the ordinary and 
the extraordinary) will be propagated outward through 
the crystal in all directions. After a given interval of 
time, imagine all the points to which the light has 
travelled in that period of time united by a surface 
(wave surface], one for each set of rays. The wave 
surface for the ordinary rays will be the surface of a 
sphere, because these rays travel with equal velocity in 
all directions. That uniting the extraordinary rays will 
be the surface of a spheroid, the shape of which will be 
the inverse of the spheroid of elasticity for the crystal 

* In positive crystals e w has a positive value; in negative 
crystals it has a negative value. (e w) is a measure of the 
double refraction. 

4 



50 PROPERTIES OF MINERALS 

in question ;* that is to say, its major axis, which is 
the maximum distance travelled in the given period of 
time, will be at right angles to the major axis of the 
spheroid of elasticity, since the latter represents the 
direction in which the vibrations take place, and is 
proportional to the velocity of propagation. 

Since in negative uniaxial crystals the axis of greatest 
elasticity coincides with the principal axis, in these 
crystals the extraordinary ray will be propagated with 
the greatest velocity in a direction at right angles 





FIG. 56. DIAGRAM OF THE WAVE FIG. 57. DIAGRAM OF THE WAVE 

SURFACES OF THE ORDINARY SURFACES OF THE ORDINARY 

AND EXTRAORDINARY RAYS IN AND EXTRAORDINARY RAYS IN 

A NEGATIVE UNIAXIAL CRYSTAL. A POSITIVE UNIAXIAL CRYSTAL. 



to the principal axis, while it will travel with its 
minimum velocity in the direction of this axis, and 
this minimum velocity will also be the velocity of the 
ordinary ray. In other words, the wave surfaces of the 
two sets of rays will be tangential at a point on the 
principal axis, and the sphere will be enclosed by the 
spheroid. 

* In other words, the wave surface of the extraordinary rays is a 
prolate spheroid for crystals, in which the spheroid of elasticity is 
oblate, and vice versa. 



PHYSICAL 51 

On the other hand, in positive uniaxial crystals, in 
which the axis of least elasticity coincides with the 
principal axis, the extraordinary ray has its greatest 
velocity in the direction of this axis, and this greatest 
velocity is equal to that of the ordinary ray. The two 
wave surfaces are therefore also tangential at a point 
on the principal axis, but the spheroid is in this case 
enclosed by the sphere. 

It is useful to note as a memoria technica that the 
shape of Fig. 56 recalls the negative sign, and that of 
Fig. 57 the positive sign. 

Biaxial Crystals. In crystals of the rhombic, mono- 
clinic, and triclinic systems, which have no principal 
axis, the elasticity is represented by an ellipsoid with 
three unequal axes (Fresnel's ellipsoid). The three axes 
of elasticity are at right angles to one another, and are 
known respectively as the axis of greatest elasticity 
(a), the axis of mean elasticity (6), and the axis of least 
elasticity (c). 

There are two directions in crystals belonging to these 
systems in which there is no double refraction ; from 
analogy with uniaxial crystals these directions are called 
optic axes, and the crystals which possess them biaxial 
crystals. In biaxial crystals the optic axes do not 
coincide with axes of elasticity, but they are in the 
plane of, and are symmetrical to, the axes of greatest and 
of least elasticity. The axis of mean elasticity is therefore 
the normal to the plane of the optic axes. When the axis 
of greatest elasticity is the acute bisectrix of the angle 
between the optic axes, the crystal is said to be negative; 



52 PROPERTIES OF MINERALS 

when the axis of least elasticity is the acute bisectrix, 
it is positive. The smaller the optic axial angle, the 
nearer does the optic character of a biaxial crystal ap- 
proximate to that of a uniaxial crystal. The angle 
between the optic axes is designated by 2V.* It varies 
slightly with the wave-length, and consequently with 
the colour of the light. Thus, for diopside 2V =- 58 52' 
for red (Li) light, 5843' for yellow (Na) light, and 583o' 
for green (Tl) light. This phenomenon is known as the 
dispersion of the optic axes. 

Every ray which enters a biaxial crystal in any 
direction other than an optic axis is separated into 
two rays, which are polarized in planes at right angles 
to one another, and travel with different velocities, and 
in different directions. The direction of vibration and the 
velocity of transmission are represented by the axes of 
the elliptical section of the ellipsoid of elasticity taken 
through the centre, and at right angles to the incident 
ray. Each of the refracted rays has a velocity of trans- 
mission which varies with the angle of incidence ; there 
is therefore no constant index of refraction. Three 
principal indices of refraction are, however, distinguished 
namely : a, the index of refraction for rays vibrating 
parallel to the axis of greatest elasticity (a) ; /3, the index 
of refraction for rays vibrating parallel to the axis of 
mean elasticity (0) ; and 7, the index of refraction for rays 

* As observed in air, the optic axes appear to include a larger 
angle on account of the refraction of the light travelling along on 
emergence into air. This apparent axial angle in air is denoted 
by 2E. 



PHYSICAL 53 

vibrating parallel to the axis of least elasticity (c) ; and 
since the velocity of transmission in these directions 
is represented by the lengths of the axes a, fc, and c, 

n = ~, 6 = -= and c = - ft is usually taken as the measure 

of refraction of a biaxial mineral,* but some authors give 

. 7 a is a measure of the double refraction. 

Crystals of the rhombic, monoclinic, and triclinic 
systems can be distinguished from one another by the 
orientation of the ellipsoid of elasticity with regard to 
the crystal axes. 

In the rhombic system each of the three axes of 
elasticity (a, fc, and c) coincides with a crystal axis, and 
the plane of the optic axes lies in one of the pinacoids. 

In the monoclinic system only one of the crystal 
axes namely, the ortho-axis (6), which is normal to 
the one plane of symmetry coincides with an axis of 
elasticity. The two remaining axes of elasticity lie in 
the clino-pinacoid (the plane of symmetry). If the 
ortho-axis (b] coincides with the axis of mean elasticity, 

* The index of refraction may be determined by measuring with 
a goniometer the angle of least deviation of light traversing a prism 
of the substance, or, for substances whose refractive index is not 
too high, by measuring the angle of total reflection when the sub- 
stance is immersed in a denser liquid or placed against a glass 
prism. It may also be determined by immersion of the substance 
in a liquid of known refractive index, one with a refractive 
index approximating to that of the substance under examination 
being selected by experiment. The behaviour of the light band at 
the contact of the two media, when tested by Becke's method, 
determines whether the substance has a higher or a lower refrac- 
tive index than the liquid in which it is immersed. 



54 PROPERTIES OF MINERALS 

the optic axial plane lies in the clino-pinacoid. If the 
ortho-axis (6) coincides with the axis of greatest, or of 
least, elasticity, the plane of the optic axes lies normal 
to the clino-pinacoid. 

In the triclinic system none of the axes of elasticity 
coincides with a crystal axis, for the reason that the 
crystal axes are chosen arbitrarily, there being no plane 
or axis of symmetry to dictate a choice. 

Absorption of Light Pleochroism. It is found as 
the result of experiment that light in passing through a 
crystal becomes partly absorbed, and, further, that the 
absorption varies with the direction of vibration within 
the crystal, or, in other words, that light of a given 
colour is more absorbed when polarized in one plane 
than in another. Since the plane of polarization varies 
with the direction of transmission, crystals which possess 
the power of absorption in a marked degree will, if white 
light be used, transmit light of a different colour in dif- 
ferent directions. Thus, a crystal of cordierite will, when 
viewed in transmitted light, appear blue in one direction, 
and yellow in another. This phenomenon is known as 
Pleochroism. If light vibrating in one plane (i.e., 
polarized light) be transmitted through a pleochroic 
biaxial mineral, parallel to each axis of elasticity in turn, 
three distinct colours are obtained, and these are know r n 
as the axial colours. In the case of cordierite they are 
Yellow for rays vibrating parallel to a. 
Light blue (\ 

Dark blue ,, ,, ,, c. 

Uniaxial crystals are dichroic only. 



PHYSICAL 55 

THERMAL, ELECTRIC, AND MAGNETIC PROPERTIES. 

Thermal Properties. The conductivity for heat of 
minerals is related to the symmetry of the system in 
which they crystallize. That it varies with the direction 
can be shown by the following experiment : if a cleavage 
surface of stibnite (a rhombic mineral with brachy- 
pinacoidal cleavage) be coated with a thin film of wax 
and touched with the point of a heated wire, the melted 
portion of the wax will be found to have the shape of an 
ellipse, the major axis of which coincides with the vertical 
axis of the crystal, and the minor axis with the brachy- 
diagonal a phenomenon consistent with the symmetry 
of a rhombic crystal. Specific heat, although a valuable 
constant of minerals, is not much used in determinative 
mineralogy, by reason of the extreme care required for 
its determination. It is defined as the ratio of the 
quantity of heat required to raise the temperature of the 
mineral one degree to fhe quantity of heat required to raise 
the temperature of an equal mass of water one degree. 
Of more service, because readily observed, is the degree 
of fusibility, or the ease with which substances fuse 
when submitted to the action of heat. A splinter of 
the mineral is held with the platinum forceps in the 
flame of a Bunsen burner or of a blowpipe. Its fusi- 
bility is determined by comparison with the following 
substances (Von Kobell's scale *) : 

i. Stibnite. A rather large fragment fuses easily in 
the gas flame. 

* As modified by Penfield. 



56 PROPERTIES OF MINERALS 

2. Chalcopyrite. A fragment of the standard size 
(diameter 1*5 millimetres) fuses rather slowly in the gas 
flame. 

3. Almandine Garnet. A fragment of the standard 
size fuses readily to a globule before the blowpipe. 

4. Actinolite. The edges of a fragment of the 
standard size are readily rounded before the blowpipe. 

5. Orthoclase. The edges of a fragment of the 
standard size are rounded with difficulty before the 
blowpipe. 

6. Bronzite. Only the finest points are rounded 
before the blowpipe. 

Electrical Properties. The electrical conductivity of 
minerals is a very variable factor. The best conductors 
are the metals, and, among compounds, those which 
possess metallic lustre, like pyrites, chalcopyrite, galena, 
haematite, etc. On the other hand, minerals like quartz, 
felspar, calcite, barytes, fluorspar, garnet, are poor con- 
ductors. A process for the separation of metallic minerals 
from their gangue is founded on these differences in con- 
ductivity. In the electrostatic separation process the dry 
mixture of metallic mineral particles and gangue mineral 
grains, such as quartz, while in a neutral electrical 
state, are brought into contact with a surface highly 
charged with electricity ; the metallic particles, because 
they conduct electricity readily, become charged to the 
same condition as the surface, and fly from it; the 
quartz particles, on the other hand, require a longer 
time to receive the charge, and therefore cling to the 



PHYSICAL 57 

surface until they have acquired the same electrical 
condition. 

This difference in the behaviour of the two classes of 
material permits a separation to be made by a suitable 
arrangement of machinery. 

Some minerals, and especially those which are hemi- 
morphic, become electrified when heated or cooled. 
This phenomenon is known as pyro-electricity. It is 
manifested by the presence of statical changes at oppo- 
site ends of the crystal positive at one, and negative 
at the other. 

Magnetic Properties. The property of being 
attracted by a magnet is possessed in greater or less 
degree by all minerals which contain iron, and this 
property can be made use of in the separation of 
pow r dered mixtures of minerals. The process of 
magnetic separation by means of the electro-magnet 
has been successfully applied to ore-dressing opera- 
tions, and the method is advantageously used in 
laboratory investigations of sands and rocks to effect 
the isolation of minerals for chemical analysis. For 
laboratory work the sands are sifted to a uniform size, 
and the rock crushed to grains not exceeding 0*25 
millimetre, the powder being removed by washing. 
Minerals having a high magnetic permeability, such as 
magnetite, pyrrhotite, and haematite, are first removed 
by a weak permanent magnet (an operation which is 
facilitated by covering the poles with a movable paper 
cap). The minerals of which iron is a constituent can 



58 PROPERTIES OF MINERALS 

then be removed from the non-ferriferous minerals, and 
also separated from one another, by regulating the 
intensity of the field of an electro-magnet by the suitable 
adjustment of movable pole-pieces.* 

In this way, for example, the pyroxene, olivine, horn- 
blende, and biotite, can be separated from the plagio- 
clase felspar with which they are associated in an 
olivine gabbro, or monazite can be separated from 
zircon and quartz in a " monazite sand." 

TASTE AND ODOUR. 

Only those minerals which are to some extent soluble 
in water have a taste. Examples of such minerals are 
salt, soda, Epsom salt, alum, nitre. 

Odour is emitted from some minerals when they are 
rubbed, struck with a hammer, or heated. Sulphur 
compounds are characterized by the familiar foetid odour 
of sulphuretted hydrogen or by the choking smell of 
sulphurous acid. Arsenic compounds (like mispickel) 
have a smell of garlic. There are also characteristic 
clayey and bituminous odours that are emitted by 
argillaceous and bituminous minerals. 

THE SURFACE ENERGY OF MINERALS. 

The condition of molecular equilibrium established 

at the free surface of a substance is known as its 

surface energy. It is a function of internal cohesion, 

and, consequently, is generally greater in solids than in 

* For details of the process, see T. Crook, Science Progress, 
1907, p. 18. 



PHYSICAL 59 

liquids. In solids it is responsible for the phenomenon 
known as the surface condensation of gases ; in liquids 
it manifests itself as surface tension. The surface 
tension of liquids tends to make them occupy the least 
space possible in proportion to their mass, and to this 
is due the inclination of liquids towards the globular 
form, w T hich reaches its maximum development in 
mercury. The surface tension of mercury is, of course, 
exceptionally high among liquids. After mercury comes 
water; oils have a low surface tension relatively to 
water. Among solids, the surface energy is greatest in 
elements, such as the metals, graphite, etc. ; and it is 
manifested in these by the power of condensing gases 
at the surface, as exemplified by spongy platinum, finely 
divided carbon, etc. Next to the elements, the sul- 
phides of the heavy metals have the highest surface 
energy; while the compounds of the non-metals and 
of the light metals (such as quartz, felspar, calcite, etc.) 
are deficient in that respect. 

When a solid substance is in contact with a liquid, 
whether it will draw up or depress the liquid in its 
immediate neighbourhood depends on the ratio between 
the surface energies of the two substances. The angle 
of contact made by the liquid with the solid is there- 
fore a measure of the ratio between the surface energies 
of the solid and the liquid. Since the phenomenon of 
wetting is brought about by the strong surface energy 
of the solid overcoming the relatively weak surface 
tension of the liquid and its internal cohesion, the 
degree of wetting is also measured by the contact angle 



60 PROPERTIES OF MINERALS 

(or " wetting angle "). When into water in which a 
small quantity of oil is present a mixture of solids is 
introduced, there is a selective wetting by the oil of the 
solid substances that have a high surface energy. On 
this principle is based the separation of metals and 
metallic sulphides from their gangue minerals by the 
so-called " oil-concentration processes." The separation 
is assisted by agitation, by means of which air or other 
gaseous bubbles are induced to form on the oily mineral 
particles, and they are thus enabled to float on the 
water in which the gangue minerals, free from oil, are 
sunk by gravity. 

The oil process can be most advantageously used 
for separating minerals whose difference in density is 
insufficient to permit of the application of the usual 
concentration by gravitation methods. Thus chalco- 
pyrite may be separated from magnetite, galena and 
blende from barytes, sulphides of copper from oxide of 
tin, etc. 

Similarly, when a diamantiferous " deposit " is 
washed over a grease-coated surface, the diamonds are 
retained, while the accompanying minerals are carried 
away by the water. 

DENSITY. 

Density is the mass of a unit volume. On the centi- 
metre-gramme-second system it is expressed as grammes 
per cubic centimetre. The specific gravity of a sub- 
stance is the ratio of its density to that of water at 
4 C. The mass of a cubic centimetre of pure water 



PHYSICAL 61 

at 4 C. and under normal pressure being 0-999973 
gramme, " specific gravity " is for all practical pur- 
poses identical with "density." It is usually deter- 
mined by weighing the body first in air and then in 
water : the difference is the weight of a mass of 
water equal in bulk to the body; and this difference 
divided into the weight in air is the specific gravity. 
Another method, which is very serviceable in cases 
where the density is not above 3*3, is to make use 
of one of the so-called " heavy liquids." The con- 
centrated solution is diluted, by the gradual addition 
of water, until a fragment of the mineral under examina- 
tion remains just suspended. Its specific gravity is 
then determined either by weighing a measured volume 
or by the Westphal balance. By an adroit adjust- 
ment of the strength of such a solution, it may be 
used as a means of separating a mixture of minerals 
of different density, such as are found associated in 
rocks. 

The best known heavy liquids are the following : 
Sonstadt's aqueous solution of iodide of potassium and 
mercury (maximum density 3*18) ; Klein's aqueous solu- 
tion of borotungstate of cadmium (maximum density 
3-28) ; Braun's solution methylene iodide (maximum 
density 3*32) ; Retger's solution thallium-mercuro- 
nitrate (liquid at 76 C. with a density of 5*0). A useful 
liquid for a preliminary removal of the light minerals is 
bromoform (maximum density 2*9). It has the advan- 
tage of being cheap, and it is cleaner and less sticky 
than some of the above-mentioned liquids. 



62 PROPERTIES OF MINERALS 

If minerals be arranged according to their density, it 
will be found that the native metals rank as the heaviest. 
Gold is 19 (sinking to 15 in proportion as it is alloyed 
with copper and silver) ; platinum, 17 ; mercury, 13*6 ; 
lead, 11*37; silver, io'6 ; copper, 8*84. Next come the 
metallic ores e.g., cinnabar, 8*1 ; galena, 7-5 ; cassit- 
erite or tinstone, 6*9 ; mispickel, 6-05 ; cuprite, 6'o ; 
chalcocite, 575 ; bornite, 5-2 ; magnetite, 5*17 ; pyrites, 
5*03 ; ilmenite, 4-84 ; chalcopyrite, 4*2 ; blende, 4*06 ; 
chalybite, 3*86 ; limonite, 3*8. 

Among minerals that are not compounds of the heavy 
metals, barytes takes the first place, with a density 
of 4*48; apatite is 3-2; fluorspar, 3-18; calcite, 272; 
gypsum, 2*32 ; rock-salt, 2*14 ; sulphur, 2*07 ; graph- 
ite, 2*16. 

The common rock-forming minerals have a density 
between 3*5 and 2*5 : thus the pyroxenes range from 
3*5 to 3*27; olivine is 3*4; the amphiboles vary from 
3*4 to 2*9 ; the micas from 3*2 to 27 ; the felspars 
from 276 to 2*56 ; while quartz is 2*65. 

The gems vary from 4*6 to 2*2 : thus zircon is 4*69 ; 
the garnets range from 4*3 to 3*15 ; corundum (sapphire 
and ruby) is 4*0 ; topaz, 3*53 ; diamond, 3*52 ; turquoise, 
275 ; emerald, 27 ; opal, 2*2. 

Thus it will be seen that the metallic ores betray 
themselves, even to the casual observer, by their great 
relative weight, a fact which is naturally of inestimable 
value. 

Most of the ore-dressing processes are based on the 
relatively high density of the valuable metallic ores as 



PHYSICAL 63 

compared with their associated gangue minerals. The 
principle of " gravity concentration " is made use of in 
a variety of ways in dollies, jigs, pulsators, percussion 
and shaking tables, and in the hydraulic sluicing of 
alluvial gold and tin-ore ; while the prospector applies 
it daily in the simple process of washing gold or other 
ores in the pan, the batea, or on the vanning shovel. 

Among precious stones density is also of great im- 
portance, since it enables the jeweller to determine with 
certainty whether a given stone is really what its colour 
and general appearance may indicate. 



CHAPTER III 
CHEMICAL COMPOSITION 

MINERALS are either simple elementary substances such 
as gold, sulphur, and diamond, or they consist of 
compounds in which the elements are in certain fixed 
proportions. 

The elements are divided by chemists into metals, of 
which the most important are gold, platinum, silver, tin, 
copper, lead, mercury, iron, nickel, cobalt, zinc, man- 
ganese, aluminium, barium, calcium, magnesium, potas- 
sium, and sodium ; and non-metals, such as oxygen, 
sulphur, nitrogen, phosphorus, chlorine, fluorine, carbon, 
silicon. Of the metals, only gold, platinum, silver, 
lead, copper, mercury, and iron (rare), occur native. 

With regard to the compound substances occurring in 
Nature, these are either compounds of the metals with 
simple non-metallic elements, such as oxygen, sulphur, 
arsenic, and the halogens (chlorine, fluorine, etc.), or 
they are oxy-salts (and sulpho-salts). The compounds 
with the simple non-metals are known as oxides, sul- 
phides (and sulphur salts), arsenides, chlorides, fluorides, 
etc. ; while the oxy-salts (and sulpho-salts) are sup- 
posed to be derived from the corresponding oxy-acids 

64 



CHEMICAL 66 

(and sulph-acids) by the replacement of their hydrogen 
by metals. Thus 

Nitrates are formed from hydrogen nitrate or nitric 
acid HNO 3 . 

Carbonates are formed from hydrogen carbonate or car- 
bonic acid H 2 CO 3 . 

Sulphates are formed from hydrogen sulphate or sul- 
phuric acid H 2 SO 4 . 

Metaborates are formed from hydrogen metaborate or 
metaboric acid HBO 2 . 

Phosphates are formed from hydrogen phosphate or 
phosphoric acid H 3 PO 4 . 

Orthosilicates are formed from hydrogen orthosilicate 
or orthosilicic acid, H 4 SiO 4 . 

Metasilicates are formed from hydrogen metasilicate or 
metasilicic acid H 2 SiO 3 . 

Hydrogen, in combination with oxygen, also enters 
into the combination of a great number of minerals 
(hydrates) as " water of constitution " and " water of 
crystallization." 

The compound which plays by far the largest part in 
the constitution of the earth's crust is silicon dioxide or 
silica (i.e., a compound of silicon with oxygen, SiO 2 ), 
which either in the free state as colloidal silica or as 
quartz, or in combination with metallic bases as sili- 
cates, forms more than half the solid crust, and enters 
into the composition of nearly all its component rocks. 
Next in importance is alumina, which is an oxide of 
aluminium. In union with silica, it constitutes the 

5 



66 PROPERTIES OF MINERALS 

basis of an important series of silicates (felspar, mica, 
clay, etc.), which are the chief ingredients of a great 
number of rocks. Carbon dioxide often appears in 
combination with lime or magnesia, forming immense 
deposits of limestone and dolomite. 

Lime (oxide of calcium), besides being present as a 
carbonate in limestone, occurs, in combination with 
sulphuric acid, in anhydrite and gypsum. Other com- 
pounds which largely enter into the composition of the 
rock-forming minerals are magnesia and the alkalies 
potash and soda. 

Iron and manganese play a part of great importance 
in many rocks. The oxides and salts of iron may be 
said to constitute Nature's colour-box ; for the rich 
shades of yellow, red, and brown, which are so effective 
in coast scenery, and are seen wherever bare rock is 
exposed, are chiefly due to iron. The deep chocolate 
colour of some soils, and the varied colours of sands, 
are to be ascribed to the same cause. Manganese is 
widely distributed in marine sediments, as has been 
demonstrated by the Challenger researches of Murray 
and Renard; and like iron, its oxides are a common 
surface deposit. 

Polymorphism. There are several instances of 
minerals that differ in crystalline form and physical 
properties, but are identical in chemical composition. 
The property by which one and the same chemical 
substance has the power of appearing in two or more 
different states of molecular aggregation is termed poly- 
morphism. Calcite and aragonite (both carbonate of 



CHEMICAL 67 

lime), and rutile, anatase, brookite (all titanic acid), are 
familiar instances. The former is a case of dimorphism, 
the latter of trimorphism. 

Isomorphism. Isomorphous minerals are those 
which, having a similar chemical composition, crystal- 
lize in identical or closely related forms, and are 
capable of forming homogeneous mixed crystals e.g., 
barytes (sulphate of barium), celestite (sulphate of 
strontium), and anglesite (sulphate of lead), all of 
which crystallize in allied rhombic forms. The phe- 
nomenon of isomorphism points to a close connection, 
between the atomic constitution of the molecule and 
the arrangement of the latter in the structure of the 
crystal. 

Pseudomorphism. In the mineral kingdom there 
are certain substances which exhibit the form of one 
mineral while possessing the chemical composition 
and molecular structure of another. These remark- 
able bodies are termed pseudomorphs. Although arising 
from various causes, they are all due to some secondary 
process which acts on the original mineral in such 
a way as to remove or decompose its substance, 
while retaining its crystal form. Pseudomorphs are 
classified according to their mode of origin. Quartz 
is sometimes found in crystal forms characteristic of 
the mineral calcite ; and tinstone, in the form of 
felspar: these are pseudomorphs produced by replace- 
ment, molecule by molecule. Galena, which is sul- 
phide of lead crystallizing in the regular system, 



68 PROPERTIES OF MINERALS 

is occasionally found presenting the crystal form of 
pyromorphite (phosphate of lead crystallizing in the 
hexagonal system) : this is a case of pseudomorphism 
produced by chemical change. Other pseudomorphs are 
produced by incrustation i.e., the deposition of one 
mineral in regular layers on the faces of already exist- 
ing crystals of another (for instance, quartz on chaly- 
bite). These are termed epimorphs. In the special case, 
where a change in molecular structure takes place with- 
out a corresponding change in chemical composition, 
such as brookite (titanium dioxide) to rutile (also 
titanium dioxide), the pseudomorphs are termed para- 
morphs. 

Classification by Chemical Composition. In the 

following list the more important mineral species are 
classified according to their chemical composition. 
Those that occur most commonly are distinguished by 
being printed in heavy type. 

I. THE NATIVE ELEMENTS. 

A. NATIVE METALS AND ALLOYS. 

Gold, Au. 
Silver, Ag. 

Electrum gold-silver alloy, Au,Ag. 

Copper, Cu. 

Iron, Fe. 

Lead, Pb. 

Platinum, Pt. 

Iridosmine indium-osmium alloy, Ir,Os. 

Mercury, or Quicksilver, Hg. 



CHEMICAL 

Amalgam silver-mercury alloy, Ag,Hg. 

Arsenic, As. 

Antimony, Sb. 

Allemontite antimony-arsenic alloy, Sb,As, 

Bismuth, Bi. 

B. NON-METALS. 

Sulphur, S. 

Diamond, C (regular). 
Graphite, C (pseudo-hexagonal). 



II. THE COMPOUNDS OF METALS WITH 

ELEMENTS OF THE ARSENIC AND 

SULPHUR GROUPS. 

A. ARSENIDE GROUP. 

Niccolite arsenide of nickel, NiAs. 
Smaltite diarsenide of cobalt, CoAs 2 . 
Chloanthite diarsenide of nickel, NiAs 2 . 
Sperrylite diarsenide of platinum, PtAs 2 . 

B. MONOSULPHIDE GROUP. 

Argentite monosulphide of silver, Ag 2 S. 

Blende monosulphide of zinc, ZnS. 

Galena monosulphide of lead, PbS. 

Chalcocite, or Copper Glance monosulphide of 
copper, Cu 2 S. 

Stromeyerite monosulphide of copper and silver 
(Cu,Ag) 2 S. 

Cinnabar monosulphide of mercury, HgS (hexa- 
gonal). 

Metacinnabarite monosulphide of mercury, HgS 
(regular). 



70 PROPERTIES OF MINERALS 

Millerite monosulphide of nickel, NiS. 
Alabandite monosulphide of manganese, MnS. 
Pyrrhotite, or Magnetic Pyrites monosulphide of 

iron, FeS. 
Realgar monosulphide of arsenic, AsS. 

C. BISULPHIDE GROUP. 

Hauerite disulphide of manganese, MnS 2 . 
Pyrites disulphide of iron, FeS 2 (cubic). 
Marcasite disulphide of iron, FeS 2 (rhombic). 
Molybdenite disulphide of molybdenum, MoS 2 . 
Sylvanite telluride of gold and silver (Au,Ag)Te 2 .* 

D. SESQUISULPHIDE GROUP. 

Stibnite, or Antimonite sesquisulphide of anti- 
mony, Sb 2 S 3 . 

Bismuthinite, or Bismuth Glance sesquisulphide of 
bismuth, Bi 2 S 3 . 

Orpiment sesquisulphide of arsenic, As 2 S 3 . 

E. ARSENOSULPHIDE GROUP. 

Cobaltite, or Cobalt Glance, CoAs 2 .CoS 2 . 
Gersdorffite, or Nickel Glance, NiAs. 2 .NiS 2 . 
Mispickel, or Arsenopyrite, FeAs 2 .FeS 2 . 

F. SULPHUR SALTS. 

Bornite, or Erubescite sulphoferrite of copper, 

3Cu 2 S.Fe 2 S 3 . 

Chalcopyrite, or Copper Pyrites sulphoferrite of 
copper, Cu 2 S.Fe 2 S 3 . 

* Included here on account of the close relationship of tellurium to 
sulphur.^ 



CHEMICAL 71 

Polybasite sulphantimonite of silver and copper, 

9 (Ag,Cu) 2 S.Sb 2 S 3 . 
Pearceite sulpharsenite of silver and copper, 

9(Ag,Cu) 2 S.As 2 S 3 " 

Stephanite sulphantimonite of silver, 5Ag 2 S.Sb 2 S 3 . 
Tetrahedrite, or Grey Copper sulphantimonite of 

copper, 3Cu 2 S.Sb 2 S 3 . 

Tennantite sulpharsenite of copper, 3Cu 2 S.As 2 S 3 . 
Bournonite sulphantimonite of lead and copper, 

2PbS.Cu 2 S.Sb 2 S 3 . 

Pyrargyrite sulphantimonite of silver, 3Ag 2 S.Sb 2 S 3 . 
Proustite sulpharsenite of silver, 3Ag 2 S.As 2 S 3 . 
Enargite sulpharseniate of copper, 3Cu 2 S.As 2 S 5 . 
Freieslebenite sulphantimonite of lead and silver, 

5 (Pb, Ag 2 )S.2Sb 2 S 3 . 



III. THE COMPOUNDS OF METALS WITH 
ELEMENTS OF THE CHLORINE GROUP. 

A. SIMPLE CHLORIDES, ETC. 

Rock-salt chloride of sodium, NaCl. 

Sylvite chloride of potassium, KC1. 

Sal Ammoniac chloride of ammonium, (NH 4 )C1. 

Kerargyrite, or Horn Silver chloride of silver, AgCl. 

Embolite bromochloride of silver, Ag(Br,Cl). 

Fluorspar, or Fluorite fluoride of calcium, CaF 2 . 

B. COMPOUND CHLORIDES, ETC. 

Cryolite double fluoride of aluminium and sodium, 

3NaF.AlF 3 . 
Carnallite hydrated double chloride of potassium and 

magnesium, KCl.MgCl 2 + 6H.,O. 



72 PROPERTIES OF MINERALS 

C. OXYCHLORIDES, ETC. 

Matlockite oxychloride of lead, PbCl 2 .PbO. 
Mendipite oxychloride of lead, PbCl 2 .2PbO. 
Atacamite hydrated oxychloride of copper, 

CuCl 2 .3Cu(OH) 2 . 



IV. THE COMPOUNDS OF METALS WITH 
OXYGEN. 

A. MONOXIDE GROUP. 

Cuprite, or Ruby Copper monoxide of copper, Cu 2 O. 
Zincite, or Spartalite monoxide of zinc, ZnO. 
Melaconite, or Tenorite monoxide of copper, CuO. 
Periclase monoxide of magnesium, MgO. 
Brucite magnesium hydrate, Mg(OH) 2 . 

B. EPITRITOXIDE GROUP. 

Spinel double oxide of magnesium and aluminium, 
MgO.Al 2 3 . 

Hercynite double oxide of iron and aluminium, 

FeO.Al 2 8 . 

Gahnite, or Zinc -spinel double oxide of zinc and 
aluminium, ZnO.Al 2 O 3 . 

Magnetite double oxide of iron, FeO.Fe 2 O 3 . 

Franklinite double oxide of iron, zinc, and man- 
ganese, (Fe,Zn,Mn)O.(Fe,Mn) 2 O 3 . 

Chromite double oxide of iron and chromium, 

FeO.Cr 2 O 3 . 

Hausmannite double oxide of manganese, MnO.Mn 2 O 3 . 

Chrysoberyl, or Alexandrite double oxide of beryllium 
and aluminium, BeO.Al 2 O a . 



CHEMICAL 73 



C. SESQUIOXIDE GROUP. 

Braunite sesquioxide of manganese, Mn 2 O 3 . 
Corundum sesquioxide of aluminium, A1 2 O 3 . 
Haematite sesquioxide of iron, Fe 2 O 3 . 
Ilmenite sesquioxide of iron and^titanium, (Fe,Ti) 2 O 3 . 
Goethite hydra ted sesquioxide of iron, Fe 2 O 3 .H 2 O. 
Turgite hydrated sesquioxide of iron, 2Fe 2 O 3 .H 2 O. 
Limonite hydrated sesquioxide of iron, 2Fe 2 O 3 .3H 2 O. 
Manganite hydrated sesquioxide of manganese, 

Mn 2 O 3 .H 2 O. 
Diaspore hydrated sesquioxide of aluminium, 



A1 2 O 3 .H 2 O. 



Bauxite hydrated sesquioxide of aluminium, 



A1 2 O 3 .2H 2 O. 



Gibbsite hydrated sesquioxide of aluminium, 

A1 2 3 . 3 H 2 0. 

Psilomelane hydrated sesquioxide of manganese and 
barium, (Mn, Ba)O.MnO 2 .H 2 O. 



D. DIOXIDE GROUP. 

Pyrolusite dioxide of manganese, MnO 2 . 
Rutile dioxide of titanium, TiO 2 . 
Anatase dioxide of titanium, TiO 2 . 
Brookite dioxide of titanium, TiO 2 . 
Cassiterite, or Tinstone dioxide of tin, SnO 2 . 
Zircon double oxide of zirconium and silicon, 

ZrO 2 .SiO, 

Tridymite dioxide of silicon, SiO 2 (rhombic). 
Quartz dioxide of silicon, SiO 2 (hexagonal). 
Chalcedony dioxide of silicon, SiO 2 . 
Opal hydrated dioxide of silicon, SiO 2 + wH 2 O. 



74 PROPERTIES OF MINERALS 

V. OXYGEN SALTS OF THE METALS. 

A. CARBONATES. 

Witherite carbonate of barium, BaCO 3 . 
Strontianite carbonate of strontium, SrCO 3 . 
Cerussite carbonate of lead, PbCO 3 . 
Aragonite carbonate of calcium, CaCO 3 . 
Calcite carbonate of calcium, CaCO 3 . 
Magnesite carbonate of magnesium, MgCO 3 . 
Dolomite double carbonate of magnesium and calcium, 

MgCa(C0 3 ) 2 . 
Ankerite carbonate of magnesium, calcium and iron, 

(Mg,Ca,Fe)C0 3 . 
Barytocalcite double carbonate of barium and calcium, 

BaCa(CO 3 ) 2 . 

Chalybite, or Siderite carbonate of iron, FeCO 3 . 
Rhodochrosite carbonate of manganese, MnCO 3 . 
Calamine carbonate of zinc, ZnCO 3 . 

B. HYDRATED CARBONATES. 

Chessylite, or Azurite hydrated basic carbonate of 

copper, 2CuCO 3 .Cu(OH) 2 . 
Malachite hydrated basic carbonate of copper, 

CuCO 3 .Cu(OH) 2 . 

Natron hydrated carbonate of sodium, Na 2 CO 3 .ioH 2 O. 
Trona hydrated carbonate of sodium, 

Na 2 C0 3 .HNaC0 3 .2H 2 0. 

C. CHROMATES AND SULPHATES. 

Crocoite chromate of lead, PbCrO 4 . 
Anhydrite sulphate of calcium, CaSO 4 . 
Celestite sulphate of strontium, SrSO 4 . 



CHEMICAL 75 

Barytes sulphate of barium, BaSO 4 . 
Anglesite sulphate of lead, PbSO 4 . 
Thenardite sulphate of sodium, Na 2 SO 4 . 

D. HYDRATED SULPHATES. 

Gypsum, or Selenite hydrated sulphate of calcium, 

CaSO 4 .2H 2 O. 

Kieserite hydrated sulphate of magnesium, MgSO 4 .H 2 O. 

Epsomite, or Epsom Salt hydrated sulphate of mag- 
nesium, MgSO 4 -7H 2 O. 

Mirabilite, or Glauber's Salt hydrated sulphate of 
sodium, Na 2 SO 4 .ioH 2 O. 

Melanterite hydrated sulphate of iron, FeSO 4 .7H 2 O. 

Chalcanthite hydrated sulphate of copper, CuSO 4 -5H 2 O. 

Brochantite hydrated basic sulphate of copper, 

CuSO 4 .3Cu(OH) 2 . 

Kalinite, or Potash alum hydrated sulphate of potassium 
and aluminium, K 2 SO 4 . A1 2 (SO 4 ) 3 + 24H 2 O. 

Alunite, or Alumstone hydrated basic sulphate of potas- 
sium and aluminium, K 2 SO 4 A1 2 (SO 4 ) 3 .2A1 2 (OH) 6 . 

E. PHOSPHATES, ARSENATES, AND VANADATES. 

Monazite phosphate of cerium, lanthanum, and didy- 

mium, (Ce,La,Di)PO 4 . 
Xenotime phosphate of yttrium, YPO 4 . 
Vivianite hydrated phosphate of iron, Fe 3 (PO 4 ) 2 .8H 2 O. 
Erythrite, or Cobalt Bloom hydrated arsenate of cobalt, 

Co 3 (As0 4 ) 2 .8H 2 0. 
Annabergite, or Nickel Bloom hydrated arsenate of 

nickel, Ni 8 (AsO 4 ) 2 .8H 2 O. 
Libethenite hydrated basic phosphate of copper, 

Cu(CuOH)PO 4 . 



76 PROPERTIES OF MINERALS 

Olivenite hydrated basic arsenate of copper, 

Cu(CuOH)AsO 4 . 
Clinoclase hydrated basic arsenate of copper, 

(CuOH) 3 As0 4 . 

Scorodite hydrated arsenate of iron, FeAsO 4 .2H 2 O. 
Torbernite hydrated phosphate of uranium and copper, 

Cu(U0 2 ) 2 (PO 4 ) ?i .8H 2 0. 
Autunite hydrated phosphate of uranium and calcium, 

Ca(U0 2 ) 2 (P0 4 ) 2 .8H 2 0. 
Wavellite hydrated phosphate of aluminium, 

2(A10H) 3 (P0 4 ) 2 . 9 H 2 0. 
Lazulite hydrated phosphate of iron and magnesium, 

(MgFe)(A10H) 2 (P0 4 ) 2 . 

Turquoise hydrated phosphate of aluminium, with small 
quantity of phosphate of copper, 2A1 2 O 3 .P 2 O 5 .5H 2 O. 
Pyromorphite chloro-phosphate of lead, 

3 Pb 3 (P0 4 ) 2 .PbCl 2 . 

Mimetite chloro-arsenate of lead, 3Pb 3 (AsO 4 ) 2 .PbCl 2 . 
Vanadinite chloro-vanadate of lead, 3Pb 3 (VO 4 ) 2 .PbCl 2 . 
Apatite (a) chlor-apatite chloro-phosphate of calcium, 
3Ca 3 (PO 4 ) 2 .CaCl 2 ; (b) fluor-apatite fluoro-phosptiate 
of calcium, 3Ca 3 (PO 4 ) 2 .CaF 2 . 



F. NITRATES. 

Nitre, or Saltpetre nitrate of potassium, KNO 3 . 
Nitratine, or Soda nitre nitrate of sodium, NaNO. 



G. BORATES. 

Borax hydrated borate of sodium, Na 2 O.2B 2 O 3 .ioH 2 O. 
Boracite chloro-borate of magnesium, 

6Mg0.8B 2 O s .MgCl 2 . 



CHEMICAL 77 

H. TlJNGSTATES AND MOLYBDATES. 

Wolfram tungstate of iron and manganese, 

(Fe,Mn)WO 4 . 

Scheelite tungstate of lime, CaWO 4 . 
Stolzite tungstate of lead, PbWO 4 . 
Wulfenite molybdate of lead, PbMoO 4 . 

