THE ISOMORPHISM AND THERMAL PROPERTIES
OF THE FELDSPARS.
PART I— THERMAL STUDY,
ARTHUR L. DAY and E. T. ALLEN.
PART II— OPTICAL STUDY, - J. P. IDDINGS.
WITH AN INTRODUCTION BY
GEORGE F. BECKER.
Washington, D. C. :
Published by the Carnegie Institution of Washington.
1905.
CARNEGIE INSTITUTION^OF WASHINGTON
Publication No. 31
PRESS OF GIBSON BROS.
WASHINGTON, D. C.
INTRODUCTION.
By GEORGE F. BECKER.
INTRODUCTION.
The prime duty of a geological survey is to make a geological map
of the country. Those who are unfamiliar with the duties of a geolo-
gist are apt to suppose that no great amount of knowledge is needed
to produce a satisfactory map of this kind. Those who have tried it
know better. The field geologist is at once confronted by the theo-
retical aspects of his science in such a manner that he is compelled to
adopt at least tentative views. He must decide what is to be mapped,
and this decision implies that he knows or assumes relations between
the various members of the series with which he has to do. All geolo-
gists worthy of the name are continually and painfully aware that
they deal largely in uncertainties or matters of opinion, and thev
can not fairly be reproached with the insufficiency of the grounds
which they sometimes have to show for the views they adopt, unless
they lay themselves open to the accusation of neglecting results
established by theory and experiment.
Geology is not a science, but the application of the sciences to the
elucidation of the history of the earth. Its best developed and
oldest branch is zoological geology or paleontology, and next in order
of development, though substantially the latest in chronological order,
is mineralogical geology as represented by petrographv. The rapid
advance in the description of rocks is due, as everyone knows, to the
introduction of the microscope and of exact optical methods in the
determinations of minerals. Less advance has been made in the wider
subject called petrology or lithology, as well as in orogeny, vulcanism,
and ore deposits. The resources of the terrestrial laboratory so far
transcend those which can be equipped by man that vast groups of
geological phenomena still await even approximate explanation.
Observations on the lithosphere alone will not suffice to elucidate
these dark regions. As Messrs. Day and Allen very properly insist,
"geological field research is a study of natural end-phenomena, of
completed reactions, but with a very imperfect record of the earlier
intermediate steps in the earth-making processes." In fact, the un-
known quantities outnumber the equations which field observation
puts at our disposal. In their present state of development the
sciences of physics and chemistry can aid the geologist only to a mod-
erate extent. We do not know in most cases whether the laws of
5
41 •
6 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
physics as established under ordinary conditions retain their validity
at temperatures exceeding iooo° C, while of the chemical behavior
of substances at these temperatures chemists can tell us little more
than that affinities are radically different from those observed at ioo°.
Similarly elasticians can discuss the small strains in a building or a
bridge with some approach to completeness, but they do not even
make the attempt to deal with deformations which are sensible to
the eye and which are almost universal in geological exposures.
It was in recognition of the need for researches in physics which would
throw light on geological problems that Dr. Carl Barus was ap-
pointed physicist on my staff in the United States Geological Survey
as far back as 1880, and that a physical laboratory was estab-
lished under that Survey in 1882. This was discontinued in 1S92,
not because its importance was underestimated by the Director,
but on account of a failure of appropriations. The laboratory was
reestablished in 1901 because it was felt that without the aid to be
derived from physical determinations the efficiency of the Survey
must suffer. There was nothing novel in the appreciation thus dis-
played of the importance of physics to geology; indeed, several great
geophysical problems have been recognized by natural philosophers
for more than a century ; and their difficulty, not their unimportance,
has stood in the way of experimental investigation.
The field geologist meets with phenomena in all the ruggedness of
their utmost complexity, and he is sometimes tempted to face and
make an assault upon the situation as he finds it. A little consider-
ation shows that in such circumstances a frontal attack must lead to
disaster. The outposts must be overcome one by one. We must
patiently begin with the simplest problems that can be devised and,
aided by the most perfect appliances known, study them exhaustively
before proceeding to more difficult and complex cases.
In a plan submitted to the Director when the new physical labora-
tory of the Survey was first contemplated, I laid especial stress upon
the study of isomorphism and eutexia. These subjects, with the
determinations of thermal constants which they imply, have occu-
pied the attention of the physical laboratory during the greater part
of the time since its reestablishment, and will continue to take the first
place in the researches there undertaken.
It would appear that the relations between liquids must be reduci-
ble to very general groups. Liquids must either be miscible or im-
miscible, and miscible liquids must exhibit either isomorphic proper-
ties or eutectic ones. It is possible that magmas are in some cases
INTRODUCTION. 7
immiscible ; thus zircons separate out in the process of consolidation
so early, so completely, and in such minute crystals as to suggest
immiscibility. On the other hand, Alexejew reached the conclusion
that in all cases where solutions do not react upon one another chemi-
cally, they become miscible above a certain temperature. Again, all
researches on the genesis of minerals from fused magmas show that,
as a rule, the crystals are precipitated from undercooled glasses or
from miscible liquids. With some possible exceptions, therefore,
which, so far as is yet known, are unimportant, the investigation of
liquid magmas reduces to the study of isomorphous mixtures (in
which the physical properties are continuous functions of the compo-
sition) and of eutectic ones.
A main aim of lithological studies for many years has been to
classify rocks. It is difficult to overestimate the importance to the
whole history of the earth of a natural and rational petrological tax-
onomy. The earliest classifications were largely chemical. After
the introduction of the microscope they became chiefly mineralogical,
but with Lagorio's famous paper on the nature of the glass base and
the processes of crystallization in eruptive magmas, 1887, chemical
considerations again become predominant. In my opinion, classifi-
cation of rocks on a basis of composition alone can never be satis-
factory or final. It is possible, of course, to classify analyses; but
rocks are at least for the most part very variable mixtures, without
analogy to definite chemical compounds, and this method thus fails to
cover the ground or to reveal the relations of parts to the whole.
On the other hand, physical chemistry seems to me to open the
road to a classification which must be helpful and may possibly be
final. Among the ten or a dozen important rock-forming minerals*
* In Bulletin 228 of the United States Geological Survey Professor Clarke has
given an estimate, based on nearly 700 analyses, of the approximate relative
abundance of the more important minerals found in igneous rocks and aggregat-
ing 94.2 per cent. Adding the more important of the minerals which eluded
separate computation in one sum, this table takes the following form, which is
very suggestive with reference to important silicate solutions:
Per cent.
Feldspar 59-5
Hornblende and pyroxene 16.8
Quartz 12.0
Biotite 3.8
Titanium minerals 1.5
Apatite 0.6
Magnetite, olivine, leucite, nepheline, etc 5.8
100.
LIBRARY sqJ
8 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
there can be only a limited number of isomorphous series and a limited
number of eutectic combinations. The most important isomorphous
series forms the subject of this publication. It is well known to be
probable also that the amphiboles, the pyroxenes, and the micas each
constitute isomorphous groups. No other isomorphous groups ap-
pear to have much lithological importance. In general, it is manifest
that isomorphism is to be expected only within groups of closely
allied compounds, and it is even a matter of surprise that the ortho-
silicate anorthite and the polysilicate albite should exhibit complete
isomorphism of the simplest type, as Messrs. Day and Allen have
shown that they do. It seems hardly possible, therefore, that a satis-
factory classification of rocks can be based on the study of isomor-
phous series ; indeed the mineralogical rock definitions of twenty-five
years ago were little else than such a classification, which has been
rejected as inadequate.
No serious attempt has yet been made to group rocks on eutectic
principles, one very sufficient reason being our ignorance of eutexia in
magmas. Professor Lagorio refers briefly to eutexia, but regards the
important solvent in magmas as a silicate of the alkalies, the glass
least subject to devitrification.* Mr. Teall, in 1888, discussed eutexia
very lucidly and showed its importance in the physics of rocks, f but
he did not propose employing it as a basis of classification. In 1901
I briefly set forth some of the advantages of such a system. %
The applicability of eutexia to rock classification depends upon the
fact that it makes the systematic discussion of magmatic mixtures
possible. Inasmuch as the subject-matter of lithology consists of
mixtures, their classification must be carried out in terms of definite
or standard mixtures, while the only mixtures possessing appropriate
distinguishing properties are the eutectics. Thus in dealing with
magmas or other heteromorphous miscible liquids the eutectics seem
to afford not only the best but the only natural and rational standards
of reference. With any eutectic as a basis, a series of magmas may
be prepared, each differing from the eutectic by containing an excess
of one or more constituents. Thus if abc represents an eutectic of
three substances, a mixture composed of a, mb, and nc may be re-
* There is no fundamental difference between fluid solvents and solutes, and no
objection to regarding the alkaline glass as the solvent if found on other grounds
expedient.
f Brit. Petrography, 1888, p. 394.
% Report on the geology of the Philippine Islands. Twenty-first Ann. Rep.
U. S. Geol. Surv., Pt. Ill, 1901, p. 519.
INTRODUCTION. 9
garded as the eutectic plus (m — i) b plus (n — i) c. In some cases
at any rate the ground mass of a rock (as Mr. Teall pointed out) repre-
sents an eutectic. This is probably not true in general but, if it
were, the scheme proposed would be to group together in one genus
all the rocks which have the same ground mass and to regard the
phenocrysts as minor or specific characteristics. The size of crystals
is no index of the rapidity of their formation, and Messrs. Day and
Allen have shown that anorthites of the size of very large pheno-
crysts may form in a few minutes, while in more viscous magmas
small feldspar crystals may form with extreme slowness. Hence
great care is requisite in deciding microscopically the question which
crystals were the last to form.
It is worthy of note that the geological behavior of an intrusive
or effusive rock is conditioned largely by the character of the eutectic.
So long as this remains liquid the phenocrysts are, mechanically speak-
ing, mere flotsam and jetsam in the stream. The character of the
eutectic must decide whether a lava pours down a gentle declivity as
does a basalt, or piles up about the orifice like a rhyolite. Now, if it be
not essential to consider such geologically important properties in the
classification of rocks, it is at all events desirable to do so. Such
properties must sooner or later be dealt with methodically by geolo-
gists, and a thoroughly rational classification of rocks will correlate
physical and chemical properties.
These last paragraphs deal with plans rather than achievements,
and have been written chiefly to emphasize the importance of the
work done by Messrs. Day and Allen as one step in a broader scheme.
Evidently every step of the larger plan involves accurate studies of
the melting points and thermal properties of the rock-forming min-
erals, and first of all that most important group, the lime-soda feld-
spars, which make up approximately one-half of the lithosphere. In
the meantime, since my proposal to use eutectics as a basis of rock
classification was printed, some valuable work has been done on
eutectics, chiefly by J. H. L. Vogt.*
Only the first step has been taken in this investigation — the study of
the triclinic feldspars in dry fusion. It has been attended with great
difficulties, many of them only touched upon in the paper which fol-
lows, but of which I have been cognizant in detail. Except for Dr.
Day's resourcefulness and experimental skill, success would not have
been achieved, but a road has now been broken out in this ultra-
* Die Silikatschmelzlosungen. Christiania, 1903. Mr. Vogt printed a pre-
liminary communication early in 1902.
IO ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
tropical jungle which will almost certainly lead to further successful
explorations.
Meantime the results reached are of great importance. The
melting points of the triclinic feldspars have been determined with
an accuracy never before attained in determinations at such tem-
peratures. Any future correction of them must be of very trifling
amount. These points, considered with reference to composition,
and the very fine series of specific gravity determinations on chemi-
cally pure feldspars, seem to settle beyond question the isomorphism
of the plagioclases. The first cogent arguments for this isomorphism
were given by Sartorius von Walthershausen in 1853, but the more
thorough investigation of Tscherrnak in 1864 has properly connected
the theory with his name. Nevertheless, some of the ablest investi-
gators have been unconvinced that the isomorhpism was complete,
and I confess to surprise that the proof is so irrefragable as Messrs.
Day and Allen have made it.
The study of the feldspars and sodium tetraborate (dehydrated
borax) have confirmed results of Professor Lagorio, which are thus
summarized by Mr. Teall (op. cit., p. 397) :
Silicate solutions differ from aqueous solutions in the readiness with which they
form amorphous glass when cooled rapidly. This appears to be connected with
the fact that they may be readily overcooled, and that when in this state they are
highly viscous, so that a rapid approach of the molecules is prevented. The melt-
ing point of glass is lower than that of the same substance in a crystalline condi-
tion. A glass, therefore, results from the solidification of an overcooled liquid.
Messrs. Day and Allen show that crystallization can be brought
about at very different degrees of overcooling or at very different
temperatures, so that the solidifying temperature of crystals out of
undercooled liquids is not a physical constant, while solidification to
the amorphous state almost or quite eludes determination by the
means found adequate to fix the melting points of crystals.*
* The properties of amorphous substances are very perplexing. It is well known
that some physicists class glasses at any temperatures as liquids, and there is no
question that it is hard to draw the line between them and liquids. On the other
hand, Mr. Spring has recently shown that mere deformation of crystalline metals
at ordinary temperatures changes their densities and electrical potentials, so that
mere derangement of crystalline particles, without any absorption of energy com-
parable with that accompanying true fusion, suffices to impart to lead, silver, bis-
muth, etc., properties analogous to those of glasses. The whole subject demands
fuller investigation which, to be successful, must harmonize thermal, electrical, and
mechanical phenomena.