I. NlOBO-TANTALATE. 

Columbite niobo-tantalate of iron and manganese, 
(Fe,Mn)O.(Nb,Ta) 2 O 5 . 

J. TlTANATE AND TlTANO-SILICATE. 
Perovskite titanate of calcium, CaO.TiO 2 . 
Sphene, or Titanite titano-silicate of calcium, 

CaO.TiO 2 .SiO 2 . 

K. SILICATES. 
(a) Orthosilicates. 

Forsterite orthosilicate of magnesium, SiO 2 .2MgO. 
Fayalite orthosilicate of iron, SiO 2 .2FeO. 
Olivine orthosilicate of iron and magnesium, 

Si0 2 .2(Fe,Mg)O. 

Tephroite orthosilicate of manganese, SiO 2 .2MnO. 
Monticellite orthosilicate of calcium and magnesium, 

SiO 2 .CaO.MgO. 

Phenacite orthosilicate of beryllium, SiO 2 .2BeO. 
Willemite orthosilicate of zinc, SiO 2 .2ZnO. 
Andalusite orthosilicate of aluminium, SiO 2 .Al 2 O 3 . 
Kyanite orthosilicate of aluminium, SiO 2 .Al 2 O 3 . 
Topaz fluoro-orthosilicate of aluminium, 

Si0 2 .Al 2 (0,F 2 )0 2 . 
Nepheline--- orthosilicate of sodium, potassium, and 

aluminium, 2SiO 2 .Al 2 O 3 .(Na,K) 2 O. 



78 PROPERTIES OF MINERALS 

Anorthite orthosilicate of calcium and aluminium, 

2SiO 2 .Al ? O 3 .CaO. 
Meionite orthosilicate of calcium and aluminium, 

6SiO 2 .3Al 2 O 3 .4CaO. 

Grossular-garnet orthosilicate of calcium and alu- 
minium, 3SiO 2 .Al 2 O 3 .3CaO. 
Pyrope orthosilicate of magnesium and aluminium, 

3SiO 2 .Al 2 O 8 .3MgO. 
Almandine orthosilicate of iron and aluminium, 

3SiO 2 .Al 2 O 3 .3FeO. 
Spessartite orthosilicate of manganese and aluminium, 

3SiO 2 .Al 2 O 3 .3MnO. 
Uvarovite orthosilicate of calcium and chromium, 

3SiO 2 .Cr 2 O 3 .3CaO. 
Melanite orthosilicate of calcium and iron, 

3SiO 2 .Fe 2 O 3 .3CaO. 
Sodalite* chloro-orthosilicate of sodium and aluminium, 

3SiO 2 .Al 2 O 3 .(AlCl)O.2Na 2 O. 
Haiiyne sulpho-orthosilicate of sodium, calcium, and 

aluminium, 3SiO 2 .Al 2 O 3 .(AlNaSO 4 )O.Na 2 O.CaO. 
Nosean sulpho-orthosilicate of sodium and aluminium, 
3SiO 2 .Al 2 O 3 .(AlNaSO 4 )O.2Na 2 O. 

(b) Metasilicates. 

Rhodonite metasilicate of manganese, SiO 2 .MnO. 
Wollastonite metasilicate of calcium, SiO 2 .CaO. 
Enstatite metasilicate of magnesium, SiO 2 .MgO. 
Hypersthene metasilicate of iron and magnesium, 

Si0 2 .(Fe,Mg)0. 
Hedenbergite metasilicate of iron and calcium, 

2SiO 2 .FeO.CaO. 

* For a discussion of the constitution of the sodalite group see 
Brogger and Backstrom, Zeits. fur Min., vol. xviii., p. 209. 



CHEMICAL 79 

Diopside metasilicate of magnesium and calcium, 

2SiO 2 .MgO.CaO. 
Tremolite metasilicate of magnesium and calcium, 

4SiO 2 .3MgO.CaO. 
Anthophyllite metasilicate of magnesium and iron, 

2SiO 2 .(Fe,Mg)O. 

Actinolite metasilicate of magnesium, iron, and cal- 
cium, 4SiO 2 .3(Mg,Fe)O.CaO. 
Acmite, or Aegirine metasilicate of sodium and iron, 

4SiO 2 .Fe 2 O 3 .Na 2 O. 
Riebeckite metasilicate of sodium and iron, 

4SiO 2 .Fe 2 O 3 .Na 2 O + 2(FeO.SiO 2 ). 

Augite metasilicate of calcium, iron, and aluminium, 
2 Si0 2 .CaO.(Mg,Fe)0 + SiO 2 .(Al,Fe) 2 O 3 .(Mg,Fe)O. 
Hornblende metasilicate of calcium, magnesium, iron, 
and aluminium, 

2Si0 2 .CaO.(Mg,Fe)0 + SiO 2 .(Al,Fe) 2 O,.(Mg,Fe)O. 
Spodumene metasilicate of lithium, sodium, and 

aluminium, 4SiO 2 .Al. 2 O 3 .(Li, Na) 2 O. 
Leucite metasilicate of potassium and aluminium, 

4 Si0 2 .Al 2 3 .K 2 0. 

Beryl metasilicate of beryllium and aluminium, 
6SiO 2 .Al 2 O 3 .3BeO. 



(c) Isomorphous Mixed Silicates. 

Felspars isomorphous mixtures of Orthoclase and 
Albite, or of Albite and Anorthite. 

Orthoclase, or Potash - felspar silicate of potassium 
and aluminium, 6SiO 2 .Al 2 O 3 .K 2 O. 

Albite, or Soda -felspar silicate of sodium and alu- 
minium, 6SiO 2 .Al 2 O 3 .Na 2 O. 

Anorthite, or Lime -felspar orthosilicate of calcium 
and aluminium, 2SiO 2 .Al 2 O 3 .CaO. 



80 PROPERTIES OF MINERALS 

ScapolitCS isomorphous mixtures of Meionite and 

Marialite. 
Meionite silicate of calcium and aluminium, 

6SiO 2 .3Al 2 O 3 .4CaO. 
Marialite chloro-silicate of sodium and aluminium, 

i8SiO 2 .3Al 2 O 3 . 3 Na 2 O.2NaCl. 
Chlorites isomorphous mixtures of Serpentine and 

Amesite. 
Serpentine hydrated silicate of magnesium, 

2SiO 2 .3MgO.2H 2 O. 

Amesite hydrated silicate of magnesium and alu- 
minium, SiO 2 .Al 2 O 3 .2MgO.2H 2 O. 

(d) Hydrated Silicates, with " Water of Constitution" 
Hemimorphite hydrated orthosilicate of zinc, 



Si0 2 .2ZnO.H 2 0. 



Dioptase hydrated orthosilicate of copper, 



Si0 2 .CuO.H 2 0. 
Chrysocolla hydrated orthosilicate of copper, 

SiO 2 .CuO.2H 2 O. 
Prehnite hydrated silicate of calcium and aluminium, 

3SiO 2 .(Al,Fe) 2 O 3 .2CaO.H 2 O. 

Zoisite hydrated orthosilicate of calcium and alu- 
minium, 6SiO 2 .3Al 2 O 3 .4CaO.H 2 O. 
Epidote hydrated orthosilicate of calcium, aluminium, 

and iron, 6SiO r 3(Al,Fc) t O r 4CaO.H 1 O. 
Piedmontite hydrated orthosilicate of calcium, alu- 
minium, and manganese, 

6SiO 2 .3(Al,Mn) 2 O 3 4CaO.H 2 O. 

Orthite hydrated orthosilicate of calcium, iron, alu- 
minium, and cerium, 

6SiO 2 .3(Al,Ce) 2 O 3 .4(Ca,Fe)O.H 2 O. 

Staurolite hydrated silicate of iron and aluminium, 
5SiO 2 .6Al 2 O 3 .2FeO.H 2 O. 



CHEMICAL 81 

Vesuvianite, or Idocrase hydrated silicate of calcium, 
magnesium, aluminium, and iron, 

Si 2 7 (Al,Fe)(OH,F)(Ca,Mg) 2 . 
Cordierite hydrated metasilicate of magnesium, iron, 

and aluminium, ioSiO 2 4(Mg, Fe)O.4Al 2 O 3 .H 2 O. 
Muscovite, or Potash -mica hydrated orthosilicate 
of potassium and aluminium, 

6SiO 2 .3Al 2 O 3 .K 2 O.2H 2 O. 
Paragonite, or Soda - mica hydrated orthosilicate of 

sodium and aluminium, 6SiO 2 .3Al 2 O 3 .Na 2 O.2H 2 O. 
Phlogopite, or Magnesian - mica hydrated silicate of 
magnesium, potassium, and aluminium, 

6SiO 2 .2Al 2 O 3 .4(Mg, Fe)O.K 2 O.H 2 O. 

Biotite, or Magnesian-iron-mica hydrated orthosilicate 

of magnesium, potassium, iron, and aluminium, 

6SiO 2 .2(Al, Fe) 2 O 3 .4(Mg, Fe)O.K 2 O.H 2 O. 

Margarite hydrated silicate of calcium and aluminium, 

2SiO 2 .2Al 2 O 3 .CaO.H 2 O. 
Chloritoid, or Ottrelite hydrated silicate of iron and 

aluminium, SiO 2 .Al 2 O 3 .FeO.H 2 O. 
Talc hydrated silicate of magnesium, 

4SiO 2 .3MgO.H 2 O. 
Kaolinite hydrated silicate of aluminium, 

2SiO 2 .Al 2 O 3 .2H 2 O. 



(e) Hydrated Silicates, with " Water of Crystallization " 
(Zeolite Group). 

Natrolite hydrated silicate of sodium and aluminium, 

3SiO 2 .Al 2 O 3 .Na 2 O.2H 2 O. 
Analcite hydrated silicate of sodium and aluminium, 

4SiO 2 .Al 2 O 3 .Na 2 O.2H 2 O. 
Chabazite hydrated silicate of sodium, calcium, and 

aluminium, 4SiO 2 .Al 2 O 3 .(Ca, Na 2 )O.6H 2 O. 

6 



82 PROPERTIES OF MINERALS 

Stilbite hydrated silicate of sodium, calcium, and alu- 
minium, 6SiO 2 .Al 2 O 3 .(Ca, Na 2 )O.6H 2 O. 

Heulandite hydrated silicate of sodium, calcium, and 
aluminium, 6SiO 2 .Al 2 O 8 .(Ca, Na 2 )O.3H 2 O. 

(/) Bovosilicates. 

Datolite hydrated boro-silicate of calcium, 

2SiO 2 .B 2 O 3 .2CaO.H 2 O. 

Axinite hydrated boro-silicate of calcium, iron, man- 
ganese, and aluminium, 

8SiO 2 .B 2 O 3 .2Al 2 O 3 .6(Ca, Fe, Mn)O.H 2 O. 
Tourmaline hydrated boro-silicate of sodium, mag- 
nesium, and aluminium. According to Clarke,* the 
tourmaline series consists of salts of an acid : 

Al 5 (Si0 4 ) 6 (B0 2 ) 2 .B0 3 H 2 .H 12 . 
* See Amer. Journ. of Set., vol. viii., 1899, p. in. 



PART II 
DESCRIPTIVE MINERALOGY 

CHAPTER I 

THE ROCK-FORMING MINERALS 

ALTHOUGH no doubt the greater number of the rocks 
that constitute the visible portion of the earth's crust 
are of sedimentary origin, it is to the crystalline rocks 
that we must look for variety in mineral composition ; 
and even those minerals that are found in the sedimen- 
tary strata are derived in great measure from rocks of 
igneous or metamorphic origin. The common con- 
stituents of the crystalline rocks are quartz, the felspars, 
the micas, the amphiboles, the pyroxenes, and the 
olivines; while chlorite, serpentine, talc, kaolinite, the 
carbonates of lime and magnesia (calcite, dolomite, 
etc.), hydrated silicates of alumina (clay), the zeolites, 
etc., are produced by their decomposition, and are 
termed secondary minerals. The relative proportion in 
which the minerals composing the superficial crust of 
the earth occur has been roughly calculated to be as 
follows : 

83 



84 DESCRIPTIVE MINERALOGY 

Felspar ... ... ... ... 48 per cent. 

Quartz ..; 35 

Mica, chlorite, talc, etc. ... ... 13 ,, 

Hornblende, augite, olivine, and 

serpentine ... ... ... i 

Carbonates of lime and magnesia i 

Clay (hydrated silicate of alumina) i ,, 
Other substances (including ores, 

salts, etc.) i 

These numbers serve to give a rough idea of the 
composition of the external layers of the earth. Lower 
zones must have a different composition ; for there the 
basic silicates, together with the heavy metals and their 
compounds, doubtless play a more important role. 

QUARTZ GROUP. 

Quartz. Pure silica. SiO 2 (silicon, 467). Crystal- 
lizes in the hexagonal (rhombohedral) system. Usual 
habit, prismatic with rhombohedral terminations. The 
prism faces have a characteristic horizontal striation ; 
occasionally they are absent, the crystals then consist- 
ing of the double pyramid of twelve faces. 

Other and rarer forms also occur; indeed, so numerous 
are they, and so varied their association, that no two 
quartz crystals can be said to be absolutely identical. 

The crystals are frequently distorted, the distortion 
being the result of the undue development of some of 
the faces at the expense of their fellows, but in every 
case the angle between any given pair of faces remains 



ROCK-FORMING MINERALS 



85 



constant. One kind of distortion gives rise to what is 
known as sceptre-quartz. 

When pure, quartz is colourless. Streak, white. 
Lustre, vitreous. Transparent. Index of refraction, 
1*551. Double refraction, weak (e &> = 0*009), positive. 
Rhombohedral cleavage, imperfect. Brittle. Fracture, 
conchoidal. Hardness, 7. Density, 2*65. 
Insoluble in acids (excepting hydrofluoric), 
in potash (distinction from opaline silica). 

The mineral may be recognized by its hardness, 



Infusible. 
Insoluble 





FIG. 58. QUARTZ. 

Bi-pyramidal crystal a com- 
bination of the rhombo- 
hedral forms R (1011) and 
z (oili). 



FIG. 59. QUARTZ. 
A combination of the rhom- 
bohedral forms R (roll), 
z (ion), and the prism M 
(lolo). 



pellucidity, and vitreous lustre. Usually it is quite 
colourless and transparent, but coloured and trans- 
lucent or even opaque varieties also occur e.g., brown 
or yellow (smoky quartz, cairngorm, jasper), pink (rose- 
quartz), purple (amethyst), green (chry sop rase), white and 
translucent (milky quartz). When pure and crystallized, 
it is known as rock-crystal; when crypto-crystalline, as 
chalcedony. 

The chief rocks in which quartz occurs are granite, * 
quartz-porphyry, felsite, rhyolite, gneiss, mica-schist, 



86 DESCRIPTIVE MINERALOGY 

quartzite, sand, sandstone, clay, and shale. It is also 
a common companion of the ores, being a frequent 
vein-stone, or gangue material. 

THE FELSPAR GROUP. 

Under the name felspar is included a series of im- 
portant rock-forming minerals, which, while varying in 
chemical composition, are similar in physical character 
and crystalline form. They are silicates of alumina, 
with one or more of the bases potash, soda, and 
lime and can be regarded as isomorphous mixtures 
of three primary minerals namely, potash -felspar 
K 2 O. Al 2 O 3 .6SiO 2 ; soda-felspar, Na 2 O.Al 2 O 3 .6SiO 2 ; and 
lime-felspar, CaO.Al 2 O 3 .2SiO 2 . These primary felspars 
exist in nature: the potash-felspar as orthoclase (pseudo- 
monoclinic) and microcline (triclinic); the soda-felspar, 
as albite (triclinic) ; and the lime-felspar, as anorthite 
(triclinic). 

There are two principal isomorphous series namely, 
a lime-soda series (the plagioclases) and a potash-soda 
series (anorthoclase, etc.), the former being the most 
important. 

If the symbol Ab be used to represent the albite 
molecule J(Na 2 O.Al 2 O 3 .6SiO 2 ), and the symbol An for 
the anorthite molecule CaO.Al 2 O n .2SiO 2 , then, as 
Tschermak has shown, the lime-soda or plagioclase 
series may be represented by the general formula 
m Ab + n An. The nature of the variation in chemical 
composition is illustrated by the following selected 
points in the series : 



ROCK-FORMING MINERALS 



87 





Percentage Composition. 








SiO 2 


A1 2 O 3 


NaaO 


CaO 


Ab 


687 


19-5 


u-8 


o 


Ab 3 Anj 


62-0 


24-0 


87 


5 '3 


Ab 2 An x 


6o-2 


25-2 


7'9 


67 


Abj An x 


55-6 


28-3 


57 


io'4 


Abj An 2 


5 J 7 


30-9 


4-0 


13-4 


Ab, An 5 


47-4 


33-8 


2'0 


16-8 


An 


43*2 


367 


O 


20' I 



For convenience, the following names are given to 
the intermediate members of the series : 

Oligoclase = Ab to Ab 3 An t 
Andesine = Ab 3 Ar^ to Ab x An x 
Labradorite = Ab,. Anj to Abj An 3 
Bytownite = Ab x A 3 to An 

The physical properties also show a progressive varia- 
tion from one end of the series to the other. Thus, the 
density and the index of refraction increase from the 
albite to the anorthite end, as will be seen in the 
following table : 



Formula. 


Density. 


Mean Index of 
Refraction. 


Ab - - 


2-605 


i'535 


Ab 3 An x - 


2-649 


J'545 


Ab 2 Ar^ 


2'66o 


i'549 


Ab-L Arij 


2-679 


i-558 


Abj An 2 


2-710 


1-567 


Abj An 5 ^ 


2733 


!'577 


An - 


2-765 


1-585 



88 



DESCRIPTIVE MINERALOGY 



The potash-soda series may be represented by the 
general formula (K,Na) 2 O.Al 2 O3.6SiO2. It is known 
as the anorthoclase or microcline-albite series. The 
first known member of this series was described by 
Forstner as soda-orthoclase from Pantelleria, near 
Sicily. Its chemical composition can be represented 
as 2(K 2 O.Al 2 O 3 -6SiO 2 ) +3(Na 2 O.Al 2 O 3 -6SiO 2 ). 

Orthoclase. K 2 O.Al 2 O 3 .6SiO 2 (potash, 17 ; alumina, 
18; silica, 65 per cent). Although now generally 
assumed to be triclinic, with pseudo-monoclinic sym- 
metry, orthoclase is still usually treated as a monoclinic 
mineral. The common forms are : 



Basal plane, p (ooi). 
Prism, / (no). 
Clinopinacoid, M (oio). 
Orthopinacoid, k (100). 

There 



Orthodome, x (101). 
Orthodome, y (201). 
Clinodome, n (021). 
Hemipyramid, o (in). 



are three chief crystal habits viz. : 
(i) elongated along the edge PM, as 
in the Baveno crystals ; (2) tabular 
along M, as in the variety known as 
sanidine from the Drachenfels ; (3) 
elongated vertically, with dominant 
prism faces, as in the variety known as 
adularia, from St. Gothard. 

Crystals of the sanidine and adularia 
habits generally have a roof-like termination produced 
by the combination of the basal plane (P) with one or 
other of the orthodomes x and y. 




ROCK-FORMING MINERALS 



Crystals of orthoclase are frequently twinned, most 
often on what is known as the Carlsbad type. In this 
the two individuals are usually united on the clino- 
pinacoid (M) ; and the basal planes (P) of the crystals 






a c 

FIG. 61. CRYSTALS OF ORTHOCLASE. 
a, Baveno habit ; b, sanidine habit ; c, adularia habit. 

are inclined in opposite directions, each being brought 
into close juxtaposition with the orthodome of the other 
individual. The relative position of two individuals 
twinned on this type can be explained by an imaginary 





FIG. 62. ORTHOCLASE CRYSTALS TWINNED ON THE CARLSBAD TYPE. 

rotation of one of them through 180 about the vertical 
axis. 

Another type of twinning is the Baveno, in which the 
crystals are twinned about the clinodome n (021), and 
united by that plane. Since the angle made by the 



90 



DESCRIPTIVE MINERALOGY 



basal plane (P), and by the clinopinacoid (M), with the 
clinodome n is, in each case, very nearly 45, crystals of 
the Baveno habit, twinned on the Baveno type, differ 
very little in shape from the untwinned individuals ; 
but that they are twins is shown by the position of the 
cleavages. 

A third type of twinning is the Manebach, in which 
the crystals are twinned about the basal plane P (ooi), 
and united by that plane. The faces M of the two 
individuals fall into the same plane. 





FIG. 63. ORTHOCLASE 
TWINNED ON THE 
BAVENO TYPE. 



FIG. 64. ORTHOCLASE 
TWINNED ON THE 
MANEBACH TYPE. 



When pure and free from inclusions, orthoclase is 
colourless and transparent (adularia, sanidine) ; but 
more frequently it is opaque, and either white, flesh- 
coloured, or red. Although this opacity and coloration 
may in some instances be due to extraneous matter 
included in the crystal during its formation, it is more 
frequently the result of alteration produced by weather- 
ing. During this process the felspar substance under- 
goes chemical change, kaolin (a hydrated silicate of 
alumina) being formed, while silica is set free. In 



ROCK-FORMING MINERALS 91 

certain cases muscovite and talc are also products of its 
decomposition. 

Streak, colourless. Lustre, vitreous. Index of re- 
fraction, 1*524. Double refraction, weak (7 - a 
= 0*006). Fracture, conchoidal. Brittle. Hardness, 6. 
Density, 2*5. Fusibility, 5. Insoluble in acids. 

There are two perfect cleavages namely, parallel to 
the basal plane, P, and to the clinopinacoid, M ; and it 
is noteworthy that the cleavage planes intersect at right 
angles. There is also an imperfect prismatic cleavage. 

Orthoclase is an essential constituent of the more^ 
acid igneous rocks, such as granite, syenite, and por-T) 
phyry, also of the foliated granitic rocks or gneisses. 
The clear fissured sanidine variety is common in the 
volcanic rocks rhyolites and trachytes. It is found 
in those sedimentary rocks which are derived directly 
from the waste of granitic rocks, such as felspathic 
grits and sandstones. Orthoclase is also common as a 
vein-stone, being a chief constituent of the pegmatites. 

Microcline. Triclinic potash felspar. Identical in 
chemical composition with orthoclase. The angle 
made by P (ooi) with M (oio) is about 89 30'. A 
characteristic feature of microcline is its polysynthetic 
twinning on both the albite and pericline types. The 
resulting twin-lamellation is in two directions at right 
angles, producing a rectangular cross-hatching, which 
is especially visible in suitably oriented thin sections 
when examined between the crossed nicols of a polarizing 
microscope. In other physical properties microcline 
does not differ from orthoclase ; and it is to be noted 



92 DESCRIPTIVE MINERALOGY 

that if we imagine microcline crystals to be polysyn- 
thetically twinned on an ultra-microscopic scale, such 
crystals would be indistinguishable from orthoclase : 
on this reasoning some authors regard orthoclase as 
triclinic, with pseudo-monoclinic symmetry acquired by 
twinning. 

Albite. Na 2 O.Al 2 O 3 .6SiO 2 (soda, ir8; alumina, 19-5; 
silica, 68*7 per cent.). Crystallizes in the triclinic 
system, the angle between the basal plane (P) and the 
brachypinacoid (M) being 86 24'. 

Common forms are : 



Basal plane, P (ooi). 
Prisms, T (110) and / (no). 
Brachypinacoid, M (oio). 
Macropinacoid, k (100). 



Macrodome, x (ioi). 
Macrodome, y (201). 
Brachydome, n (021). 
Pyramid, o (ill). 




Twinning on the Carlsbad, Baveno, and Manebach 
types, as in orthoclase; but the charac- 
teristic feature of albite, and, indeed, of 
all plagioclase felspars, is the Albite type 
of twinning, in which the crystals are 
twinned about the brachypinacoid (M), and 
FIG 65 united on that plane. 

In this type there is usually a repeated 
or polysynthetic arrangement of individuals, the crystal 
being divided into a number of parallel lamellae, each of 
which is in the twinning position with regard to its 
immediate neighbours ; alternate lamellae, however, are 
similarly situated. The structure of the twinned crystal 
can be explained by an imaginary rotation of alternate 



ROCK-FORMING MINERALS 



lamellae about an axis normal to the brachypinacoid (M). 
The basal planes of all the lamellae lie on the same side 
of the crystal, but slope alternately towards and away 
from one another so as to produce a series of parallel 
ridges and depressions, the general effect of which is a 
parallel striation parallel to M, which is best seen on the 
surfaces produced by the basal cleavage. 

In the Pericline type the crystals are twinned about 
a plane normal to the brachydiagonal, and the relative 
position of the two individuals may be imagined as 





FlG. 66. POLYSYNTHETIC 

TWIN OF PLAGIOCLASE. 
("Albite-type.") 



FIG. 67. DUAL TWIN OF 
ALBITE. 

P, Basal plane of first individual ; 
P!, basal plane of second indi- 
vidual. 



produced by rotation through 180 of one of them about 
the brachydiagonal. In this type the M faces of the 
two individuals do not fall into the same plane. 

Albite is usually colourless and transparent. Streak, 
colourless. Lustre, vitreous. Index of refraction, 1*533. 
Double refraction, positive and moderate (7 - a = 0*008). 
Basal cleavage, perfect. The brachypinacoidal and 
prismatic cleavages, imperfect. Fracture, uneven. 
Brittle. Hardness, 6. Density, 2*63. Fusibility, 4. 
Insoluble in acids. 



94 DESCRIPTIVE MINERALOGY 

Albite and orthoclase occur in intimate intergrowth, 
the albite being in narrow lamellae intercalated in the 
orthoclase along planes parallel to the orthopinacoid. 
In sections parallel to the basal plane or to the clino- 
pinacoid, the included albite appears as strips and 
patches, which are distinguishable from the orthoclase 
by their twin striation. Such intergrowths are known 
as perthite and microperthite. 

Anorthite. CaO.Al 2 O 3 .2SiO 2 (lime, 2O'i; alumina, 
367 ; silica, 43*2 per cent.) Crystallizes in the triclinic 
system, the angle between the basal plane (P) and the 
brachypinacoid (M) being 85 50'. The habit of the 
crystals is varied. Twinning occurs on the Albite, 
Pericline, Manebach, and Carlsbad types. Colourless. 
Streak, colourless. Lustre, vitreous. Transparent. 
Index of refraction, 1*585. Double refraction, negative 
and moderate (7 a = 0*013). Basal cleavage, perfect. 
Brachypinacoidal cleavage, less perfect. Fracture, 
conchoidal. Brittle. Hardness, 6*5. Density, 275. 
Fusibility, 5. Decomposed by hydrochloric acid with 
gelatinization. 

The isomorphous series of plagioclase felspars formed 
by albite and anorthite has already been dealt with 
(p. 86). 

The plagioclase felspars occur in many igneous rocks 
e.g., in the intrusive diorites, gabbros, dolerites, and 
in the volcanic andesites, porphyrites, and basalts. The 
variety oligoclase often accompanies orthoclase in 
granite and trachyte. Oligoclase and albite are also 



ROCK-FORMING MINERALS 95 

frequent constituents of the schists, occurring in them 
in a clear granular form in association with quartz 
(secondary or granulitic felspar). 

Like orthoclase, plagioclase is prone to decom- 
position, giving rise to epidote, zoisite (in the so-called 
saussurite), calcite, and kaolin. 

THE FELSPATHOID GROUP. 

Nepheline. Orthosilicate of soda, potash, and 
alumina: (NaK) 2 O.Al 2 O 3 .2SiO 2 . Pure sodium-nepheline 
which has been prepared artificially has the composition: 
SiO 2 = 42*3, Al 2 O 3 = 35*g, Na 2 O = 2i*8; but in nepheline, 
as it occurs naturally, the proportion of Na 2 O to K 2 O 
is usually about 5 : i. Crystallizes in the hexagonal 
system in small six-sided prisms (combination of hex- 
agonal prism and basal plane). Colourless to white 
or grey. Transparent to translucent. Lustre, vitreous 
to resinous. Streak, white. Index of refraction, 1*54. 
Double refraction, negative and weak (co e = '005). 
Basal and prismatic cleavages, imperfect. Hardness, 
5*5-6. Density, 2*6. Fusibility, 4. Soluble in hydro- 
chloric acid with separation of gelatinous silica, the 
solution giving cubes of common salt, when evaporated. 

Nepheline is found in the cavities of volcanic ejected 
blocks (Monte Somma, Laacher See) ; as an essential 
constituent of certain lavas (phonolite, nepheline- 
basalt, tephrite, etc.) ; and as a constituent of the 
soda-series of the plutonic rocks (syenites and alkali- 
gabbros). 



96 DESCRIPTIVE MINERALOGY 

Leucite. Metasilicate of alumina and potash : K 2 O. 
Al 2 O 3 .4SiO 2 (silica, 55; alumina, 23-5; potash, 21-5 
per cent.) Pseudo-regular. 

This mineral occurs crystallized in icositetrahedra, 
and no doubt at the temperature at which it was formed 
crystallized in the regular system, but on cooling broke 
up into rhombic or monoclinic sectors. In consequence 
the crystals exhibit weak double refraction ; but when 
heated to 500 C. they become optically isotropic. 
Index of refraction, 1*508. Colour, dirty white or grey. 
Lustre, vitreous. Transparent to opaque. Streak, 
white. Prismatic cleavage very imperfect. Brittle. 
Fracture, uneven to conchoidal. Hardness, 5 - 6, 
Density, 2*45 -2*50. Infusible. Slowly decomposed 
by hydrochloric acid with separation of silica. 

Leucite occurs as a constituent of the more recent 
volcanic rocks leucitophyre, leucite-tephrite, and leuci- 
tite; also in members of the alkali series of the plutonic 
rocks. 

Sodalite. Chloro-orthosilicate of aluminium and 
sodium: 3SiO 2 .Al 2 O 3 .(AlCl)O.2Na 2 O. Regular, with 
cubic habit ; also massive. Colourless to yellowish ; 
greenish white; or pale blue. Index of refraction, 1*484. 
Lustre, vitreous. Transparent to opaque. Streak, 
white. Cubic cleavage, fair. Fracture, conchoidal to 
uneven. Hardness, 5'5 - 6. Density, 2*2- 2*4. Fusi- 
bility, 3*5 - 4. Gelatinizes easily with hydrochloric 
acid. 

Occurs in blue, greenish, or colourless grains in 
syenites and in volcanic ejectamenta. 



ROCK-FORMING MINERALS 97 

Haiiyne and Nosean. Isomorphous sulpho-ortho- 
silicates of alumina, lime, and soda. The soda end of 
the isomorphous series with little or no lime is nosean 
= 3SiO 2 .Al 2 O 3 .(AlNaSO 4 )O.2Na 2 O, with SiO 2 = 317, 
SO 3 = 14-1, A1 2 O 3 = 26-9, and Na 2 O = 27-3. When 
Na 2 : Ca = 3 : 2, the composition of haiiyne is SiO 2 = 
32*0, SO 3 = 14*2, A1 2 O 3 = 27*2, CaO = 10*0, Na 2 O = 16*6. 

These minerals crystallize in the regular system. 
Habit, dodecahedral. Colour, blue (haiiyne) or grey 
(nosean). Lustre, vitreous. Transparent to opaque. 
Streak, white. Index of refraction, 1*496. Dodeca- 
hedral cleavage, fair. Fracture, sub - conchoidal to 
uneven. Hardness, 5-6. Density, 2'25-2*5. Gelatinize 
easily with hydrochloric acid ; on evaporation, needles 
of gypsum are formed in the case of haiiyne, none in 
the case of nosean. 

Haiiyne and nosean are essentially volcanic minerals 
occurring in volcanic ejectamenta and in phonolites, 
andesites, and basalts. 

Melilite. Silicate of alumina, iron, lime, magnesia, 
and soda: i2(Ca,Mg)O.2(Al,Fe) 2 O 3 .9SiO 2 . Tetragonal; 
occurring in small square tables and prisms, also in 
irregular grains. Colour, white to yellow. Lustre, 
vitreous. Translucent. Index of refraction, 1*629. 
Double refraction, weak. Hardness, 5-5*5. Density, 
2'g-3'i. Fracture, conchoidal to uneven. Brittle. 
Fusibility, 4. Gelatinizes easily with hydrochloric acid. 

Occurs as a constituent of certain basalts (melilite- 
basalt) and of nepheline and leucite rocks. 



98 DESCRIPTIVE MINERALOGY 

THE SCAPOLITE GROUP. 

Silicates of alumina, lime, and soda + sodium chloride. 
The scapolite group, like the lime -soda plagioclase 
group, may be regarded as isomorphous mixtures of 
two molecules viz., the meionite (Me) molecule 
(4CaO.3Al 2 O 3 .6SiO 2 ), with silica = 40*5, alumina = 34*4, 
and lime = 25*1 per cent, and the marialite (Ma) mole- 
cule (Na 4 Al 3 Si 9 O24Cl), with silica = 63-9, alumina = 18*1, 
soda = 147, and chlorine = 4*20 per cent, (oxygen for 
chlorine to be deducted). Wernerite includes scapolites 
with Me : Ma ranging from 3 : i to i : i ; Mizzonite 
those with Me : Ma ranging from i : 2 to i : 3. Couse- 
ranite and Dipyre are varieties of mizzonite. 

The scapolites crystallize in the tetragonal system, 
with prismatic habit. Colourless to white, also bluish, 
greenish, or reddish. Streak, white. Lustre, vitreous. 
Index of refraction, 1*55-1*59. Cleavage parallel to 
the prism of the second order (100), fair. Fracture, 
conchoidal to uneven. Brittle. Hardness, 5 - 6. 
Density, 2*57-2*74. 

Of infrequent occurrence in igneous rocks; oftener 
in gneisses and crystalline schists and in contact-altered 
limestones, calc-silicate rocks, etc. 

THE MICA GROUP. 

The micas are hydrated * silicates of alumina and the 
alkalies, potash, soda, or lithia, with which iron and 

* Most of the water is only given off at a high temperature, and 
must be regarded as water of constitution.] 



ROCK-FORMING MINERALS 99 

magnesia are associated in some varieties. They 
crystallize in the monoclinic system, but possess pseudo- 
hexagonal symmetry. The crystals consist of six-sided 
tablets, of which the six sides are made up of the four 
faces of a prism, and two of the clino-pinacoid, the 
broad terminal faces being those of the basal plane. 
Mica has a very perfect cleavage parallel to the basal 
plane, permitting of its separation into laminae of 
extraordinary thinness ; it is characteristic for mica, as 
distinguished from other allied minerals (talc, chlorite), 
that these laminae are elastic, and cannot therefore 
be permanently bent. The density 
varies from 2*76 to 3*2. Hard- 
ness, 2-3. 

For practical purposes the micas FIG. 68. MICA- 
may be conveniently separated into CRYSTAL. 

i i i j ji i -i Basal plane- M 

two broad subdivisions: the white or prism- A clino- 

light-coloured. and the black or dark- pinacoid;'*,clino- 

dome. 

coloured varieties. Chief among the 
former is muscovite, or potash-mica, which is essenti- 
ally a hydrated silicate of alumina and potash. This 
variety of mica is not attacked by hydrochloric acid. 
It is generally pale-coloured to silvery white, with 
pearly lustre on the cleavage surfaces. It occurs in 
flakes, scales, and laminae, in many granites, gneisses, 
and phyllites. Fragments are often present in sand- 
stones and shales (derived, no doubt, originally from 
granitic rocks) ; and by their parallel arrangement 
impart to these rocks a fissile character. 




100 DESCRIPTIVE MINERALOGY 

Less common than muscovite are the following light- 
coloured micas : 

Paragonite, or soda-mica. 
Lepidolite, or lithia-mica. 

Biotite. The most important dark mica ; essentially 
a hydrated ferro-magnesian and aluminous silicate. 
Unlike muscovite, this mica is attacked by hot hydro- 
chloric acid. It has a dark brown to black colour and 
sub-metallic lustre, and is transparent to opaque. It 
occurs in granites and mica-traps and in certain syenites, 
diorites, trachytes, and andesites. Loose crystals of a 
reddish-brown biotite (vubellan) are frequently found im- 
bedded in volcanic tuff. By decomposition mica readily 
passes into chlorite, assuming then a green colour. 

Those varieties in which there is much magnesia and 
little iron are distinguished as phlogopite. They are 
generally somewhat lighter in colour than biotite. 
Those which are rich in lithium and iron are known as 
zinnwaldite. 

The micas may also be conveniently classified by 
the relation of their percussion-figures to the position 
of the plane of the optic axes as shown by the inter- 
ference figure obtained by the examination of a cleavage 
flake under the microscope in convergent polarized light. 
The percussion-figure is a 6-rayed star obtained by 
driving a needle into a cleavage flake by means of 
a sharp blow with a light hammer. It will be found 
that one of the rays is either parallel to, or at right 
angles to, the plane of the optic axes. This, therefore, 



ROCK-FORMING MINERALS 



101 




gives the direction of the plane of symmetry. Micas 

in which the plane of the optic axes is perpendicular 

to the plane of symmetry are 

known as micas of the first class, 

and include all the alkali-micas; 

those in which the optic axial 

plane is parallel to the plane of 

symmetry are known as micas 

of the second class, and include 

biotite, phlogopite, and zinnwal- 

dite. Those anomalous ferro- 

magnesian micas which are 

found to belong to the first class 

are named anomite by Tscher- 

mak, to whom this classification owes its origin. 



FIG. 69. MICA, SHOWING 
THE PERCUSSION FIGURE 
(THE 6- RAYED STAR, OF 
WHICH THE LONGEST RAY 
is PARALLEL TO THE 
PLANE OF SYMMETRY, 
b), AND THE PRESSURE 
FIGURE (THE 6-RAYED 
STAR, SHOWN BY PECKED 
LINES). 



THE AMPHIBOLE AND PYROXENE GROUPS.* 

These minerals are silicates, mainly of magnesia and 
lime; but some varieties contain iron and alumina or 
manganese, soda or lithia in addition. All the varieties 
resist the action of acids excepting hydrofluoric. Their 
density varies from 2*90 to 3-55 ; their hardness from 
5 to 6. The commoner species of both groups crystal- 
lize in the monoclinic system, but rhombic and triclinic 
varieties also occur. The main feature distinguishing 
the amphiboles from the pyroxenes is the angle between 

* " Amphibole " and " pyroxene " are used here as group names, 
while "hornblende" and " augite " are reserved for the specific 
rock-forming varieties which crystallize in the monoclinic system. 



102 



DESCRIPTIVE MINERALOGY 



the faces of the prism, which in the former measures 
124, in the latter 87. 

The crystals are usually short-columnar, and consist 
of prisms and pinacoids, terminated by a pair of 
pyramidal faces. In the amphiboles the prism-faces 
usually predominate over the pinacoids, the ortho- 
pinacoidal faces being often even absent ; in the 
pyroxenes, on the other hand, they are about equally 
developed. The cross-section of an amphibole crystal 





FIG. 70. CROSS-SECTION OF AN 
AMPHIBOLE CRYSTAL, SHOWING 
THE LINES OF PRISMATIC 
CLEAVAGE INTERSECTING AT AN 
ANGLE OF 124. 



FIG. 71. CROSS-SECTION OF A 
PYROXENE CRYSTAL, SHOWING 
THE LINES OF PRISMATIC 
CLEAVAGE INTERSECTING AT 
AN ANGLE OF 87. 



is consequently lozenge-shaped ; that of a pyroxene 
octagonal. 

The crystals are occasionally twinned namely, on 
the orthopinacoid (see Fig. 71). A cleavage exists in 
both minerals parallel to the faces of the prism. In 
the amphiboles it approaches a high degree of perfection, 
the cleaved surfaces being smooth and lustrous ; but in 
the pyroxenes it is far less perfect, and the cleavage 
surfaces are consequently uneven. 



ROCK-FORMING MINERALS 



103 



The common variety of both groups is a black 
mineral ; but green, blue, and white varieties also occur. 

The chemical constitution of the more important 
pyroxenes and amphiboles is given in the following 
table : 



PYROXENES. 

Rhombic. 

Enstatite MgO.SiO 2 . . 
Hypersthene 
(Mg,Fe)O.Si0 2 . 

Monoclinic. 
Diopside MgO.CaO.2SiO 2 



Augite 

CaO.MgO.2SiO 2 

+ MgO.(Al,Fe) 2 3 .Si0 2 . 
JEgmne (Acmite) 

Na 2 O.Fe 2 O 3 .4SiO 2 . 