INTRODUCTION. 1 1
Mr. Roozeboom's discussion of isomorphous mixtures seems
admirably verified by this investigation of the feldspars. When
considered in connection with the high viscosity of the materials, it
also explains the fact that the curve of melting points closely follows
Kuster's rule. It woidd seem, therefore, that not only concentrated
solutions but isomorphous ones form exceptions to the accepted laws
of dilute aqueous solutions. Such isomorphous solutions as those of
the feldspars here dealt with could in fact hardly be considered as
dilute, and some of them (as for instance AbiAiii) must be very
concentrated solutions, whichever of the components is considered as
solvent.
The specific volumes of the feldspars seem to bear a relation to the
composition so nearly linear that the differences may be ascribed
to unavoidable errors in synthesis and analysis. It should not be
forgotten, however, that the specific volumes are determined at
something like iooo° below the temperature of crystallization, and
that, since the coefficients of contraction of Ab and An doubtless
differ to some extent, variations in density as determined at 250
might be due not to a lack of isomorphism, but to the difference in
contraction of the two components.
The artificial feldspars prepared at the cost of great labor are pure,
while natural crystals are not so. Hence lithologists, in making
separations by heavy solutions, should substitute the densities here
found for those hitherto employed. The changes are not great, but
they are sufficient in some cases to affect conclusions.
A very noteworthy result of the investigation is the apparent super-
heating of the albitic feldspars. It is pointed out by Messrs. Day and
Allen that this may be only apparent and due to the extreme viscosity
of the melt. In fact, the separation of molecules in melting and their
deorientation must be successive processes, so that in any fusion, if
the operation could be instantaneously arrested, a layer of molecules
would be found separated from the solid mass but not yet deoriented.
Such material would differ from a liquid crystal by not being in a
condition of stable equilibrium.
Prof. J. P. Iddings kindly undertook the detailed examination of
the slides made from the feldspar preparations. He shows in his
report that to one per cent, or less, the feldspars correspond optically
to the mixtures prepared. A closer correspondence could not be
hoped for in materials so viscous that diffusion afforded scarcely any
aid in attaining homogeneity. He has discussed many interesting
features of the crystallization of the feldspars, most of them familiar
12 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
in effusive rocks. Spherulitic structure in particular is brilliantly
illustrated in the slides, and in the nature of the microlites of artificial
feldspar he has found nothing to suggest essential differences between
the experiments and natural processes the results of which constantly
come under the observation of lithologists. Professor Iddings has
also examined the refractive indices of the artificial feldspars and has
found them accordant with isomorphism.
I can not conclude this review without a mention of the tireless
energy and watchfulness which Messrs. Day and Allen have exercised,
and of which I have been a daily witness, in a most laborious task
attended by so many difficulties that it sometimes seemed almost
hopeless.
The vastness of the field open to geophysical research is partially
indicated in the preceding pages, and I have recently endeavored to
enumerate somewhat more fully the pressing problems of geophysics.*
The Government of the United States has of late years pursued the
enlightened policy of making yearly grants for chemical and physical
researches under the Geological Survey, but the appropriations are
inadequate for the more difficult and costly studies in this field.
This is not strange, for though geophysics has already proved techni-
cally valuable, so that mining engineers display a hearty interest in it,
and although it will assuredly lead to the solution of certain well-
defined economic problems of the first importance, its fundamental
researches are somewhat remote from industry, and large public
appropriations are hardly to be hoped for in the near future.
The Trustees of the Carnegie Institution of Washington, recogniz-
ing these facts, have supplemented the Congressional appropriations
by grants to Dr. Day and to myself, and the work described in this
paper, though begun under the Survey, was completed at the expense
of the Carnegie Institution. In these circumstances the Director of
the Survey consented that it should be offered to the Institution for
publication with this due recognition of the cooperation of the Survey.
The Institution has accepted it as its first contribution to geophysics
and defrays the cost of publication. It is surely a matter of con-
gratulation that the Government and a private institution should
cooperate in the advancement of knowledge. Such an alliance
brightens the prospects of Science.
* International Scientific Congress of St. Louis, 1904, printed in Science, Octo-
ber 28, 1904.
Part I.
The Isomorphism and Thermal Properties
of the Feldspars.
BY
ARTHUR L. DAY and E. T. ALLEN.
i LISSR^
THE ISOMORPHISM AND THERMAL PROPERTIES
OF THE FELDSPARS.*
The investigation here recorded is the first chapter in a rather com-
prehensive plan for the study of the rock-forming minerals at the
higher temperatures. In its broader outlines, at least, it is by no
means a new plan. Mr. Clarence King and Dr. George F. Becker
were inspired by a desire to reach the mineral relations from the ex-
perimental side, which is recorded in the very earliest records of the
U. S. Geological Survey, and much of the remarkable ground-breaking
work of Prof. Carl Barus was undertaken in furtherance of a carefully
prepared scheme of research along these lines. The matter has been
advanced but little in the intervening years. The present renewal
of the effort in this direction is again due to Dr. Becker and has had
the benefit of his wide field experience and enthusiastic and effective
cooperation throughout.
In October, 1900, one of the authors was called from the Reichsan-
stalt to equip a laboratory in the U. S. Geological Survey in which the
exact methods and measurements of modern physics and physical
chemistry should be applied to the minerals. The ultimate purpose
was geological, to furnish a better basis of fact for the discussion of the
larger problems of geology, but it appeared highly probable also that
a quantitative study of the thermal phenomena in this class of sub-
stances would offer new relations of intrinsic interest and of consider-
able theoretical value. This inference has been happily substantiated
quite recently through the publication by Tammann of an extended
treatise on melting and crystallization,! in which he offers some very
interesting speculations on the conditions of equilibrium for sub-
stances above and below the melting temperature under different
pressures. The behavior of crystalline minerals which melt at tem-
peratures considerably higher than he was able to command offer
peculiarly advantageous opportunities for verifying the truth of his
inferences and of contributing further to the knowledge of this most
important change of state of matter.
* A preliminary paper containing the chief results of this investigation was
read before the Geological Society of Washington, March 25, 1904, and a brief
abstract published in Science (vol. xix, p. 734), May 6, 1904. A second abstract
appeared in the American Journal of Science (4), 19, p. 93, 1905.
f Tammann, " Krystallisiren and Schmelzen." Leipzig, 1903.
*5
1 6 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
TEMPERATURE MEASUREMENTS.
It is only a short time since it became possible to measure even
moderately high temperatures with certainty and to express them in
terms of a well-established scale. Temperature is a peculiar function
in that it is not additive. Two bodies, each at a temperature of 500,
can not be united to obtain a temperature of ioo°, nor can any num-
ber of bodies at a temperature of 500 or below give us information
about the temperature 510 or above. Furthermore, temperature is
not independently measurable ; we can only measure phenomena like
the expansion of gases or the conductivity of platinum wire or the
energy of thermal radiation, which we have good reason to suppose
will vary with the temperature uniformly or according to a known
law.