Triclinic. 
Rhodonite MnO.SiO 9 . 



AMPHIBOLES. 
Rhombic. 

Anthophyllite (Mg,Fe)O.SiO 2 . 

Monoclinic. 
Tremolite CaO.3MgO.4SiO 2 . 

Actinolite 

Ca0.3(Mg,Fe)0.4Si0 2 . 

Hornblende 

CaO.3(MgFe)O.4SiO 2 

+ Ca0.2MgO.Al 2 O 3 .3SiO 2 . 
Arfvedsonite 

4(Na 2 ,Ca,Fe)O.4SiO 2 

+ 2(Ca,Mg)O.2(Al J Fe) 2 O 3 .2SiO 2 . 

Triclinic. 

^nigmatite (Cossyrite) 

2Na 2 O.9FeO.(Al,Fe) 2 O 3 .i2SiO 2 . 



The following facts emerge from a study of the 
above table : (i) that these minerals are, in the main, 
metasilicates ; (2) that the pure metasilicate of mag- 
nesia is rhombic, that of lime and magnesia, monoclinic,* 
and that of manganese, triclinic; (3) that the mono- 
clinic varieties, known as augite and hornblende, are 
characterized by the presence of the sesquioxides of 

* The pure lime metasilicate, wollastonite, is monoclinic. By 
some authors it is considered to be a pyroxene, although it does 
not possess the characteristic prismatic cleavage. 



104 



DESCRIPTIVE MINERALOGY 



aluminium and iron ;* (4) that in the monoclinic class 
the amphibole molecule is double that of the pyroxenes, 
which explains why, at high temperatures, augite is 
more stable than hornblende. 

Diopside Augite. Crystallizes in the monoclinic 
system with short - columnar habit in the direction 
of the vertical axis. 

The common forms are : 



Orthopinacoid, a (100). 
Clinopinacoid, b (oio). 



Prism, m (no). 
Hemipyramid, s (in). 



M : m = 92 50'. 

Twinning on the orthopinacoid, a (100). 
Colour, various shades of green to colourless (diop- 
side) ; also brown to black (augite). Lustre, vitreous. 





FIG. 72. AUGITE. 

a, Orthopinacoid ; b, clinopinacoid ; 
s, hemipyramid ; m, prism. 



FIG. 73. TWINNED CRYSTAL 
OF AUGITE. 



Transparent to opaque. The plane of the optic axes 
coincides with clinopinacoid. Index of refraction, 1*7 ; 
double refraction, positive and strong (7 -a = 0*030). 
Streak, white to grey. Prismatic cleavage, fairly perfect. 

* Augite may be regarded as a combination of ;;/ molecules of 
diopside with n molecules of Tschermak's silicate 
(MgFe)0.(AlFe)-A.SiO 2 . 



ROCK-FORMING MINERALS 



105 



Parting, parallel to the orthopinacoid (diallage). Frac- 
ture, uneven to subconchoidal. Hardness, 5-6. Density, 
3-2-3-6. Fusibility, 4. Insoluble in acids. 

Common augite occurs in dolerite, basalt, and in 
certain trachytes and andesites. The green and white 
varieties (diopside) are found in peridotites, and as an 
accessory constituent in some metamorphic limestones. 
Diallage is an essential constituent of gabbro; while the 
rhombic pyroxenes occur in some varieties of diorite, 
gabbro, dolerite, andesite, and peridotite. 

Actinolite Hornblende. Crystallizes in the mono- 
clinic system, with long or short columnar habit, due 





FIG. 74. HORNBLENDE. 

m, Prism ; b, clinopinacoid ; 
r, hemipyramid. 



FIG. 75. TWINNED CRYSTAL 
OF HORNBLENDE. 



to the predominance of the prism, m (no), frequently 
without the orthopinacoid, a (100), but seldom without 
the clinopinacoid, b (oio) (see Fig. 72). 

Twinning about the orthopinacoidal plane is frequent. 

Colour, various shades of green to almost colourless 
(actinolite) ; also dark brown to black (hornblende). 
Streak, white to grey. Lustre, vitreous. Transparent 
to opaque. Index of refraction, i '64. Double refraction 



106 DESCRIPTIVE MINERALOGY 

strong, negative. Optic axial plane, the clinopinacoid. 
Prismatic cleavage, perfect. Fracture, uneven to sub- 
conchoidal. Brittle. Hardness, 5-6. Density, 2*9-3-4. 
Fusibility, 3-4. Insoluble in acids. 

Common hornblende occurs in certain varieties of 
granite and syenite, also in diorite, trachyte, and ande- 
site. Actinolite is found in blades, needles, and fibres 
in schists, amphibolites, and epidiorites. Nephrite, a 
variety of actinolite, forms a closely knit plexus of 
minute fibres and blades in the hard and tough sub- 
stance which is so much prized under the name of jade. 
Tremolite occurs in metamorphic limestones. 

THE OLIVINE GROUP. 

The olivines are ortho-silicates of lime, magnesia, 
iron, and manganese. They crystallize in the rhombic 
system, and form an isomorphous series, of which the 
following are the chief members : 

Forsterite: 2MgO.SiO 2 . 
Monticellite : CaO.MgO.SiO 2 . 
Fayalite: 2FeO.SiO 2 . 
Tephroite: 2MnO.SiO 2 . 
Common Olivine : 2(Mg,Fe)O.SiO 2 . 

Common olivine occurs in tabular or prismatic com- 
binations of pinacoids and domes ; also in irregular 
grains. Twinning on the brachydome (on). Colour, 
black, olive-green, or yellow. Lustre, vitreous. Trans- 
parent to translucent. Index of refraction, 1-678. 



ROCK-FORMING MINERALS 107 

Double refraction, positive and strong (7 -a = '036). 
Optic axial plane (ooi). Brachy-pinacoidal cleavage, 
imperfect. Brittle. Fracture, conchoidal. Infusible. 
Decomposed by hydrochloric acid with 
separation of gelatinous silica. 

Certain yellowish-green and leek- 
green varieties of olivine are used as 
gem-stones, under the names of chryso- 
lite and peridote. FlG - 76.-OLivi NE . 

,. . . c, Basal plane ; a, 
As a rock constituent, ollVine IS macro - pinacoid ; 

characteristic of the basic and ultra- 




basic rocks, occurring in basalts, dole- m, prism ; tf, macro- 

1 i f dome. 

rites, and gabbros, and being the cruet 

constituent of the peridotites. Under the influence of 

the weather and percolating waters it is peculiarly liable 

to alteration, giving rise in some cases to serpentine, 

with separation of iron ore, in others to dolomite or 

calcite. 

SECONDARY MINERALS. 

The rock constituents described above have been 
produced by crystallization from a molten, or at least 
a plastic, condition ; but a considerable proportion of 
the rocks that make up the earth's crust are composed 
of minerals that have been accumulated and deposited 
by water. Such rocks, for instance, are the sandstones, 
clays, shales, and many limestones. The minerals com- 
posing these rocks are of earlier origin than the rocks 
themselves, being derived from pre-existing mineral 
aggregates by processes of disintegration and denuda- 



108 DESCRIPTIVE MINERALOGY 

tion. These mineral aggregates, of course, may also 
have been of aqueous origin, but if we could trace back 
the history of such a series of changes sufficiently far, 
we should arrive finally at a primary crust, which must 
have had an igneous birth. The part played by water 
in the disintegration and decomposition of rocks, and 
in the distribution and rearrangement of the materials 
thus produced, is best exemplified by a specific case. 
Granite is a rock composed chiefly of quartz, felspar, 
with one or more micas or other ferro-magnesian con- 
stituent. Submitted to meteoric influences (rain, frost, 
percolating water, etc.), it decomposes into a loose, 
crumbling mass, which is ultimately washed away to 
form new combinations. Let us endeavour to trace 
the history of the three constituent minerals. First 
the quartz : this mineral, though chemically unaffected, 
becomes mechanically separated from its associates, 
and reduced by trituration to small partially rounded 
grains, in which condition it goes to form deposits of 
sand, and these when consolidated give rise to sand- 
stone. Mixed with the quartz, one would naturally 
expect to find occasional fragments of felspar and of 
light mica ; and such indeed is the case, as" may be 
seen in certain felspathic and micaceous varieties of 
sandstone. 

But most of the felspar is decomposed under the 
influence of acid surface waters : its alkalies are re- 
moved in solution as carbonates, and there remain 
behind certain hydrated silicates of alumina, which, 
although they constitute several distinct minerals, are 



ROCK-FORMING MINERALS 109 

for convenience generally referred to as kaolin. These 
substances are also found in clays, which are in great 
part derived from the decomposition of felspathic 
rocks. 

Finally, the dark mica, or the ferro-magnesian con- 
stituent, is first converted into chlorite, which may in 
turn undergo decomposition, the products being re- 
moved in solution. 

With regard to the dissolved portions, the alkalies 
(potash and soda) and alkaline earths (lime and 
magnesia) unite with materials derived from other 
sources to form chemical precipitates of various 
salts e.g., calcite, dolomite, magnesite, gypsum, rock- 
salt, etc. 

Most of these minerals are included with the salts 
in Chapter III. (p. 208); but a few notes on chlorite, 
serpentine, talc, kaolinite, epidote, and the zeolites are 
appended here. 

Chlorite Group. The minerals included under this 
head are hydrated silicates of magnesia, iron and 
alumina, the water of which (about 12 per cent.) is 
only given off at a high temperature. Monoclinic, with 
pseudo-hexagonal symmetry. Generally in small scales 
or fibres, sometimes aggregated to spherular or spiral 
forms. 

The well-crystallized varieties are known as ortho- 
chlorites ; those that occur in scales and fibres, leptochlo- 
rites. The orthochlorites are regarded by Tschermak as 
isomorphous mixtures of a serpentine molecule (Sp) 
and an amesite molecule (At). 



110 DESCRIPTIVE MINERALOGY 

The isomorphous series may be represented thus : 

Serpentine : Sp = 2H 2 O.3(Mg,Fe).O.2SiO 2 . 

Penninite : Sp. At. 

Prochlorite : Sp 3 , At l7 . 

Corundophilite : Sp. At 4 . 

Amesite : At = 2H 2 O.2(Mg,Fe)O.Al 2 O 3 .SiO 2 . 

The chlorites are dark green in colour. Streak, 
white. Index of refraction, r6. Double refraction, 
weak (y - a = 0*003). Basal cleavage, perfect. The 
cleavage flakes are pliable, not elastic, as with mica. 
Hardness, 2-3. Density, 2*6-3*0. Fusible with diffi- 
culty. Decomposed by sulphuric acid. 

In one form or another the chlorites are a very 
frequent alteration product, especially in the more basic 
igneous rocks. The green colour of many of the latter 
(greenstones) is due to the presence of the mineral. 

Serpentine. Hydrated silicate of magnesia and iron : 
2H 2 O.3(Mg,Fe)O.2SiO 2 . Crystal system, uncertain. 
Occurs massive as an aggregate of blades, scales, and 
fibres. Colour, dull green, often stained red and yellow 
by iron oxides. Lustre, dull to resinous. Translucent 
to opaque. Streak, white to grey. Hardness, 3-4. 
Density, 2'5-2'7. Index of refraction, rtf. Double 
refraction, negative, moderate (y - a = o*oi). Fusi- 
bility, 6. Decomposed by hydrochloric acid. 

Occurs as an alteration product of olivine and other 
ferro-magnesian silicates. Veins of fibrous serpentine 
(chrysotile) are worked as a source of commercial 
asbestos. In these the direction of the fibres is at 



ROCK-FORMING MINERALS 111 

right angles to the vein, whereas in fibrous hornblende, 
which is also known and used as asbestos, the long axis 
of the fibres is parallel to the seams in which they occur. 

Talc, steatite, or soapstone, is a hydrated silicate 
of magnesia (H 2 O.3MgO.4SiO 2 ) crystallizing in the 
monoclinic system, but with pseudo-hexagonal sym- 
metry. It is a pale green or colourless mineral, very 
soft, and with a greasy feel. Lustre, pearly. Trans- 
lucent. Index of refraction, 1*55. Double refraction, 
strong (y -a = 0*040). Hardness, i. Density, 27-2*8. 
Cleavage, basal; flakes, non-elastic. Fusibility, 6. In- 
soluble in acids. Talc occurs as an alteration product 
of magnesian minerals in igneous and metamorphic 
rocks. 

Kaolinite is a constituent of kaolin, and probably 
of all clays. It is a hydrated silicate of alumina 
(2H 2 O.Al 2 O 3 .2SiO 2 ), crystallizing in the monoclinic 
system, but with pseudo - hexagonal symmetry. It 
occurs in minute colourless six-sided plates and scales. 
Hardness, 1-2. Density, 2-34-2-57. 

Kaolinite is one of the minerals produced in the 
alteration of felspar. Under the influence of the 
change known as kaolinization, felspar loses its glassy 
appearance, becomes dull, and finally crumbles down 
into a white mealy powder (kaolin), which contains 
kaolinite. Kaolin mixed with quartz is, consequently, 
often found in the immediate neighbourhood of granite. 

Epidote. Silicate of lime, alumina, and iron : 
H 2 O.4CaO.3(Al,Fe) 2 O3,.6SiO 2 . Monoclinic with elon- 



112 DESCRIPTIVE MINERALOGY 

gated habit parallel to the orthodiagonal axis. Colour, 
yellowish green. Lustre, vitreous. Translucent. 
Streak, grey. Index of refraction, 1*75. Double re- 
fraction, negative, strong (y a = 0-04), Perfect cleavage 
parallel to the basal plane. Hardness, 6*5. Density, 
3*4. Fracture, uneven. Fusibility, 3*5. Partially de- 
composed by hydrochloric acid. 

Epidote occurs as a frequent alteration product of 
the ferro-magnesian minerals, in gabbros, diorites, 
epidiorites, and hornblendic and chloritic schists. 
It is also found veining these rocks, or associated 
with other secondary minerals in the amygdules of 
old lavas. 

The Zeolite Group. A group of hydrated silicates 
of various bases : alumina, potash, soda, lime, baryta, 
and strontia. They are secondary products, occurring 
in igneous rocks as the infillings of amygdaloidal cavities 
(especially of melaphyres and basalts), or as pseudo- 
morphs after decomposed minerals (e.g., nepheline). 

Generally speaking, the zeolites are colourless to 
white, and occur in fibrous and radiate aggregates. 
The index of refraction is low. All of them also 
exhibit low double refraction. They are easily decom- 
posed by hydrochloric acid, with separation of gelatinous 
silica. 

Some of the more commonly occurring varieties are : 
heulandite, natrolite, analcime, phillipsite, laumontite, 
scolecite, and apophyllite. The composition of these 
is given in the following tables : 



ROCK-FORMING MINERALS 



113 



Name. 


Crystallographic 
System. 


Formula. 


Heulandite ... 


Monoclinic 


H 4 CaAl 2 (SiO 3 ) 6 + 3H 2 O 


Natrolite 


Rhombic 


Na 2 Al 2 Si 3 O l0 + 2H 2 O 


Analcime 


Regular 


NaAl(SiO 3 ) 2 + H 2 O 


Phillipsite 
Laumontite ... 
Scolecite 


Monoclinic 
Monoclinic 
Monoclinic 


(K 2 ,Ca)Al 2 (Si0 3 ) 4 + 4^H 2 
H 4 CaAl 2 Si 4 O 14 + 2H 2 O 
Ca(AlOH) 2 (SiO 3 ) 3 + 2H 2 O 


Apophyllite ... 


Tetragonal 


H 7 KCa 4 (Si0 3 ) 8 + 4irH 2 



NT 


Percentage Composition. 




so* 


A1 2 3 . 


CaO. 


Na 2 0. 


K 2 0. 


H 2 0. 


Heulandite 


59-2 


16-8 


9'2 


_ 


_ 


14-8 


Natrolite 


47*4 


26-8 




16-3 





9'5 


Analcime 


54'5 


23-2 





14-1 





8-2 


Phillipsite 


48-8 


207 


7'6 




6-4 


i6'S 


Laumontite 


51-1 


217 


11-9 







I5'3 


Scolecite 


45*9 


26*0 


i4'3 








irs 


Apophyllite 


537 


~ 


25-0 


5-2 





16-1 



MINERALS OF THE GRANITE CONTACT ZONE. 

Andalusite. Silicate of alumina : Al 2 O 3 .SiO 2 (silica, 
36*8; alumina, 63*2 per cent.). Rhombic; occurring in 
square thick-set prisms, terminated by the basal plane. 

Crystals of this mineral usually appear dark-coloured 
(reddish-brown), owing to the presence of included 
carbonaceous matter. In thin section, however, the 
grains are either colourless or pink. Lustre, vitreous. 
Translucent. Streak, colourless. Index of refraction, 



114 DESCRIPTIVE MINERALOGY 

1*638. Double refraction, negative, moderate (y-a 
on). Cleavage prismatic. Fracture, uneven. Brittle. 
Hardness, 7-7*5. Density, yi-^'z. Infusible. In- 
soluble in acids. Occurs in slates and shales that have 
undergone metamorphism in contact with granite ; also 
in gneisses and crystalline schists, and as an accessory 
constituent of granite (e.g., at the Cheesewring in 
Cornwall). Chiastolite is a variety of andalusite, con- 
taining graphitic material arranged along the diagonals 
of the prism. It is found in small light-coloured prisms 
in chiastolite-slate in the neighbourhood of granite (e.g., 
Skiddaw). 

Sillimanite. Silicate of alumina : Al 2 O 3 .SiO 2 (silica, 
36*8; alumina, 63*2 per cent.). Rhombic; with pris- 
matic habit, without definite terminal faces, often in 
long and slender crystals ; sometimes fibrous (fibroUte). 
Colour, brown to greyish-green ; in thin section, colour- 
less. Lustre, vitreous. Transparent. Streak, white. 
Index of refraction, fairly high ( 1*667) ; double refrac- 
tion, moderate, stronger than that of andalusite. 
Cleavage parallel to the brachypinacoid, perfect. Hard- 
ness, 6-7. Density, 3^23 - 3*24. Infusible. Insoluble 
in acids. Occurs in gneisses and crystalline schists, 
often in the aureoles of metamorphism around granite, 
in association with cordierite, corundum, andalusite, 
and kyanite. 

Kyanite. Silicate of alumina: Al 2 O 3 .SiO 2 (silica, 
36*8 ; alumina, 63*2 per cent.), like sillimanite and 
andalusite. Triclinic ; usually in long prismatic crystals. 



ROCK-FORMING MINERALS 115 

Colour, white to blue. Lustre, vitreous. In thin 
section, colourless to pale blue, with weak pleochroism. 
Index of refraction, high (172). Double refraction, 
negative, strong (y-a = o'oi6). Cleavage parallel to 
the macropinacoid, perfect, with partings parallel to 
the basal plane. Fracture, fibrous. Brittle. Hard- 
ness =4 -7. Density = 3*5-37. Kyanite occurs in 
gneisses and crystalline schists in association with 
garnet, staurolite, and sillimanite ; often in the aureoles 
of metamorphism round granite. It is also found in 
sands and clays in association with rutile, tourmaline, 
zircon, etc. According to Vernadsky, kyanite, when 
heated to 1300 C., is converted into sillimanite. 

Staurolite. Hydrated silicate of alumina, iron, and 
magnesia: 2H 2 O.6(Fe,Mg)O.i2Al 2 O 3 .nSiO 2 . Rhombic. 
In prismatic forms terminated by the basal plane; 
commonly twinned, forming symmetrical Maltese and 
St. Andrew's crosses. Colour, reddish-brown. Lustre, 
vitreous to resinous. Translucent to opaque. Index 
of refraction, high (174). Double refraction, moderate 
but slightly stronger than quartz. Brachypinacoidal 
cleavage, perfect. Fracture, conchoidal to uneven. 
Hardness, 7-75. Density, 3'3-3'8. Infusible. Un- 
attacked by acids. Occurs in the crystalline schists, 
and in rocks of the granite contact-zone. 

Cordierite. Hydrated silicate of alumina, iron, and 
magnesia : H 2 O.4(Mg,Fe)O.4Al 2 O 3 .ioSiO 2 . Rhombic ; 
with pseudo-hexagonal symmetry. Often twinned. 
Colour, dark blue ; in thin section usually colourless. 



116 DESCRIPTIVE MINERALOGY 

Lustre, vitreous to resinous. Transparent to trans- 
lucent. Pleochroic. Refractive index, 1*536. Double 
refraction (y- = '007), slightly lower than that of quartz. 
Brachypinacoidal cleavage perfect. Parting parallel 
to basal plane. Fracture, subconchoidal. Hardness, 
7-7*5. Density, 2*6. 

Cordierite occurs in granites and gneisses (cordierite- 
gneiss) ; more rarely in volcanic rocks (e.g., basalt, 
andesite). It alters easily into mica-like decomposition 
products (pinite, esmarkite, praseolite, gigantolite, etc). 

Idocrase or vesuvianite. Hydrated silicate of lime 
and alumina ; probably 2H 2 O.i2CaO.3Al 2 O 3 .ioSiO 2 , 
Small quantities of manganese, iron, magnesium, and 
alkalies, are often present. Tetragonal. In square 
prisms, with basal plane and pyramid. Colour, dark 
green, brown, red, or yellow. Lustre, vitreous. Index of 
refraction, high (172). Double refraction, weak (w - e 
= *ooi). Prismatic cleavage imperfect. Fracture, 
uneven. Hardness, 6-7. Density, 3.35-3.45. Fusi- 
bility, 3. Scarcely attacked by acids. Occurs in lime- 
stones that have undergone alteration by contact with 
igneous rocks. 

Axinite. A borosilicate of calcium and aluminium : 
7CaO.2Al 2 O 3 .B 2 O 3 .8SiO 2 , in which lime maybe partially 
replaced by manganese, iron, and magnesium. Tri- 
clinic. In broad crystals with acute edges. Colour, 
honey-yellow to clove-brown. In thin section, colourless 
to pale yellow or violet. Lustre, vitreous. Transparent 
to translucent. Index of refraction, high (1*677). 



ROCK-FORMING MINERALS 117 

Double refraction, moderate (y-a = -oo9). Brachy- 
pinacoidal cleavage, distinct. Fracture, conchoidal to 
uneven. Brittle. Hardness, 6-5-7-0. Density, 3*271- 
3*29. Occurs in the contact-aureoles of granite. 

Topaz. A silicate and fluoride of aluminium : 
Al 2 O 2 (OH,F) 2 .SiO 2 . Rhombic. In short prismatic 
crystals with pyramidal and basal terminations. Colour- 
less, wine-yellow or tinted blue, red, or green. In thin 
section, colourless. Lustre, vitreous. Transparent to 
translucent. Index of refraction, 1*62. Double refrac- 
tion, moderate (y-a = *oog). Basal cleavage, perfect. 
Hardness, 8. Density, 3*4-3*6. Infusible, and un- 
attacked by acids. Occurs in some granites and 
pegmatites ; also in the contact-aureoles of granite. 

Datolite. A basic orthosilicate of calcium and boron : 
H 2 O.2CaO.B 2 O 3 -2SiO 2 with SiO 2 = 37-6, B 2 O 5 = 21-8, CaO 
= 35'0 and H 2 O = 5*6 per cent. Monoclinic. In stumpy, 
prismatic forms, or irregular grains. Colourless to 
white. Lustre, vitreous. Transparent to translucent. 
Index of refraction, moderate (1*65). Double refraction, 
very strong (y a= -0448). No defined cleavage. Frac- 
ture, subconchoidal. Hardness, 5. Density, 2*9-3*0. 
Fusibility, 2-2*5. Not attacked by acids. Occurs in 
association with zeolites and calcitein the amygdaloidal 
cavities of basalt ; also in the zones of contact-meta- 
morphism. 



CHAPTER II 

THE ORES 

THE ores are those minerals from which the metals can 
.be profitably extracted. They consist of the oxides, 
sulphides, chlorides, carbonates, sulphates, etc.. of the 
metals, and, in a few cases, of the native metals them- 
selves e.g., gold, platinum, silver, and copper. Although 
they exceed the rock-forming minerals in number, 
in bulk they constitute an insignificant fraction of the 
earth's crust, occurring as ore-bodies in its fissures and 
cavities, or as small particles and grains disseminated 
through its constituent rocks. Only exceptionally are 
masses of ore encountered that are large enough to 
bear comparison with the rocks themselves e.g., in 
the case of certain iron ores. 

It is outside the scope of this small book to discuss 
either the mode of occurrence or the genesis of the ore- 
deposits. For the present purpose it must suffice to 
distinguish between (i) ore -deposits formed in situ, 
whether occurring as lodes, veins, masses, or beds, and 
of whatever mode of origin whether formed by mag- 
matic differentiation (i.e., concentration in an igneous 
magma), pneumatolysis (deposition from vapours), 

118 



ORES 119 

hydatogenesis (deposition from water), or metaso- 
masis (chemical replacement) ; and (2) detrital or placer 
deposits, in which the minerals have been derived by 
erosion from pre-existing formations, and have been 
accumulated in hill-talus, river-gravels, or sea-beaches 
e.g., alluvial gold, stream tin, and iron sand. 

The first-named is by far the larger of the two classes 
of ore-deposits, and is in general also the more important 
as a source of the metals. For, although a larger 
quantity of gold or tin may in any given district be 
temporarily won from superficial detrital deposits 
(' placers '), these become comparatively soon ex- 
hausted, or their working is prohibited by State legisla- 
tion on account of the injury done to the soil by the 
disposal of the debris from the gold-washings ;* and the 
miner must then perforce turn his attention to the more 
lasting veins and beds of ore that extend deep down 
into the crust of the earth. Here again a further dis- 
tinction is necessary between the primary ores, found in 
those parts of the lodes and beds that are below the 
permanent water-level, and the secondary, oxidized ores 
that characterize those superficial portions within the 
belt of weathering. Whereas the latter are remarkable 
for their variety and complexity, and comprise the bulk 
of the multifarious minerals classed as ores, the former 
are in general limited to a few simple sulphides. 

* Thus, Central California, which at one time had an output of 
gold from placer-mining, valued at three and a half millions sterling 
per annum, has, in consequence of the action of the State, entirely 
ceased to produce gold from this class of deposit. 



120 DESCRIPTIVE MINERALOGY 

Thus, the great class of secondary copper ores are 
derived in the main from the primary sulphide of 
copper and iron known as chalcopyrite or copper pyrites. 
Similarly, both lead and silver ores mostly take their 
origin from primary sulphides of these metals, which 
often occur in isomorphous association in the same 
mineral (galena). Again, blende (sulphide of zinc) is the 
primary source of zinc ores, and also of rather rare 
cadmium minerals, the two metals being closely allied 
and frequently associated. 

Gold, also, although not chemically combined with, 
is closely associated with, and even mechanically in- 
cluded in, pyrites, chalcopyrite, and mispickel, and is only 
set free by the breaking down (oxidation) of these pyritic 
ores when brought within the zone of weathering by 
the natural lowering of the ground-water level during 
the ordinary process of denudation. 

In the following pages a brief description is given 
of the metals : platinum, gold, mercury, copper, 
silver, lead, zinc, nickel, cobalt, iron, manganese, bis- 
muth, antimony, arsenic, vanadium, tin, titanium, molyb- 
denum, tungsten, uranium, and aluminium. 

ORES OF PLATINUM. 

The native metal is the sole source of commercial 
platinum. A compound with arsenic (sperrylite) is 
known, but it has only been found in one or two places. 
On account of its infusibility and the difficulty with 
which it is attacked by acids, platinum constitutes a 
valuable material for chemical vessels crucibles, 



ORES 121 

dishes, etc. It is also largely used in the electric light 
industry and in the dental and photographic trades, as 
well as by manufacturing jewellers. At 8 per ounce, 
it is almost twice as valuable as gold. 

Native Platinum occurs in small flattened grains and 
scales, but occasionally in larger nuggets. It crystal- 
lizes in the regular system ; but crystals are rare, only 
small cubes having been occasionally found. Its colour 
lies between a steel grey and a silver white. It 
takes a higher polish than silver. Hardness, 4*5-5 ; 
fracture, hackly ; density, 14-19, amounting in chemi- 
cally pure platinum to 21*5. Platinum is thus one of 
the heaviest metals known. Malleable and ductile. 
Infusible before the blowpipe, except in the very thinnest 
wire. Insoluble in acids, except aqua regia, in which 
it is easily dissolved to platinum chloride. It is also 
attacked by caustic alkalies. 

Platinum occurs as a primary constituent of the peri- 
dotites of the Urals (Nischne Tagilsk, Mount Solovief) ; 
but the chief source of supply are the placers in the 
valleys of the rivers (Issa, Wyja, Tura, and Njassma) 
draining the same districts. It is associated in the 
sands of these placers with chromite and magnetite. 
The ore consists of a mixture of platinum with 
osmium-iridium, and the metal is besides alloyed with 
palladium, rhodium, iridium and smaller quantities of 
osmium and iridium. Iron is invariably present, in 
quantities from 4 to 13 per cent. The output of the 
Russian deposits amounts to 200,000 ounces per 
annum. 



122 DESCRIPTIVE MINERALOGY 

Outside Russia, the State of Colombia (districts of 
Choco and Barbacoas) is the largest producer (7,000 
ounces per annum). It is also found in British 
Columbia (Tulameen River), Northern California, 
Brazil (Minas Geraes), Assam, Borneo, and New 
Zealand (River Tayaka"). 

Sperrylite. Arsenide of platinum : PtAs 2 (platinum 
56*47 per cent). Crystallizes in the regular system, with 
pentagonal hemihedrism. Habit, cubic or octahedral. 
Colour, tin white. Opaque. Lustre, metallic. Streak, 
black. Fracture, conchoidal. Brittle. Hardness, 
6-7. Density, 10*6. Infusible. Soluble in aqua regia. 

A rare mineral, and only interesting as the one ore 
of platinum known besides the native metal. Occurs 
in Ontario, Canada (Vermilion Mine), and North 
Carolina (Cowee Valley). 

ORES OF GOLD. 

The chief supply of the noble metal is native gold, 
other ores being comparatively rare. It is true the 
tellurides (sylvanite, krennerite, calaverite, petzite, etc.) 
are worked in a few places, but, in proportion to the 
production of the whole world, the supply from this 
source is extremely small. Gold amalgam (a compound 
of the noble metal with mercury) is of no importance 
as an ore. 

The original source of the bulk of native gold is in 
auriferous quartz veins, and in conglomerate beds 
(" banket ") ; but a small proportion is possibly a 



ORES 123 

primary syngeftetic constituent of ingenous rocks. Within 
the belt of weathering the gold of these deposits occurs 
free, and the ore is " free-milling " i.e., it is amenable 
to amalgamation when, after suitable crushing, the ore 
mixed with water (the pulp) is passed over copper plates 
coated with quicksilver ; but below the permanent 
water-level it is closely associated with, and to a 
considerable extent mechanically included in, pyrites 
(auriferous pyrites), and in the treatment of such ores 
(" pyritic ores ") it is essential to reduce them to a 
sufficient state of fine subdivision to enable the gold 
to be extracted by cyanide solution. What the exact 
genetic relation is between the pyrites and the gold 
has not been satisfactorily settled, but it is clear that 
a community of origin is indicated. 

Besides being associated with pyrites, gold, being iso- 
morphous with silver, lead, and copper, is almost 
always present in the ores of these metals, and it 
is usually an important by - product both in the 
metallurgy of silver and of copper. Sometimes, how- 
ever, the gold is, from the economic standpoint, the 
dominant constituent, as in the gold-copper pyrrhotite 
veins of Rossland in British Columbia, and in the gold- 
silver telluride veins of Transylvania, of Cripple Creek 
in Colorado, of Tonapah in Nevada, of Mexico, and of 
Kalgoorlie in West Australia. 

A large quantity of gold is won from transported 
material. Such are the screes and talus debris of 
mountain slopes (drift gold), the sands of the rivers that 
drain them (alluvial gold), and the beach gravels that 



124 DESCRIPTIVE MINERALOGY 

accumulate near the mouths of the rivers (beach placer 
gold). Transported or placer gold is derived from the 
disintegration of the gold-bearing quartz veins, so 
common in the old crystalline rocks of mountain dis- 
tricts. The metal is also found in mudstones, sand- 
stones and conglomerates, in which it may have either 
accumulated at the time of the formation of the deposit 
or been introduced subsequently by deposition from 
solution. 

The most important gold-fields are situated in 
Australasia (Western Australia, Victoria, New South 
Wales, Queensland, New Zealand), the United States 
(California, Nevada, Arizona, Montana, Colorado, and 
Alaska), Canada (British Columbia and the Yukon), 
Mexico, Colombia, Venezuela, Guiana, Chili, Peru, Brazil, 
South Africa (Witwatersrand, Barberton, and Lyden- 
burg, in the Transvaal, and Rhodesia), West Coast of 
Africa, East Indies (Sumatra, Java, Borneo), British 
India (Kolar gold-field in Mysore), Russia (Siberia and 
the eastern slopes of the Ural Mountains). 

Native Gold crystallizes in the regular system ; but 
crystal forms Cas a rule, octahedral or dodecahedral) 
are rare, indistinct, and distorted. More usually it 
occurs in branched or wiry aggregates, in leaf-like ex- 
pansions, or in a finely divided condition as mustard 
gold, paint gold, and sponge gold ; also as minute grains, 
generally in intimate association with pyrites dissemi- 
nated through quartz. It is also found in gravels and 
sands as dust, and in loose grains and nuggets. Large 
nuggets are occasionally found : thus, one from Upper 



ORES 125 

California weighed 161 pounds ; and others have been 
found in Australia, one of which yielded 2,268 ounces 
of gold. 

Gold is the most malleable and the most ductile 
of all metals. It is soft, having a hardness of only 
2*5-3. Its fracture is hackly. Its colour and streak 
vary from a reddish to a brassy yellow, the variation 
being caused by the presence of small quantities of 
silver and copper. The amount of silver present in 
native gold varies from i to 40 per cent. ; alloys 
with over 20 per cent, are termed electrum. The 
density of native gold ranges from 15*6 to 19*4, being 
less with increasing percentage of silver. The density 
of pure gold is 19*37. The metal is insoluble in single 
acids, but dissolves readily in a mixture of nitric and 
hydrochloric acids (aqua regia). Fusible with ease in 
the flame of the blowpipe (2*5-3 on Von KobelPs 
scale). 

Auriferous Pyrites. In the deeper-seated portions 
of quartz veins, the gold is often found in intimate 
but mechanical association with iron, copper, arsenical, 
or magnetic pyrites ; and no doubt a considerable pro- 
portion of the so-called " free gold " occurring in the 
oxidized (weathered) zone of gold-bearing deposits has 
been liberated by the decomposition of these minerals. 

Gold Amalgam. An alloy of gold with mercury. In 
soft, yellowish-white grains and balls. Density, 15*5. 
Occasionally accompanies native gold in California and 
Colombia. 



126 DESCRIPTIVE MINERALOGY 

Calaverite. Telluride of gold: AuTe 2 (Au 44*03, 
Te 55*97, per cent.). As a rule silver is also present. 
Occasionally crystallized in striated crystals of prismatic 
habit, but more commonly massive. Colour, bronze 
yellow, with metallic lustre. Fracture, uneven to semi- 
conchoidal. Brittle. Hardness, 2-3. Density, 9. Fuses 
easily (T on Von Kobell's scale), yielding on charcoal 
before the blowpipe a globule of gold. Soluble in aqua 
regia. Occurs in California (Calveras County), Colorado 
(Cripple Creek) and Western Australia (Kalgoorlie). 

Sylvanite. Telluride of gold and silver: AuAgTe 4 
(Au 24-45, Ag 13*39, Te 62-16, per cent.). Crystal- 
lizes in the monoclinic system, with tabular habit 
after the clinopinacoid (oio), or with predominant 
orthodome (101) and basal plane (ooi), or pseudo- 
rhombic with dominant pinacoids (oio) and (100). 
Often twinned and reticulated (graphic tellurium), and 
in scaly to granular aggregates. Colour and streak, 
silver white to steel grey, with metallic lustre. 
Cleavage, perfect parallel to the clinopinacoid. Frac- 
ture, uneven. Sectile. Hardness, 1-2. Density, 7*9-8*3. 
Easily fused (i on Von Kobell's scale). Imperfectly 
soluble in nitric acid (with separation of gold) ; soluble 
in aqua regia, with ' separation of silver chloride. 
Occurs in Transylvania in Hungary, Colorado (Cripple 
Creek), Western Australia (Kalgoorlie). 

Krennerite. Telluride of gold and silver : (Au, Ag)Te 2 
(the percentage of gold varies from 24-45 to 44'O3). 
Crystallizes in the rhombic system, with prismatic 



ORES 127 

habit and basal termination. Vertically striated. 
Colour, silver white to light brassy yellow, with metallic 
lustre. Cleavage, perfect parallel to the basal plane. 
Fracture, uneven to semi-conchoidal. Hardness, 2-3. 
Density, 8*35. Easily fusible (i on Von Kobell's scale). 
Occurs in Transylvania (Nagyag), Colorado (Cripple 
Creek), and West Australia (Kalgoorlie). 

Petzite. Telluride of silver and gold: (Ag,Au) 2 Te. 
When free from gold, petzite contains theoretically 
63*27 per cent, of silver ; it may contain, however, as 
much as 25 per cent, of gold. With Ag : Au=3 : I, 
the percentages are silver, 42 ; gold, 25*5. Crystal- 
lizes in the regular system, in cubes or distorted forms ; 
also massive or granular. Colour, leaden grey to steel 
grey, with metallic lustre. Hardness, 2-3. Density, 
8'3-g'o. Fracture, uneven to semi-conchoidal. Sectile 
to brittle. Easily fusible (1*5). Occurs in the Altai, 
Transylvania, California, Colorado (Cripple Creek), and 
other places. 

ORES OF MERCURY. 

The only important source of mercury is the sulphide, 
cinnabar ; but the metal also occurs, though rarely, in 
the native state, globules of quicksilver being found 
as an alteration product in the oxidation zone of quick- 
silver deposits. Metacinnabarite, the black sulphide, 
is also an alteration product of cinnabar, and other 
rare associates are the chloride, the telluride, and the 
selenide of mercury. A gold amalgam (Au,Hg) and 



128 DESCRIPTIVE MINERALOGY 

a silver amalgam (Ag,Hg) are also known, but on 
account of their rarity they are of no importance as 
ores. Mercury finds a variety of uses in the arts : it is 
greatly used in the extraction of gold and silver from 
their ores (in the so-called "amalgamation" processes), 
in dentistry, and in the manufacture of scientific in- 
struments (e.g., the barometer and thermometer) ; its 
salts are also employed in medicine. 

Cinnabar. Sulphide of mercury: HgS (mercury 
86'2 per cent.). Crystallizes in the hexagonal system, 
with rhombohedral hemihedrism. Occurs in thick 
tabular crystals, composed of the basal plane in com- 
bination with a series of rhombohedral faces. More 
frequently, however, it is found massive or as an earthy 
incrustation. Colour, a bright crimson or cochineal red. 
Streak, scarlet. Transparent to translucent. Lustre, 
adamantine. Prismatic cleavage, perfect. Fracture, 
conchoidal to uneven. Sectile. Hardness, 2 - 2*5. 
Density, 8-8'2. Volatile before the blowpipe. Heated 
carefully in the open tube, yields metallic mercury and 
fumes of sulphur. Decomposed by aqua regia, with 
separation of sulphur. 

Cinnabar is important as the sole source of the 
mercury of commerce. It occurs in veins, irregular 
masses, or disseminated in grains through sandstone, 
and is often accompanied by pyrites, marcasite, chalco- 
pyrite, stibnite, realgar, and mispickel. 

The chief mines are in Southern Spain (Almaden), 
Austria (Idria in Carniola), Italy, Russia, California 



ORES 129 

(New Almaden, New Idria, Sulphur Bank, Clear Lake), 
Mexico (Guad alcazar, Huitzuco), Peru (Huancavelica), 
China (Kweichou). 

ORES OF COPPER. 