The measure of temperature now generally accepted as standard is
the expansion of hydrogen gas between the melting point of ice and
the normal boiling point of water, divided into 100 equal increments
or degrees. Temperatures above this point have been determined by
continuing the expansion of hydrogen or nitrogen* in the same units, as
far as it has been found possible to provide satisfactory containing ves-
sels for the expanding gas. Such determinations are then rendered per-
manent and available for general use by establishing fixed points, such
as the melting temperatures of easily obtainable pure metals, at con-
venient intervals. Beyond 11500 no trustworthy gas measurements
have been made, and we have, therefore, no standard scale. For
higher temperatures it is usual to select some convenient phenomenon
which is measurable up to the temperature desired, to compare it with
the gas scale as far as the latter extends, and then to continue on the
assumption that the law of its apparent progression below 1 1500 will
continue to hold above that point. In this way we obtain degrees
which, if not identical with the degrees of the gas scale, approximate
very closely to them, and can receive a small correction if necessary,
whenever the gas scale shall be extended or another scale substituted.
The application of measurable high pressures at the higher tem-
peratures has never been successfully accomplished, and until some-
thing can be done in this direction, our knowledge of the rock-forming
minerals, and in fact all the generalizations relating to equilibrium
* To 6oo°, Chappuis et Harker, Travaux et memoires du bureau international
des poids et mesures, 12, 1902. To 1 1500, Holborn and Day, Ann. der Physik, 2,
505, 1900. English translation, Am. Journ. Sci., (4), 10, p. 171, 1900.
GENERAL PLAN — RELATION TO GEOLOGICAL RESEARCH. I J
between the states of matter, which have been established for mode-
rate temperatures, must be regarded as more or less tentative and
subject to eventual revision. We have been accustomed to assume,
both in geology and in physics, with rather more confidence than
scientific experience justifies, that established relations for ordinary
temperatures and pressures will hold in comparable ratio for the higher
temperatures and pressures also. Experimentation under extreme
conditions is slow and technically difficult, and it is, therefore, not
strange that simple relations which are verifiable within easily acces-
sible conditions should now and then be accorded the dignity of
natural laws without sufficient inquiry into the more remote con-
ditions.
GENERAL PLAN.
Our plan on entering this field was to study tne thermal behavior
of some of the simple rock-making minerals by a trustworthy method,
then the conditions of equilibrium for simple combinations of these,
and thus to reach a sound basis for the study of rock formation or
differentiation from the magma. Eventually, when we are able to
vary the pressure with the temperature over considerable ranges, our
knowledge of the rock-forming minerals should become sufficient to
enable us to classify many of the earth-making processes in their
proper place with the quantitative physico-chemical reactions of the
laboratory.
RELATION TO GEOLOGICAL RESEARCH.
The relation which this plan bears to general geological research
may perhaps be expressed in this way. Geological field research is
essentiallv a study of natural end-phenomena, of completed reactions,
with but a very imperfect record of the earlier intermediate steps in
the earth-making processes. The records of the splendid laboratory
experiments in rock synthesis which have already been made are also
of this character. The final product has been carefully studied, but
the temperatures at which particular minerals have separated out of
the artificial magma, and the conditions of equilibrium before and
after such separation, have not been determined. In fact, except for
a limited number of determinations of the melting points of natural
minerals, no exact thermal measurements upon minerals or cooling
magmas have been made, and it is in this direction that a beginning
is to be attempted. The temperatures of mineral reactions under
atmospheric pressures are nearly all within reach of existing labora-
tory apparatus and methods.
i8
ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
EXISTING METHODS.
Furthermore, the methods which have been used hitherto in deter-
mining these mineral melting points seem to the authors to be open
to serious objection, both in principle and in application. They
depend, almost without exception, upon the personal judgment of the
observer, and not upon the actual measurement of any physical con-
stant. For this reason, perhaps, more than any other, the results
obtained by different observers upon the same mineral from the same
source do not agree within considerable limits, much larger than can
be properly ascribed to impurities in the specimens. Familiar
examples will best illustrate this point. Among the determinations
of the mineral melting points, two have received much more general
acceptance than others — those of Joly* and of Doelter.f
The melting temperatures which they obtained for some of the
typical feldspars are as follows :
Meldometer measurement.
Thermoelectric measurement.
Joly, 1891.
Cusack, 1896.
Gas furnace.
Doelter, 1901.
Electric furnace.
Doelter, 1902.
Microcline
Albite
Oligoclase
Labradorite
Anorthite
"75°
"75
1220
1230
1169°
1172
1235
"55°
1 103
1 1 10
1 1 19
1 1 10
"55°
1 1 10
1 1 20
1125
1132
The determinations agree in recording higher melting points toward
the calcic end of the series, but the differences between corresponding
melting points by the two methods is greater than the observed differ-
ences between different feldspars.
Joly's method was novel. He stretched a thin strip of carefully
prepared platinum foil between suitable clamps, placed a few grains
of the powdered mineral upon it, and mounted a small microscope
above, so as to be readily trained on any part of the strip. The foil
was then heated by an electric current which could be very gradually
increased, and the temperature measured from the linear expansion
of the strip at the moment when the observer at the microscope
noticed the first signs of melting. The author of this method was
able to obtain concordant results with it to within about 50 C, but
* J- JolY. Proc. Royal Irish Acad., 3, 2, p. 38, 1891. R. Cusack, Proc. Royal
Irish Acad., 3, 4, p. 399, 1896.
tC. Doelter, Tschermak Min. u. Petr. Mitth., 20, p. 210, 1901; 21, p. 23, 1902.
DETAILED PLAN. 1 9
differences several times greater than 50 appeared in our observations
with the Joly apparatus, unless the grains were prepared with the
greatest care and all the observations made by the same observer.
The size and form of the grains, the care used in locating them
exactly in the middle of the strip, every draught of air, but most of
all the judgment of the observer as to when the substance appeared
to melt, all entered into the result to a very considerable degree.
There is also another source of error with which we afterward became
familiar, which may serve to account for the very large differences
between Joly's results and our own later values with some of the
well-known minerals, though not with all. In certain of the minerals,
after melting, the resistance to change of' shape, due to viscosity, is of
the same order of magnitude as that due to the rigidity of the crystal
just before melting, a fact which may well have led to large errors of
judgment in this method of detecting melting points.
The possibility of working very expeditiously with minute quanti-
ties of a substance led us to study this method with great care, and we
were fortunate enough to possess an instrument of Professor Joly's
own model, made by Yeates & Son, Dublin, but the results obtained
with it, even under most favorable conditions, are more in the nature
of personal estimates than of exact measurements of the change of
state. Its value for qualitative study, and in eases where only a very
minute quantity of a substance is available, is unquestioned.
Doelter has employed electric furnaces, modeled after that in use at
the Reichsanstalt by Holborn and Day, for the determination of the
melting points of the metals. He measured his temperatures with
thermoelements, and used several grams of material in his determina-
tions, but he also judged of the approach of the melting point by the
appearance of the charge and usually recorded two temperatures — the
first approach of viscous melting and the point where the material
appeared to have gone over into a thin liquid.
DETAILED PLAN.
We determined from the first to get rid of this personal factor.
However carefully such observations may be made, and however well
supported by the reputation of a particular scientist for skilful and
exact work, they can not have a permanent value. Melting points of
pure minerals are not different, in principle at least, from the melting
points of other chemical compounds or of metals. They occur at less
accessible temperatures and involve some complicating phenomena,
as we shall see presently, but the change of state of a solid crystalline
20 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
mineral to a liquid must of course be defined by an absorption of heat.
Whether the appearance of the mineral charge in the furnace will
offer a trustworthy index through which to locate this absorption
may well be expected to differ with different substances. Nearly all
observers have recorded the fact that many substances of this class
remain very viscous after melting, and that the transition is not well
marked in the appearance of the material.
We therefore planned an apparatus which should be as sensitive as
possible to heat changes over a long range of temperatures, and then
prepared to examine the thermal behavior of simple minerals of
natural or artificial composition when gradually heated or cooled.
Changes of crystalline form (Umwandlungen) or of state (melting and
solidifying) must involve a more or less sharply marked absorption
or release of heat, and be recorded as breaks in a smooth curve in the
same way as in the determination of metal melting points or the
singularities of any of the well-known chemical compounds at lower
temperatures.
APPARATUS.
The apparatus used in these determinations may be assumed to be
fairly well known. It is the same in all essential particulars as that
used by Holborn and Day* in establishing the high-temperature scale
with the gas thermometer at the Reichsanstalt. And yet it is plain
that such a scale requires some care in the transplanting, particularly
as the authors were without a gas thermometer and were, therefore,
not in position to make direct comparisons with the gas scale.
THERMO-ELEMENTS.
The temperatures were measured with thermo-elements exclusively.
We obtained from Dr. Heraeus (Hanau, Germany) a set of four ele-
ments cut successively from the same roll of wire, which, when joined
together, proved to be identically alike in their readings over the
range of temperatures covered by the gas scale of the Reichsanstalt
(2500 to 1150° C.) within the limits of observation error. Through
the courtesy of Prof. Holborn these were taken to the Reichsanstalt
and measured in the original melting-point furnace with the same
elements in terms of which the gas-thermometer scale had been ex-
pressed, and five careful comparisons made. These were the melting
points of the pure metals, cadmium (in air), zinc (in air), antimony
(reducing atmosphere), silver (reducing atmosphere) and copper (in
* Ludwig Holborn and Arthur L,. Day, Am. Journ. Sci (4), 8, p. 165, 1S99
TEMPERATURE SCALE. 21
air). A fortunate circumstance made it possible to send these care-
fully calibrated elements to Washington by messenger, which made
it certain that they suffered nothing in transit.
The elements were then further compared in an electric furnace,
which will be described below, and the melting points of the same
group of metals again determined in our laboratory. The metals
used, however, were from other sources than those which had served
for the calibration at the Reichsanstalt. When this test was finished,
we were able to assure ourselves that, although all the constants in
the measuring apparatus — thermo-elements, resistances, standard
cells, metals, etc. — had been changed in the transfer from the Reich-
sanstalt to the Geological Survey at Washington, the aggregate error
nowhere exceeded i° over the entire range from 2500 to 11500. It
will be remembered that i° was about the accuracy which the stand-
ard gas thermometer showed at 10000. Our thermo-electric system
is, therefore, now doubly established — (1) by direct comparison and
(2) through an independent series of metal melting points — upon the
gas-thermometer scale of the Reichsanstalt within the limits of error of
the latter, and can be verified at any time with the help of two of the
elements which have been laid aside for this purpose, or the melting
points of the metals. The scale is, therefore, permanent.
TEMPERATURE SCALE.
As the introduction of the standard high temperature scale of the
Reichsanstalt into this country and its establishment by proper
fixed points may be a matter of considerable interest to other
investigators, some further details regarding the metals chosen for
these fixed points are added here. We tried to find metals which
should not only be of the purity necessary for such standards, but
which should be easily obtainable in uniform quality. With four of
the five metals of the Reichsanstalt series — cadmium, zinc, silver, and
copper — no difficulty was experienced, but we were not able to find
satisfactory antimony in this country. This need not prove an
obstacle, for the four points mentioned will serve most purposes with-
out a fifth, while if the needs of an experiment are so exacting as to
require an intermediate melting point, antimony can be imported
from Kahlbaum, of Berlin, without great delay or excessive cost, in
the same purity as that originally used at the Reichsanstalt.
The cadmium and zinc in our series were taken from the regular
listed chemicals of Eimer & Amend (zinc, " C. P. in sticks ;" " cadmium,
metal sticks"). The silver was the well-known test silver of the
22
ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
Philadelphia Mint laboratory, while the copper was also from Eimer &
Amend ("C. P. copper drops, cooled in hydrogen").
Careful analyses of samples of the cadmium, zinc and copper follow :
Zinc.
Cadmium.
Copper.
(Eimer & Amend's "C P.
(Eimer & Amend's in
(Eimer & Amend's "C P.
in sticks. ")
sticks )
drops, cooled in hydrogen.")
Per cent.
Per cent.
Per cent.
As None
As None
Te and Se None
Cu Trace
Cu Trace
Sb Trace
Pb 0.0412
Pb 0.0860
As None
Cd 0.0021
Zn Trace
Bi None
Fe 0.0053
Fe 0.0025
Ag 0.018
Co and Ni None
Co and Ni None
Pb O . OO I
S 0 . 0005
S 0 . 0005
Co and Ni None
Zn 0.010
Total impurities 0 . 0S90
Fe *o.oi 1
Total impurities 0.040
Total impurities.. 0.049 1
* This figure is doubtless somewhat too high.
It was not deemed necessary to make an analysis of the silver, as
we were assured that it contained no impurity which could be quanti-
tatively determined.
The melting temperatures of cadmium and zinc are relatively low,
and those of silver and copper comparatively high on the gas scale,
with a long interval between, so that it sometimes becomes very
desirable to have an intermediate point. The two melting points
which are most conveniently located are aluminium and antimony.