Copper, one of the earliest metals known to man, 
was much prized by the ancients on account of its 
toughness. It was used by them, for instance, in an 
alloy with one-tenth of its weight of tin, for the manu- 
facture of weapons and tools. The alloy with zinc 
(brass) was also in great use for ornamental work. 
There are a great number of minerals containing 
copper, but comparatively few are of commercial 
importance as a source of the metal. In the upper 
weathered portion of the lodes are found the oxidized 
ores, which, besides the oxides, cuprite and melaconite, 
include the carbonates, malachite and chessylite ; the 
silicate, chrysocolla ; the sulphate, chalcanthite ; the 
sulphide, covellite ; and native copper. In the zone of 
secondary enrichment which occurs immediately below 
the belt of weathering are found the rich sulphides, 
chalcocite and bornite, and the sulpharseniate, enargite ; 
while in the deepest parts of the deposits the copper 
is confined to the primary ore, chalcopyrite, either alone 
or in association with iron pyrites (cupriferous pyrites). 
Besides these simple compounds, copper is also obtained 
in considerable quantity from certain complex ores, 
such as the sulphantimonite and sulpharsenite of 
copper, silver, iron, and zinc (e.g., fahl-ore and ten- 
nantite). 

9 



130 DESCRIPTIVE MINERALOGY 

Copper is one of the most valuable metals employed 
in the arts, and perhaps its greatest application is as 
a conductor of electricity. A great quantity also goes 
into consumption in the form of sheet copper or as 
castings, and the metal is largely used in the manu- 
facture of brass, bronze, and other alloys, in elec- 
trolysis, and as a chemical agent in the form of blue 
vitriol (copper sulphate) and other salts. 

The production of copper is about 847,000 tons (of 
2,240 pounds) per annum (1910), distributed as follows : 

Tons. 

United States ... ... ... 483,000 

Mexico ... ... ... 59,000 

Spain and Portugal ... ... 48,000 

Australasia ... ... ... 42,000 

Chili ... ... ... ... 41,000 

Japan ... ... ... ... 41,000 

Germany ... ... ... 24,000 

Russia ... ... ... 23,000 

Canada ... ... ... 22,000 

Peru ... ... ... ... 20,000 

Africa ... ... ... ... 16,000 

Other countries ... ... ... 28,000 



847,000 

Native Copper. Regular. When crystallized, this 
metal occurs in small and large crystals, having the 
forms of the octahedron, the cube, and the rhombic 
dodecahedron, all of which are usually much distorted, 
and aggregated to irregular branching masses. Most 



ORES 131 

frequently, however, no crystalline form is visible, the 
metal occurring in irregular lumpy masses, in wiry and 
mossy coils, or as foil and in plates. Its colour is the 
familiar copper-red, but the surface is often tarnished, 
and is then of a dirty yellow or brown colour. Lustre, 
metallic. The metal is malleable, ductile, and tenacious. 
No cleavage. Fracture, hackly. Its hardness is 2*5-3 ; 
its density, 8*5-8*9 ; and its fusibility, 3 on Von Kobell's 
scale. Native copper is usually chemically pure, but it 
sometimes contains iron and silver. Soluble in nitric 
and hydrochloric acids. 

Copper occurs, chiefly in association with secondary 
ores,"* in Cornwall and many places in Europe, Siberia, 
Brazil, Chili, Bolivia (Corocoro), Australia and Tas- 
mania, the United States (e.g., in the copper-mines of 
Lake Superior). A great mass, weighing 400 tons, 
was discovered in one of the mines at the last-named 
locality, and forty men were employed for twelve 
months in its extraction. 

Cuprite, or red copper ore. Copper monoxide or 
cuprous oxide : Cu 2 O (copper 88*8 per cent.). Crys- 
tallizes in the regular system, in well -formed octa- 
hedra, either alone or in combination with the cube 
and the rhombic dodecahedron. It is also found 
massive and granular, or as a brick- red earth (tile ore). 
It has a brilliant cochineal-red colour, which is best 
seen in transparent, or at least translucent, crystals, 

* Metallic copper is easily produced from cuprite by the action 
of sulphuric acid : Cu a O + H 2 SO4 = Cu + CuSO4 + H 2 O. 




132 DESCRIPTIVE MINERALOGY 

or on reducing opaque specimens to powder. Lustre, 

metallic to adamantine, and streak, brownish - red. 

Its octahedral cleavage is fairly perfect. Fracture, 
conchoidal to uneven. Brittle. Hard- 
ness, 3*5-4. Density, 5*7-6-2. Fusi- 
bility (Von KobelPs scale), 2*5 - 3. 
Before the blowpipe on charcoal it 
yields a metallic globule of copper. 
The ease with which it can be reduced 

: 10.77- E - ma k es ft one of the best ores for the 

Octahedron. ... . . 

extraction of the metal, but it is only 
found as a decomposition product in the upper or 
oxidized portions of copper sulphide lodes. 

Of widespread occurrence, if limited in quantity 
for instance, in Cornwall (Liskeard, Redruth), 
France (Chessy), Nassau, Harz, Saxony, Silesia, 
Bohemia, Hungary, Italy, Spain, Siberia, Australia 
(Wallaroo, Moonta, Burra Burra, and Cobar), Tas- 
mania (Mount Lyell), Namaqualand in South Africa, 
Arizona (Clifton, Morenci, Bisbee, and Globe), Lake 
Superior, Alaska (Mount Wrangell), Mexico (Boleo), 
Peru, Chili, Bolivia (Corocoro), etc. 

Melaconite, or tenorite. Black oxide of copper, or 
cupric oxide : CuO (copper 79-86 per cent.). Crystal- 
lizes in the triclinic system, with pseudo-monoclinic 
symmetry and tabular habit (100). Twinning parallel 
to the macropinakoid (100) and the brachydome (on). 
Also massive, powdery, earthy, scaly, and cellular. 
Colour, black to grey. Lustre, metallic to dull. 



ORES 188 

Opaque. Streak, dirty green. Cleavage, basal. Frac- 
ture, conchoidal to uneven. Thin flakes are elastic. 
Hardness, 3-4. Density, 5'8-6'3. Fusibility, 3. Soluble 
in hydrochloric and nitric acids. 

Occurs in the oxidized portion of copper lodes as a 
decomposition product of chalcopyrite, bornite, etc., 
but not in sufficient quantity to be of great importance 
as a source of copper. It is found on Vesuvius in lava 
as a sublimation product. Other localities are Corn- 
wall, Spain (Huelva), Siberia (Bogoslowsk and Nischne 
Tagilsk), Japan, Australia, Bolivia, Mexico (Boleo), 
Chili, Peru, United States (Tennessee, Arkansas, and 
Michigan). 

Chalcocite, copper glance, or redruthite. Sulphide 
of copper: Cu 2 S* (copper 79*83 per cent.). Crys- 
tallizes in the rhombic system. The crystals have a 
pseudo-hexagonal symmetry, occurring often in flat 
six-sided tablets, composed of 
pyramids and brachydomes, or of 
prisms and brachypinacoids, ter- 
minated in both cases by the FlG " 78CoppER-GLANCE. 

T i i ^, f A Basal plane ; z, pyra- 

basal plane. They are frequently m id ; e , brachydome. 
twinned, the twinning plane being 

a face of the prism. More frequently, however, the ore 
is massive, platy, or nodular. Colour, dull black. Lustre, 
metallic. Superficially often iridescent or tarnished by 
incipient alteration to covellite or bornite. Streak, 

* The impure sulphide, CuS (with copper 66*9 per cent.), 
Covellite, is a blue microcrystalline mineral (hexagonal), and 
rather rare. 



134 DESCRIPTIVE MINERALOGY 

blackish-grey. Opaque. Prismatic cleavage, imperfect. 
Fracture, conchoidal. Slightly sectile. Hardness, 
2-5-3. Density, 5'5-5'8. Fusibility, 2-2-5. After care- 
ful washing and rinsing with carbonate of soda, yields, 
before the blowpipe, a globule of copper. 

Chalcocite is a rich ore of copper, of frequent occur- 
rence in copper lodes, especially in the zone of secondary 
enrichment. Notable examples are to be found in 
Cornwall, Saxony, Hungary (Kapnik, Rezbanya, Ora- 
vicza), Italy (Monte Catini), Caucasus (Kiadebek), 
Eastern Russia (Bogoslowsk, Nischne Tagilsk, Spassky), 
South-West Africa (Namaqualand), Transvaal (Mes- 
sina), Australia (Wallaroo, Moonta, Burra Burra), 
Japan (Ashio, Besshi), Alaska (Mount Wrangell), Cali- 
fornia (Shasta County), Montana (Anaconda, near 
Butte), Arizona (Bisbee, Jerome, Clifton, etc.), Bolivia 
(Corocoro). 

Chalcopyrite, or copper pyrites. Sulphide of 
copper and iron, or sulphoferrite of copper: Cu 2 S.Fe 2 S 3 
(copper 34*56, iron 30*52 per cent.). Tetragonal, the 
commonest form being the hemihedral sphenoid ; but 
the crystals are usually small and distorted, and conse- 
quently difficult to determine. Twinning on various 
types; most frequently with (in) as twinning plane 
(see Fig. 77). Mostly, however, it occurs massive, in 
large nodular, kidney-shaped, or botryoidal masses, and 
in scattered grains and specks. Its colour is brassy to 
golden yellow, being a stronger yellow than that of iron 
pyrites. The surface of the mineral is often iridescent 
in blue or red tints, which are the result of tarnish 



ORES 135 

(peacock ore). Streak, black. Cleavage, parallel to 
(201), imperfect. Fracture, conchoidal to imperfect. 
Hardness, 3*5-4. Density, 4'i-4*3. Fusibility, 2. 
Before the blowpipe, on charcoal, yields black mag- 
netic globule ; when mixed with carbonate of soda yields 
a ferruginous copper globule. Dissolves in nitric acid, 
with separation of sulphur. It may be distinguished 
from iron pyrites by its less hardness, and from gold 
by its brittleness, since it crumbles under the point of 
the knife. 

Chalcopyrite is the most widely distributed of all 





FIG. 79. TWINNED CRYSTALS OF CHALCOPYRITE. 

copper ores, and is responsible for the bulk of the 
world's output of copper. It occurs as a mineral of 
sedimentary origin, as a product of magmatic con- 
centration in igneous rocks, and as a true vein deposit. 
A well-known example of the sedimentary ores is the 
Kupferschiefer, a thin bed of bituminous shale in 
the Zechstein (Permian) formation of the Southern 
Harz, in which chalcopyrite occurs as a fine dust in 
association with bornite, chalcocite, iron pyrites, and 
galena. These sulphides have been deposited from 
a solution of the metallic sulphates by the reducing 



136 DESCRIPTIVE MINERALOGY 

action of decomposing animal matter. Chalcopyrite is 
invariably found in copper lodes when depths below the 
zones of weathering and of secondary enrichment are 
reached. It is the primary copper mineral, from which 
the other sulphides (chalcocite and bornite), and the 
oxides and carbonates that characterize the upper 
portions of the lodes, are derived. It is impossible to 
enumerate the localities for chalcopyrite, since it occurs 
wherever copper is mined ; the mention of a few of the 
copper-mining districts where this mineral bulks largely 
must suffice: Germany (Mansf eld, Rammelsberg), Saxony 
(Annaberg), Spain and Portugal (Huelva District, Rio 
Tinto, Tharsis, etc.), Ural Mountains (Bogoslowsk), 
Caucasus (Kiadabek), Altai (Tschudak and Songatof), 
Finland (Pitkaranta), Norway (Sulitelma, R0ros, Vigs- 
nas, Foldal, Ytter0), Sweden (Falun), Italy (Monte 
Catini, Massa Marittima, Mossetana, Boccheggiano), 
Austro- Hungary (Kitzbiihel in the Tyrol, Schmollnitz 
in Hungary, and Rezbanya in the Banat), South Africa 
(Ookiep, Spectakel, Nababeep, and Kopeberg, in Little 
Namaqualand), Australia (Great Cobar in New South 
Wales ; Wallaroo, Moonta, and Burra Burra, in South 
Australia; Mount Morgan in Queensland), Tasmania 
(Mount Lyell), Japan (Ashio, Ani Osarisawa, Besshi, 
and Kosaka), China (Yunnan and Kweichou Provinces), 
Canada (Rossland, Sudbury, Capeltown), Montana 
(Anaconda and other mines at Butte), Arizona (United 
Verde, Copper Queen, and other mines in the Jerome, 
Bisbee, Clifton, Morenci, and Globe Districts), Cali- 
fornia (Mountain Copper in Shasta County), Alaska 



ORES 



137 




CHRISTIANA 



MAP SHOWING COPPER ORE DEPOSITS IN NORWAY. 



138 DESCRIPTIVE MINERALOGY 

(Copper Mountain, Mount Wrangell), many places in 
the Appalachian States, Mexico (Cananea, Boleo, and 
Montezuma), Northern Chili, Bolivia (Corocoro), Peru 
(Cerro de Pasco). 

Bornite, erubescite, purple ore, or horseflesh ore. 
Sulphide of copper and iron, or sulphoferrite of copper : 
3Cu 2 S.Fe 2 S 3 (copper 55*57, iron 16*36 per cent.)- Crys- 
tallizes in the regular system. Habit, cubic. Usually 
occurs massive or in disseminated grains. Colour, 
reddish-brown to copper-coloured. Often iridescent. 
Opaque. Lustre, metallic. Streak, greyish-black. 
Octahedral cleavage, very imperfect. Fracture, sub- 
conchoidal to uneven. Not very brittle. Hardness, 3. 
Density, 4*9-5-4. Fusibility, 2*5. With carbonate of 
soda on charcoal yields a globule of copper. Soluble in 
nitric or concentrated hydrochloric acid, with separa- 
tion of sulphur. 

Bornite is a valuable ore of copper, occurring in 
the oxidized or in the secondarily enriched portion of 
the lodes for example, in Cornwall, Norway, Harz, 
Saxony, Siberia (Spassky), South Africa (Namaqua- 
land), Chili, Peru, Bolivia, Mexico, United States 
(Shasta County in California and Butte in Montana), 
and Canada. 

Enargite. Sulpharseniate of copper: As 2 S r 3Cu 2 S 
(copper 48*36 per cent.). Crystallizes in the rhombic 
system. Habit, columnar in the direction of the ver- 
tical, with vertical striation, or tabular parallel to the 
basal plane. Also massive, granular, or columnar. 



ORES 139 

Colour, grey to iron black. Lustre, metallic. Streak, 
greyish - black. Cleavage, perfect, parallel to the 
prisms (no). Fracture, uneven. Brittle. Hardness, 3. 
Density, 4'4-4'5. Fusibility, i. On charcoal with car- 
bonate of soda yields a globule of copper. Heated in 
the open tube, gives off arsenical and sulphurous fumes. 
Soluble in aqua regia. 

Enargite occurs, as an ore of copper in the United 
States (Anaconda mine at Butte in Montana, Tintic 
mines in Utah, etc.), Argentine (Sierra Famatina), 
Philippines (Luzon). 

Tetrahedrite, fahl - ore, or grey copper ore. 
Sulphantimonite of copper, with a variable amount of 
copper replaced by silver, iron, and zinc: 3(Cu 2 ,Ag 2 , 
Fe,Zn)S.(Sb,As) 2 S 3 . The corresponding sulpharsenite 
of copper, 3Cu 2 S.As 2 S 3 , is tennantite. The pure 
sulphantimonite of copper contains theoretically 46*84 
per cent, of copper; the sulpharsenite of copper, 52*64 
per cent. Crystallizes in the regular system, with 
tetrahedral habit. Twinned parallel to a face of the 
octahedron. Colour, steel grey to iron black. Opaque. 
Lustre, metallic. Streak, black to reddish. No cleavage. 
Fracture, conchoidal to uneven. Very brittle. Hard- 
ness, 3-4. Density, 4*4-5'i. Fusibility, 1*5. A valuable 
ore of copper and of silver (of which it may contain 
up to 30 per cent.). 

Tetrahedrite occurs in Cornwall, Harz (Andreas- 
berg), Saxony (Freiberg), Hungary (Kremnitz), Silesia, 
Bohemia (Przibram), Nassau (Dillenburg), Spain, Russia 
(Bogoslowsk), Chili, Bolivia (Huanchaca, where it is 



140 DESCRIPTIVE MINERALOGY 

worked as a silver ore), Peru, Mexico, and United 
States (Arkansas, Utah, Nevada, California), Australia 
(Broken Hill). 

Malachite. The green hydrated basic carbonate of 
copper : CuCO 3 .Cu(OH) 2 , or 2CuO.CO 2 .H 2 O (copper 
59*3 per cent.). Crystallizes in the monoclinic system, 
but usually occurs massive or with a smooth mamillary 
surface and concentric fibrous internal structure. Colour, 
bright green. Streak, pale green. Lustre, silky to dull. 
Hardness, 3*5-4- Density, 4. Fusibility, 3. Reduced 
on charcoal before the blowpipe to globule of copper ; 
colours the flame green. Gives off water when 
heated in the closed tube. Dissolves in acids with 
effervescence. 

Malachite is of universal occurrence in the upper part 
of copper lodes, together with the other oxidized ores 
of copper, and is a valuable ore, when in sufficient 
quantity for profitable extraction. Well-known occur- 
rences are the following : Cornwall, Chessy in France, 
Spain, Siberia (Nischne Tagilsk, especially at the mine 
Mednoroudiansk), South Australia (Burra Burra), Ka- 
tanga in Central Africa, United States (especially 
Arizona), Mexico (Boleo), Chili, etc. 

Chessylite, or azurite. The blue hydrated basic 
carbonate of copper: 2CuCO 3 .Cu(OH) 2 (copper 55*3 
per cent.). Crystallizes in the monoclinic system, but 
also occurs in massive or in earthy forms. Colour, deep 
blue. Streak, pale blue. Lustre, vitreous. Fracture, 
conchoidal. Hardness, 3*5-4. Density, 3*5-3*8. Fusi- 



ORES 141 

bility, 3. Behaviour before the blowpipe and with 
acids same as for malachite. It accompanies malachite 
in the oxidized form of copper lodes, but is of less 
frequent occurrence. For localities, see the list given 

under malachite. 

Chrysocolla. Hydrated silicate of copper: CuO.SiO 2 . 
2H 2 O (copper 36 per cent.). Amorphous. Massive 
and compact. Opaline to earthy. Vitreous to greasy 
lustre, or dull. Translucent to opaque. Colour, green 
to blue. Streak, bluish-white. Fracture, conchoidal. 
Brittle. Hardness, 2-4. Density, 2-2*2. Infusible. 
Mixed with carbonate of soda on charcoal before the 
blowpipe, yields metallic copper. Decomposed by acids, 
with separation of silica. Heated in the open tube, 
yields water. Occurs as a decomposition product of 
copper ores in the belt of weathering of lodes. Russia 
(Bogoslowsk, Nischne Tagilsk), South Africa (Nama- 
qualand), United States (Michigan, Arizona, California), 
Mexico (Boleo), Chili. 

Dioptase. Hydrated silicate of copper: CuO.SiO 2 . 
H 2 O (copper 40*2 per cent.). Crystallizes in the 
hexagonal-rhombohedral system, with short columnar 
habit, with rhombohedral terminations. Colour, emerald 
green. Streak, green. Lustre, vitreous. Translucent 
to transparent. Rhombohedral cleavage perfect. Frac- 
ture, conchoidal to uneven. Brittle. Hardness, 5. 
Density, 3*3. Double refraction, strong, positive. 
Before the blowpipe turns black, but does not melt. 
Colours the flame green. With carbonate of sodium 



142 DESCRIPTIVE MINERALOGY 

on charcoal yields a globule of copper. Decomposed 
by hydrochloric acid with separation of silica. 

Dioptase is of rather rare occurrence. A well-known 
locality is the Kirghese Steppes, where it occurs in lime- 
stone. It has also been found in Chili, Peru, Arizona, 
and Central Africa (Congo). 

Chalcanthite. Hydrated sulphate of copper : CuSO 4 . 
5H 2 O (copper 25*4). Crystallizes in the triclinic system. 
Also occurs massive or as incrustations. Colour, 
blue. Streak, white. Lustre, vitreous. Brittle. Hard- 
ness, 2*5. Density, 2'2. Fracture, conchoidal. Fusi- 
bility, 3. Soluble in water. Yields water when heated 
in the open tube. Occurs in small quantities only, as 
a decomposition product of chalcopyrite. 

Atacamite. Hydrated oxychloride of copper : 
CuCl 2 -f 3Cu(OH) 2 (copper 59*4). Crystallizes in the 
rhombic system. Also occurs massive. Colour, dark 
green. Lustre, vitreous. Streak, light green. Hard- 
ness, 3-3-5. Density, 37-3*8. Fusibility, 3-4. Yields a 
globule of copper on charcoal before the blowpipe. 
Gives off water when heated in the open tube. Soluble 
in acids. Occurs chiefly in the dry desert regions of 
Chili and Peru (Atacama), where it is worked as an ore 
of copper ; also in Mexico (Boleo). 

ORES OF SILVER. 

Silver occurs in the native state, but this is not an 
important source of supply. The largest amount of the 
metal is obtained from argentiferous galena, (Pb,Ag 2 )S, 



ORES 



143 



which may be regarded as an isomorphous mixture of 
argentite (Ag 2 S) and galena (PbS), both crystallizing 
in the regular system. A rhombic sulphide of silver 
acanthite also exists, and has been found, for example, 
in the silver-mines of Freiberg. If this mineral be 
regarded as isomorphous with the rhombic sulphide 
of copper chalcocite an explanation is afforded of 
the constitution of argentiferous chalcocite or stromeyerite, 
(Cu,Ag) 2 S, which also occurs at Freiberg and in the 
Altai. Silver also replaces copper in tetrahedrite, some 
varieties of which contain up to 30 per cent, of silver, 
and are then regarded as silver ores. Other important 
ores of silver are the sulphantimonites and sulph- 
arsenites of silver, lead and copper. Their chemical 
relationship is shown in the following table : 



Name. 


Chemical Formula. 


Cystallographic 
System. 


Argentite 


Ag 2 S 


Regular 


Argentiferous galena 


(Pb,Ag 2 )S 


Regular 


Acanthite 


Ag 2 S 


Rhombic 


Stromeyerite 


(Cu,Ag) 2 S 


Rhombic 


Polybasite 


9(Ag,Cu) 2 S.Sb 2 S 3 


Monoclinic 


Pearceite 


9(Ag,Cu) 2 S.As 2 S 3 


Monoclinic 


Stephanite 


5Ag 2 S.Sb 2 S 3 


Rhombic 


Pyrargyrite ... 


3Ag 2 S.Sb 2 S 3 


Hexag-rhombohedral 


Proustite 


3Ag 2 S.As 2 S 3 


Hexag-rhombohedral 


Tetrahedrite 


3 (Cu,Ag) 2 S.Sb 2 S 3 


Regular 


Freieslebenite 


5(Pb,Ag 2 )S.2Sb 2 S 3 


Monoclinic 



Of the haloid compounds of silver, the chloride, 
kerargyrite, is important ; the chloro-bromide, embolite, 
less so ; while the iodide, iodyrite, is quite rare. 



144 DESCRIPTIVE MINERALOGY 

Silver ores occur in veins belonging to two main 
groups : (i) Those associated with volcanic rocks of late 
Mesozoic or Tertiary age ; and (2) those of much earlier 
age. 

The first group is well illustrated by occurrences in 
the Carpathians of Transylvania (Nagyag-Verespatak) 
and Hungary (Schemnitz, Kremnitz and Nagybanya- 
Kapnik) ; in the Andes of Bolivia (Potosi, Huanchaca, 
Oruro), Peru (Cerro de Pasco), and Colombia (Tolima) ; 
in the Sierras of Mexico (Durango, Fresnillo, Zacatecas, 
Guanajuato, Puchuca) and of Arizona ; in the Sierra 
Nevadas of California (San Bernardino) and Nevada 
(Comstock, Esmeralda, etc.) ; in the Wahsatch Range 
of Utah (Hornsilver, etc., in Beaver County) ; in the 
Rockies of Colorado (Boulder, San Juan, Silver Cliff, 
Rosita, etc.) ; in the Coromandel peninsula (Hauraki) 
of New Zealand ; and finally in Japan (Akita and 
the island of Sado). 

The second group is illustrated by occurrences in 
the silver-lead deposits of Saxony (Freiberg, Annaberg, 
and Schneeberg), of the Harz (Clausthal and Andreas- 
berg), Bohemia (Przibram), and Norway (Kongsberg). 

A large amount of silver is now obtained from Cobalt 
in Northern Ontario, Canada, where silver ores (native 
silver and argentite) are associated with ores of nickel 
and cobalt (smaltite, niccolite, chloanthite, and cobalt - 
ite), as well as bismuth and mispickel. 

The world's output of silver amounts to close on 
220,000,000 ounces per annum, having a value of, 
roughly, 23,500,000. 



ORES 145 

Native Silver. Regular. Native silver, when crys- 
tallized, presents cubical or octahedral forms, but the 
crystals are usually distorted or united to divergent 
branching masses. Most frequently, however, the 
metal occurs in strings and wiry coils, occasionally 
even assuming a moss-like character ; it is also found 
as plate or foil, or in massive lumps. One such 
mass from Kongsberg in Norway, which is preserved 
in the Copenhagen Museum, weighs about 5 hundred- 
weight. 

Silver is very malleable and ductile, ranking next to 
gold in these qualities. In hardness it lies between 
gold and copper, its position in Mohs' scale ranging 
from 2*5 to 3. The density of native silver varies from 
10* i to ii ; that of pure silver is 10*5. Although 
naturally of a white colour, it is often tarnished super- 
ficially to a red, brown, or blackish colour. Lustre, 
metallic; fracture, hackly. The native ore contains 
traces of copper, arsenic, antimony, and iron. It is 
soluble in nitric acid, and gives a precipitate with 
hydrochloric acid. Fusibility, 2 (Von Kobell's scale). 

Native silver occurs in veins, associated with the 
other ores of silver, or intermingled with native copper 
as at Lake Superior. Other occurrences in the United 
States are in Arizona, Nevada (Comstock Lode), Colo- 
rado, North Carolina, etc. It is also found in Ontario 
(Cobalt), and in Australia. Large deposits occur in the 
mines of Peru and Mexico. In Europe it is found in 
Norway (Kongsberg), the Harz, Saxony, Silesia, Hun- 
gary, Spain (Sierra Morena), and the Dauphine. 

10 



146 DESCRIPTIVE MINERALOGY 

Argentite. Sulphide of silver : Ag 2 S (silver 87 per 
cent.). Crystallizes in the regular system in cubes, 
octahedra, and rhombic dodecahedra, but also occurs 
massive. Colour and streak, a dull black or lead grey. 
Lustre, metallic. Opaque. Cubic cleavage imperfect. 
Fracture, conchoidal. Sectile. Hardness, 2-2*5. Den- 
sity, 7-7*4. Fusibility, 1*5. Yields a globule of silver 
on charcoal. 

An important ore of silver in many silver-mines, as, 
for example, those of Saxony, Bohemia, Hungary, and 
Norway, in Europe; those of Peru and Chili, in South 
America; those of Mexico, Arizona, Idaho, Colorado, 
Nevada (Comstock Lode), Utah, and Ontario (Cobalt), 
in North America ; and those of Australia and 
Tasmania. 

Stephanite, or brittle silver ore. A sulphide oi 
silver and antimony, or sulphantimonite of silver 
Represented by the formula 5Ag 2 S.Sb 2 S 3 , which gives 
a silver percentage of 68*36. Il 
occurs massive, or crystallized ir 
thick six-sided tablets or in shorl 
FIG. 80. STEPHANITE. prisms of the rhombic system 
c, Basal plane ; />, pyra- This mineral has an iron-blacl 

mid ; d, brachydome. . 

colour, metallic lustre, is soft (hard 
ness, 2-2*5), an d nas a density of 6-2-6*3. It cleaves 
parallel to the brachypinacoid (oio), has an uneven tc 
semi-conchoidal fracture, and is brittle. Before the 
reducing flame of the blowpipe on charcoal it yield: 
a button of silver. 

Stephanite occurs with other silver ores in Saxon] 




ORES 147 

(Freiberg), Bohemia, Hungary (Schemnitz, Kremnitz, 
and Hodritsch), Chili, Peru, Mexico, and Nevada 
(Comstock Lode). 

Pyrargyrite, or dark ruby silver ore. A sulphide 
of silver and antimony, or sulphantimonite of silver : 
3Ag 2 S.Sb 2 S 3 (silver 59*97, antimony 22*21, sulphur 
17*82, per cent.). Crystallizes in the hexagonal-rhom- 
bohedral system (hemimorphic). Habit, short columnar 
with manifold rhombohedral and scalenohedral termina- 
tions. Often twinned. Also occurs massive. Cleavage, 
rhombohedral (1011). Fracture, conchoidal to uneven. 
Brittle. Hardness, 2-3. Density, 577-5*86. Lustre, 
metallic-adamantine. Translucent in thin splinters. 
Colour in reflected light, black or grey-black ; in trans- 
mitted light, deep cochineal red. Streak, red. Fusi- 
bility, i (Von Kobell). Before the blowpipe gives off 
dense antimonial fumes; yields a globule of silver 
when fused with carbonate of soda on charcoal. Occurs 
with other silver ores, and frequently with galena, 
the gangue being often calcite. 

Pyrargyrite occurs in Saxony, Bohemia, Hungary 
(Schemnitz, Kremnitz), Transylvania, the Harz (Andreas- 
berg), Norway, Spain, Montana, Nevada (Comstock 
Lode), Mexico, Chili, Peru, Bolivia, Canada (Cobalt). 

Proustite, or light ruby silver ore. A sulphide 
of silver and arsenic, or sulpharsenite of silver : 
3Ag 2 S.As 2 S 8 (silver 65*4, arsenic 15*17, sulphur 
19*43, per cent.). Crystallizes in the hexagonal-rhombo- 
hedral system (hemimorphic), in similar forms to pyrar- 



148 



DESCRIPTIVE MINERALOGY 



FIG. 81. PYRARGYRITE. 

P, Rhombohedron ( + R) ; 
z, a more obtuse rhom- 
bohedron (-% R) ; h, a 
scalenohedron ; n, prism. 



gyrite, with which it is isomorphous. Also massive. 

Cleavage, rhombohedral (1011). Fracture, conchoidal 

to uneven. Brittle. Hardness, 2. Density, 5 '55-5*64. 
Lustre, adamantine. Transparent 
to translucent. In reflected light, 
black or grey -black; in trans- 
mitted light, proustite has a 
brighter colour than pyrargyrite, 
inclining to scarlet-red. Streak, 
red. Fusibility, i (on Von 
Kobell's scale). Heated on char- 
coal before the blowpipe, gives 
off arsenical fumes (smelling of 

garlic), and yields a globule of silver with carbonate of 

soda. 

Occurrence, same as pyrargyrite. Chailarcillo, a 

mine in Chili, is a noted locality. 

Polybasite. A sulphide of silver, copper, and anti- 
mony, or sulphantimonite of silver and copper : 
9(Ag,Cu) 2 S.Sb 2 S 3 (with 62 to 75 per cent, silver and 
from o to 10 per cent, copper). The corresponding 
arsenical compound is also known (pearceite). Mono- 
clinic, in thin six-sided tables with pseudo-rhombo- 
hedral symmetry. Also occurs in scaly aggregates. 
Metallic lustre. Colour, iron black; in thin fragments 
by transmitted light cherry red. Basal cleavage, perfect. 
Fracture, uneven. Hardness, 2-3. Density, 6-6'2. 
Fusibility, I (Von Kobell's scale). Before the blow- 
pipe gives off antimonial fumes ; with carbonate of 
soda on charcoal yields globule of cupriferous silver. 



ORES 149 

Occurrence with other silver sulphides, in Saxony 
(Freiberg), Harz (Andreasberg), Bohemia (Joachims- 
thai, Przibram) Hungary (Schemnitz, Kremnitz, 
Hodritsch), Colorado, Nevada (Comstock Lode), Mon- 
tana, Arizona, Mexico, Peru, Chili. 

Kerargyrite, chlorargyrite, or hornsilver. Chloride 
of silver : AgCl (silver 7.5-3 per cent.)- Crystallizes in 
the regular system, with cubic habit, but usually occurs 
massive or in scales and plates. Colour, whitish-grey. 
Lustre, resinous to adamantine. Translucent. Mal- 
leable. Sectile. Hardness, 1-2. Density, 5*58-5-6. 
Easily fused (fusibility, i), yielding a globule of silver 
on charcoal. A valuable ore of silver, but not of very 
common occurrence. The largest deposits are in Mexico 
(Oajaca), Chili, and Peru. It is also found in Nevada 
(Comstock Lode), California (Pine Hill), and New South 
Wales (Broken Hill). 

Embolite. Chlorobromide of silver: Ag(Cl,Br) 
(silver 65 per cent.). Crystallizes in the regular system, 
but occurs in small disseminated particles. Colour, 
greenish. Translucent. Sectile. Hardness, 2-3. Den- 
sity, 579-5*80. Fusibility, i. Yields a globule of silver 
on charcoal. Of rare occurrence. Mexico (Oajaca), 
Chili (Chanarcillo). 

ORES OF LEAD. 

Lead occurs native, but it is a rare mineral. The 
bulk of the ore mined is galena. This, the simple 
monosulphide of lead, is the primary lead mineral, and 



150 DESCRIPTIVE MINERALOGY 

is invariably encountered in lead ore deposits when the 
mines are carried below the belt of weathering. In the 
upper oxidized portions of the deposits it has been re- 
placed, although rarely completely, by the carbonate 
(cerussite) and the sulphate (anglesite). The red oxide, 
minium (Pb 3 O 4 ), also occurs, but is unimportant as 
an ore. 

Lead ores occur as vein deposits of hydatogenetic 
origin and as metasomatic replacements. In the vein 
deposits the dominant gangue material may be quartz, 
dolomite, or barytes. In the quartz veins the galena is 
accompanied by chalcopyrite, and by silver ores in the 
dolomite and barytes veins; while zinc-blende is present 
in all three types. 

The bulk of the metasomatic deposits were originally 
formed by the replacement of limestone by galena, the 
sulphide of lead being carried in solution by alkaline 
sulphides. In most cases a secondary concentration 
has accumulated the ores in fissures, cavities, joints 
and bedding planes, whence they have spread out into 
the adjoining country rock. Such deposits occur in 
limestones of all ages Archaean, Ordovician, Devonian, 
Carboniferous, Triassic, Cretaceous, and Tertiary. 

Lead has a multifarious application in the arts : it is 
used in the form of sheets, pipe, shot, glazier's lead, 
wire; for type metal and other alloys; and in the 
manufacture of the pigments white lead (basic carbo- 
nates) and red lead (oxide, Pb 3 O 4 ). Litharge (PbO) 
and lead acetate (sugar of lead) are also important 
articles of commerce. 



ORES 151 

The world's production of pig lead amounts to a 
little over one million tons per annum. 

Galena. Sulphide of lead : PbS (lead 86'6 per cent.). 
Crystallizes in the regular system, generally as a com- 
bination of the cube and octahedron ; the cubical habit 
is usually dominant, with frequent twinning parallel to 
the octahedron (m). It also occurs massive in granular 
aggregates. 

In argentiferous galena enough silver is present to 
make the mineral a valuable source of that metal. 





FIG. 82. GALENA. 

a, Cube ; o, octahedron ; d, rhombic dodecahedron ; 
e, icosi-tetrahedron. 

A small amount of gold is usually associated with the 
silver. 

Colour, lead grey. Streak, greyish-black. Opaque. 
Lustre, metallic. When massive, dulL Tarnishes on 
exposure to air. Cubical cleavage, perfect. Fracture, 
even. Hardness, 2-2*5. Density, 7'4-7'6. Fusibility, 2. 
On charcoal with sodium carbonate, yields a globule of 
lead. Soluble in concentrated nitric acid, with separa- 
tion of sulphur. 

Galena is the most widely distributed, the most 
abundant, and the most important, ore of lead. The 
following are localities of some of the more im- 



152 DESCRIPTIVE MINERALOGY 

portant lead -mining districts in which galena occurs 
below the oxidation zone, generally in association with 
blende : Cardigan and Montgomery (Welsh Potosi, 
Van, Cwm Ystwith), Flintshire (Halkyn), Denbigh- 
shire (Minera), Isle of Man (Laxey and Foxdale), 
Anglesey (Parys, Mona), Cornwall, Northumberland, 
Cumberland (Threlkeld), Durham (Weardale), York- 
shire (Swaledale), and Derbyshire ; Saxony (Freiberg, 
Schneeberg, Annaberg, Altenberg), Silesia (Katzbach), 
Nassau (Ems, Holzappel), Harz, Bohemia (Przibram, 
Pilsen, Kuttenberg), Styria (Graz), Carinthia (Raibl and 
Bleiberg), Carniola, Bosnia, Tuscany, Sicily, Sardinia, 
Spain (Murcia, Linares, Ciudad Real, Sierra Morena), 
Sweden (Sala), North Africa (Tunis, Constantine), 
Colorado (Leadville), Idaho (Coeur d'Alene), Utah 
(Frisco, Wisconsin, Oquirrh, Bingham), Illinois, Dakota, 
Arkansas, Nevada (Eureka), Missouri (Joplin), Mexico 
(Sierra Mojada), Canada (Ontario), New South Wales 
(Broken Hill), Tasmania (Zeehan, Mount Read), South 
Africa (Transvaal). 

Cerussite. Carbonate of lead : PbCO 3 (lead 77*5 per 
cent.). Crystallizes in the rhombic system, being iso- 
morphous with aragonite. The crystals are partly of a 
pyramidal habit, partly tabular, and are often twinned 
on a face of the prism (no). They are usually colour- 
less and pellucid ; but less transparent and white, or 
slightly tinted, varieties occur, occasionally forming 
delicate silky and fibrous aggregates. The crystals are 
characterized by a brilliant adamantine lustre. Pris- 



ORES 153 

matic cleavage, imperfect. Fracture, conchoidal. Brittle. 
Hardness, 3-3*5. Density, 6-4-6*6. Fusibility, 1*5. 
Yields a globule of lead when fused on charcoal with 
carbonate of soda. Soluble in dilute nitric acid, with 
effervescence. 

Cerussite is an important ore occurring in the 
oxidized zone of lead deposits. Well - crystallized 
specimens, represented in most collections, are usually 
from Cornwall (Pentire Glaze), Nassau (Friedrichsegen 
mine, near Ems), Bohemia (Mies), Siberia (Nert- 





FIG. 83. SIMPLE AND TWINNED CRYSTALS OF CERUSSITE. 

t, Pyramid ; m, prism ; e, brachy prism ; 
6, brachypinacoid. 

schinsk), New South Wales (Broken Hill), Colorado 
(Leadville), Rhodesia (Broken Hill). 

Anglesite. Sulphate of lead : PbSO 4 (lead 68*3 per 
cent.). Crystallizes in the rhombic system, being iso- 
morphous with barytes and celestite. Habit, tabular. 
Also occurs massive. Colourless. Streak, white. Lustre, 
adamantine to resinous. Transparent. Cleavage, pris- 
matic and basal, fair. Brittle. Fracture, conchoidal. 
Hardness, 3. Density, 6-3. Fusibility, 2. Yields 
globule of lead with carbonate of soda on charcoal. 
Soluble with difficulty in nitric acid. Anglesite occurs 



154 DESCRIPTIVE MINERALOGY 

in the oxidation zone of lead ores as an alteration pro- 
duct of galena, and often constitutes an important ore 
of lead. It derives its name from Anglesey, where it 
is found at the Parys copper mine. Other well- 
known localities are Sardinia (Monte Poni), Westphalia 
(Siegen), Harz, Pennsylvania (Phcenixville), and many 
lead-mines in the United States. 



Pyromorphite. Chlorophosphate of lead : 3Pb 3 
(PO 4 ) 2 .PbCl 2 (lead 76*4 per cent). Crystallizes in 
the hexagonal system. Habit, prismatic. Also occurs 
in uniform and botryoidal aggregates. Colour, green, 
yellow, or brown. Streak, white. Lustre, resinous. 
Translucent. Prismatic cleavage, imperfect. Fracture, 
subconchoidal. Brittle. Hardness, 3*5-4. Density, 
6*5-7'i. Fusibility, 2. On charcoal, with carbonate of 
soda, yields a globule of lead. Soluble in nitric acid. 
Occurs in association with other ores of lead. 