Aluminium, on account of its low density, and perhaps because it
has been less successfully purified than the other metals, does not
give a sharp and satisfactory melting point. The melting point of
Kahlbaum's antimony, of which a recently published analysis is
reproduced here, serves this purpose excellently. It rarely solidifies
without considerable undercooling, but the point to which the tem-
perature rises after crystallization begins is sensibly identical with
the melting point.
Antimony (Kahlbaum, Berlin). f
Fe 0.012%
004
003
Cu
Pb
•019%
The C. P. antimony obtained from Eimer & Amend and from
Merck & Co., in a careful analysis, for which we are indebted to Dr.
I Fritz Henz, Inaugural Dissertation, Zurich. Published Leipzig, 1903.
TEMPERATURE SCALE.
23
W. F. Hillebrand of the Geological Survey, each showed about one per
cent of the sulphide still present and traces of other impurities. The
melting temperatures of these varied under different conditions as
much as 150, and were totally unsuited to this work.
Inasmuch as the melting points of these metals were determined
with thermo-elements which Professor Holborn had just calibrated
with the metals in use at the Reichsanstalt for this purpose, a compari-
son of the values obtained will show the accuracy with which one may
reproduce the Reichsanstalt scale entirely from local sources :
Metal.
Reichsanstalt.
Day and Allen.
Differ-
ence.
Source.
Melting
point.
Source.
Melting
point.
Cadmium....
Zinc
Silver
Copper (in
air)
Copper (re-
ducing at-
mosphere.)
Kahlbaum
Do.
Gold u. Silber
Scheideanstalt.
Haddernheim
Kupferwerk.
Do.
32 1. 7°
419.0
961.5
1064. 9
1084. 1
Eimer & Amend
Do.
Philadelphia Mint
Eimer & Amend
Do.
321. 7°
420.0
962 . 2
1065.3
*ioS3.6
o.o°
I .0
0.7
04
05
* A single determination with one element; all others are mean values with two
or more elements.
For the method of extrapolation of the scale and further informa-
tion regarding the use and accuracy of thermo-elements at these
temperatures, reference is made to the papers of Holborn and Day
already cited.
For everyday use, four more elements were prepared and calibrated
in the same way. Of these, two are of the usual form (fig. 1 ) and two
are of a new design which has proved very effective in the determi-
nation of the melting points of non-metallic substances. It will be
seen from the diagram of the insulated element that the hot junction
is protected from the melting charge by a casing of platin-iridium
(0.5 mm. thick) and by a protecting tube of refractory Berlin (Mar-
quardt) porcelain (1.5 mm. thick) . Very early in our experiments upon
the mineral silicates we became aware that the conductivity of these
materials for heat would be much poorer than in similar charges of
metal. Furthermore, the charge of mineral which the furnaces could
carry was only one-fourth to one-third as great as the metal charges
used in the calibrations, because of the great difference in specific
gravity and the limited space which could be heated to a fairly uni-
form temperature. For these reasons the changes of state would be
less sharply marked upon the heating and cooling curves than metal
melting points, and it was feared that the readings of the protected
24
ISOMORPHISM AND THERMAL PROPERTIES OF EEEDSPARS.
element might prove too high or too low through inability to take on
the temperature of the surrounding mass promptly. It was to dis-
cover and obviate this possible source of error that the form of thermo-
element indicated in the adjoining diagram (fig. 2) was devised. It
really amounts to nothing more than the ordinary
form of platinum — platin-rhodium element with the
platinum wire insulated from the other by a very
slender porcelain (Marquardt) tube and the platin-
rhodium wire broadened out and wrapped around
this tube like a cap over the portion which dips into
the charge. The hot junction is
then the lower extremity of the cap
where the platinum wire emerges
from its insulating tube and is welded
inside the platin-rhodium cap.
This form was dictated entirely by
experience to meet conditions where
an exposed element might be neees-
sarv or desirable. The wires of an
ordinary element, if embedded with-
out protection, are rather frail for the
wear and tear of breaking or drilling
mineral charges out of the crucibles
after the measurements, and they can
not be strengthened without intro
ducing a greater error through the
amount of heat conducted away from
the junction than the one which it is
desired to obviate.
It has furthermore been the almost
invariable experience of one of the
authors* that when an element,
through exposure to combustion products or otherwise, no longer
gives normal readings, the seat of the trouble lies in the 5 or 6 centi-
meters of the platinum wire immediately adjacent to the hot junc-
tion, and not in the alloy. The pure platinum sometimes seems to
absorb enough volatile or other contact products, when unprotected in
a furnace at very high temperatures, to alter both its resistance and
its thermo-electric potential, f Changes of this kind are not serious
when a number of control elements are constantly available, and they
Fig. i. — Thermoelement (standard
form) in position.
Fig. 2. — A new form of thermo-ele-
ment.
Day.
f Holborn& Day, Am. Journ. Sci. (4), 10, p. 171, 1900.
FURNACE. 25
are usually permanently corrected by half an hour's glowing in the open
air at full white heat. The glowing must be done by passing a suitable
current through from end to end and not with a Bunsen burner or
gas blast.
In event of a serious accident involving an exposure of the element
which can not be corrected by glowing, cutting out the exposed por-
tion of the platinum wire and reconnecting will almost always restore
the normal readings.
The new form of element seeks to avoid both these difficulties; it
offers the advantage of an exposed junction without exposing the
platinum wire, and by making the platin-rhodium cap project but
little above the surface of the melting charge it avoids excessive loss
of heat by conduction away from the hot junction. In fact, in this
latter particular the new form enjoys a distinct advantage over the
usual form of heavily protected element. It has the disadvantage of
being more frail to handle, but there is little danger of anything more
serious happening than the breaking of the porcelain tube, which is
readily replaced.
These elements are calibrated in metal baths like the others by
inclosing in a porcelain protecting tube.
It need only be added that all the more important temperatures
throughout this work were separately determined with three different
elements. One of these was always from the Reichsanstalt set (pro-
tected) and one usually an unprotected element of the new form.
No systematic differences between the readings of the two tvpes
have ever been found.
FURNACE.
The furnace, in plan, differed but little from that in use for melting-
point determinations at the Reichsanstalt. In the working out, two
important changes were introduced, in order to enable it to reach the
higher temperatures of the mineral melting points. A more refrac-
tory and better insulating material (a mixture of magnesite and
corundum) was substituted for fire clay in the hotter parts and the
coil was wound on the inside of the oven tube instead of outside. The
latter involved some little mechanical ingenuity and skill in winding,
but the gain in economy and in the rapidity with which changes could
be effected or constant conditions established more than repaid any
additional labor in preparation.
A diagram of the furnace in section is shown in fig. 3. It could be
used for any temperature up to 16000 C. without any difficulty or
especial precautions and could be regulated to maintain a constant
temperature at a particular point for long periods of time.
26
ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
Heating
coil -,$
Fig.
3. — The furnace, showing ther-
moelement and charge.
The coil, which was obtained from Dr. Heraeus, was of platin-
iridium wire (90 parts Pt., 10 parts Ir.), 1.5 mm. in diameter, and
required about 3000 watts to
maintain a constant temperature
of 16000 C. The furnace was
carried at times on a 1 10-volt
direct-current street main, but
accurately constant temperatures
could not be depended on without
the storage battery.
The insulation in these furnaces
was so perfect that shutting off or
reversing the heating current at
the highest temperatures did not
produce a quiver in the galvanom-
eter to which the thermo-element
was connected, although the sen-
sitiveness of the system was such
that a leakage amounting to a
single micro-volt (corresponding to
less than o.i°) at 16000 would have caused a displacement of more
than two millimeters on the scale.
STANDARDS.
The thermo-electrical potential was measured upon a potentiom-
eter (Wolff, Berlin, Reichsanstalt calibration) in terms of a standard
cadmium cell (saturated) prepared by ourselves. Two of these
cells were used interchangeably during the earlier measurements.
Toward the close of the series four fresh cells were prepared for com-
parison with the earlier ones and were found to agree with them within
0.0001. One of these later cells (the readings of the four were iden-
tical to the fifth significant figure) was verified bv Dr. Wolff, of the
Bureau of Standards, by comparison with the standard Clark cells of
that institution and found to be 1.0195 V at 200 C, assuming the legal
value (United States) of the Clark cell, 1.434 V, at 150 C. Substitut-
ing the Reichsanstalt value, Clarke = 1.4328,* our cells would give
a normal potential difference of 1.0186 at 200. The temperature
determinations which follow are, therefore, calculated in terms of this
number.
With the apparatus here described, the authors were enabled to
command any furnace temperature up to 16000 conveniently, to regu-
* Jaeger u. Kahle, Wied. Ann., 65, p. 926, 1898.
FIRST GROUP OF MINERALS INVESTIGATED. 27
late it quickly and with great exactness, or to hold it constant for long
intervals. An oxidizing or reducing atmosphere could also be easily
introduced whenever desired. It is, however, undesirable to expose
either coil or thermo-element too freely to oxygen at very high tem-
peratures on account of the considerable losses by sublimation to
which the platinum metals are subject.
With the help of the standard metals mentioned, which are readily
obtainable and can be used repeatedly, thermo-elements or resistance
pyrometers can be calibrated in any laboratory, and used for all
measurements up to the limit of the Reichsanstalt scale ( 1 1 500 C. ) with
no greater error than that inherent in the scale itself. Above this
temperature up to 16000 the continuation of the thermo-electric scale
probably still furnishes the most convenient and trustworthy extra-
polation which has yet been perfected.
The uniformity and certainty of this extrapolation will best be
illustrated by the measurements upon anorthite (the highest melting
point we measured). The melting temperature of a mineral of very
poor conductivity for heat and relatively low specific gravity is much
more difficult to measure than that of a metal, but the agreement of
the results tabulated below (see Anorthite, p. 37) is sufficiently good
to demonstrate the accuracy of the extrapolation. The thermo-
electric potential, therefore, appears to deserve entire confidence for
consistent extrapolation through the 4500 immediately above the
present Reichsanstalt scale.
FIRST GROUP OF MINERALS INVESTIGATED.
The particular group of minerals chosen for the first investigation
was the soda-lime feldspar series, and orthoclase. The reasons for
this choice will be fairly obvious. Aside from its being altogether the
most important group of rock-forming minerals, unusual interest has
been attracted to it through Tschermak's theory that these feldspars
bear a very simple relation to one another, that they are (orthoclase
excepted, of course) in fact merely isomorphous mixtures of albite
and anorthite. This hypothesis has given occasion for serious and
extended study, both from the optical and thermal sides.
A complete review of the literature of the feldspars will not be
attempted here. Although opinion is still somewhat divided,* it is
probably fair to say that the optical researches have not yet definitely
established or disestablished the isomorphism of the albite-anorthite
* Fouque et Levy, Synthesedes Mineraux et des Roches, p. 145, 1882; C. Viola,
Tschermak Min. & Petr. Mitth., 20, p. 199, 1901; Lane, Journ. Geol., XII, 2, p.
83, 1904; J. H. L. Vogt, " Die Silikatschmelzlosungen," Christiania, 1903.
28 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
group. Investigation from the thermal point of view has been even
less satisfactory by reason of the subjective methods employed, to
which reference has already been made, though the recorded results
indicate with reasonable unanimity that the melting point of anorthite
is above that of albite and that the intermediate feldspars will prob-
ablv fall between the two.* Beyond this conclusion, the great body
of evidence is more or less contradictory and sometimes contro-
versial in character.
Orthoclase (Preliminary).
Somewhat unluckily, our measurements began with natural ortho-
clase (microcline) from Mitchell County, North Carolina, a quantity
of which was placed at our disposal by the U. S. National Museum.
The material was powdered so as to pass readily through a ioo-mesh
sieve, and placed in ioo cc. or 125 cc. platinum crucibles, sometimes
open and sometimes covered, in charges of from 100 to 150 grams.
These charges were heated slowly in the electric furnace from 6000 to
above 14000 C, but, although the thermal apparatus was sufficiently
sensitive to detect an unsteadiness of a tenth of a degree with certainty
not the slightest trace of an absorption or release of heat was found.
The charge at the beginning of the heating was a dry crystalline
powder which was prodded from time to time with a stout platinum
wire to ascertain its condition as the heating progressed. At about
10000 traces of sintering were evident; at 10750 it had formed a solid
cake which resisted the wire, at 11500 this cake had softened suffi-
ciently to yield to continued pressure, and at 13000 it had become a
viscous liquid which could be drawn out in glassy.- almost opaque
threads by the wire. Under the microscope the opacity was seen to
be due to fine included bubbles, the material being entirely vitreous.
The cooling was equally uninstructive ; the vitreous mass solidified
graduallv without recrystallization or the appearance of any thermal
phenomenon. Frequent repetitions with fresh charges and varied
conditions added nothing to our knowledge of the melting tempera-
ture, and the matter began to look very unpromising.
We also reheated charges of the resulting glass, which was some-
times repowdered and sometimes in the cake as it had cooled. But
except to observe that the glass powder began to sinter earlier (8oo°),
no new facts appeared. f
* J. H. L. Vogt, loc. cit., p. 154, expresses the opinion that the soda-lime feld-
spars fall under Type III of Roozeboom's types of isomorphous series with a
minimum between anorthite and albite.
f These sintering temperatures varied within considerable limits with the fine-
ness of the material and, therefore serve only in a very rough way to define the
state of the charges.