ORES OF ZINC. 

Blende, the monosulphide of zinc, is the source of 
the bulk of the zinc of commerce (spelter). It is the 
primary zinc ore, and is invariably found in the deeper 
parts of the deposits ; while within the belt of weather- 
ing it is replaced by the carbonate (calamine), the 
silicates (smithsonite and willemite), and the oxide 
(zincite). 

As in the case of lead, zinc ores occur both as veins 
and as metasomatic replacements. 



ORES 155 

The same three types of vein deposit hold for zinc 
ores as for lead ores, blende and galena being frequently 
associated in the same veins. 

The metasomatic ores have been formed by the 
replacement of limestone, sulphide of zinc being de- 
posited as blende from solution in alkaline sulphides. 
The subsequent distribution of the ore along fissures 
and other openings is regulated by solution, redeposi- 
tion, and concentration. Where oxidation is possible, 
secondary zinc ores are also formed. 

The chief uses of zinc are in the manufacture of 
galvanized iron (sheet iron coated with zinc), in electro- 
zincing, and for alloys (especially brass, which is an 
alloy of zinc with copper, and German silver, in which 
zinc is alloyed with copper and nickel). Its compounds 
are also used as pigments (zinc oxide in zinc white, 
and zinc sulphide in admixture with barium sulphate in 
lithopone), and for many other purposes. 

The world's output of spelter amounts to close on 
800,000 tons per annum. 

NOTE. The metal cadmium is also obtained from 
zinc ores. Zinc and cadmium are closely allied metals, 
and their compounds occur in isomorphous inter- 
mixture. Thus, blende often contains from 0*3 to 3 per 
cent, of cadmium sulphide (CdS), as at Ouro Preto in 
Brazil, and in Kentucky, Illinois, and the Joplin District 
of Missouri, in the United States. Cadmium is used 
in fusible alloys for soft solders, electric fuses, and (in 
alloy with mercury) in dental amalgam. It may also 
take the place of bismuth in cliche metal. 



156 DESCRIPTIVE MINERALOGY 

Zinc-Blende, sphalerite, or "Black Jack." Sul- 
phide of zinc: ZnS (zinc 67*06 per cent.). Iron is a 
common constituent of blende, especially of the darker- 
coloured varieties,* and cadmium is frequently present. 
Crystallizes in the regular system, with tetrahedral sym- 
metry. Habit, usually dodecahedral. Twinned crys- 
tals frequent, the twinning axis being the normal to 
a face of the octahedron (see Fig. 82). Also occurs 
massive, or in granular, nodular, and botryoidal aggre- 
gates. Colour, usually brown to black (" Black Jack"), 
but also yellow to colourless. Transparent to trans- 






FIG. 84. TWINNED CRYSTALS OF BLENDE. 
o, Octahedron ; a, cube ; d, rhombic dodecahedron. 

lucent. Lustre, resinous to adamantine. Streak, 
yellow or brown. Dodecahedral cleavage, perfect. 
Conchoidal fracture. Brittle. Hardness, 3-4. Den- 
sity, 3'g-4'i. Fusibility, 5. Gives off sulphur fumes 
when heated. Soluble in nitric acid, with separation 
of sulphur. 

Blende is the most widely distributed and most 
abundant ore of zinc, being found, in association with 
galena, below the oxidation zone of all zinc-lead ore 

* The iron is probably present as sulphide of iron (FeS) in 
isomorphous admixture with blende (ZnS). 



ORES 



157 



deposits. For localities, see those given for galena. A 
few may be specially mentioned : North of England 
(Alston Moor), Belgium, Sardinia, the Alps, Westphalia 
(Iserlohn), Upper Silesia, Hungary (Schemnitz), Carin- 




MAP OF THE MISSOURI-KANSAS ZINC AND LEAD FIELDS. 

thia (Raibl), Greece (Laurium), Northern Spain (As- 
turias), Algeria, New South Wales (Broken Hill), 
Missouri-Kansas (Webb City, Joplin, Galena, Duenweg, 
Oronogo, Granby, Alba Neck, Badger, Miami, and 
Carthage). 



158 DESCRIPTIVE MINERALOGY 

Blende also occurs in association with chalcopyrite, 
as in the complex ores of Huanchaca in Bolivia. 

Calamine, or zinc spar (smithsonite of Dana). 
Carbonate of zinc : ZnCO 3 (zinc 52 per cent.). Crys- 
stallizes in the hexagonal system, with rhombohedral 
symmetry, being isomorphous with calcite. When 
crystallized, it occurs in small crystals with curved 
faces. Usually, however, it is found in kidney-shaped 
and botryoidal aggregates, or massive. Though colour- 
less when pure, it is often tinted grey, yellow, brown, 
or green. Lustre, vitreous to pearly. Streak, white. 
Hardness, 5. Density, 4-3-4-35. Infusible. Soluble 
in warm dilute hydrochloric acid, with effervescence. 

An important ore of zinc in those deposits, or por- 
tions of deposits, which lie within the belt of weather- 
ing. It is largely worked in the zinc-mines of Siberia 
(Nertschinsk), Aix-la-Chapelle (Altenberg), Northern 
Spain (Santander), Greece (Laurium), Virginia (Austin 
Mines), Illinois (Jo Davies County), Arkansas, Iowa, 
Kansas, Kentucky, Rhodesia (Broken Hill). 

Willemite. Silicate of zinc : SiO 2 .2ZnO (zinc 
58*7 per cent.). Crystallizes in the hexagonal system, 
with rhombohedral symmetry, occurring in small crys- 
tals. More usually it is found massive, granular, or in 
kidney-shaped aggregates. Colour, white, yellow, or 
brown. Transparent to translucent. Lustre, vitreous. 
Cleavage, basal. Fracture, subconchoidal. Hardness, 
5-6. Density, 4-02-4*18. Infusible. Gelatinizes with 
hydrocloric acid. 



ORES 



159 



In Europe it occurs at Aix-la-Chapelle (Altenberg) 
and a few other localities ; but it is only found in 
workable quantities in New Jersey in the United 
States (Franklin Furnace and Sterling Hill), where it 
occurs in association with zincite and franklinite, and 
constitutes a valuable ore of zinc. 

Hemimorphite (calamine of Dana). Hydrated sili- 
cate of zinc : SiO 2 .2ZnO.H 2 O (zinc 41 per cent.). 
Crystallizes in the rhombic system, with hemimorphic 
development i.e., the crystals present different com- 
binations at the two ends of the vertical axis (hence 



9 9 




FIG. 85. CRYSTALS OF HEMIMORPHITE. 

c, Basal plane ; b, brachypinacoid ; a, macropinacoid ; g, prism ; 
o, and^, macrodomes ; m, brachydorae. 

its name). The crystals are usually small tablets 
(parallel to oio), which are combined to fan-shaped, 
spherical, and kidney-shaped aggregates. Occurs also 
in fibrous and granular masses. Twinning parallel 
to the basal plane. Although often colourless or 
white, it is also grey, yellow 7 , brown, or even of 
a green or blue tint. The crystals are usually 
transparent, and have a glassy lustre. Streak, white. 
Prismatic cleavage, perfect. Fracture, uneven. Brittle. 
Hardness, 4-5. Density, 3*4-3*5. Strongly pyro- 
electric. Infusible. Loses water at a red heat only. 



160 DESCRIPTIVE MINERALOGY 

An ore of zinc, often associated with the car- 
bonate (calamine), as at Aix-la-Chapelle (Altenberg), 
Westphalia (Iserlohn and Siegen), Carinthia (Bleiberg 
and Raibl), Spain (Santander), Siberia (Nertschinsk), 
Virginia (Austin Mines), Missouri (Granby, Duenweg, 
Joplin, Spring City, Aurora, Sarconie). 

Zincite. Oxide of zinc: ZnO (zinc 8031 per cent.)- 
Crystallizes in the hexagonal system, with hemimorphic 
development. Habit, columnar, with prism (1010), 
pyramid (1011), and basal plane (oooi). Occurs 
usually massive or in granular aggregates. Colour, 
dark red. Transparent to translucent. Lustre, ada- 
mantine to metallic. Streak, orange to yellow. Basal 
cleavage, perfect. Fracture, subconchoidal. Brittle. 
Hardness, 4-5. Density, 5*4 "5*7- Infusible. Soluble 
in acids. 

Occurs as an ore of zinc in association with 
willemite and franklinite in the zinc-mines of New 
Jersey (Franklin Furnace and Sterling Hill). 

Franklinite. An epitritoxide of zinc, manganese, 
and iron: (Zn,Fe,Mn).O(Fe,Mn) 2 O 3 (percentage of zinc 
variable). A member of the spinel group. Crystal- 
lizes in the regular system, with octahedral habit. 
Usually occurs massive or granular. Colour, iron 
black or dark brown. Streak, brown. Opaque. Lustre, 
metallic. Fracture, uneven. Brittle. Hardness, 6. 
Density, 5*15. Slightly magnetic. Infusible. Soluble 
in hydrochloric acid. 

Occurs as an ore of zinc in association with zincite 



ORES 161 

and willemite in the zinc-mines of New Jersey (Franklin 
Furnace and Sterling Hill), where the zinc is first 
extracted, and the residue then treated as an iron ore. 

ORES OF NICKEL. 

The chief source of nickel is nickeliferous pyrrhotite 
(see under pyrrhotite), which occurs in the marginal 
portions of large intrusions of norite near Sudbury* in 
Ontario, Canada. In this ore the nickel appears to be 
present in the form of pentlandite (Fe,Ni)S. Another 
source is the indefinite hydrated silicate of nickel known 
as garnierite, which is mined at Noumea in New Cale- 
donia. Linnceite, the sulphide of cobalt and nickel, is 
also a source of nickel. The remaining nickel ores 
viz., niccolite, chloanthite, and gersdorffite are responsible 
for the production of a comparatively small amount of 
the metal ;t while the nickel minerals millerite, anna- 
bergite, and zaratite only occur as decomposition 
products in the weathered portions of the lodes. Nickel 
is used principally in the manufacture of certain white 
alloys e.g., German silver, which is an alloy of nickel 
with copper and zinc also for nickel-plating. 

Pentlandite. A sulphide of iron and nickel: (Fe,Ni)S 
(nickel 10 to 39 per cent.). Crystallizes in the regular 
system, but usually occurs massive. Colour, yellowish- 

* Sudbury is responsible for more than half the world's annual 
production of nickel. 

t The nickel derived from this source of supply is chiefly con- 
tained in the silver ores shipped from the Cobalt district of Ontario, 
Canada. 

11 



162 DESCRIPTIVE MINERALOGY 

bronze. Streak, black. Lustre, metallic. Octahedral 
cleavage. Fracture, uneven. Hardness, 3*5-4. Den- 
sity, 4*95-5. Fusibility, 5. Soluble in nitric acid. 

This mineral occurs in association with pyrrholite and 
chalcopyrite in the so-called nickeliferous pyrrhotite of 
Sudbury, Ontario, which is the most important source 
of the metal nickel. 

Niccolite. Arsenide of nickel : NiAs (nickel 44*1 
per cent.). Crystallizes in the hexagonal system, but 
occurs mostly massive or in granular and botryoidal 
aggregates. Colour, copper red. Opaque. Lustre, 
metallic. Streak, brownish-black. No cleavage. Frac- 
ture, conchoidal to uneven. Fairly brittle. Hardness, 5. 
Density, 7*3-7*7. Fusibility, 2. On charcoal, yields a 
white, brittle, metallic globule, and gives off arsenical 
fumes. Soluble in aqua regia and in nitric acid, with 
separation of sulphur. 

Occurs as an ore of nickel in association with silver 
and cobalt ores. Cornwall, Harz (Andreasberg), Saxony 
(Schneeberg, Annaberg), Bohemia (Joachimsthal), Nor- 
way, Sweden, Chili (Chanarcillo, and Huasco), Ontario 
(Cobalt). 

Chloanthite. Arsenide of nickel: NiAs 2 (nickel 
28*1 per cent.). Crystallizes in the regular system, 
with pentagonal hemihedrism. Usual form, the cube. 
Also massive or granular. Colour, tin white to steel 
grey. Opaque. Streak, greyish-black. Lustre, metallic. 
Octahedral cleavage, imperfect. Fracture, uneven. 
Brittle. Hardness, 5-6. Density, 6*3-7. Fusibility, 2, 



ORES 163 

yielding magnetic globule on charcoal, with production 
of arsenical fumes. Decomposed by nitric acid, yield- 
ing a green solution. 

Occurs as an ore of nickel in the Erzgebirge, 
Harz, Saxony, Chili, Peru, United States, Canada 
(Cobalt), etc. 

Gersdorffite. Arsenosulphide of nickel: NiAsS 
(nickel 35*31 per cent.). Crystallizes in the regular 
system, with pentagonal hemihedrism. Usual form, the 
octahedron, often in combination with the cube. Also 
granular and massive. Colour, silver white to steel 
grey. Streak, greyish-black. Opaque. Lustre, metal- 
lic. Cubic cleavage, fair. Fracture, uneven. Brittle. 
Hardness, 5-5*5. Density, 5'6-6'2. Fusibility, 2. Gives 
off sulphurous and arsenical fumes on heating. Decom- 
posed by nitric acid. 

Occurs as an ore of nickel in Westphalia, Rhine 
Province, Nassau, Harz, Saxony, Hungary, Canada 
(Ontario). 

Millerite. Sulphide of nickel : NiS (nickel 64/69 
per cent). Crystallizes in the hexagonal system, in 
acicular or capillary prisms or in fibrous aggregates. 
Colour, brassy yellow. Lustre, metallic. Opaque. 
Streak, greenish-black. Rhombohedral cleavage, per- 
fect. Fracture, uneven. Brittle. Hardness, 3-4. Den- 
sity, 5*3-5*6. Fusibility, 1*5-2. Yields a magnetic 
globule. Soluble in nitric acid, with separation of 
sulphur. 

Millerite is not itself of importance as an ore of nickel, 



164 DESCRIPTIVE MINERALOGY 

but occurs with other nickel ores, as at the Gap Mine, 
Lancaster County, Pennsylvania," and at Cobalt, 
Ontario. 

Garnierite, genthite, noumeite. Rather indefinite 
hydrated silicates of nickel, magnesium, and iron : 
2(Mg,Ni)O.3SiO 2 + wH 2 O (nickel 15-30 per cent.). No 
crystal form ; occurs massive, often as an incrustation. 
Colour, apple green. Translucent to opaque. Lustre, 
resinous. Streak, greenish-white. Fracture, uneven. 
Hardness, 3-4. Density, 2*2-2 '8. Infusible. Decomposed 
by hydrochloric acid, with separation of silica. Usually 
occurs in serpentine, sometimes in association with 
chromite. 

It is worked as an ore of nickel at Noumea in 
New Caledonia, where it is found in a decomposed 
serpentine rock ; also, but to a smaller extent, near 
Frankenstein in Silesia. 

Annabergite, or nickel-bloom. Hydrated arseni- 
ate of nickel: Ni 3 (AsO 4 ) 2 .8H 2 O or 3NiO.As 2 O 5 .8H 2 O 
(nickel 29*5 per cent.). Not crystallized. Occurs as 
earthy deposit, resulting from the decomposition of 
nickel ores. Colour, apple green. Streak, greenish- 
white. Hardness, i'5-2'5. Fusibility, 4. 

A decomposition product of nickel ores. Itself of no 
importance as an ore. Original locality, Annaberg in 
Saxony. 

Zaratite, or emerald-nickel. Hydrated nickel car- 
bonate : NiCO 3 .2Ni(OH) 2 .4H 2 O or CO 2 .3NiO.6H 2 O 

(nickel 26 per cent.). Not crystallized. Massive. 



ORES 165 

Usually as an incrustation. Colour, emerald green. 
Lustre, vitreous. Translucent. Streak, pale green. 
Hardness, 3-3*2. Density, 2'6-2'j. Brittle. Infusible. 
Dissolves with effervescence in hydrochloric acid. 

Of no importance as an ore. A decomposition product 
of other nickel ores. Locality, Texas in Pennsylvania. 

ORES OF COBALT. 

The chief ores of cobalt are linnceite (the sulphide), 
smaltite (the arsenide), and cobaltite (the arsenosulphide). 
Glaucodot (the arsenosulphide of cobalt and iron) is 
of less importance; while erythrite or cobalt-bloom (a 
hydrated arseniate of cobalt) only occurs as a decomposi- 
tion product in the superficial portions of the lodes. 
Most nickel ores contain some cobalt, and the 
metal also occurs in mispickel (q.v.). Both cobalt 
and nickel are found in some copper ores. 

The main supply of cobalt is from the mines of 
Ontario and of New Caledonia. It is largely obtained 
as a by-product in the smelting of the silver ores 
from Cobalt in Ontario. The principal use of cobalt 
is as a pigment, especially in its application to 
glass- making and pottery. Z offer, a roasted cobalt 
ore, and the oxide, arsenate and phosphate of cobalt, 
are all used for imparting a blue colour to glass, or in 
glazing and painting on porcelain and glass. 

Linnaeite, a sulphide of cobalt and nickel: (Co,Ni) 3 S 4 
(cobalt and nickel 57*88 per cent.). Crystallizes in the 
regular system, with octahedral habit. Also occurs 



166 DESCRIPTIVE MINERALOGY 

massive or granular. Colour, light steel grey. Streak, 
blackish-grey. Opaque. Lustre, metallic. Cubical 
cleavage, imperfect. Fracture, uneven. Brittle. Hard- 
ness, 5-6. Density, 4*8-5. Fusibility, 2. Soluble in 
nitric acid. 

An important ore of cobalt and nickel, found in 
association with galena, chalcopyrite, and pyrites, as 
in Westphalia, Sweden, and Missouri in the United 
States. 

Smaltite. Arsenide of cobalt : CoAs 2 (cobalt 28*2 
per cent.). Crystallizes in the regular system, with 
pentagonal hemihedrism. Usual form, the cube. Also 
massive or granular. Colour, tin white to steel grey. 
Opaque. Streak, grey-black. Lustre, metallic. Octa- 
hedral cleavage, imperfect. Fracture, uneven. Brittle. 
Hardness, 5-6. Density, 6-3-7. Fusibility, 2-3. Yields 
a magnetic globule on charcoal, and gives off arsenical 
fumes. Decomposed by nitric acid, yielding a pink 
solution. 

Occurs as an ore of cobalt in the Erzgebirge, Harz, 
Saxony, Chili, Peru, Ontario (Cobalt), United States, 
Transvaal. 

Cobaltite. Arsenosulphide of cobalt : CoAsS (cobalt 
35*5 P er cent.). Crystallizes in the regular system. 
Usual form, the pentagonal dodecahedron, modified by 
the cube. Also granular or massive. Colour, silver 
white, with a reddish tint. Opaque. Streak, greyish- 
black. Lustre, metallic. Cubical cleavage, fair. 
Fracture, uneven. Brittle. Hardness, 5-6. Density, 



ORES 167 

6-6*4. Fusibility, 2-3. Yields a magnetic globule on 
charcoal. Decomposed by nitric acid. 

Occurs in crystalline schists, together with chalco- 
pyrite. Also in ore veins, as in Westphalia (Siegen), 
Hungary, Caucasus, Norway (Skutterud and Snarum), 
Sweden (Tunaberg), Chili, Ontario (Cobalt). 

Glaucodot. Arsenosulphide of cobalt and iron : 
(Co,Fe)AsS (cobalt 4 to 25, iron 12 to 33 per cent.). 
Rhombic, with habit similar to that of mispickel. 
Colour, tin white to reddish silver white. Opaque. 
Metallic lustre. Streak, black. Basal cleavage, per- 
fect. Fracture, uneven. Brittle. Hardness, 5. Den- 
sity, 5*9-6. Fusibility, 2-3. Yields a magnetic globule 
on charcoal. Decomposed by nitric acid, with separa- 
tion of sulphur. Of little importance as an ore of 
cobalt. 

Occurs in Norway (Skutterud, Sulitelma), Sweden 
(Hakansboda), United States (New Hampshire). 

Erythrite, or cobalt -bloom. Hydrated arseniate 
of cobalt : Co 3 (AsO 4 ) 2 .8H 2 O or 3CoO.As 2 O 5 .8H 2 O 
(cobalt 29*5 per cent.). Crystallizes in the monoclinic 
system, in slender prismatic needles. Also massive, 
earthy, or as an incrustation. Clinopinacoidal cleavage, 
perfect. Colour, crimson to peach colour. Lustre, 
pearly to vitreous or dull. Streak, pale red. Trans- 
lucent. Sectile. Hardness, 1*5 -2*5. Density, 2*95. 
Fusibility, 2*5. 

Occurs as a decomposition product of cobalt ores. 
Of no importance as an ore. 



168 DESCRIPTIVE MINERALOGY 

ORES OF IRON. 

The ores of iron are abundant and of widespread 
occurrence. Those in chief use for the extraction of 
the metal are the oxides, magnetite and hematite, the 
group of hydrated oxides embraced under the general 
term limonite, and the carbonate, chalybite. The sul- 
phides pyrites, marcasite,pyrrhotite, etc. are not mined 
for their iron, but for their sulphur content* ; but these 
minerals are the original source of much of the oxidized 
iron ore which is found within the belt of weathering of 
the earth's crust,f although the largest proportion of 
the oxides is derived, in the first instance, from the 
decomposition of the numerous iron-bearing minerals 
(ferromagnesian silicates, etc.) that occur in igneous 
rocks and are especially abundant in the more basic 
subdivisions. Chalybite is an intermediate product, 
and haematite and limonite ores often pass downwards 
into spathic ores (chalybite), from which they have 
been derived by oxidation and hydration ; while the 
chalybite itself has, in some cases, been traced to the 
decomposition of pyrites. Of the oxides, magnetite 
alone occurs in considerable quantity as a mineral of 
igneous origin, the deposits of this ore near the peri- 
phery of large intrusions of basic igneous rocks, having 
been formed by magmatic concentration, while the rock 

* The iron oxide residue, which remains after the iron sulphides 
have been roasted in the manufacture of sulphuric acid, is frequently 
used in blast-furnaces for the production of pig-iron. 

t For example, the limonite gossan deposits. 



ORES 169 

was still in a state of igneous fusion. Chromite, and in 
a less degree haematite, also occur as minerals of igneous 
origin. 

Native iron occurs as a constituent of meteorites, and 
as small nodules in certain basalts (e.g., Disco Island in 
West Greenland). There seems to be some doubt 
whether it occurs in the latter as an original or as a 
secondary constituent. The existence of iron in the 
native state is only of scientific interest : native iron 
is not an ore of the metal. 

The world's annual output of iron ore amounts at the 
present time to about 133,000,000 tons, from which some 
60,000,000 tons of pig-iron are produced. The dis- 
tribution of the ores mined is shown by the following 
table, which summarizes the output (for 1909) of the 
ten principal producers : 

United States ... ... 53,034,000 tons 

Germany ... ... 25,095,000 ,, 

United Kingdom ... ... 14,980,000 ,, 

France ... ... ... 12,254,000 

Spain ... ... ... 9,056,000 ,, 

Sweden ... ... 3,823,000 

Austria ... ... ... 2,450,000 ,, 

Canada ... ... ... 239,000 ,, 

Belgium ... ... 203,000 ,, 

An interesting estimate of the actual and potential 
ore reserves of the world has been published by the 
Eleventh International Geological Congress, which sat 
in Stockholm in 1910. It is quoted here, as it serves 



170 



DESCRIPTIVE MINERALOGY 



SCALE or ENGLISH MILES 

. , 1 1 9 



:.. ; L A P L A N D 
O 




IRON ORE OCCURRENCES IN SCANDINAVIA." 



ORES 171 

admirably to illustrate the distribution of the known 

iron ores of the world : 

Actual Potential 

Reserves. Reserves. 

Million Tons. Million Tons. 

Europe ... 12,032 41,029 

America ... 9,855 81,822 

Australia ... 136 69 

Asia ... ... 260 457 

Africa... ... 125 not estimated 

Total ... 22,408 123,377* 

Magnetite, or magnetic iron ore. The epitritoxide 
of iron : Fe 2 O 3 .FeO (iron 72*4 per cent.). Crystallizes 
in the regular system, occurring in very perfect octa- 
hedra or rhombic dodecahedra. 
Twinning on the spinel type : twin- 
ning plane (m). More frequently, 
however, it is found in granular 
and compact masses ; in minute 
particles, scattered through many 
igneous rocks and crystalline schists ; FlG - 86. MAGNETITE. 
or in loose rounded granules (" iron 
sand ") that have been washed out of igneous rocks. 
It possesses an iron-black colour, black streak, and 
metallic lustre. Its fracture is conchoidal to uneven. 
There is no cleavage, but planes of parting parallel to 
the octahedron are characteristic. Hardness, 5-5- 6-5. 
Density, 4^9 -5*2. Its magnetic properties are pro- 
nounced, and it was these qualities that caused it to 
an unknown amount. 




172 DESCRIPTIVE MINERALOGY 

be esteemed by the ancients under the name of the 
lodestone. When powdered it is easily soluble in hydro- 
chloric acid. Before the blowpipe it fuses only with 
difficulty (fusibility, 5-5*5). 

Magnetite is often associated with plutonic igneous 
rocks of basic composition, and such association denotes 
a community in origin. Important deposits occur in 
Sweden (Kirunavaara, Luossavaara, Gellivare, and 
Taberg), Norway (Sydvaranger and Dunderland), 
Russia (Gora Blagodat and Gora Magnitnaja in the 
Urals and the Caucasus), Spain (Lugo), Transvaal 
(Bushveld). In England, the ore has been worked at 
Rosedale in Yorkshire. 

Haematite. Sesquioxide of iron : Fe 2 O 3 (iron 70 per 
cent.). Hexagonal-rhombohedral. This iron ore com- 
prises two varieties : the crystallized or specular iron 
ore, and the amorphous and earthy material known as 
red hczmatite. 

The crystals are partly of a rhombohedral and pyram- 
idal habit, partly tabular, according as rhombohedral 
and pyramidal faces or the basal plane predominates. 
The prismatic faces are always subordinate. The rhom- 
bohedral faces are often curved, passing gradually over 
into the basal plane. Specular iron ore also occurs in 
granular and scaly aggregates, this variety being known 
as micaceous hematite. The ordinary red iron ore is not 
crystallized, its structure being either crypto- crystalline 
or earthy. It occurs in nodular and botryoidal masses, 
having a smooth exterior and a radiating internal struc- 
ture (kidney ore], and it also forms stalactitic aggregates. 



ORES 



178 



The earthy variety is termed red ochre, and is used 
as a paint. Lustre, metallic to dull. Colour, iron 
black to dark steel grey. Streak, cherry red. The 





FIG. 87. SOME COMMON FORMS OF HEMATITE. 

r, Rhombohedron ; 5, a more obtuse rhombohedron ; e, a 
negative rhombohedron ; n, a deutero-pyramid. 

colour of the earthy varieties is red. No true cleavage. 
Fracture, conchoidal to uneven. Infusible before the 
blowpipe. Soluble in concentrated hydrochloric acid. 
Haematite is an important source of iron, and the ore 




FIG. 88. KIDNEY IRON ORE. 
A variety of haematite. (From a photograph.) 

has a very widespread distribution. Small quantities are 
found in Cornwall, Devon, and South Wales (Forest of 
Dean) ; but the largest English mines are in North Lanca- 



174 DESCRIPTIVE MINERALOGY 

shire and Cumberland (Ulverston and Whitehaven). The 
chief foreign mining centres are France (Normandy 
and the Pyrenees), Spain (Bilbao, Almeria, Oviedo), 
Italy (Elba), Germany (Lahn and Dill districts, El- 
bingerode and Hiittenrode in the Harz), Southern 
Russia (Kvivoj-Rog), United States (Marquette, Gogebic, 
and Menominee ranges on the south side, and Vermilion 
and Mesabi ranges on the north side, of Lake Superior, 
Adirondack district, Clinton mines in Alabama), Cuba, 
Brazil (itabirite ores). 

Goethite. Hydrated oxide of iron : Fe 2 O 3 .H 2 O (iron 
80*91 per cent.). Crystallizes in the rhombic system. 
Habit, columnar to acicular or capillary, with 
striated prism faces, or tabular parallel to the brachy- 
pinacoid (oio). Also in kidney-shaped or botryoidal 
masses with radial, fibrous, and concentric structures, 
or in fibrous or scaly aggregates. Colour, yellowish 
or reddish-brown to black. Translucent to opaque. 
Lustre on smooth faces, adamantine to metallic, also 
dull to silky. Streak, yellow to reddish-brown. Brachy- 
pinacoidal cleavage, perfect. Fracture, uneven. Brittle. 
Hardness, 5. Density, 3'8-4*4. Fusibility, 5-5*5. Yields 
water in the closed tube. Slowly soluble in concentrated 
hydrochloric acid. 

An ore of iron, but not so common as haematite or 
limonite. Occurs in Cornwall (Lostwithiel, Botallack, 
Redruth), Scotland, Westphalia, Nassau, Harz, Saxony 
(Schneeberg, Freiberg), Silesia, Bohemia (Przibram), 
Russia (Ekaterinburg), United States (Marquette, 
Mesabi, etc., in the Lake Superior district). 



ORES 175 

Limonite, or brown iron ore. Hydrated oxide of 
iron: Fe 2 O 3 + wH 2 O. Does not occur crystallized, but 
has a crypto-crystalline to earthy texture, and forms 
nodular, botryoidal, kidney-shaped, or stalactitic masses, 
frequently with an internal fibrous structure. The 
amount of water is variable. Some authors use the 
water -content as a means of distinction between the 
following mineral species : turgite, with 5*3 per cent, 
of water, corresponding to 2Fe 2 O 3 .H 2 O ; limonite, with 
14*5 per cent, of water, corresponding to 2Fe 2 O 3 .3H 2 O ; 
xanthosiderite, with 18*4 per cent, of water, corresponding 
to Fe 2 O 3 .2H 2 O; limnite, with 25*2 per cent, of water, 
corresponding to Fe 2 O 3 .3H 2 O. But it is somewhat 
doubtful whether these definite hydrates really exist. 

The different varieties of limonite have been classified 
by Tschermak, according to texture, as follows : fibrous 
limonite (including turgite) ; compact brown iron ore, in- 
cluding oolitic ironstone and the decomposition products 
of chalybite and pyrites; ochreous limonite, including 
xanthosiderite and the various ochres, umbers, and 
siennas; pitchy limonite (stilpnosiderite), characterized 
by a black colour, glazed surface, and conchoidal 
fracture ; earthy and sandy limonite, including limnite, 
bog iron ore, and lake iron ore, found as a brownish- 
yellow deposit from ferrous carbonate in marshes, wet 
moorland, and shallow lakes ; pisolitic and oolitic limonite, 
including the so-called "minettes " ; limonitic cement and 
impregnation, found in clays, sandstones, conglomerates, 
and as concretions. 

Limonite varies in colour from yellow to blackish- 



176 DESCRIPTIVE MINERALOGY 

brown, and has a yellowish - brown streak. It is 
opaque, and has a dull to silky lustre. Its hardness 
is from I to 5*5, according to the variety, and its density 
from 3*3 to 4. Fracture, according to the variety, fibrous, 
conchoidal, or earthy. Brittle. Fusible with difficulty. 
Heated in the closed tube, yields water. Limonite is an 
important iron ore on account of its abundance and wide- 
spread distribution. The percentage of iron depends, 
of course, upon the amount of sandy and clayey im- 
purity. The considerable percentage of phosphoric 
acid depreciated the value of some of these ores until 
the introduction of the basic process. 

Limonitic ores occur in the British Isles : in South 
Wales (Forest of Dean) and Antrim. Large supplies 
are also derived from the oolitic iron ores of Jurassic 
age in North Yorkshire (Cleveland district), Northamp- 
tonshire, and Lincolnshire. Similar oolitic ores (the 
so-called " minette " ores) furnish large supplies in 
Lorraine, Luxemburg, and Belgium. Mention must 
also be made of the deposits, of recent age, of 
bog and lake iron ore of Silesia, the Banat, Fin- 
land, and Scandinavia (Smaland, Vestragotland, and 
Dalarne). Other important iron-mining centres where 
limonite ores are exploited are Prussia (Ilsede and 
Salzgitter, north of the Harz), Bavaria, Wiirtemberg, 
Spain (Lugo, Santander, Murcia, Oviedo), Russia 
(Nischne-Novgorod, Kaloga, and Kertch), United States 
(Appalachian Mountains and the Marquette, Gogebic, 
Menominee, Vermilion, and Mesabi ranges of the Lake 
Superior district). 



ORES 177 

Chalybite, siderite, or spathic iron ore. Car- 
bonate of iron : FeCO 3 (iron 48*2 per cent.). Crystallizes 
in the hexagonal system, usually in flat rhombohedra 
with curved faces. Also occurs massive or mixed 
with clay (clay ironstone), and often contains carbonates 
of lime, magnesia, and manganese. Cleavage, rhombo- 
hedral. Colour, buff or fawn. Streak, white. Hard- 
ness, 3*5-4*5. Density, 3*7-3*9. Soluble in acids, with 
liberation of carbon dioxide, but not so readily 
soluble as calcite. Fusibility, 4*5-5. 

The nodules of clay ironstone, which are found in the 
shales of the Coal Measures, are termed sph&rosiderite. 
This variety of the ore forms the staple material of the 
British iron production. The varieties used in smelting 
contain from 25 to 35 per cent, of iron. The blackband 
ironstone is an impure carbonate of iron interstratified 
in thin seams in the Coal Measures of the North of 
England. It is especially valuable on account of its 
contained coaly matter. 

Clay ironstone is worked in all the principal coal- 
fields of the British Isles e.g., those of Scotland, 
Northumberland and Durham, Derbyshire and York- 
shire, North Staffordshire, South Wales and Coalbrook- 
dale. The Jurassic ironstones of Yorkshire, North- 
amptonshire, and Lincolnshire also consist largely of 
spathic ores (chalybite) in the deeper parts of the 
deposits. Other centres of the iron -mining industry 
where spathic ores are worked are France (Normandy 
and the Pyrenees), Spain (Leon), Germany (Sieger- 
land), Austria (Styrian Erzberg and the Huttenberger 

12 



178 DESCRIPTIVE MINERALOGY 

Erzberg of Carinthia), Hungary (Vares), United States 
(the Appalachian coalfields of Pennsylvania, Ohio, 
Kentucky, etc.). 

Chromite, or chrome iron ore. Oxides of iron 
and chromium : FeO.Cr 2 O 3 * (oxide of chromium 
68 per cent.). Crystallizes in the regular system, but 
usually occurs massive, granular, or compact ; also in 
loose grains. Colour, iron black to brownish - black. 
Opaque. Streak, brown. Lustre, submetallic. Fracture, 
uneven. Hardness, 5*5. Density, 4*4. Infusible. Im- 
parts a characteristic green coloration to the borax bead. 

This mineral is valuable on account of its chromium 
content, which is required for a variety of purposes 
e.g., the manufacture of ferro-chrome for chromium 
steels, and of refractory furnace linings ; and the 
preparation of the compounds used as pigments, 
mordants, oxidizing agents, etc. The ore is widely 
distributed in serpentines, and as peripheral concentra- 
tions in the ultrabasic rocks (peridotites) from which the 
serpentines are derived. It is also found in sands. 
Examples of its occurrence are Silesia, Bohemia, 
Greece, Russia (Ural Mountains), New South Wales, 
New Zealand, British Columbia, United States, Trans- 
vaal, Rhodesia, New Caledonia, and Asia Minor, the 
two latter being the largest producers. 

Ilmenite, menaccanite, or titaniferous iron ore. 
Oxides of iron and titanium : FeO.TiO 2 , with or with- 

* Ferrous oxide is replaceable to some extent by magnesia (MgO), 
and chromic oxide by alumina (A1 2 O 3 ). 



ORES 179 

out Fe 2 O 3 . Crystallizes in the hexagonal system, with 
rhombohedral symmetry. Isomorphous with haematite. 
Habit, tabular parallel to the basal plane, with dominant 
rhombohedral faces. Also occurs massive ; in granular 
and scaly aggregates ; and as rolled grains (inenaccanite 
sand). Colour, iron-black. Streak, black to brown. 
Opaque. Lustre, semi-metallic metallic on freshly 
fractured faces. Although there is no true cleavage, the 
mineral separates along basal and rhombohedral twin- 
ning planes. Fracture, conchoidal. Hardness, 5-6. 
Density, 4'5-5'3 The density increases with the Fe 2 O 3 
content. As a rule non-magnetic. Infusible. Soluble 
when finely powdered in boiling hydrochloric acid, the 
solution heated with tinfoil giving a violet colour. 

A very common mineral, especially as an accessory 
constituent of igneous rocks ; also in sediments, schists, 
and as sand. Of no value as an iron ore. Original 
locality, the Ilmen Mountains in the Urals. A 
common companion of the diamond in the kimberlite 
pipes of South Africa. The black sands in the beach 
placers of the United States and New Zealand are rich 
in ilmenite. 

Iron Pyrites. Disulphide of iron : FeS 2 (iron 
46*64, sulphur 53*36, per cent.). Crystallizes in the 
regular system. The cube, either alone or combined 
with the faces of the pentagonal dodecahedron and the 
octahedron, is the most frequent form. The faces of 
the cube are usually striated, in consequence of the 
tendency to oscillatory combination of that form with 
the pentagonal dodecahedron. The latter form also 



180 DESCRIPTIVE MINERALOGY 

occurs alone, in which case its faces are striated by 
oscillation with the cube. Twinning parallel to a face 
of the rhombic dodecahedron. 

The crystals are of all sizes, and occur either singly 
or grouped. Nodular, botryoidal, kidney-shaped, com- 
pact masses, round pellets, and irregular grains all 
these forms are of common occurrence. The colour of 
unaltered pyrites is brassy yellow ; but the surface is 
frequently tarnished, and the mineral alters easily into 
a dark brown limonite, the change being one of oxida- 
tion and hydration. The streak is brownish - black. 




FIG. 89. CRYSTALS OF IRON PYRITES. 

Lustre, metallic. Opaque. Fracture, conchoidal to 
uneven. A cubic cleavage is scarcely perceptible. 
Brittle. Hardness, 6-6*5. Density, 4*9-5* i. The 
superior hardness is an important distinction from 
copper pyrites and from gold. Pyrites strikes fire with 
steel. Fusibility, 2*5-3. Yields a magnetic globule 
before the blowpipe. Decomposed by nitric acid, with 
separation of sulphur. 

Iron pyrites is very widely diffused through the crust 
of the earth, occurring both as an accessory constituent 
of rocks and * in veins and large masses, either in- 
dependently or as a very common companion of other 



ORES 181 

sulphide ores ; but, in spite of its high percentage of 
iron, it is not used for the production of that metal. The 
chief use of pyrites is for the manufacture of sulphuric 
acid, for which purpose it is imported from Spain, 
Portugal, and Belgium, besides being exploited largely in 
British metalliferous mines. Pyrites is often associated 
with gold, and in such cases the latter is usually 



m 







FIG. 90. IRON PYRITES (AFTER J. M. BONTWELL, U.S. GEOL. 
SURVEY). 

mechanically included within its crystals (auriferous 
pyrites) . 

On account of its ubiquity, it is useless to give 
localities for pyrites : it is associated with marine and 
brackish water sediments of every age, appearing as grains 
disseminated through shales and clays or in seams and 
layers, as in the Coal Measures ; or, again, as con- 
cretionary nodules, as in certain zones of the Chalk or 
in the Oxford and Kimeridge clays. By oxidation it 



182 DESCRIPTIVE MINERALOGY 

is converted into the oxides of iron, the liberated sul- 
phuric acid producing gypsum in calcareous clays, and 
alum shale in those free from lime. 

Marcasite, or cockscomb pyrites. Bisulphide of 
iron : FeS 2 (same composition as pyrites). Crystallizes 
in the rhombic system, with tabular habit parallel to 
the basal plane (ooi), or with dominant brachydomes ; 
or, again, prismatic, parallel to (no). Twinning parallel 
to the prism face, often repeated. Also occurs in 
botryoidal, nodular, and globular forms (sometimes with 
radial fibrous structure), or massive. Colour, brassy 
yellow. Metallic lustre. Opaque. Streak, dark greyish - 
green. Cleavage distinct, parallel to the prism (no). 
Fracture, uneven. Brittle. Hardness, 6. Density, 
4-65-4-88. Behaviour before the blowpipe and with 
acid, same as pyrites. 

Occurrence similar to pyrites, but less widely dis- 
tributed. It is frequently present as concretions in 
clays, marls, and limestones (e.g , in the chalk of 
Dover and Folkestone). 

Pyrrhotite, or magnetic pyrites. Monosulphide of 
iron : FeS (iron 63*61 per cent.). Crystallizes in the 
hexagonal system, with tabular habit parallel to the 
basal plane. Twinning parallel to a pyramid face (1012). 
Usually occurs massive or granular. Colour, bronzy 
yellow to copper-coloured. Lustre, metallic. Streak, 
greyish -black. Basal cleavage, imperfect. Fracture, 
uneven. Brittle. Hardness; 3-4. Density, 4*5-4'6. 
Magnetic i.e., the fine powder is attracted by the 



ORES 183 

magnet. Fusibility, 2*5-3. On charcoal yields a black 
magnetic globule. Soluble in hydrochloric acid, with 
separation of sulphur. 

Pyrrhotite is of common occurrence as an acces- 
sory constituent of igneous rocks, of limestones and 
of crystalline schists ; it is also found with other 
sulphides in ore veins, and in large independent 
lenticular masses in the crystalline schists (e.g., in the 
Huronian of the Appalachian Mountains of the Eastern 
United States). When it contains nickel (up to 5 per 
cent., average 3 per cent. nickeliferous pyrrhotite), it 
constitutes a valuable ore of nickel, as at Sudbury in 
Ontario, Canada,* at the Lancaster Gap mine in 
Pennsylvania, and at Erteli in Norway. In Rossland, 
British Columbia, it is associated with gold. 

ORES OF MANGANESE. 

Manganese is a widely-distributed metal, occurring 
as a constituent of at least a hundred different 
minerals. Only some half a dozen (mainly oxides and 
hydrates of manganese), however, are of importance as 
ores ; and these are all secondary compounds that owe 
their origin to the decomposing action of percolating 
alkaline and carbonated waters on primary manganese 
minerals in igneous and metamorphic rocks. As 
examples of such primary minerals, the manganese- 
pyroxene (rhodonite), the manganese-olivine (tephroite), 
and the manganese-garnet (spessartite) , may be quoted. 

* Sudbury is responsible for half the world's annual output of 
nickel. 



184 DESCRIPTIVE MINERALOGY 

The manganese thus derived is carried in solution 
as a bicarbonate, and reacting on rocks within the 
zone of weathering (in other words, in the presence of 
free oxygen) forms replacement deposits. The com- 
monest ores produced in this way are pyrolusitc 
(peroxide of manganese), psilomelane (hydrated oxide of 
manganese), and braunite (an oxide and silicate of 
manganese). Wad or bog-manganese is an indefinite 
mixture of various hydrated oxides that occur at 
the surface, especially as lateritic deposits. Other 
less frequent ores are manganiie (the hydrated sesqui- 
oxide) and rhodochrosite (the carbonate). Franklinite, 
which is an ore of zinc occurring at Franklin Furnace 
in New Jersey, contains sufficient manganese to make 
its extraction from the zinc-residuum profitable. 

Manganese is chiefly used for the preparation of 
certain alloys of iron and manganese (spiegeleisen and 
ferromanganese) which are used in the manufacture of 
steel ; but the oxide is also used in the manufacture 
of chlorine, bromine, and oxygen, as a dryer in paints 
and varnishes, and as a decolorizer of glass. 

Pyrolusite. Peroxide of manganese: MnO 2 (manga- 
nese 63*22 per cent.). When it occurs in crystals it is 
pseudomorphous after manganite, but it is mostly 
found as earthy masses, or in botryoidal, kidney-shaped, 
and nodular aggregates, with radiate fibrous structure. 
It also occurs as dendritic markings on the bedding 
and parting planes of rocks. Colour, steel grey 
to iron black and opaque, with dull, but sometimes 
silky, lustre. Streak, black, soiling the fingers. Hard- 



ORES 185 

ness, 2*25. Density, 4*7-5. Infusible. Soluble in 
hydrochloric acid with evolution of chlorine. 

A widely-distributed mineral, and often an im- 
portant ore of manganese. It is frequently found 
as an alteration product of manganite and other 
manganese-bearing minerals, and largely as a replace- 
ment deposit within the zone of weathering, where, 
together with hydrated oxides of iron and alumina, it 
forms ores of lateritic origin. It thus often associates 
with psilomelane and with wad, but it may often be dis- 
tinguished from these rather similar oxides of manganese 
by the possession of a crystalline or fibrous structure. 
It is also of inferior hardness to psilomelane. Examples 
of occurrence are : Nassau, Moravia, Caucasus, United 
States, India, etc. 

Manganite. Hydrated sesquioxide of manganese : 
Mn 2 O 3 .H 2 O (manganese 62*5, water 10*23 P er cent.). 
Crystallizes in the rhombic system, with dominant 
prismatic habit (i 10) . The basal plane is striated parallel 
to the macrodiagonal. Twinning parallel to the brachy- 
dome (on). Also occurs in columnar to fibrous and 
radiating aggregates. Colour, dark steel grey to iron 
black. Opaque. Lustre, submetallic. Streak, dark 
brown. Brachypinacoidal cleavage, perfect. Fracture, 
uneven. Brittle. Hardness, 3-4. Density, 4'2-4'4. 
Infusible. Gives off water when heated in the closed 
tube. Often partially altered to pyrolusite. 

Manganite occurs in veins at Ilfeld in the Harz, and 
with other manganese ores in Nassau and at many 
other localities. 



186 DESCRIPTIVE MINERALOGY 

Psilomelane. A mineral of rather indefinite com- 
position, but probably a hydrated manganese manganate 
in which a portion of the manganese is replaceable by 
barium and potassium, corresponding to the formula 
(H 2 ,K 2 ,Mn,Ba) 2 MnO 5 . The highest percentage of 
manganese possible, in accordance with the formula 
Mn 2 MnO 5 , is 67*35 P er cent. Psilomelane does not 
occur crystallized, being massive, botryoidal, reniform, 
or stalactitic. Colour, iron black to dark steel grey. 
Opaque. Streak, brownish-black, shining. Lustre, 
submetallic to dull. Fracture, uneven. Hardness, 5-6. 
Density, 3*7-4*7. Infusible. Soluble in hydrochloric 
acid, with evolution of chlorine. 

Psilomelane is widely distributed, and constitutes 
the most important ore of manganese, occurring both 
as a replacement deposit in rocks within the zone of 
weathering, and, together with the hydrated oxides of 
iron and aluminium, in ores of lateritic origin. It 
is mined in India, Russia (Caucasus), Spain, Brazil, and 
the United States. In India it constitutes, together 
with braunite, go per cent, of the manganese ore 
exported. 

Wad. The name given to an indefinite mixture of 
various oxides, chiefly of manganese, but also of iron, 
aluminium, barium, etc., together with a little silica and 
10 to 20 per cent, of water. It occurs in amorphous 
earthy or compact masses or as incrustations. Colour, 
dull black to brownish-black. Streak, brownish-black 
to black. Usually very soft, soiling the fingers. Density, 
3-4*3. Infusible. Soluble in hydrochloric acid. 



ORES 187 

Wad is a widely-distributed and abundant surface 
deposit of manganese, and often still possesses the 
original slaty or schistose structure of the rocks which it 
has replaced. It is frequently associated with psilo- 
melane, but may be distinguished from it by its inferior 
hardness, and from pyrolusite by the absence of crystal- 
line or fibrous structures. 

Braunite. An oxide and silicate of manganese, corre- 
sponding to the general formula wMnMnO 3 + wMnSiO 3 . 
It may be regarded as an isomorphous mixture of 
manganese manganite and manganese silicate. The ratio 
of m : n is usually 3:1; but it may be also 7 : 2 or 
4 : i (with m : n = 3 : i, manganese = 63*6 per cent.). 
Braunite crystallizes in the tetragonal system, with 
dominant pyramid of the first order (in). Also occurs 
massive and granular. Colour, dark brownish-black to 
steel grey. Opaque. Lustre, submetallic. Streak, 
dark brown. Cleavage parallel to the pyramid (m), 
perfect. Fracture, uneven to subconchoidal. Brittle. 
Hardness, 6-6*5. Density, 475-4*82. Slightly magnetic. 
Infusible. Soluble in hydrochloric acid, with evolution 
of chlorine, and yielding a residue of gelatinous silica. 
An important ore of manganese ; as at Ilfeld in the 
Harz, Telemark in Norway, and at many places in 
India, etc. 

Rhodochrosite, or dialogite. Carbonate of manga- 
nese : MnCO 3 (manganese 47*83 per cent.). Crystal- 
lizes in the hexagonal system, with rhombohedral 
symmetry, being isomorphous with calcite ; but usually 



188 DESCRIPTIVE MINERALOGY 

occurs massive. Rhombohedral cleavage, perfect. 
Colour, pink, rose red and light brown. Lustre, pearly. 
Translucent. Streak, white. Brittle. Hardness, 
3'5-4'5- Density, 3*45-3*6. Infusible. Soluble in 
hydrochloric acid, with effervescence. Distinguished 
from rhodonite (MnSiO 3 ), which it resembles in colour, 
by its inferior hardness and behaviour with hydro- 
chloric acid. 

Rhodochrosite is not widely distributed, but is an 
important ore of manganese when in sufficient quantity. 
It has been mined in the French Pyrenees and at a 
few other places. 

ORES OF BISMUTH, ANTIMONY, AND ARSENIC. 

Native bismuth is the principal source of the metal ; 
but the sulphides of the metal are also worked, and 
there are several compounds of bismuth with selenium, 
tellurium, silver, gold, copper, and lead, as well as 
oxides, carbonates, etc., that are of no importance as 
ores. Antimony occurs native ; but the principal supply 
of the metal is derived from the sulphide stibnite. 
It is also found in numerous sulphide compounds, such 
as antimonial tetrahedrite, pyrargyrite, etc., and in the 
oxidation zone as oxides and oxysulphides (senannon- 
tite, cervantite, and kermesite). Besides occurring in the 
native state, arsenic is found in two forms as sulphides, 
realgar and orpiment, and in numerous complex ores of 
silver, copper, lead, etc.; but the principal supply is 
obtained from the arsenosulphide of iron mispickel. 



ORES 189 

Bismuth and antimony are chiefly valuable in the arts 
for their alloys with lead, tin, and copper. Bismuth 
is used in fusible alloys and for cliche metal ; antimony, 
in alloy with lead (antimonial lead), as type metal, 
babbitt metal, etc., and arsenic, for the manufacture 
of arsenious oxide (white arsenic). 

Native Bismuth. Bi. Crystallizes in the hexagonal 
system, but usually occurs in crystalline, granular, or 
scaly masses or as botryoidal incrustations. Colour 
and streak, silver white, generally iridescent at the 
surface. Opaque. Lustre, metallic. Basal cleavage, 
perfect. Brittle. Fracture, uneven. Hardness, 2. 
Density, 9'7'9'8. Fusibility, i. Soluble in nitric acid, 
the solution giving a white precipitate with water. 

Native bismuth, the most important source of the 
metal, occurs in association with cobalt and silver 
ores, as in Saxony (Schneeberg), Bohemia (Erzgebirge), 
Sweden (Dalarne), Bolivia (Tazna, Illampa), Ontario 
(Cobalt), New South Wales (Cobar). 

Bismuthinite. Sesquisulphide of bismuth: Bi 2 S 3 
(bismuth 81*22 per cent.). Crystallizes in the rhombic 
system, with acicular habit. Usually massive, scaly, 
or fibrous. Colour, lead grey to tin white. Iridescent 
on the surface. Opaque. Streak, grey. Lustre, 
metallic. Brachypinacoidal cleavage, perfect. Hard- 
ness, 2. Slightly sectile. Density, 6*4-6*5. Fusi- 
bility, i. Yields a globule of bismuth in the reducing 
flame. Soluble in hot nitric acid, the solution giving 
a white precipitate with water. 



190 DESCRIPTIVE MINERALOGY 

Bismuthinite occurs in Cornwall, Saxony (Schnee- 
berg), Hungary (Rezbanya), France (Meymac), Sweden 
(Riddarhyttan), Australia, Bolivia (Tazna, Chorolque), 
Utah (Beaver City). 

Native Antimony. Sb. Crystallizes in the hex- 
agonal system, with rhombohedral habit ; but usually 
occurs massive or as granular or botryoidal incrusta- 
tions. Colour, tin white. Opaque. Lustre, metallic. 
Basal cleavage, perfect. Brittle. Fracture, uneven. 
Hardness, 3. Density, 6*6-6*7. Fusibility, i. Volatile. 
Soluble in aqua regia. 

Native antimony occurs in Sweden (Sala), Germany 
(Andreasberg in the Harz), Bohemia (Przibram), Borneo 
(Sarawak), Chili (Huasco), Peru, New Brunswick (York 
County). 

Stibnite, antimonite, or antimony glance. Sesqui- 
sulphide of antimony : Sb. 2 S 3 (antimony 71*76 per cent.). 
Crystallizes in the rhombic system, occurring in diver- 
gent aggregates of long prismatic or acicular crystals, 
with pyramidal terminations. The prism faces are 
characterized by a well - developed vertical striation. 
Colour, lead grey. Streak, lead grey. Opaque. 
Lustre, metallic, especially bright on cleavage faces. 
Cleavage parallel to the brachypinacoid (oio), perfect. 
Fracture, subconchoidal. Slightly pliable. Hardness, 2. 
Density, 4*6-4*7. Easily fusible (fusibility on von 
Kobell's scale, i). Volatilizes. Decomposed by hydro- 
chloric acid. 

Stibnite occurs in ore deposits in association with 



ORES 191 

galena, blende, cinnabar, and the gangue minerals 
barytes and quartz. It is of widespread occurrence in 
Europe e.g., Westphalia, Harz, * 

Hungary, Bohemia, Italy, Corsica, /AF\ 

^vxS-^^n 

Sardinia, France, Algeria, Spain, 
Russia. The bulk of the world's 
supply, however, is now mined in 
Japan (Ichinokawa mines in the 
island of Shikoku), China, and 
Australia (New South Wales, Vic- FIG. 91. ANTIMONITE 

toria, and Tasmania). It is also A Pyramid ; w, prism ; 

b, macropmacoid. 
worked in the United States, 

Mexico, Peru, Borneo (Sarawak), and New Zealand. 

Mispickel, arsenical pyrites, or arsenopyrite. 
Arsenosulphide of iron : FeAsS (iron 34*34, arsenic 
46*01, sulphur 19*65, per cent.). Crystallizes in the 
rhombic system, with prismatic habit parallel to (no), 
or brachydomatic parallel to (012). Twinning parallel 
to the macrodome (101) or to the prism (no). Also 
occurs massive and granular. Colour, tin white to 
steel grey. Opaque. Lustre, metallic. Streak, greyish- 
black. Cleavage parallel to the prism (no). Fracture, 
uneven. Brittle. Hardness, 5*5-6. Density, 5*9-6'2. 
Strikes fire with steel, with smell of garlic. Fusible 
before the blowpipe to a magnetic globule, with emission 
of arsenical fumes. Fusibility, 2. Decomposed by nitric 
acid, with separation of sulphur. 

Mispickel is a common companion of ores e.g., 
those of silver, cobalt, and nickel, in which it is 
associated with blende, galena, pyrites, and chalco- 



192 DESCRIPTIVE MINERALOGY 

pyrite ; it also accompanies tin ore together with wolf- 
ramite, fluorspar, and quartz. Together with pyrites 
it is often associated with gold in gold-quartz veins. 
Mispickel is the chief source of the arsenic of com- 
merce, and is largely worked as an arsenic ore in 
Cornwall, at Schneeberg in Saxony, and in Ontario. 

Realgar. Monosulphide of arsenic : AsS (arsenic 
70*08 per cent.). Crystallizes in the monoclinic system. 
Habit, short columnar with vertical striation. Also 
occurs massive or in granular aggregates, and as in- 
crustations. Colour, red to orange red. Streak of the 
same colour, but lighter in tint. Transparent to trans- 
lucent. Lustre, resinous. Clinopinacoidal cleavage, 
fair. Fracture, subconchoidal. Sectile. Hardness, 1-2. 
Density, 3*56. Fusibility, I. Volatile, with smell of 
garlic. Decomposed by aqua regia, with separation of 
sulphur. 

Realgar occurs in association with silver and lead 
ores: Harz (Wolfsberg), Hungary (Tajova, Nagy- 
banya, Felsobanya, Kapnik), Transylvania (Nagyag), 
Italy (Casa Testi, Vesuvius, Etna), Corsica, Chili? 
Peru, Bolivia. 

Orpiment. Sesquisulphide of arsenic : As 2 S 3 (arsenic 
60*96 per cent.). Crystallizes in the rhombic system, 
with short columnar habit ; but occurs more frequently 
massive, in granular or fibrous aggregates, sometimes 
with a botryoidal surface. Colour, orange yellow. 
Streak, the same, but lighter. Translucent. Lustre, 
resinous. Brachypinacoidal cleavage, perfect. Flexible. 



ORES 193 

Sectile. Hardness, 1-2. Density, 3*4-3*5. Fusibility, I. 
Very volatile. Soluble in aqua regia. Occurrence the 
same as realgar. 

ORES OF VANADIUM. 

Vanadium is found in dechenite (vanadate of lead 
PbO.V 2 O 5 ) and vanadinite (chlorovanadate of lead 
gPbO.3V 2 O 5 .PbCl 2 ), and other rare minerals occurring 
in Spain, Sweden, the Argentine, Mexico, and Colorado; 
but the only commercial source is an occurrence of 
vanadinite in Spain. It is used in the form of ferro- 
vanadium in the manufacture of steel alloys. The 
addition of from o'2 to 0*5 per cent, of vanadium to 
steel increases its tensile strength, its ductility, and its 
elasticity. 

Vanadinite. Chlorovanadate of lead : 9PbO.3V 2 O 5 . 
PbCl 2 (V 2 O 5 19*36 per cent.). Crystallizes in the 
hexagonal system, being isomorphous with pyro- 
morphite (chlorophosphate of lead) and apatite (fluoro- 
phosphate of calcium). Colour, red. Streak, nearly 
white. Lustre, resinous. Translucent. Cleavage, none. 
Brittle. Fracture, uneven. Hardness, 3. Density, 
6*9 -7*1. Fusibility, 1*5. Partly soluble in acids. 
Occurrence as above. 

ORES OF TIN. 

The only important ore of tin is the oxide, cassit- 
erite, the sulphide, stannite, being of little practical 
value. A large proportion of the tin ore mined is 

13 



194 DESCRIPTIVE MINERALOGY 

from detrital deposits, in which cassiterite occurs as a 
sand in association with quartz, topaz, tourmaline, 
axinite, garnet, epidote, wolfram, scheelite, fluorspar, 
monazite, ilmenite, and magnetite, these minerals 
sharing with cassiterite the property of resisting de- 
struction by weathering. Tin ore is also found in 
the veins, chimneys, impregnations, etc., which occur 
at or near the margin of large granite intrusions, 
and are generally considered to be of pneumatolytic 
origin i.e., to have been derived from the granite 
magma through the agency of vapours and gases dis- 
solved in it at the time of intrusion, and given off 
during consolidation. In these original deposits 
cassiterite is associated with tourmaline, fluorspar, 
axinite, wolfram, and topaz, to all of which, on 
account of their peculiar composition, pneumatolytic 
origin is ascribed. 

Tin is of great use in the arts on account of its 
valuable alloys, such as soft solder (an alloy with lead), 
bronze, gun-metal, bell-metal, specular metal (all alloys 
with copper), Britannia metal (an alloy with antimony), 
and babbitt and other friction metals (alloys with anti- 
mony and copper). Its largest application is in the 
tin-plate industry, in which sheet iron is coated with 
a layer of tin. It is also used for tinning copper 
utensils, and in the form of tinfoil. 

The subchloride of tin (tin-salt) is used as a mordant 
in dyeing. It is a valuable reducing agent. 

Cassiterite, tinstone, or black tin. Dioxide of 
tin : SnO 2 (tin 78*82 per cent.). Crystallizes in the 



ORES 



195 



tetragonal system, the common form being the pyramid, 
combined with the prism, of the first order. The 
edges of both pyramid and prism are, however, some- 
times truncated by the faces of the pyramid and prism 
of the second order. The faces of the first pyramid 
show a horizontal, those of the prism a vertical, stria- 
tion. Habit, usually stout columnar. The crystals are 
often twinned, the twinning plane being a face of the 
deutero-pyramid (101). Repetitions of the twinning 
produce ring or star-shaped aggregates of three to five 




FIG. 92. CASSITERITE. 

s, Proto-pyramid ; m, proto-prism ; 
e, deutero-pyramid ; a, deutero- 
prism. 




FIG. 93. CASSITERITE TWINNED 
CRYSTAL). 



individuals. Tinstone also occurs massive in fibrous 
nodules (wood-tin) in grains dispersed through granite, 
or in pegmatite veins, but most frequently in loose 
rounded fragments, mingled with river gravels (stream- 
tin). Colour, usually brown or black, but yellow and 
grey tints also occur. Lustre, adamantine to resinous. 
Cleavage parallel to (100), imperfect. Fracture, sub- 
conchoidal to uneven. Brittle. Hardness, 6-7. Den- 
sity, 6*8-7'i. Infusible. Yields a globule of tin with 
carbonate of soda on charcoal. Insoluble in acids. 



196 



DESCRIPTIVE MINERALOGY 



Becomes coated with metallic tin when placed on zinc 
with hydrochloric acid. 

Cassiterite was formerly extensively mined in Corn- 




MAP OF THE TIN-MINING DISTRICTS OF THE FEDERATED MALAY 
STATES AND DUTCH EAST INDIES. 

wall, and tin ore is still a staple product of that county. 
The historical tin deposits of Altenberg and Zinnwald 
in Saxony also deserve mention ; but the main supplies 
are now derived (i) from the tin deposits (largely of 



ORES 197 

alluvial origin) of the Malay Peninsula (Perak, Selangor, 
Nigri-Sembilan) and of the Dutch East Indies (Banka 
and Billiton) ; (2) from Australasia Queensland 
(Herberton), New South Wales (Vegetable Creek, 
etc.), West Australia, Tasmania (Mount BischofT 
and Briseis) and (3) from Bolivia (La Paz, 
Oruro, Potosi, and Chorolque). Other producers are 
Mexico, California, Dakota, Alaska (Seward Penin- 
sula), Japan, and China. An increasing production 
is being made in South Africa (Swaziland, Transvaal), 
and extensive alluvial deposits are being explored in 
Nigeria. 

Stannite, tin - pyrites, or bell - metal ore. Sul- 
phostannate of iron and copper : Cu 2 FeSnS 4 or SnS 2 . 
Cu 2 S.FeS (tin 27*68, copper 29*5, iron 13*02, per 
cent.). Crystallizes in the tetragonal system, with 
sphenoidal hemihedrism. Habit, pseudo-regular. Twin- 
ning axis the normal to (m). Generally occurs 
massive or in granular aggregates. Colour, steel grey 
to iron black. Streak, black. Opaque. Lustre, 
metallic. Basal and prismatic cleavages, indistinct. 
Fracture, subconchoidal to uneven. Brittle. Hardness, 
3-4. Density, 4*3-4*5. Fusibility, 1*5. Gives off 
sulphur fumes when heated. Decomposed by nitric 
acid, with separation of sulphur and oxide of tin. 

Stannite is rarely of importance as an ore of tin. 
It occurs in Cornwall, Bohemia (Zinnwald), Bolivia 
(Oruro and Potosi), Dakota (Black Hills), Tasmania 
(Zeehan), Japan. 



198 DESCRIPTIVE MINERALOGY 

ORES OF TITANIUM. 

Titanium occurs as an oxide (TiO 2 ) in the three 
minerals rutile, anatase, and brookite, of which the two 
first named are tetragonal, and the last rhombic. Its 
occurrence in ilmenite or titaniferous iron ore has been 
already dealt with on p. 178. The only important source 
of the metal is rutile, which can be reduced in the 
electric furnace by the aid of aluminium, the resulting 
ferrotitanium (10-20 per cent, of titanium) being used 
for increasing the strength of cast iron and of steel. 

Rutile. Dioxide of titanium : TiO 2 (titanium 60 per 
cent.). Crystallizes in the tetragonal system, in pris- 
matic forms similar to those of 
cassiterite. Twinned on the pyra- 
mid of the second order (101). 
Also occurs massive. Colour, 
reddish-brown. Streak, yellowish- 
brown. Lustre, metallic to ada- 
mantine. Translucent. Refrac- 
tive index high ; co = 2'6i6. Double 

FIG. 94. RUTILE refraction, strong, positive (e-o> = 
(REPEATED TWINNING). . 

0*287). Prismatic cleavage, good. 

Fracture, uneven. Brittle. Hardness, 6*5. Density, 4*3. 
Infusible. Insoluble in acids. A good test for titanium 
is to fuse the mineral with sodium bisulphate, acidify 
the aqueous solution with sulphuric acid, and add 
hydrogen peroxide. A deep orange colour betrays the 
presence of titanium. 

Rutile is worked as an ore of titanium at Risor and 
other places in Norway, also in the United States. 




ORES 199 

ORES OF MOLYBDENUM. 

Only two compounds of molybdenum are of com- 
mercial importance : the sulphide, molybdenite, and the 
molybdate of lead, wulfenite. The commercial ores 
should not contain less than 42 per cent, of the metal, 
and should be free from other metallic minerals. 
Molybdenum in the form of ferromolybdenum (con- 
taining from 75 to 87 per cent, of molybdenum) and 
nickel - molybdenum (containing 75 per cent.) is used 
for the manufacture of steel alloys. The effect of 
the addition of from 2 to 4 per cent, of molybdenum 
to steel is to increase the hardness, toughness, and 
elongation, without any corresponding deterioration 
when the steel is heated or welded. 

Molybdenite. Sulphide of molybdenum : MoS 2 
(molybdenum, 59*96 per cent.). Crystallizes in the 
hexagonal system. Habit, six-sided tabular (basal plane, 
prism, and pyramid). Also occurs in scales and grains. 
Colour, lead grey. Streak, greenish -grey. Opaque. 
Lustre, metallic. Basal cleavage, perfect. Pliable. 
Sectile. Hardness, I. Greasy to the touch. Soils 
the ringers. Density, 47-4*8. Infusible. Heated in 
open tube yields sulphur fumes. Soluble in warm 
aqua regia. The powdered mineral, moistened with 
sulphuric acid and evaporated to dryness in a porcelain 
crucible, yields a characteristic blue colour. 

Molybdenite occurs in small quantities in many 
granites, pegmatites, gneisses, crystalline limestones, 
and schists. It is often associated with tin ore. 



200 DESCRIPTIVE MINERALOGY 

Wulfenite. Molybdate of lead: PbMoO 4 (molyb- 
denum 26*2 per cent.). Crystallizes in the tetragonal 
system, with tabular habit. Colour, red. Streak, 
white. Translucent. Lustre, adamantine to resinous. 
Pyramidal cleavage, fair. Fracture, subconchoidal. 
Brittle. Refractive index, 2*402. Double refraction, 
strong. Hardness, 3. Density, 67. Fusibility, 2. 
Decomposed by hydrochloric acid. 

Wulfenite occurs in the oxidized zone of lead ores 
for example, in Utah, Nevada, Arizona, and New 
Mexico. 

ORES OF TUNGSTEN. 

Tungsten is obtained from the ores wolframite (tungs- 
tate of iron and manganese) and scheelite (tungstate of 
calcium), which are associates of cassiterite in nearly all 
tin-mining districts. The pure tungstate of manganese 
(hubneriU) and the pure tungstate of iron (ferberite), 
which are isomorphous with wolframite, are also known, 
and in places occur in sufficient quantity to be mined 
as ores.* The chief use for tungsten is in the manu- 
facture of steel alloys for lathe tools. A small percentage 
of tungsten increases the elastic limit and the tensile 
strength of steel. Tungsten steel is also self-hardening. 
On account of its high fusing-point (3080 C.), tungsten 
has also come into use as a filament in incandescent 
lamps. The world's production of tungsten ores is 
about 6,000 tons per annum, based on 60 per cent. ore. 

* A useful chemical test for tungsten is Digest the pulverized 
mineral in strong hydrochloric acid; the solution boiled with 
metallic zinc yields a blue coloration when tungsten is present. 



ORES 201 

Wolframite. Tungstate of manganese and iron : 
(Fe,Mn,)WO 4 (WO 3 76 per cent.). Crystallizes in 
the monoclinic system. Colour, black. Streak, dark 
reddish-brown Lustre, metallic to adamantine. Opaque. 
Clinopinacoidal cleavage, perfect. Brittle. Fracture, un- 
even. Hardness, 5'5. Density, 7-3. Fusibility, 3. De- 
composed by hydrochloric acid. 

Wolframite is a common associate of tin ore, as in 
Cornwall, Spain, Bohemia, Straits Settlements, Queens- 
land, New South Wales, United States, Bolivia. 

Scheelite. Tungstate of calcium : CaWO 4 (WO 3 
80 '6 per cent.). Crystallizes in the tetragonal system, 
with bipyramidal habit. Colour and streak, white. 
Lustre, vitreous to adamantine. Translucent. Index 
of refraction 1*919. Double refraction, strong. Py- 
ramidal cleavage, imperfect. Brittle. Fracture, uneven. 
Hardness, 4*5. Density, 6. Fusibility, 5. Decomposed 
by hydrochloric acid. 

Scheelite occurs in association with tin ore e.g., 
in Cornish, Bohemian, and Australian tin -mining 
districts. 

ORES OF URANIUM. 

The principal ores of uranium are pitchblende, torber- 
nite, and autunite. The rather doubtful mineral, carno- 
tite, appears to be a vanadium analogue of autunite. 
Uranium compounds are used in the coloration of glass 
and in porcelain - painting ; but they have recently 
acquired importance as the material from which radium 
compounds are prepared. 



202 DESCRIPTIVE MINERALOGY 

Pitchblende, or uraninite. Oxide of uranium : 
UO.U 2 O 3 (contains lead and rare elements, including 
minute quantities of helium and radium). Crystallizes 
in the regular system, but usually occurs massive, 
botryoidal, or in grains. Colour, black, grey, or brown. 
Streak, greyish-black. Lustre, submetallic to resinous. 
Hardness, 5*5. Density, 9-97. Infusible. Soluble in 
dilute sulphuric acid. 

Pitchblende occurs in Cornwall in association with 
cassiterite ; also in Bohemia (Joachimsthal and Przi- 
bram), Saxony, Colorado (Gilpin County), and other 
places. 

Torbernite. Hydrated phosphate of uranium and 
copper: Cu(UO 2 ) 2 (PO 4 ) 2 .8H 2 O. Crystallizes in the 
tetragonal system, with tabular habit. Colour, emerald 
green. Transparent to translucent. Streak, pale 
green. Lustre, subadamantine to pearly. Basal 
cleavage, perfect (like mica). Non-flexible. Sectile. 
Hardness, 2-2*5. Density, 3*4-3*6. Fusibility, 3. 

Torbernite occurs in Cornwall, Bohemia (Joachims- 
thai), Saxony (Schneeberg). 

Autunite. Hydrated phosphate of uranium and 
calcium : Ca(UO 2 ) 2 (PO 4 ) 2 .8H 2 O. Crystallizes in the 
rhombic system, with tabular habit similar to torber- 
nite. Colour, citron to sulphur yellow. Translucent. 
Streak, yellowish. Lustre, subadamantine to pearly. 
Basal cleavage, perfect. Hardness, 2-2*5. Density, 
3*05-3*2. Fusibility, 3. 

Autunite occurs in Cornwall. 



ORES 203 

ORES OF OTHER RARE METALS. 
The minerals monazite and xenotime are esteemed 
is a source of ceria and thoria for incandescent gas 
nantles; but monazite alone occurs in sufficient 
quantity to be of economic importance. Columbite is 
^alued as a source of tantalum for the manufacture of 
the metallic filaments of electric lamps, and zircon 
for the zirconium used in the electric lamps of the 
Nernst type. 

Monazite. Phosphate of cerium and lanthanum and 
didymium, with some thorium: (Ce,La,Di)PO 4 . Crystal- 
lizes in the monoclinic system. Colour, yellow to 
brown. Transparent to translucent. Lustre, resinous. 
Basal parting. Fracture, uneven. Hardness, 5-5*5. 
Density, 5'2-5'3. Index of refraction, i'8n. Double 
refraction, very high. Infusible. 

Monazite occurs in brown crystals in the granite of 
Arendal in Norway; but the commercial material is 
obtained from alluvial sands in Brazil (Prado), and 
North and South Carolina. 

Xenotime. Phosphate of yttrium : YPO 4 . Crystal- 
lizes in the tetragonal system. Colour, yellow to brown. 
Transparent to translucent. By alteration it becomes 
opaque. Lustre, resinous to vitreous. Prismatic 
cleavage, perfect. Fracture, uneven. Hardness, 4-5. 
Density, 4-55-5' i. Refraction and double refraction, 
very high. 

Xenotime occurs in granite and in sands, but less 
frequently than monazite. 



204 DESCRIPTIVE MINERALOGY 

Columbite. A niobo-tantalate of iron and man- 
ganese : (Fe,Mn)O.(Nb,Ta) 2 O 5 . Crystallizes in the 
rhombic system, in short, prismatic crystals terminated 
by the basal plane. Colour, iron black. Lustre, sub- 
metallic to adamantine. Streak, black. Opaque. 
Fracture, uneven. Hardness, 6. Density, 5'2-6'o. 
Fusibility, 5-5*5. 

Columbite occurs in granitic rocks in Sweden, West 
Greenland, Ilmen Mountains of Siberia, and in Brazil 
(Minas Geraes). 

Orthite, or allanite (a member of the epidote group). 
Silicate of calcium, iron and aluminium, with small 
proportions of cerium, lanthanum, didymium, yttrium 
and erbium. Crystallizes in the monoclinic system. 
Colour, black. Streak, grey. Opaque to translucent. 
Lustre, vitreous to resinous. Fracture, uneven to con- 
choidal. Hardness, 5'5-6. Density, 3-4. Fusibility, 2*5. 

Orthite occurs in granites, pegmatites and crystalline 
schists e.g., in Sweden, Siberia, and the United 
States. 

ORES OF ALUMINIUM. 

Aluminium compounds are among the most widely 
distributed of minerals ; thus, the felspars, micas, and 
most of the hornblendes and pyroxenes, which con- 
stitute the bulk of the igneous rocks, are all aluminium 
silicates ; and the same minerals, together with quartz 
and other less complex silicates of aluminium, compose 
the great group of clays, shales, and slates. The only 
compounds of aluminium, however, which can be con- 



ORES 205 

sidered as ores-or, in other words, from which the metal 
-an under present economic conditions be profitably 
>xtracted-are bauxite (a hydrated oxide of aluminium) 
md cryolite (a double fluoride of aluminium and sodium). 
Bauxite is probably not a definite mineral, but rather 
a trade name given to any substance in which there are 
large quantities of aluminium hydrate, in contradistinc- 
tion to ordinary clay, in which the aluminium is com- 
bined with silica, and cannot be profitably extracted. 

Aluminium is esteemed on account of its low 
density, its rigidity, its malleability, and the fact that 
it takes a high polish. It is largely used, for instance, 
in the motor industry (for crank -cases, gear-boxes, 
radiators, etc.). Its high conductivity for electricity 
makes it a competitor with copper for the transmission 
of power. It also forms valuable alloys with nickel, 
copper, zinc, and magnesium. 

Bauxite. Hydrated oxide of aluminium: A1 2 O 3 . 
2 H 2 0.* (aluminium 39 per cent.). Does not occur 
crystallized, but in concretionary, pisohtic, or earthy 
masses. Colour, white, yellow, brown, or red. Opaque. 
Lustre, dull or earthy. Hardness most variable. 
Density, 2-55. Infusible. Insoluble in acids. 

Bauxite derives its name from Baux, near Aries, 
in France, where it is largely mined as an ore of 
aluminium. The only other large producer 

* Holland regards bauxite as an intimate mixture of two 
mmerals-viz., gibbsite, having the formula A1 2 O , 3 H.O : and 
diaspore, with the formula A1 2 O 5 .H 2 O (" Rec. Geol. S 
vol. xxxii., p. 176). 



206 DESCRIPTIVE MINERALOGY 

United States, where the ore is mined in Alabama, 
Arkansas, Georgia, and Tennessee. 

Cryolite. A fluoride of aluminium and sodium : 
Al 2 F 6 .6NaF (aluminium 12*85 per cent., sodium 3279 
per cent., and fluorine 54*36 per cent.). Crystallizes 
in the monoclinic system. It forms tabular crystals 
or occurs massive. Colourless to greyish -white, 
yellow, or brown. Streak, white. Lustre, vitreous. 
Transparent to translucent. Index of refraction low 
(/9= 1*36). Double refraction, weak. Cleavage, basal 
and prismatic, perfect, the cleavage appearing nearly 
cubical on account of the close approximation to 90 of 
the angle between the two prism faces and between the 
latter and the basal plane. Fracture, uneven. Brittle. 
Hardness, 2*5. Density, 2*97. Fusibility, 1*5. Decom- 
posed by sulphuric acid. 

Cryolite occurs as a large bed or vein in gneiss at 
Ivigtut in Arksukjord (West Greenland). 



APPENDIX TO THE ORES. 

THE VEINSTONES OR GANGUE MINERALS. 

The ores are accompanied in their lodes and veins 
by a number of minerals, which, from an economic 
point of view, are either intrinsically worthless, or are 
regarded as valueless in comparison with the ore for 
which the particular vein is exploited. The commonest 
veinstones are quartz and its congeners jasper, opal, 
chalcedony; the carbonates of calcium calcite and 



APPENDIX TO THE ORES 207 

aragonite ; the double carbonate of calcium and mag- 
nesium dolomite ; the carbonate of iron chalybite ; 
the sulphate of barium barytes ; the sulphates of 
calcium anhydrite and gypsum; the iron-oxides 
magnetite, hematite and limonite; the oxides of manga- 
nese wad, etc.; the sulphides of iron pyrites, marcasite, 
and pyrrhotite ; the arsenosulphide of iron mispickel ; 
and many other sulphides, which under more favour- 
able circumstances are themselves regarded as ores 
e.g., blende, galena, stibnite and molybdenite. Veins of 
pneumatolytic origin are characterized by the presence 
of tourmaline, fluorspar and topaz. Many rock-forming 
minerals also occur as veinstones: examples are, the 
felspars orthoclase and albite ; the hornblendes, pyroxenes 
and micas and their decomposition products chlorite, 
serpentine, sericite and talc; the zeolites, such as analcime, 
laumontite and prehnite ; lastly, the so-called accessory 
rock-forming minerals, such as garnet, apatite, and sphene. 
Descriptions of all these minerals are given in their 
appropriate chapters. 



CHAPTER III 

THE SALTS AND USEFUL MINERALS OTHER THAN 

ORES 

ALTHOUGH every combination of an acid with a base 
may be termed a salt, only the carbonates, sulphates, 
nitrates, chlorides, phosphates and borates of the alkalies 
and alkaline earths are included here.* 

These substances occur frequently as beds that have 
been deposited from the waters of a former lake or 
inland sea ; but many of them are found also as veins, 
either alone or forming the gangue material of ores of 
the heavy metals. Nitrates are formed by the decompo- 
sition of organic matter in saliferous and rainless dis- 
tricts ; while phosphatic deposits have in part been 
produced by the accumulation of bones or of the 
coprolites of fishes. 

An appendix to the chapter contains a description of 
the non-metallic elements sulphur and carbon. 

CARBONATES. 

A great number of carbonates occur as natural salts. 
Some of them namely, the carbonates of the heavy 

* The compounds formed by the union of some of the acids with 
the heavier metallic bases have been already referred to among the 
ores, 

208 



SALTS, ETC. 



209 



metals have been mentioned among the ores. The 
carbonates now to be described are the following : 

calcite,aragonite, dolomite, magnesite,witherite, strontianite, 
natron, and trona. 

Calcite. Carbonate of lime : CaCO 3 (lime 56, 
carbon dioxide 44, per cent.). Crystallizes in the 



FIG. 95. CALCITE: PRISMATIC FORMS. 
c, Basal plane; #, pyramid ; r, rhombohedron ; a, prism. 

hexagonal system, with rhombohedral hemihedrism. 
Common forms are flat or acute rhombohedra ; sharp- 
pointed scalenohedra (dog-tooth spar) ; prisms, terminated 




FIG. 96. CALCITE: RHOMBOHEDRAL FORMS. 

r t Rhombohedron (R) ; e, a more obtuse 
rhombohedron ( - \ R). 



FIG. 97. CALCITE. 

Twinned crystal, the 
twin-plane being a 
face of the rhombo- 
hedron, e(-\ R). 



either by the basal plane or by rhombohedral faces; 
and flat tables with dominant basal plane. There are 
several types of twinning : that in which the twin-plane 

14 



210 



DESCRIPTIVE MINERALOGY 



is the obtuse rhombohedron (e), is the commonest, and 
is responsible for the polysynthetic lamellation char- 
acteristic of calcite when viewed in thin sections under 
the microscope. Colourless to white, yellow, red, and 
brown. Transparent to opaque. Lustre, vitreous. Streak, 
white. Index of refraction, moderate (co = 1*658). 
Double refraction, negative, very strong (to e = O'ij2). 
Rhombohedral cleavage, perfect. Fracture, conchoidal. 
Brittle. Hardness, 3. Density, 27. Infusible. Soluble, 
with effervescence in acids. 





FIG. 98. CALCITE : SCALENOHEDRAL FORMS. 
y, Scalenohedron (R 3 ). 

Calcite is a frequent veinstone, a common secondary 
constituent in rocks, and the chief component of meta- 
morphic limestones (marbles). The purest variety 
Iceland spar is used in the manufacture of optical 
instruments. 

Aragonite. Carbonate of calcium : CaCO 3 (lime 56, 
carbon dioxide 44, per cent.). Crystallizes in the 
rhombic system, in tabular and prismatic forms; its 
frequent pseudo-hexagonal habit is due to repeated 
twinning on the prism (m). Occurs also as fibrous 



SALTS, ETC. 



211 



aggregates. Colourless. Transparent. Lustre, vitreous. 
Index of refraction high (/3 = 1*682). Double refraction, 
negative, very strong (7 - a-=OT56). Brachypinacoidal 
and prismatic cleavages, imperfect. Fracture, sub- 
conchoidal. Brittle. Hardness, 3*5. Density, 2*9. 
Infusible. Soluble in acids, with effervescence. 




\ 

FIG. 99. ARAGOMTE. 

b, Brachypinacoid ; m, prism 
k, brachydome. 




FIG. loo. DIAGRAM ILLUSTRATING THE 
MODE OF TWINNING IN ARAGONITE : 
FOUR INDIVIDUALS TWINNED ON A 
FACE OF THE PRISM. 

The striation indicates the direction 
of the brachydiagonal. 



Aragonite occurs as a gangue material of ore deposits ; 
also as stalactitic incrustations, and as spheroidal con- 
cretions as in the Sprudelstein of Carlsbad in Bohemia. 

Dolomite, or pearl spar. Double carbonate of 
calcium and magnesium : CaMg(CO 3 ) 2 (lime 30*4, 
magnesia 217, carbon dioxide 47*9, per cent.). 
Crystallizes in the hexagonal system, with rhombo- 
hedral hemihedrism. Saddle-shaped crystals with 
curved faces are characteristic. Colourless to white. 
Transparent. Lustre, vitreous. Streak, white. Index of 
refraction high (a> = r682). Double refraction, negative, 
very strong (o> e = 0*179). Rhombohedral cleavage, 
perfect. Fracture, subconchoidal. Brittle. Hard- 



212 DESCRIPTIVE MINERALOGY 

ness, 3*5. Density, 2-85. Infusible. Soluble with 
effervescence in warm acids. 

Dolomite occurs as a veinstone in association with 
ores, and massive in magnesian limestones. 

Magnesite. Carbonate of magnesium : MgCO 3 
(magnesia 47*62, carbon dioxide 52*38, per cent.). 
Crystallizes in the hexagonal system, with rhombohedral 
symmetry. More usually, however, it occurs massive, 
in granular, fibrous, or compact aggregates. Colour- 
less, white, or yellow. Transparent to opaque. Rhom- 
bohedral cleavage, perfect. Hardness, 4-4*5. Density, 
2*9-3. Infusible. Soluble in acids, with effervescence, 
when in the state of powder and warmed. 

Magnesite occurs as a decomposition product of ferro- 
magnesian silicates, and is found associated with ser- 
pentine, which is of similar origin. It is exploited for 
the manufacture of the sulphate of magnesia (Epsom 
salt). 

Witherite. Carbonate of barium : BaCO 3 (oxide of 
barium 77*7, carbon dioxide 22*3, per cent.). Crystal- 
lizes in the rhombic 
system. Habit, pseudo- 
hexagonal and bipyra- 
midal (see Fig. 101). 
Twinning on the prism. 
Also found massive or in 
FIG. IOI.-WITHERITE. botryoidal or kidney- 

Pseudo-hexagonal bipyramid com- 
posed of t, pyramid ; w, brachy dome . shaped aggregates. 

Colourless to white, 
grey or yellowish-grey. Transparent. Lustre, vitreous. 




SALTS, ETC. 213 

Streak, white. Index of refraction, moderate (/3= 1*531). 
Double refraction, negative, weak. Brachypinacoidal 
and prismatic cleavages, imperfect. Fracture, uneven. 
Brittle. Hardness, 3-3*5. Density, 4*2-4*3. Fusibility, 
2. Colours the flame green. Soluble in hydrochloric 
acid. 

Witherite accompanies certain ore deposits as a vein- 
stone, and is exploited to a small extent^ for use in the 
manufacture of plate-glass. 

Strontianite. Carbonate of strontium : SrCO ;{ (oxide 
of strontium, 70*17, carbon dioxide 29*83, per cent.). 
Crystallizes in the rhombic system, in varieties of habit 
resembling aragonite, but usually found in fibrous aggre- 
gations. Colourless to white, grey, yellow, brown, or 
green. Transparent. Lustre, vitreous. Streak, white. 
Prismatic cleavage, imperfect. Fracture, uneven. Hard- 
ness, 3*5-4. Density, 3*6-3*8. Infusible. Colours the 
flame red. Strontianite occurs as a veinstone, and is 
exploited for use in the refining of sugar. 

Natron. Hydrated carbonate of sodium : Na 2 CO 3 . 
ioH 2 O. Crystallizes in the monoclinic system, but 
usually found massive or as an earthy incrustation. 
Colourless to grey or white. Lustre, vitreous. Streak, 
white. Basal cleavage, imperfect. Hardness, 1-1*5. 
Density, 1*4-1*45. Fusibility, I. Colours the flame 
yellow. Soluble in water. 

Natron occurs as an evaporation product of the so- 
called soda-lakes in Africa and America, and is exploited 
for the manufacture of soda salts. 



214 DESCRIPTIVE MINERALOGY 

Trona. Hydrated carbonate of sodium : Na 2 CO 3 . 
HNaCO 3 . 2H 2 O. Crystallizes in the monoclinic 
system, but usually occurs massive or as earthy incrus- 
tations. Colourless to white or grey. Transparent. 
Lustre, vitreous. Orthopinacoidal cleavage, perfect. 
Fracture, uneven. Hardness, 2*5-3. Density, 2* 1-2*15. 
Fusibility, 1*5. Colours the flame yellow. Soluble in 
water. 

Trona occurs as an incrustation from the evapora- 
tion of lakes ; e.g., at Borax Lake, San Bernardino Co., 
California. 

SULPHATES. 

Several important sulphates occur as natural salts. 
Of these, the sulphates of the heavy metals have been 
referred to among the ores ; but the following sulphates 
of the alkalies and alkaline earths are treated here : 
anhydrite, gypsum, barytes, celestite, mirabilite, epsomite, 
alum, and alunite. 

Anhydrite. Sulphate of lime: CaSO 4 (lime 41*16, 
sulphur trioxide 58*84, per cent.). Crystallizes in the 
rhombic system, in thick tabular crystals; but more 
often found massive, in coarsely crystalline aggre- 
gates resembling marble. Colourless, white, or bluish- 
white. Transparent to opaque. Pinacoidal cleavages, 
perfect. Fracture, uneven. Brittle. Hardness, 3-3*5. 
Density, 2*8-3. Fusibility, 3. Soluble in hydrochloric 
acid. 

Anhydrite occurs in beds associated with gypsum 
and rock-salt, as, for example, in the New Red Marl 



SALTS, ETC. 



215 



of this country, and in the Stassfurt salt deposits of 
Central Germany. 

Gypsum, or selenite. Hydrated sulphate of lime : 
CaSO 4 .2H 2 O (lime 32*5, sulphur trioxide 46*6, water 
20*9, per cent.)- Crystallizes in the monoclinic 
system, usually in stout or slender prisms, but also in 
flat rhomboid tables, that owe their shape to the domin- 
ance of the clinopinacoid. Occasionally the crystals are 
lenticular, with curved faces. Also occurs in fibrous 
masses (satin spar) ; or granular (alabaster). Twinning 




FIG. 102. GYPSUM. 

/, Negative hemi-pyramid ; n, posi- 
tive hemi-pyramid ; f, prism ; 
b, clinopinacoid. 



FIG. 103. GYPSUM : CRYSTAL 
TWINNED ON THE ORTHO- 

PINACOID. 



on the orthopinacoid is common, producing a swallow 
tailed form. Usually colourless and water-clear, 
but also white or tinted. Streak, white. Lustre, 
vitreous. Transparent. Index of refraction, 1*522. 
Double refraction, moderate (7 a = o'oi). Clinopina- 
coidal cleavage, perfect. Sectile. Flexible. Hardness, 2. 
Density, 2'2-2'4. Fusibility, 3. Soluble in hydrochloric 
acid. 

Gypsum is a very common mineral occurring in beds 
and veins in marls and clays in the New Red Marl, 



216 DESCRIPTIVE MINERALOGY 

Oxford Clay, Purbeck Beds, Gault, London Clay, etc. 
It is used to improve soils or to make plaster of Paris. 
The compact granular variety (alabaster) is in request 
for vases, statuary, etc. 

Barytes, or heavy spar. Sulphate of barium : BaSO 4 
(oxide of barium 657, sulphur trioxide 34*3, per cent.). 
Crystallizes in the rhombic system, with tabular or 
prismatic habit, the former being due to the dominance 
of the basal plane (c) in combination with the prism (m), 
the latter to elongation either along the 6-axis with 
dominant macrodome (d), or along the a-axis with 
dominant brachydome (o). Also compact, granular, or 
fibrous. Colourless to reddish-white, grey, or yellow. 
Lustre, vitreous. Transparent. Streak, white. Index 
of refraction, high (/3= 1-637). Double refraction, 
positive, moderate (7 - a = croi2). Basal and pris- 
matic cleavage, perfect. Fracture, uneven. Brittle. 
Fusibility, 3. Colours the flame green. Not decom- 
posed by acids. 

Barytes often forms the gangue material of ores ; but 
it also occurs alone in veins, and is exploited for mixing 
with white lead. 

Celestite, or celestine. Sulphate of strontium : 
SrSO 4 (oxide of strontium 56*52, sulphur trioxide 43*48, 
per cent.). Crystallizes in the rhombic system, being 
isomorphous with barytes. Habit, prismatic, due to 
elongation along the 6-axis, with dominant macrodome ; 
or tabular, with dominant basal plane. Colourless to 
bluish-white or grey. Transparent. Lustre, vitreous. 



SALTS, ETC. 217 

Streak, white. Index of refraction, high (/3= 1*624). 
Double refraction, positive, moderate (7 a = 0*009). 
Basal cleavage, perfect ; prismatic, good. Fracture, 
uneven. Hardness, 3-3*5. Density, '3*9-4. Fusible. 
Colours the flame red. Insoluble in acids. 

Celestite is exploited for the preparation of strontium 
compounds, which are used for fireworks. 

Mirabilite, or Glauber's salt. Hydrated sulphate of 
soda: Na 2 SO 4 ioH 2 O (soda 19*3, sulphur trioxide 24*8, 
water 55*9, per cent.). Crystallizes in the mono- 
clinic system, but occurs in Nature mostly in the form 
of an efflorescent incrustation. Colourless to white. 
Lustre, vitreous. Transparent. Streak, white. Pina- 
coidal cleavage, perfect. Hardness, 1*5-2. Density, 
1*48. Fusibility, 1*5. Colours the flame yellow. 
Soluble in water. 

Occurs in the so-called soda-lakes of Wyoming and 
in Salt Lake, Utah. 

Epsomite, or Epsom salt. Sulphate of magnesium : 
MgSO 4 .7H 2 O (magnesia 
16*26, sulphur trioxide 32*52, 
water 5 1*22 per cent.). Crystal- 
lizes in the rhombic system, 
with dominant prism (m) and 

brachydome (z), but occurs 

XT . ,1 r-i FIG 104 ARTIFICIAL CRYSTAL 

in Nature mostly as a fibrous 4 OF EpsoMITE . 

efflorescence. Colourless to 

white. Transparent. Lustre, vitreous. Cleavage, 

brachypinacoidal. Fracture, conchoidal. Hardness, 



218 DESCRIPTIVE MINERALOGY 

2-2*5. Density, i'7-r8. Fusibility, i. Soluble in 
water, to which it imparts a bitter taste. 

It occurs on the steppes of Siberia, in Catalonia, and 
as a deposit from the soda-lakes of Wyoming in the 
United States. It also exists in solution, as in the 
springs at Epsom and in sea water. 

Alums. The alums constitute an isomorphous group 
of sulphates of alumina and of the alkalies, magnesia, 
iron, and manganese, each member of which crystal- 
lizes in the regular system, with twenty-four molecules 
of water. Some of the more important members of the 
group are the following : 

Kalinite or potash-alum : 

K 2 S0 4 + A1 2 (S0 4 ) 3 +24H 2 0. 
Mendozite or soda-alum : 

Na 2 SO 4 + A1 2 (SO 4 ) 3 + 24H 2 O. 
Tschermigite or ammonia-alum : 

(NH 4 ) 2 S0 4 + A1 2 (S0 4 ) 3 



These salts are easily soluble in water, and have a 
sweetish astringent taste. 

Although the alums exist in small quantities as 
natural efflorescences, the alum of commerce is 
chiefly prepared from the mineral alunite, or alum- 
stone, a hydrated sulphate of alumina and potash : 
(K 2 O.3A1 2 O 3 .4SO 5 .6H 2 O), and from aluminous shales 
containing pyrites, the latter mineral supplying the 
sulphuric acid. 



SALTS, ETC. 219 

NITRATES. 

The important nitrates occurring as natural salts are 
those of the alkalies, potash, and soda viz., nitre and 
nitratine. They are used in the manufacture of nitric 
acid and of gunpowder, and are also of great value as 
artificial manures. 

Nitre, or saltpetre. Nitrate of potash: KNO 3 (potash 
46*58, nitrogen peroxide 53*42 per cent.)- Crystal- 
lizes in the rhombic system, in silky needle-shaped 
prisms, but also occurs as an incrustation. Colour- 
less, white, or grey. Lustre, vitreous. Transparent. 
Cleavage, prismatic. Fracture, conchoidal. Hard- 
ness, 2. Density, 1*9-2*1. Fusibility, i. Easily soluble 
in water. Like all potash salts, imparts a violet 
coloration to the flame of a spirit-lamp or Bunsen 
burner. 

Nitre is found as an efflorescent crust, or mixed with 
the soil of certain rainless districts in Spain, Algeria, 
India, Quito and is used principally for the prepara- 
tion of gunpowder. 

Nitratine, soda-nitre, or Chili saltpetre. Nitrate of 
soda: NaNO 3 (soda 36*49, nitrogen peroxide 63*51, per 
cent.). Crystallizes in the hexagonal system, with 
rhombohedral hemimorphism ; but is usually found as 
an efflorescent incrustation. Colourless, but also white 
or grey. Lustre, vitreous. Transparent. Rhombo- 
hedral cleavage, perfect. Fracture, conchoidal. Hard- 
ness, 1-5-2. Density, 2*i-2'2. Deliquescent and easily 



220 DESCRIPTIVE MINERALOGY 

soluble in water. Fusibility, I. In common with all 
soda salts, it colours the flame yellow. 

Nitratine is found mixed with clay and sand (caliche), 
on the rainless pampas of Chili (Tarapaca and Anto- 
fagasta) and Peru, and is exploited for the preparation 
of nitric acid and of nitre, and as a manure, being 
too deliquescent for the direct manufacture of gun- 
powder. 

CHLORIDES AND FLUORIDES. 

There are a great number of naturally - occurring 
chlorides. The following only are selected for de- 
scription : rock-salt, sylvite, sal ammoniac, carnallite, and 
the fluoride, fluorspar. 

Rock-Salt, or halite. Chloride of sodium : NaCl 
(sodium 39-3, chlorine 607, per cent.)- Crystallizes 
in the regular system, usually in 
cubes, but also occurs in granular 
masses and in fibrous aggregates. 
When pure, colourless or white, 
but often tinted yellow, red, blue, 
FIG. 105. ROCK-SALT. e tc., by the presence of a small 
Skeleton-cubes. quantity o f some impurity. Trans- 

parent to opaque. Lustre, vitreous. Streak, white. 
Cubic cleavage, perfect. Fracture, conchoidal. Brittle. 
Hardness, 2-2*5. Density, 2-1-2-3. Fusibility, 1-5. 
Soluble in water, and easily recognizable by its saline 
taste. 

Rock-salt is widely distributed among lacustrine 




SALTS, ETC. 



221 



deposits. Thus it occurs with gypsum in the Trias of 
Cheshire and Worcestershire; and there are thick 
deposits at Sperenberg near Berlin, at Wieliczka, 
in Austrian Poland, and at Parajd in Transylvania. In 
the Permian of Stassfurt, in Central Germany, it 
occurs in association with gypsum and numerous 



FIG. 106. ROCK-SALT CRYSTALS FROM STASSFURT (AFTER 
G. P. MERRILL). 

sulphates and chlorides of potassium and magnesium, 
the exploitation of which constitutes an important 
industry. Salt is also found as an efflorescence in 
the rainless districts of Chili and Africa, and as a 
deposit from brine in the " salt pans " of the Northern 
Transvaal. 



222 DESCRIPTIVE MINERALOGY 

Sylvite, or sylvine. Chloride of potassium: KC1 
(potassium 52*4, chlorine 47*6, per cent.). Crystallizes 
in the regular system, with cubic habit. Colourless. 
Lustre, vitreous. Transparent. Streak, white. Cubic 
cleavage, perfect. Brittle. Fracture, uneven. Hard- 
ness, 2. Density, 1*9. Fusibility, 1*5. Soluble in 
water, to which it imparts a saline, bitter taste. 

Sylvite occurs with rock-salt at Stassfurt in Central 
Germany, and at Kalusz in Galicia. 

Sal Ammoniac. Chloride of ammonium: NH 4 C1 
(chlorine 66*26, nitrogen 26*25, hydrogen 7*49, per 
cent.). Crystallizes in the regular system, but usually 
in stalactitic aggregations or as an incrustation. 
When pure, colourless or white, but often stained 
yellow by chloride of iron. Hardness, 1*5-2. Density, 
1*52. Soluble in water, to which it imparts a pungent 
saline taste. 

Sal Ammoniac occurs as an incrustation on lava at 
Vesuvius, Etna, and in other volcanic districts. 

Carnallite. Double chloride of magnesium and 
potash: MgCl 2 .KCl + 6H 2 O (MgCl 2 , 34*2, KC1, 26*8, 
H 2 O 39, per cent.). Crystallizes in the rhombic 
system. Colourless to white or red. Transparent. 
Lustre, vitreous. Fracture, conchoidal. Hardness, i. 
Density, 1*6. Fusibility, 1-1*5. Soluble in water, and 
deliquesces when exposed to air. 

Carnallite occurs at Stassfurt in Central Germany. 

Fluorspar or fluorite. Fluoride of calcium : CaF 2 
(fluorine 48*72, calcium 51*28, per cent.) Crystallizes in 



SALTS, ETC. 



223 



the regular system, occurring in cubes and octahedra, 
sometimes in combination with the rhombic dodeca- 
hedron and icositetrahedron. Interpenetration twins 




FIG. 107, THE STASSFURT DEPOSITS. 

a, Rock-salt ; b, polyhalite ; c, kieserite ; d, carnallite ; e, kainite ; 
/, impervious clay ; g t anhydrite ; h, gypsum ; k, sandstone. 

common. The crystals are transparent, and have a 
glassy lustre. Colour, blue, violet, green, and yellow. 




FIG. 108. FLUORSPAR : CUBE 
WITH OCTAHEDRON. 



FIG. 109. FLUORSPAR : CUBE 
WITH ICOSITETRAHEDRON. 



Transparent. Lustre, vitreous. Streak, white. Index 
of refraction, low (/* = i'43). Octahedral cleavage, 
perfect. Fracture, subconchoidal. Hardness, 4. 



224 



DESCRIPTIVE MINERALOGY 



Density, 3-2. Fusibility, 3. Phosphorescent when 
heated. Decomposed by sulphuric acid. 

As a veinstone, fluorspar" is associated with tin ore 
in Saxony, Bohemia, and Cornwall, with lead ores in 
Derbyshire, Cumberland, and Northumberland, and 
with silver ores in Saxony, the Harz, and Norway 
(Kongsberg). The more beautiful varieties of the spar 
(" Blue John ") are used for the manufacture of orna- 
mental vases, while the commoner varieties are used 
as a flux in metallurgical processes, and for the prepara- 
tion of hydrofluoric acid. 

PHOSPHATES. 

The only phosphates of commercial importance are 
those of lime, known variously as apatite, phosphorite, 
coprolites, and guano. 

Apatite. Chlorophosphate of calcium, 3Ca 3 (PO 4 ) 2 . 

CaCl 2 , or fluorophosphate of calcium, 3Ca 3 (PO 4 ) 2 . 

CaF 2 . The fluorine variety 
contains 42*26 per cent, of 
phosphorus pentoxide, and 3*77 
per cent, of fluorine ; while 
the chlorine variety contains 
40*92 per cent, of the oxide, 
and 6-8 2 per cent, of chlorine. 
Crystallizes in the hexagonal 

system. Habit, short prismatic or thick tabular. 

The smaller crystals are composed of numerous pyra- 

mid and prism faces, usually terminated by the basal 




FIG. 
c, Basal plane ; x, proto 



SALTS, ETC. 225 

plane. Large opaque crystals and compact or fibrous 
nodular masses (phosphorite) also occur. Colourless 
to white, but usually tinted green, blue, violet, or red. 
Lustre, vitreous to resinous. Transparent to opaque. 
Streak, white. Refractive index, moderately high 
(= 1*646). Double refraction, negative, weak (o> e = 
0*004). Basal and prismatic cleavages, imperfect. Frac- 
ture, conchoidal. Brittle. Hardness, 5. Density, 
3'i6-3*22. Fusibility, 5. Decomposed by hydrochloric 
acid. 

Mineral phosphates are exploited for use as fertilizers 
in Cornwall, Spain, Germany, Norway, Russia, the 
United States, and Canada. Many of these deposits 
occur as nodular concretions which have been formed 
by the concentration of phosphatic material of organic 
origin. Guano is a phosphatic deposit of recent organic 
origin, occurring in regions where abundant animal life 
and small rainfall combine to assist its accumulation, 
as in Chili, Peru, Bolivia and Africa. 

BORAXES. 

Two borates are here described viz., borax and 
boracite. 

Borax. Hydrated borate of sodium : Na 2 O.2B 2 O 3 . 
ioH 2 O (soda 16*23, boron trioxide 36*65, water 47*12, 
per cent.). Crystallizes in the monoclinic system, 
but usually occurs massive (tinkal). Colourless to 
white. Transparent. Lustre, vitreous. Streak, white. 
Index of refraction, low (/3=i*47). Double refraction, 

15 



226 DESCRIPTIVE MINERALOGY 

negative, weak (7 a= 0*004). Orthopinacoidal cleavage, 

perfect ; prismatic cleavage, 
good. Fracture, conchoidal. 
Brittle. Hardness, 2-2-5. 
Density, 1*7. Fusibility, 1-1*5. 
Soluble in water, to which it 

imparts a sweetish alkaline 
FIG. in. BORAX. 

o, Orthopinacoid : m, prism, taste. 

Borax is found as a chemical 

deposit on the shores of lakes in Thibet, California, and 
Nevada, and is exploited for its antiseptic properties. 

Boracite. Chloroborate of magnesium : 6MgO. 
2B 2 O 3 .MgCl 2 (magnesia 26*9, boron trioxide 62*5, 
chloride of magnesium 10*6, per cent.). Crystallizes 
apparently in the regular system, with tetrahedral 
development. According to a recent view, the crystals 
are considered to be compound twins of rhombic or 
of monoclinic individuals. 
Colourless to white. Lustre, 
vitreous. Transparent to trans- 
lucent. Streak, white. Index of 
refraction, moderate (ft = 1-667). 
Double refraction, moderate FIG. 112. BORACITE. 

( 7 -a = 0'OIl). Tetrahedral a > Cube : rf > rhombic dodeca- 
hedron ; t, tetrahedron. 

cleavage, imperfect. Fracture, 

conchoidal. Brittle. Hardness, 7. Density, 2'g-3. 

Fusibility, 3. Soluble in hydrochloric acid. 

Boracite is found enclosed in gypsum and anhydrite 
at Luneburg and Segeberg, and in carnallite at Stass- 
furt in Germany. 




OTHER USEFUL MINERALS 



227 



Streak, 
Double 




APPENDIX. 

OTHER USEFUL MINERALS. 

Sulphur, S. Crystallizes in the rhombic system, with 
bipyramidal habit. Also occurs in nodular, kidney- 
shaped or stalactitic masses or as a mealy deposit or 
incrustation. Colour, pale yellow to dark brown. 
Lustre, resinous to adamantine. Transparent 
white. Index of refraction, high (^=2*04). 
refraction, very strong (7 a= 0*290). 
Basal and prismatic cleavage, very 
imperfect. Fracture, conchoidal. 
Brittle. Hardness, r'5-2'5. Density, 
1*9. Easily fusible. Volatile and 
combustible. Insoluble in acids. 

Sulphur is produced by sublima- 
tion in volcanic districts, by deposi- 
tion from sulphurous springs, or by the alteration of 
beds of gypsum. It occurs in deposits of industrial 
importance in Sicily and Japan. 

Graphite, Carbon, C. Crystallizes probably in the 
monoclinic system, although its six-sided crystals have 
a decidedly hexagonal habit. Most frequently it occurs 
massive or in scales disseminated through rocks. 
Colour, black. Lustre, metallic. Streak, grey. Opaque. 
Unctuous to the feel, soiling the fingers. Pliable. 
Hardness, 0*5-1. Density, i'9-2'3. Infusible. Insoluble 
in acid. 

Graphite occurs in scales disseminated through lime- 
stone, slate, gneiss, mica-schist, or in larger pocket-like 
deposits and veins, and it is exploited for " black- 



FIG. 113. SULPHUR; 
RHOMBIC PYRAMID. 



228 DESCRIPTIVE MINERALOGY 

lead " or plumbago. It is found in deposits of indus- 
trial importance in Borrowdale, near Keswick, in the 
Lake District, in Finland and Siberia, in Austria, in 
India and Ceylon, and in the United States and Canada. 

Coal, mineral pitch (asphaltum), bitumen, mineral oil 
(petroleum), mineral wax (ozokerite), mineral resin (amber), 
are other mineral substances of which the chief con- 
stituent is carbon. They are not, however, properly 
speaking, mineral species, since they have neither an 
unvarying chemical composition nor definite physical 
properties. Petroleum consists of an homologous series 
of hydrocarbons of the general formula CH 4 + nCH 2 , 
which in Nature occur mixed in all proportions. By 
fractional distillation it can be separated into heavy 
lubricating oils, light oils, spirits (petrol, benzol, etc.). 



CHAPTER IV 

GEMS 

UNDER the head of gems are included certain rather 
rare minerals which are used for the purpose of orna- 
mentation and personal adornment. 

The two chief physical features in a precious stone 
are hardness and brilliancy. While a sufficient degree 
of the former insures durability by preventing deteriora- 
tion under wear, it is the latter which determines the 
beauty of the gem-stone. But brilliancy is produced 
by a combination of properties which may be variously 
developed in a stone. These properties are the pellu- 
cidity, colour, refractive index, dispersive power, and 
pleochroism. 

The pure and delicate colours possessed by gems are 
a great feature of their beauty ; and in many cases the 
jeweller's names are founded entirely on differences in 
colour. Thus the ruby and the sapphire are red and 
blue varieties respectively of the mineral corundum, and 
the emerald and the aquamarine are green and blue 
varieties of beryl. Although most gems are perfectly 
limpid, a few are only translucent or opaque (opal, 
turquoise, etc.). The dispersive power (see p. 47) 

229 



230 DESCRIPTIVE MINERALOGY 

imparts the quality of emitting brilliant flashes of 
variously coloured light, which determines the " fire " 
of a gem. Many gems possess in greater or less degree 
the property of transmitting differently coloured light 
when viewed in different directions. This is a phe- 
nomenon of pleochroism (see p. 54). 

Among the minerals mentioned in this chapter will 
be found several that have been described in other 
parts of the book. Thus some varieties of quartz, 
felspar, and olivine, which are among the commonest 
rock-forming minerals, are used as gem-stones. 

Diamond, Carbon, C. Crystallizes in the regular 
system, the usual habit being the octahedron alone, or 






FIG. 114. CRYSTALS OF DIAMOND. 

this form in combination with the rhombic dodecahe- 
dron and other regular forms. The faces are generally 
curved, producing rounded crystals. Twinning on the 
octahedron. Colourless ; but also yellow, straw-coloured, 
or brown, and very occasionally green and blue. Trans- 
parent. Lustre, adamantine. Index of refraction, very 
high (//. = 2*417). Dispersion, very strong. Octahedral 
cleavage, perfect. Fracture, conchoidal. Brittle. Hard- 
ness, 10. Density, 3*52. Infusible. Insoluble in acid. 
Burnt in oxygen, yields carbon dioxide. 

Diamonds are found in alluvial sands, in association 



GEMS 



231 




FIG. 115. THE CULLINAN 
DIAMOND. 

Half natural size. 



with other precious stones, and with gold and platinum 
in the Deccan of India, Brazil 
(Minas Geraes), Borneo, Sum- 
atra, and Australia. The chief 
source of the present supply is 
at Kimberley, in South Africa, 
where stones of the first water 
and of good size are found 
imbedded in a dark-coloured 
ultrabasic volcanic breccia 
(kimberlite), filling volcanic 
vents or " pipes." 

Corundum. Oxide of alu- 
minium, A1 2 O 3 (aluminium 

52*9 per cent.). Crystallizes in the hexagonal system, 
with rhombohedral symmetry. Habit, tabular, pris- 
matic, or pyramidal, the first -named being deter- 
mined by the dominance of the basal plane. Hori- 
zontal striation on pyramid and prism faces. Also 
occurs in rounded crystals and as rolled pebbles and 
grains in the so-called " gem-sands." Colourless when 
pure. Generally coloured : blue (sapphire), red (ruby), 
yellow (oriental topaz), grey, purple (oriental amethyst), 
or green (oriental emerald). Streak, colourless. Lustre, 
vitreous to adamantine or resinous. Transparent to 
translucent. Index of refraction high (w = 1768). 
Double refraction, negative, moderate (o> - e= 0*008). No 
true cleavage. Separation (due to twinning) parallel to 
the basal plane and to the rhombohedron r (1011). Frac- 
ture, conchoidal. Brittle. Hardness, 9. Density, 3*9-4'!. 



2&2 DESCRIPTIVE MINERALOGY 

Corundum occurs as an accessory constituent in 
igneous rocks rich in alumina, both as an allogenic 
and as an autogenic product. It also occurs in granite 
contact zones, especially in argillaceous limestones. 
The dark granular variety, known as emery, occurs in 
admixture with iron ores (magnetite and haematite) 
in association with schists and gneiss on the island of 
Naxos. A massive variety of red corundum is exploited 
for use as a polishing material at Salem in Madras. 





FIG. 116. CORUNDUM: CHARACTERISTIC CRYSTALS. 

d, Basal plane; r, rhombohedron (R) ; o, rhombohedron (-2R); 
n, deutero-pyramid (|P2) ; e, deutero-pyramid (f P2) ; I, deutero- 
prism. 

Rubies occurring in a matrix of limestone are worked 
in Burmah. Many of the best gems are found loose in 
gravels and sands, as in Ceylon (rubies), Siam (sapphires 
and rubies), Montana (sapphires). 

Spinel. A double oxide of aluminium and magne- 
sium : MgO,Al 2 O 3 (magnesia 28-2, alumina 71*8, per 
cent.). Crystallizes in the regular system. Habit, octa- 
hedral. Twinned on the octahedron. Colour, red. Streak, 
white. Lustre, vitreous. Transparent. Index of re- 
fraction, high (yii=r7i5). Octahedral cleavage, im- 
perfect. Fracture, conchoidal. Brittle. Hardness, 8. 



GEMS 



Density, 3*5. Infusible. Soluble with difficulty in 
sulphuric acid. 

Spinel occurs in crystalline limestones and dolomites, 
also in igneous and metamorphic rocks. It is of frequent 





FIG. 117. SPINEL: OCTA- 
HEDRON. 



FIG. 118. SPINEL: CRYSTAL 
TWINNED ON AN OCTAHEDRAL 
FACE. 



occurrence in sands e.g., the so-called " gem sands " of 
Ceylon. 

Beryl. Metasilicate of beryllium and aluminiurn : 
3BeO.Al 2 O 3 .6SiO 2 (oxide of beryllium 14, alumina 19, 
silica 67, per cent.). Crystallizes in the hexagonal 
system, forming long six-sided prisms, 
terminated by the basal plane. In 
addition pyramidal faces are some- 
times developed. The prismatic 
planes usually show a vertical stria- 
tion. Colourless to white, yellowish 
or greenish white, pale pink, honey- 
yellow, and various shades of green 
and blue. The emerald-green to 
apple-green varieties are known as emerald, while the 
blue or sea-green varieties are distinguished as aqua- 
marine. Lustre, vitreous. Transparent to translucent. 



FIG. 119. BERYL. 

c, Basal plane ; P, 
pyramid ; a, prism ; 
r, deutero-pyramid 

(2P2). 



234 



DESCRIPTIVE MINERALOGY 



Streak, white. Index of refraction, moderate (&> = I "584). 
Double refraction, negative, weak (&> e=o'oo6). Basal 
cleavage, distinct. Fracture, conchoidal to uneven. 
Brittle. Hardness, 7*5-8. Density, 2'6-2'S. Fusibility, 
5'5. Insoluble in acids. 

Beryl is found imbedded in granite and pegmatite 
veins ; also in mica-schist, limestone, and clay slate. 
The best-known localities are the Ural Mountains 
(Mursinka, Ekaterinburg, and Miask), the Altai, 
Colombia (Muzo, Cosquez and Somondoco mines), and 
New South Wales (Emmaville). 

Garnet. The garnets are silicates of aluminium, iron, 
manganese, chromium, calcium, and magnesium, having 
the general formula 3MO.R 2 O 3 .3SiO 2 , in which M stands 
for metals like calcium, magnesium, etc., forming prot- 
oxides, and R for metals like aluminium and chromium, 
forming sesquioxides. According to the variation of 
MO and R 2 O 3 , the following varieties may be dis- 
tinguished : 





Constituent Bases. 






Name. 




Colour. 


Density. 


Index of 
Refraction. 








MO. 


R 2 3 . 








Grossularia 
Almandine 
Pyrope 


CaO 

FeO 
FeO, MgO, CaO 


AUO, ' Pale green 
AUG. Claret 
A1. 2 O,, Cr 2 O, | Blood red 


3-4-3-6 
3-9-4-3 
3*7-3 '8 


i'747 
i 807 

1*745 


Melanite 


CaO 


Fe 2 O 3 


I Blackish-. 
\ brown ' 


3-6-4-3 1-856 


Spessartine 


MnO 


A1 2 O 3 


{reith} 4-0-4-3 .'Sic 


Uvarovite 


CaO 


Cr 3 O 3 


Green 3'4~3'5 1*838 



Other existing varieties are isomorphous mixtures of 
these. They crystallize in the regular system, the most 



GEMS 235 

common habit being the rhombic dodecahedron, with 
edges sometimes truncated by icositetrahedral faces. 
Colour, as above. Streak, white. Lustre, vitreous to 
resinous. Transparent to opaque. Index of refrac- 
tion, as above. Rhombic dodecahedral cleavage, im- 
perfect. Fracture, uneven. Hardness, 6*5-7. Density, 
as above. Brittle. Fusibility, 3 for all varieties except 
uvarovite, which has a fusibility of 6. Attacked with 
difficulty by hydrochloric acid. 

The garnets are frequent accessory constituents of 





FIG. 120. GARNET: COMMON FORMS, RHOMBIC DODECAHEDRON, 
ICOSITETRAHEDRON, ALONE AND IN COMBINATION. 



igneous rocks, such as granite, microgranite, aplite, 
trachyte, and andesite. They are also found in gneisses 
and other crystalline schists, and in ultrabasic rocks, 
such as peridotites, eclogite, serpentine, kimberlite, etc. 
They are a frequent constituent of the so-called "gem 
sands." It would serve no purpose to give localities for 
so common a mineral ; but the occurrence of pyrope 
in Bohemia, of melanite at Frascati, near Rome, and 
uvarovite in the Ural Mountains, may be mentioned. 

Topaz. Fluorsilicate of aluminium : A1(F.OH) 2 . 
AlSiO 4 (alumina 55*44, SiO 2 32*61, fluorine 20*65, per 



236 



DESCRIPTIVE MINERALOGY 



cent.)- Crystallizes in the rhombic system. Habit, 
prismatic, terminated by the basal plane alone or 
together with faces of various brachydomes, and those 
of the proto-pyramid. One end alone shows perfect 
development, the faces constituting the opposite termi- 
nation being usually rudimentary. The prisms show 
a vertical striation. Colourless, wine-yellow, brown, 
or tinted blue, red, or green. Lustre, vitreous. Trans- 
parent to translucent. Index of refraction, 1*62. Double 
refraction, moderate (7 a = 0*009). Basal cleavage, 




M 



FIG. i2i. TOPAZ. 

P, Basal plane ; s and 0, pyramids ; x, brachypyramid ; M , prism ; 
/, brachyprism ; n andjy, brachydomes. 

perfect. Hardness, 8. Density, 3'4-3'6. Infusible and 
unattacked by acids. 

Topaz occurs in drusy cavities of granitic rocks or 
as loose and rolled pebbles. Well-known localities 
are St. Michael's Mount in Cornwall, the Mourne 
Mountains in Ireland, Saxony (Schneckenstein), Brazil 
(Minas Geraes), United States, Siberia, Ceylon. 

Tourmaline. Hydrated borosilicate of sodium, 
magnesium, and aluminium. According to Clarke, the 
tourmaline series consists of salts of an acid which 



GEMS 237 

may be represented by the formula Al 6 (SiO 4 ) 6 (BO 2 ) 2 . 
BO 3 H 2 .H 12 . Crystallizes in the hexagonal system, with 
rhombohedral symmetry and hemimorphic develop- 
ment. Habit, prismatic, with rhombohedral termina- 
tions, and the prisms vertically striated. Also occurs 
in fibrous veins and stellate aggregates. Colour, usually 
black or dark brown, but various tints of red, blue, and 
green, are common, and even colourless crystals are 
known. Transparent to translucent. Lustre, vitreous. 
Index of refraction, fairly high ( = 1*64). Double 
refraction, negative, strong (o)-e= 0*017). Strongly 
dichroic, with marked absorption of the ordinary ray. 
Rhombohedral and prismatic cleavages, imperfect. 
Fracture, subconchoidal. Brittle. Hardness, 7. Den- 
sity, 3-3*2. Fusibility, 3-5. Insoluble in acids. 

Tourmaline occurs in the contact zones of granite 
and in pegmatites. It is also a frequent constituent 
of sands and sandstones. The best gems come from 
Brazil, Ceylon, Siberia, and North America. 

Zircon. Silicate of zirconium : ZrO 2 .SiO 2 (ZrO 2 
67-2, SiO 2 32*8, per cent.). Crystallizes in the tetragonal 
system, being isomorphous with cas- 
siterite and rutile. Habit, prismatic 
or pyramidal. The prisms are of 
both orders; the pyramid, of the 
first order only. Colour, usually 
dark brown, also orange, yellow, FIG. 122. ZIRCON. 

red (jacinth), pale green, and grey p > Pyramid; m, prism; 

J J a, deutero-prism. 

(jar goon}. Streak, colourless. Lustre, 

adamantine. Transparent to opaque. Index of refrac- 




238 DESCRIPTIVE MINERALOGY 

tion, high (co=J'g^). Double refraction, positive, very 
strong (e o> = 0*062) . Dispersion, strong. Pyramidal and 
prismatic cleavages, imperfect. Fracture, conchoidal. 
Brittle. Hardness, 7*5. Density, 47. Infusible. 
Only attacked with difficulty by hot sulphuric acid. 

Zircon is a constituent of most granites, syenites, 
etc., and of the sands derived from them. The gems 
are derived from Ceylon, New South Wales, and 
Queensland. 

Sphene, or titanite. Titanosilicate of calcium : 
CaO.TiO 2 .SiO 2 (oxide of titanium 40*8, oxide of cal- 
cium 28*6, silica 30*6, per cent.). Crystallizes in the 
monoclinic system. Habit, cuneate. Colour, olive- 
brown. Transparent. Lustre, adamantine. Streak, 
white. Index of refraction, high (ft = 1*894). Double 
refraction, positive, very strong (7 a = 0*121). Dis- 
persion, very strong. Prismatic cleavage, distinct. 
Fracture, subconchoidal. Hardness, 5. Density, 3*5. 
Fusibility, 3. Decomposed with difficulty by hydro- 
chloric acid. 

Sphene occurs in many igneous rocks and crystalline 
schists, also in limestones. Its inferior hardness mili- 
tates against its employment as a gem. 

Turquoise, or callaite. A hydrated and basic phos- 
phate of alumina: A1 2 (OH) 3 PO 4 .H 2 O. Only known 
massive, occurring as veins and nodules in igneous 
rocks. Colour, cerulean or peacock blue. Opaque. 
Lustre, resinous. Fracture, conchoidal. Hardness, 6. 
Density, 2*7. 



GEMS 



239 



The turquoise used in jewellery is found near 
Nishapur in Persia, and in the Burro and Jarilal 
mountains in New Mexico. 

Chrysoberyi. A double oxide of beryllium and alu- 
minium, BeO.Al 2 O 8 (oxide of beryllium 19*8, alumina 
80-2, per cent.). Crystallizes in the rhombic system, 
in forms similar to those of olivine. Often in twins 
and triplets, the latter termed alexandrite, which was 
once considered to be a distinct mineral. Habit, short 
prismatic or thick tabular, with vertical striations. 




FIG. 123. CHRYSOBERYL. 

a, Macropinacoid ; b, brachy- 
pinacoid ; i, brachydome. 




FIG. 124. COMPOUND TWINNED 
CRYSTAL (ALEXANDRITE). 



Colour, yellowish-green to olive-green. Lustre, vitreous. 
Transparent. Streak, colourless. Index of refraction, 
high (/3=i'75). Double refraction, positive, moderate 
(7-a=o'oog). Pleochroic. Cleavage, imperfect. Frac- 
ture, conchoidal. Brittle. Hardness, 8*5. Density, 3*7. 
Infusible. Insoluble in acids. 

Chrysoberyi is known to the jewellers under the name 
of the cafs-eye (cymophane) and oriental chrysolite, and 
is chiefly found in Brazil, Ceylon, Ural Mountains, and 
in Connecticut in the United States. 



240 DESCRIPTIVE MINERALOGY 

Peridot. A beautiful transparent, deep olive- 
green variety of olivine (see p. 106), is used as a 
gem-stone. 

Opal. Hydrated silica: SiO 2 +^H 2 O. Amorphous, 
occurring in botryoidal or stalactitic masses. Colour, 
bluish, yellowish, or milk white. Translucent. Streak, 
white. Lustre, vitreous or resinous. Index of refrac- 
tion, low (/x = 1*455). The " fire " of " precious opal " 
is the result of reflection and diffraction of light 
from internal surfaces. Fracture, conchoidal. Brittle. 
Hardness, 6. Density, 2*1. Infusible, but yields water 
on heating. Soluble in potash. 

Hyalite is a transparent, colourless variety of opal. 

Chalcedony. Silica : SiO 2 . Occurs in concretionary, 
botryoidal, or stalactitic masses which have an internal 
fibrous and crystalline structure. Colourless to white. 
Transparent to translucent. Lustre, resinous. Fracture, 
uneven to splintery. Hardness, 7. Density, 2'6-2'6/j.. 

Carnelian is a red variety of chalcedony ; Sard, a 
brownish-red variety ; Plasma, a leek-green variety ; 
Chrysoprase, an apple-green variety ; Agate and Onyx are 
banded and variegated varieties which occur as the 
infilling of amygdaloidal cavities in certain basic lavas ; 
Sardonyx is a variety of onyx which contains layers of 
carnelian or sard. 

Quartz. Certain coloured varieties of quartz (see 
p. 84) are used for decorative purposes. 

Jasper is an opaque red-coloured variety in which 
there is much admixed iron oxide ; Amethyst is a violet- 



GEMS 241 

coloured quartz often used as a gem-stone; Smoky 
Quartz, Cairngorm, Rose Quartz, are smoke-coloured 
yellow and pink varieties of quartz; Tiger's Eye is 
a golden - yellow replacement by quartz of fibrous 
crocidolite. 

Felspar. Some varieties of felspar (see p. 86) are 
used as gem-stones. 

Moonstone is an opalescent variety of orthoclase found 
in Ceylon; Amazon-stone is a green variety of micro- 
cline ; Labrador Spar is an iridescent variety of labra- 
dorite, which when polished exhibits a magnificent play 
of colours. The iridescence is due to interference 
produced by a lamellar structure. 



INDEX 



ACANTHITE, 143 

Acicular habit, 4 
Actinolite, 79, 105 
Adamantine lustre, 42 
Adularia, 88, 90 
Agate, 240 
Alabaster, 215, 216 
Albite, 79, 86, 92, 207 
Alexandrite, 72, 239 
Alkali-micas, 101 
Allanite, 204 
Almandiiie, 78, 234 
Alum, 58, 218 
Aluminium, 64, 121, 204 
Alumstone, 75, 218 
Alunite, 75, 218 
Amazon-stone, 241 
Amber, 228 
Amesite, 80, no 
Amethyst, 85, 240 

Oriental, 231 
Ammonia-alum, 218 
Amorphous bodies, 35 

State, 3 

Amphiboles, 83, 108 
Analcime. 81, 112, 113, 207 
Anatase, 67, 72, 198 
Andalusite, 77, 113, 114 
Andesine, 87 

Angle, Law of Constant, 8 
Anglesite, 75, 156, 153 
Anhydrite, 74, 207, 214, 226 
Anisotropic media, 43, 44 
Annabergite, 75. 161, 164 
Anomite, 101 
Anorthic system, n, 23 
Anorthite, 78, 79, 86, 94 
Anorthoclase, 88 



Antimonite, 70, 190 
Antimony, 69, 120, 188 

Native^ 190 
Antimony glance, 190 
Apatite, 76, 193, 207, 224 
Apophyllite, 112 
Aquamarine, 229, 231, 233 
Aragonite, 66, 75, 207. 209, 210 
Argentite, 69, 143, 144, 146 
Ar senates, 75 
Arsenic, 64, 69, 120, 188 
Arsenical Pyrites, 191 
Arsenide group, 69 
Arsenious oxide, 189 
Arsenopyrite, 70, 191 
Arsenosulphide group, 70 
Asbestos, in, 241 
Asphaltum, 228 
Asterism, 43 
Asymmetric system, n 
Atacamite, 72, 142 
Augite, 79, 10 1 n., 103, 105 
Autunite, 76, 201, 202 
Avanturine, Lustre of, 43 
Axes, 8 

Optic, 48 
Dispersion of, 52 
Plane of, 51 

of elasticity, 50, 51, 53, 54 

of symmetry, 6 
Axial angle, Apparent, 52 n. 

Colours, 54 
Axinite, 82, 116, 194 
Azurite, 74, 140 

BABBITT METAI,, 194 
Barium, 64 

Sulphate, as Veinstone, 207 



242 



INDEX 



243 



Barytes, 75, 216 

Bauxite, 73, 204 

Baveno type of Twinning, 88, 89 

Becke's method, 53 n, 

Bell-metal ore, 197 

Benzol, 228 

Beryl, 79, 233 

Biaxial Crystals, 5 1 

Biotite, 81, 100, 101 

Bismuth, 69, 144, 188 
Native, 188, 189 

Bismuthinite, 70, 189 

Bitumen, 228 

Blackband Ironstone, 177 

"Blackjack," 155 

Black Oxide of Copper, 132 

Black sands, 179 

Black Tin, 193, 194 

Blende, 69, 120, 154, 207 

" Blue John " (fluorspar), 224 

Bog-manganese, 184, 186 

Boracite, 76, 225, 226 

Borates, 76, 225-6 

Borax, 76, 225 

Bornite, 70, 129, 135, 138 
Borosilicates, 81 
Botryoidal, 4 
Brachy-axis, 19 
Brachy-pinacoid, 21 
Braunite, 73, 184, 187 
Braurts solution, 6 1 
Brilliancy of Gems, properties 

causing, 229 
Brittle Silver Ore, 140 
Brittleness, 39 
Bronzite, 42, 43, 56 
Brookite, 67, 73, 198 
Brown Iron Ore, 168, 175 
Brucite, 72 
Bytownite, 87 

CADMIUM, 120, 155 
Cairngorm, 85, 241 
Calamine, 74, 154, 158, 160 
Calaverite, 122, 126 
Calcite, 66, 67, 74, 83, 95, 107, 

109, 209 
Calcium, 64 

Carbonates, 210 
as Veinstones, 206 

Fluorophosphate of, 193 



Calcium (continued) : 

Sulphates, as Veinstones, 207 

Tungstate of, 200, 201 
Callaite, 238 
Capillary habit, 4 
Carbon (see Diamond and 
Graphite), 64, 208, 230 
Carbonates, 65, 74, 208 
Carlsbad type of Twinning, 89 
Carnallite, 71, 222, 226 
Carnelian, 240 
Carnotite, 201 
Cassiterite, 73, 193, 194 
Cat's-eye, 239 

Celestite or Celestine, 74, 216 
Centro symmetry, 6 

Law of, 24 
Cerium, 203, 204 
Cerussite, 74, 80, 152 
Cervantite, 188 
Chalcanthite, 75, 129, 142 
Chalcedony, 73, 85, 206, 240 
Chalcocite, 69, 129, 133, 135 

Argentiferous, 143 
Chalcopyrite, 70, 120, 128, 129, 

134. : 58 

Chalybite, 67, 74, 168, 177, 207 
Change or play of Colours, 43 
Chatoyancy, 43 
Chemical Change, 68 
Chessylite, 74, 129, 140 
Chiastolite, 114 
Chili Saltpetre, 219 
Chloanthite, 144, 161, 162 
Chlorargyrite, 149 
Chlorides, 64, 220 
Chlorite, 80, 83, 100, 109, no, 

207 

Chloritoid, 81 
Chlorobromide of Silver (see 

Embolite), 143 
Chromates, 74 
Chromite, 72, 121, 164, 169, 178 

et alibi 

Chrysoberyl, 72, 239 
Chrysocolla, 80, 129, 141 
Chrysolite, 107 
Oriental, 239 
Chrysoprase, 85, 240 
Chrysotile, no 
Cinnabar, 69, 127, 128 



244 



INDEX 



Classification 

by Chemical Composition, 68 
et seq. 

of Crystals, 9 
Clay(s), 66, 83, 204 
Clay Ironstone, 177 

Slate, 234 
Cleavage, 35 
Clino-axis, 22 
Clinodome, 88 
Clino-pinacoid, 22, 88, 104 
Coal, 228 
Cobalt, 165 

Cobalt-bloom, 165, 167 
Cobalt Glance, see Cobaltite 
Cobaltite, 70, 144, 165, 166 
Cockscomb pyrites, 182 
Cohesion, Molecular, 35 
Columbite, 77, 203, 204 
Columnar habit, 4 
Compound Crystals, Twinning 

in, 33 

Concentric Laminated arrange- 
ment, 5 

Conchoidal Fracture, 38 
Conductivity 

Heat, 55 

Electricity, 56 
Conglomerate beds, Gold from, 

122 

Couseranite, 98 
Constant Angle, Law of, 8 
Copper, 64, 68, 129 

Grey, see Tetrahedrite 

Native, 129 
Ruby, see Cuprite 

Sulphides, Separation, 60 
Copper glance, 69, 133 
Copper Pyrites, 70, 120, 128, 129, 

134, 158 

Coprolite, 224, 225 
Cordierite, 80, 114, 115 
Axial colours of, 54 
Corundophilite, no 
Corundum, 73, 114, 231 
Coyellite, 129, 133;*. 
Critical angle, 45 
Crocidolite, 241 
Cross-shaped twins, 32 
Cryolite, 71, 204, 205, 206 
Crystal Axes, 8 



Crystalline Matter, 3 

Rocks, 83 
Crystals 

Classification of, 9 

Cubic System of, 1 1 

External habit, 3-4 

Internal structure, 4-5 

Faces of, 5 

Mixed, 67 

Positive and Negative, 5 1 -52 

Regular system of, n 

Simple, Twinning of, 33 

Symmetry of, 5 
Cube form, 11,12 

Four-faced, 14 

Pyramidal, 14 
Cubic System, 1 1 
Cubical cleavage, 37 
Cupric Oxide, 132 
Cuprite, 72, 129, 131 and ;/. 
Cuprous Oxide, 131 
Curve of Hardness, 39 
Curved faces, 5 
Cymophane, 239 

DATOWTE, 82, 117 
Dendritic habit, 4 
Density, 60 

of Gems, 62, 63 

Rock-forming minerals, 62 
Dechenite, 193 
Detrital deposits, 1 19 
Deutero-prism, 17 
Deutero -pyramids, 16 
Diallage, 105 
Diallogite, 187 
Diamond, 64, 69, 230 
Diaspore, 204 ;/. 
Didymium, 204 
Dihexagonal prism, 1 7 
Dihexagonal pyramids, 17 
Dimorphism, 67 
Diopside, 78, 104 
Dioptase, 141 
Dioxide group, 73 
Dipyre, 98 
Dispersion of Optic axes, 52 

of Rays, 45 

Dispersive power in Gems 229 
Disulphide group, 70 
Dodecahedron, see Rhombic 



INDEX 



245 



Dolomite, 67, 74, 83, 107, 109, 

209, 211 

Domatic cleavage, 37 
Domes, 20 
Double Refraction and Polariza- 

tion, 47 
Drusy faces, 5 
Ductility, 39 



42 
Elasticity, 39 

Axes of, 51,53, 54 

Ellipsoid of, 53 
Electric Properties,^ 55 
Electro-magnetic Separation, 57 
Electrostatic separation, 56 
Electrutn, 68 
Elementary substances, 64 
Elements, Surface tension in, 

59 

Ellipse >& 
Ellipsoid of Elasticity, 53 

of Fresnel, 51 

Triaxial, 35 
Embolite, 71, 143, 149 
Emerald-nickel, 164 
Emery, 232 
Enargite, 71, 129, 138 
Eustatite, 78 

Epidote, 80, 95, 104, in, 112 
Epimorphs, 61 
Epitritoxide group, 72 
Epsomite, 75, 212, 217 
Epsom Salt, 75, 212, 217 
Erubcscite, 70, 138 
Erythrite, 75, 165, 167 
Esmarkite, 116 
Even Fracture, 38 
Extraordinary Ray, 48, 49 

FACES of Crystals, 4 
Fahl ore, 129, 139 
Fayalite, 77, 106 
Felspar, 66, 79, 83, 86, 204, 207, 
230, 241 

Microcline, 91 

Oligoclase, 87, 94 

Orthoclase, 86, 88, 91, 94, 
207 

Plagioclase, 86, 94 



Felspathic rocks, 109 
Felspathoid group, 95 
Ferberite, 200 

Ferro-magnesian Micas, 100 
Ferromolybdenum, 199 
Ferro titanium, 198 
Fibrolite, 114 
Fibrous Iron ore, 175 
Fluorides, 64, 220, 222 
Fluorine, 64 
Fluorite, see Fluorspar 
Fluorspar, 71, 194, 207, 222, 

224 

Form, 6 

Forsterite, 77, 106 
Fracture, 35, 38 
Franklinite, 72, 159, 160, 184 
Freieslebenite, 71, 143 
Fundamental Intercepts, 26-8 
Fusibility, 55 



A, 69, 120, 135, 143, 149, 

151, 155, 207 
Argentiferous, 142, 151 
Gangue Minerals, 206 
Garnet, 78, 115, 194, 207, 234 
Garnierite, 161, 164 
Gems, 229 et seq. 

Density, 62, 63 

Physical features, 229 

Colour, 41 

Gem-sands, Ceylon, 233, 235 
Geniculated forms, 32 
Genthite, 164 
Gersdorfite, 161, 163 
German silver, 155, 161 
Gibbsite, 73, 204 n. 
Gigantolite, 116 
Glaucodot, 165, 167 
Glauber's salt, 217 
Glimmering' lustre, 42 
Glistening lustre, 42 
Globular form, 4 
Goethite, 73, 168, 174 
Gold, 64, 68, 118, 122 

Alloys of, 125 

Associated minerals, 120, 123 

Alluvial, 119, 123 

Drift, 123 

Dust, 124 

Free, 120, 122 



246 



INDEX 



Gold (continued} : 

Native, 122, 124 

Mustard, 124 

Paint, ibid. 

Sponge, ibid. 

Nuggets, 124-5 

Placer, 124 

Beach placer, 124 

Pyritic, 123, 181 

Telluride of, 122, 126 
Gold Amalgam, 122, 125 
Gold fields, principal, 124 
Gold and Silver, Tellurides 

of, 123, 126, 127 

Goniometer, reflecting, 5 & ., 9 
Granular structure, 4, 5 
Graphic Tellurium, 126 
Graphite, 69, 227 
Gravitation methods of separa- 
tion, 60 

Gravity concentration, 63 
Gravity, Specific, 60 
Greasy lustre, 42 
Grey Copper ore, 139 
Grossularia, 234 
Guano, 225, 225 

Gypsum, 75, 109, 207, 214, 215, 
226, 227 

HABIT of Crystals, 4 
Hackly, Fracture, 38 
Haematite, 73, 168, 169, 172 

Itabarite ores, 174 

Kidney ore, 172 

Micaceous, 172 

Veinstone, 207 
Hair-like habit, 4 
Halite, 220 
Halogens, 64 
Haloid, compounds of silver, 

Hardness, 35, 39 

Average, scale of, 40 
Curve of, 39 
Relative scale of i 
of Gems, 229 

Haiiyne, 78, 97 

Heart-shaped twins, 32 

Heat- 
Conductivity of,f55 
Specific, 55, 



Heavy Metals, Sulphides, Sur- 
face Energ} 7 , 59 
Heavy solutions, 61 
Heavy spar, 216 
Hemihedrism, u, 29 
Hemimorphism, 30, 31 
Hemimorphite, 79, 159 
Hemi-pyramid term, 22, 88, 104 
Heulandite, 81, 113 
Hexagonal system, 10, 15, 44 
Hexakis-octahedron, u, 14 
Holohedral, 22, 31 n. 
Hornblende, 79, 101 n., 103, 204 

Actinolite, 105 

Fibrous, in 

Veinstone, 207 
Horn Silver, 71, 143, 149 
Horseflesh ore, 138 
Hiibnerite, 200 

Hydatogenetic ore-deposits, 150 
Hyalite, 240 
Hydrates, 65 
Hypersthene, 78 

ICEI/AND Spar, 210 

Cleavage, 35 

! Icosi- tetrahedron, n, 13 
Idocrase, 81, 116 
Ilmenite, 73, 178, 194, 198 
Incrustation, 68 
Index of Refraction, 44 

Determination of, 53 n, 
Intercepts, Fundamental, mul- 
tiples of, 26-8 
Rational, Law of, 20 
Iridescence, 43, 241 
Iridium, 121 
Iridosmine, 68 
Irisation, 43 
Iron (see also Haematite), 64, 

68, 168 
Output, 169 
Sources, 168 et seq. 
Arsenosulphide of, as Vein- 
stones, 207 

Carbonate of, as Veinstone, 207 
Native, 169 
Ores, 168 

Brown, 175 
Chrome, 178 
Ochreous, 175 



INDEX 



247 



Iron (continued] : 
Ores (continued) : 
Oolitic, 175, 176 
Pisolitic, 175 
Reserves of, 169-70 
Spathic, 177 
Titaniferous, 178, 198 
Oxides, 65, 168 

as Veinstone, 207 
Pig-, 168 
Pyrites, 129, 135, 168, 179 

Forms, 30 
Salts, 66 
Sand, 119 
Sulphides of, as Vein- 

stones, 207 
Tungstate of, 200 
Ironstone, Blackband, 177 

Clay, 177 
Isomorphism, 67 
Isomorphous Mixed Silicates. 79 
Isotropic, 43, 44 
Itabarite ores, 174 

JACINTH, 237 
Jade, 38 
Jargoon, 237 
Jasper, 85, 206, 240 



, 75, 218 
Kaolin, 91, 95, 109 
Kaolinite, 75, 81, 83, in 
Kaolinization, 90, in 
Kerargyrite, 71, 143, 149 
Kermesite, 188 
Kidney ore, 172 
Klein's solution, 61 
Krennerite, 122, 126 
Kyanite, 77, 114 

LABRADOR Spar, 241 

Labradorite, 87 

Lamellar Twinning, 33 

Lateritic deposits, 184 

Laumontite, 112, 113 

Laws of 

Centra-symmetry, 24 
Constant Angle, 8 
Rational intercepts, 20 
Refraction, 44 
Symmetry, 8 

Lead, 64, 68, 149, 150 



Leafy form, 4 

Lepidolite, 100 

Leptochlorites, 109 

Leucite, 79, 96 

Light, Phenomena relating to, 

35, 4i, 54 
Lime, 1 66 

Phosphates of, 224 
Lime-felspar, 79, 86, 94 
Lime-mica, 80, 100 
Limestone, 65 

Argillaceous, gems in, 232 

Contact-altered, 98 

Crystalline, gems in, 233 

Graphite in, 227 

Magnesian, 214 

Matrix of Rubies from, 232 

Metamorphic, 106 
Limonite, 73, 168, 175, 207 
Linnseite, 161, 165 
Lithium-mica, 100 
Loadstone, 172 

MACRO-AXIS, 19 

Macro-pinacoid, 21 

Magnesia, 66 

Magiiesian-Iron mica, 80 

Magnesian mica, 80 

Magnesite, 74, 209, 212 

Magnetic Iron -ore, see Mag- 
netite 

Magnetic Permeability, 57 
Properties, 55, 57 

Magnetite, 72, 168, 171, 194, 207 

Malachite, 74, 129, 140 

Malay States and Dutch East 
Indies, Tin - mining 
Districts in, Map, 196 

Malleability, 39 

Mammillated form, 4 

Manganese, 64, 183 

Manganese-garnet (Spessartite), 

183 

Manganese-olivine (Tephroite), 

183 

Manganese - pyroxene (Rhodo- 
nite), 183 

Manganite, 184, i8g 

Manebach type of Twinning, 90 

Marcasite, 70, 128, 168, 182, 207 

Margarite, 80, 100 



248 



INDEX 



Marialite, 79 
Massive Structure, 5 
Meionite, 78, So 
Melaconite, 72, 129, 132 
Melanite, 234, 235 
Melilite, 97 
Menaccanite, 178 
Mendozite, 218 
Mercury, 62, 64, 68, 127 et seq. 

Sulphide, see Cinnabar 
M elaborates, 65 
Metacinnabarite, 127 
Metalli^ Lustre, 42 
Metasilicates, 65, 78-9 
Metasomatic deposits, 150, 154 
Meteorites, 169 
Mica, 66, 83, 98, 204, 207 

Percussion figure, 38, 100-1 
Microcline, 86, 91-2 
Microperthite, 94 
Millerian system of Symbols, 24 
Millerite, 70, 161, 193 
Mineral Oil, 228 

Pitch, 228 

Resin, 228 

Wax, 228 

'Minettes,' 175, 176 
Minium, 150 
Mirabilite, 75, 217 
Mispickel, 70, 120, 128, 144, 165, 

188, 189, 191, 207 
Mizzonite, 98 
Molecular Cohesion, 35 
Molybdates, 77 
Molybdenite, 70, 199 
Molybdenum, Ores of, 199 
Monazite, 73, 194, 203 
Monoclinic System, n, 22, 44 
Monosulphide group, 69 
Monoxide group, 72 

of Lead, see Galena 
Monticellite, 77, 106 
Moonstone, 241 
Morphological Characters, 3 
Mossy habit 4 
Muscovite, 80, 91, 99, 100 

NATIVE ELEMENTS, List of, 

68-9 
Native Metals and Alloys, List 

of, 68-9 



Natrolite, 81, 112 

Natron, 74, 209, 213 

Naumanrfs Symbols, 24 

Needle-shaped habit, 4 

Nepheline, 77, 95, 112 

Nephrite, 106 

Niccolite, 69, 144, 161, 162 

Nickel, 64, 161 

Nickel-bloom, 75, 164 

Nickelmolybdenum, 199 

Nitrate of soda, 2 1 9 

Nitrates, 65, 76, 219 

Nitratine, 76, 219 

Nitre, 58, 76, 219 

Nodular form, 4 

Norway, Copper deposits in, 136 

Nosean, 78, 97 

Noumeite, 164 

OBLATE SPHEROID, 49 
Ochres, 175 

Octahedral cleavage, 37 
Octahedron, u, 12, 29 

Pyramidal, 14 

Six- faced, 14 
Odour, 58 
Oil, Mineral, 228 
Oligoclase Felspars, 87, 94 
Olivines, 77, 83, 106 
Onyx, 240 

Opacity of Gems, 229 
Opal, 73, 240 
Opalescence, 43 
Optic axis, 48 

Dispersion of, 52 

Plane of, 5 1 

Optical properties of Crystals 34 
Ordinary Ray, the, 48, 49 
Ores, \\%etseq. 

Deposits 

Detrital or Placer, I 19 
formed in situ, 118 

Primary, 119 

Secondary, oxidized, 119 
Orpiment, 70, 188, 192 
Orthite, 80, 204 
Ortho-axis, 22, 53 
Orthoclase, 79, 86, 88, 94, 207 
Orthochlorites, 109 
Orthodome, 88 
Orthopinacoid, 88, 104 



INDEX 



249 



Orthorhoinbic System, 19 

Orthosilicates, 65, 77 

Oscillation of faces, 5 

Osmium, 121 

Osmium-indium, 121 

Ottrelite, 81 

Oxides, 64, 118 

Oxidized ores, Copper from, 129 

Oxy-acids, 64 

Oxychlorides, 72 

Oxygen, 64 

Compounds of Metals with, 72 
Salts of the Metals, 74 

Oxy-salts, 64 

Ozokerite, 228 

PALLADIUM, 121 

Paragon ite, 80, 100 

Parameters, 20 

Parametral form, 20 

Paramorphs, 68 

Pearceite, 71, 143 

Pearlspar, 211 

Pearly, lustre, 42 

Pellucidity of Gems, 229 

Penninite, no 

Pentagonal Dodecahedron, 29 

Pentlandite, 161 

Percussion figures, 38 

Peridot, 105, 107, 240 

Perth ite, 94 

Petroleum, 228 

Petzite, 122, 127 

Phillipsite, 112 

Phlogopite, 81, 100, 101 

Phosphates, 65, 75-6, 224 

Phosphorite, 225 

Physical Properties of Minerals, 

34 

Piedmontite, 80 
Pinacoidal cleavage, 37 
Pinacoids, 19, 21 
Finite, 116 
Pitchblende, 201, 202 
Pitch, Mineral, 228 
Placer deposits, 119 
Gold, 124 

Sands, Platinum from, 121 
Plagioclase Felspars, 86, 94; see 

also under Albite and 

Anorthite 



Plagioclase Felspars (continued) : 

Decomposition products, 95 

Twinning in, 33 

Formulae, 86, 87 

Isomorphous series, 94 
Planes 

Basal, 16, 88 

of Composition, 32 

Minimum cohesion, 36 ' 

Optic axes, 51 

Parting, 38 

Symmetry, 6-7, 9 

Twinning, 31 
Platinum, 64, 68, 120 

Native, 121 

Spongy, surface energy of, 59 
Platy habit, 4 
Plasma, 240 
Pleochroism, 54 

of gems, 229, 230 
Plumbago, 228 
Pneumatolytic veins, 194, 207 
Polybasite, 71, 143, 148 
Positive and Negative Crystals, 

49 andn. 

Polymorphism, 66 
Poly synthetic Twinning, 5, -3 
Polarization and Double Refrac- 

twn, 47 

Potash, Nitrate of, 219 
Potash-alum, 75, 218 
Potash-felspar, 86, 88, 91 
Potash-mica, 80, 99 
Praseolite, 116 
Prehnite, 80, 207 
Principal Axis, 15 
Prisms, 16, 17, 88, 104 

Dihexagonal, 17 
Prismatic cleavage, 37 

Habit, 4 
Prochlorite, no 
Prolate Spheroid, 49 
Proto-prism, 16 
Proto-pyramid, 16 
Proustite, 71, 143, 147 
Psilomelane, 184, 186 
Pseudo -hexagonal symmetry, 33 
Pseudomorphism, 67 
Pseudomorphs, 67, 68, 112 
Psilomelane, 73 
Purple ore, 138 



250 



INDEX 



Pyramidal cleavage, 37 
Pyramids 

Dihexagonal, 17 

of the First and Second 

orders, 16 

Pyrargyrite,7i, 143, 147, 188 
Pyrites, 70, 128 

Arsenical, 191 

Auriferous, 123, 125. 18 

in Chalk, 182 

Cockscomb, 182 

Copper, 120, 134 

Cupriferous, 129 

Iron, 129, 135, 168, 179 

Magnetic, 182 

Tin, 197 

Pyritic Ores, 123 
Pyroelectricity, 31, 57 
Pyrolusite, 73 184 
Pyromorphite, 76, 154, 193 
Pyrope, 78, 234, 235 
Pyroxenes, 83, 101 and ., 
105, 204 

Rhombic, 105 
Pyrrhotite, 70, 168, 82 

Nickeliferous, 161, 162, 183 

QUARTZ, 63, 73, 83, 84, 95, 

204 

Milky, 85 
Rose, 41, 85, 241 
Sceptre, 85 
Smoky, 85, 241 

euartz group, 84-6 
uartz Veins, Auriferous, 122 
Quartz Veinstone, 86, 207 

RADIUM, source of, 201 
Rational Intercepts, Law of, 

20 
Ray, Extraordinary, 48 

Ordinary, 48 
Realgar, 70, 128, 188, 192 
Retger's solution, 61 
Red Copper ore, 131 
Red ochre, 173 
Redruthite, 133 
Reflection, Total, 46 
Refraction, 43 

Double, and Polarization, 47 



Refraction (continued) : 
Index of, 44, 45, 55 . 
Index of, determination of, 

53 

In Relation to Critical 
Angle, 46 

Law of, 44 

Refractive index of Gems, 229 
Regular System, n 

Ellipsoid in, 34-5 

Isotropic, 44 

Symbols, 26 
Reniform, 4 
Replacement, 67 
Resin, Mineral, 228 
Rhodium, 121 
Rhodochrosite, 74, 184, 187 
Rhodonite, 78, 183 
Rhombic dodecahedral Cleavage, 

Rhombic Dodecahedron, n, 12 

Pyroxenes, 105 
Rhombic System, n, 19, 44 
Ellipsoid in, 35 
Symbols, 27 
| Rhombohedral cleavage, 37 

System, n 

i Rhombohedt on, 27, 30 
! Rock-crystal, 85 
! Rock-forming Minerals, 83 
Rock salt, 71, 109, 214, 220 
Rubellan, 100 
Ruby, 229 
Ruby Silver ores 
Dark, 147 
Light, 147 
Colour, 44 
Rutile, 67, 73,115, 198 

SAI, AMMONIAC, 71, 220, 222 
Salt, see also Epsom, Glauber, 
& Rock-Salt 

Stassfurt deposits, 215 
Salt Lake, Utah, 217 
Saltpans, 221 
Saltpetre, 76, 219 

Chili, 219 
Salts, 227 
Sand, Cassiterite as, 194 

Menaccanite, 179 
Sanidine, 88, 90, 91 



INDEX 



251 



Sapphire, 229, 231, 232 

Asterism, 43 
Sard, 240 
Sardonyx, 240 
Satin Spar, 215 
Saussurite, 95 
Scalar Properties oj Crystals, 

34. 35 
Scale no hedron, 30 

Scapolite group, 79, 98 
Scheelite, 76, 194, 200, 201 
Schiller lustre, 43 
Schists, 95, 106, 204 

Corundum in, 232 

Crystalline, 98. 114, 1 15 et alibi 

Gems in, 238 
Scolecite, 112 
Secondary Minerals, 83, 107 

et seq. 

Sectility, 39 
Selenite, 215 
Senarmontite, 188 
Sericite, 201 
Serpentine, 80, 83, 107, no, 164, 

178, 212 

Scsquioxide group, 73 
Sesquisulphide group, 70 
Shining lustre, 43 
Siderite, 74, 177 
Siennas, 175 
Silica, 65, 240 
Silicates, Isomorphous, Mixed, 

77 79 

Metasilicates, 78-9 
Ortho silicates, ib. 
Silky lustre, 42 
Sillimanite, 114, 115 
Silver, 62, 64, 68, 142 
Chloride of, 149 
Chlorobromide of, 149 
Haloid compounds of, 143 
Iodide of, 143 
Native, 144, 145 
Ores, 142 
Sulphide of, 146 
and Gold, Tellurides of, 

126, 127 

Silver-lead and Copper, Sulph- 
antimonites & Sulph- 
arsenates of (table], 
H3 



Smaltite, 69, 144, 166 

Smithsonite, 154 

Soapstone, in 

Soda, 66 

Soda-alum, 218 

Soda-felspar, 79, 86, 88, 92 

Sodalite, 78, 96 

Soda-mica, 80, 100 

vSoda-nitre, 76, 219 

Soda Salts, 213 

Solids, Surface Energy in, 58, 

59 

Sonstadfs solution, 61 

Spartalite, 72 

Spathic ores, 30, 168, ,77 

Specific gravity, 60 
Heat, 55 

Spelter, 154 
Output of, 155 

Sperrylite, 69, 120, 122 

Spessartine, see Spessartite 

Spessartite, 78, 183, 234 

vSphaerosiderite, 177 

Sphalerite, 156 

Sphene, 77, 238 

Spinel, 232 

Splendent lustre, 42 

Splintery Fracture, 38 

Spodumene, 79 

Sprudelstein, 211 

Stalactitic habit, 4 

Stannite, 193, 197 

Stassfurt deposits, 215; see 
Fig. 104, 223 

Staurolite, 80, 115 

Steatite, in 

Stephanite, 71, 140, 143 

Stibnite, 128, 188, 190 

Stilbite, 8 1 

Stilpnosiderite, 175 

Streak, 41 

Sttiated faces, 5 

Strontianite, 74, 209, 213 

Stromeyerite, 69, 143 

Structure, 4, 5 

Subconchoidal Fracture, 38 

Submetallic lustre, 42 

Sulph-acids, 65 

Sulphantimonites and Sulph- 
ar^eniates of Silver 
and Copper, 143 



252 



INDEX 



Sulphates, 65, 74-5, 214 et seq. 

Hydrated, 75 
Sulphides, 64 
of Copper, 60 
of Lead, see Galena 
Primary, of Metals, 120 
Rhombic, of 
Copper, 133, 143 
Silver, 143 
Sidpho-salts, 64 
Sulphur, 64, 69, 208, 227 

Compounds, 58 
Sulphur Salts, 64, 70-1 
Sulphuretted Hydrogen, Odour, 

58 

Sulphurous Acid, Odour, 58 
Surface Energy (Tension] in 
Liquids, 59 
Solids, 58, 59 
Sylvanite, 70, 122, 126 
Sylvite or Sylvine. 71, 220, 222 
Symbols, 24 

Millerian System, 24 
Naumann's, 24 
Symmetry of Crystals, 5 

Angles of, 7 
Axes, 6 

Binary, 16 
Senary, 16 
Law of, 8 

Modifications in, 28 
Physical properties in relation 

to, 34 
Plane of, 6, 7, 9 

Parallelism of, 7 
Pseudo-hexagonal, 33 

TABULAR Habit of Crystals, 4 
Talc, 8 1, 83, 91, 109, in, 207 
Taste, 58 

Telluride of Gold, 122, 126 
Tellurium, Graphic, 126 
Tenacity, 35, 38 
Tennantite, 71, 139 
Tenorite, 121, 132 
Tension, Surface, 59 
Tephroite, 77, 106, 183 
Tetragonal System, 10, 17, 44 

Symbols, 27 
Tetrahedrite, 29, 71, 139, 143 

Antimonial, 188 



Tetrahedron, 29 

Tetrakis - hexahedron, 11, 14, 

Thermal Properties, 53 
Tile ore, 131 
Tin, 64, 193 

Tenacity, 39 
Uses, 194 

Pyrites, 197 

Stream, 119, 195 
Tinkal, 225 
Tin ores, 194 
Tinstone, 193, 194 
; Titaniferous iron ore, 178, 198 
Titanite, 238 
Titano-silicate, 77 
Topaz, 77, 117, 194, 207, 235 

Oriental, 231 
Torbernite, 76, 201, 202 
Total Reflection, 46 
Tourmaline, 82, 115, 194, 236 
j Translucency of Gems, 229 
Tremolite, 79, 106 
Triakis-octahedron, n, 13 
Triclinic System, 1 1 , 23, 44 

Axes of Elasticity, 54 

Ellipsoids in, 35 
Trigonal System, 1 1 
Trimotphism, 67 
Trona, 209, 214 
Tschermigite, 218 
Tungstates, 76 
Tungsten, 200 
Turgite, 175 
Turquoise, 76, 238 
Twinning, 31 

Forms produced by, 323 
Types of 

Baveno, 88, 89 
Carlsbad, 89 
Manebach, 90 

Plane of, 32 

Polysynthetic, 5 

UMBERS, 175 

Uneven Fracture, 38 

Uniaxial Crystals, 47 

Uranite, 201, 202 

Uranium (see also Autunite and 

Torbernite, 201 
Uvarovite, 78, 234, 235 



INDEX 



253 



VAXA DATES, 75-6 
Vanadinite, 76, 193 
Vanadium, 193 
Vector properties, 34 
Veinstone or Gangue Minerals, 

91, 206 

Vesuvianite, 81, 116 
Vicinal faces, 5 
Vitreous lustre, 42 

WAD, 184, 186, 207 
Warmth, Effect of on Sym- 
metry, 34 
'Water of Constitution,' 65, 79 

of Crystallization,' 65, 81 
Wave Surface, 49 
Wavellite, 76 
Wax, Mineral, 228 
Weathering, 90, 120, 194 et alibi 
Wernerite, 98 
Wetting angle, 60 
White Arsenic, 189 
Willemite, 77, 154, 158 
Wiry habit, 4 
Witherite, 74, 209, 212 



Wolfram, 77, 194 
Wolframite, 200, 201 
Wollastonite, 78, 103 ;/. 
Wood-tin, 195 
Wulfenite, 77, 199, 200 

XANTHOSIDERITE, 175 
Xenotime, 75, 203 

YTTRIUM, 204 

ZAFFER, 165 
Zaratite, 161, 164 
Zeolite Group 81, 83, 112 
Zinc, 64, 119, 154 

Deposits, forms of, 154-5 
Zinc-blende, 156 
Zinc Spar, 158 
Zincite, 72, 154, 159, 160 
Zinnwaldite, 100, 101 
Zircon, 73, 115, 203, 237 
Zirconium, 203 
Zoisite. 86, 95 
Zones, 9 



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