BORAX. • 29
Then we tried by various means to recrystallize the melted ortho-
clase. We mixed crystalline powder with the glass, we applied suc-
cessive quick shocks to the cooling liquid for several hours with an
electric hammer below the crucible, we varied the rate of cooling and
even tried rapid see-sawing between 8oo° and 13000. We circulated
air, water vapor, and carbonic dioxide through the charge throughout
the heating, and finally introduced a rapid alternating current sent
directly through the substance while cooling, but no trace of crystalli-
zation resulted. An extremely viscous, inert mass always remained,
which gradually hardened into a more or less opaque glass. It ap-
peared somewhat translucent if very high temperatures had been
reached, but was never clear.
Following orthoclase, a number of specimens of natural albite were
tried under similar conditions and with entirely similar results.
Later on, when more experience had been acquired, these minerals
were taken up again and a satisfactory explanation for their behavior
was found. But for the moment all the defining phenomena ap-
peared to be so effectively veiled by some property, presumably the
viscosity, that we were constrained to look about for some similar
compound which should give us a better insight into the behavior of
mineral glasses and their thermal relations, and to lay aside the feld-
spars until they could be more successfully handled.
This outline of our unsuccessful experiences is given here in some
detail, in order to show the actual difficulties which confront the
student in working with the feldspars, in the face of which it is cer-
tainly not surprising that uncertain and contradictory conclusions
have been reached.
Borax.
The substance chosen for this preliminary work was ordinary
anhydrous borax (sodium tetraborate). We chose this merely be-
cause it was a simple glass and unlikely to undergo chemical change.
It is easily obtainable pure and its thermal phenomena are within
easy reach. The study of borax proved to be most instructive.
It gave us an effective insight into the behavior of this class of sub-
stances, and in particular served to define the phenomena of melt-
ing and solidifying in substances which undergo extreme under-
cooling and which recrystallize with difficulty, or not at all. The
results of this study of borax were, therefore, of much interest in them-
selves and were given in a paper before the National Academy of
Sciences at its spring meeting in Washington last year (April 21,1 903) ,
but were not printed at that time.
30 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
The borax glass upon which our measurements were made was
prepared in the usual way by heating the crystals until the water of
crystallization had been driven off and the viscous mass was reason-
ably free from bubbles. If the borax is pure, the anhydrous product ,
when cooled, is a brilliant, colorless glass, isotrophic, of conchoidal
fracture, and specific gravity 2.37. The specific gravity was deter-
mined in the fraction of kerosene boiling above 1850 C. About 100 g.
of this glass were then broken up and placed in a platinum crucible
in the electric furnace. The thermo-element was placed in position
as indicated in fig. 3, the heating current properly regulated, and ob-
servations of the temperature made at intervals of one minute, while
the glass softened and passed gradually over into a thin liquid (8oo°).
Then the current was reduced and the cooling curve observed in the
same way. These observations gave an unbroken curve, both for the
heating and cooling, as in the case of all the glasses,* without a definite
melting or solidifying point, although the arrangements for detecting
an absorption or release of heat were very sensitive. Prodding at in-
tervals with a platinum rod showed the change to be perfectly gradual
from a clear, hard cake through all degrees of viscosity to a fairly thin
liquid and back again. This observation is of considerable interest
as showing that the absence of bounding phenomena between the cold
glass, which fulfills the mechanical conditions for a solid very perfectly,
and the liquid, is not confined to mixtures of complicated chemical
composition, but is exhibited also by true chemical compounds of
undoubted purity. It is, therefore, not conditioned by composition,
but by the physical nature of the substance.
Having verified this behavior of anhydrous borax by several repeti-
tions of the experiment, various disturbing influences were applied to
the slowly cooling liquid in the hope that some temperature or range
of temperature would be found within which the vitreous condition
would prove unstable and crystallization be precipitated. The jar
produced by an electric hammer pounding upon the outside of the
furnace during cooling proved to be sufficient to bring down the entire
charge as a beautiful crystalline mass of radial, fibrous structure,
brilliant luster, rather high refractive index, and increased volume.
Fig. 4 will give a good idea of the appearance of the anhydrous crys-
talline borax in the crucible. Its specific gravity proved to be 2.28
as compared with 2.37 for the glass, a somewhat unusual relation, f
which may, in part, account for the quasi stability of the vitreous form
during cooling.
* See Tammann, loc. cit.; also Roozeboom, "Die heterogenen Gleichgewichte,
etc.," Braunschweig, 1901.
+ Tammann, loc. cit., p. 47 et seq.
m ■
FIG. 4.
CHEMICALLY PURE BORAX (ANHYDROUS),
CRYSTALLIZED FROM THE GLASS.
jO,, Boston.
ot liquids which undercool in solidifying. We next varied the experi-
ment by first cooling quietly to about ioo° below the melting point
* See Tammann, loc. cit.; also Roozeboom, "Die heterogenen Gleichgewichte,
etc.," Braunschweig, 1901.
+ Tammann, loc. cit., p. 47 et seq.
BORAX.
31
Observations were then undertaken upon the crystalline borax with
a thermo-element as before, to determine the melting temperature and
solid modifications, if such existed, but none of the latter were found.
The charge melted uniformly at 7420 and the melting point was well
defined. A curve showing the minute-to-minute observations on the
crystalline borax between the temperatures 6500 and 7750 is shown in
fig. 5, a.
Having determined the melting point of crystalline anhydrous
borax satisfactorilv, we examined more closely into the conditions
7000
6900
(765.°)
6800
6700
6600
6500
6400
6300
6200
6100
6000
5900
5800
(6 60")
5700
5600
\
a
\
1
1
\
\
J
\
b\
\
\c
\
\
1
\
/
\
d
/
\
1
1
/
/
/
\
/
\
1
7
Time - 1 division = 10 minutes
Fig. 5. — -a, Melting-point curve; b, c, d, curves showing undercooling and
crystallization at different temperatures.
under which it solidified. As has been said, if the melted charge was
allowed to cool slowly, undisturbed, no return to the crystalline state
occurred. It merely thickened gradually into a transparent glass
without releasing the "latent" heat which it had taken on in melting
(fig. 7, b). If it was subjected to the jarring produced by the electric
hammer on the furnace wall, it cooled down a few degrees below the
melting point and then began to crystallize, the heat of fusion was set
free, and a rise in temperature immediately appeared, represented by a
hump upon the cooling curve, as shown in the figure (fig. 5, b, c, d) . Up
to this point the phenomenon differs but little from the usual behavior
of liquids which undercool in solidifying. We next varied the experi-
ment by first cooling quietly to about ioo° below the melting point
32
ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
7000
(775°)
6500
and then introducing a few crystal fragments or starting the pound-
ing. Crystallization and release of the latent heat followed at once.
In fact over a range of some 2500 immediately below the melting point
it proved to be within our power to precipitate the crystallization of
the undercooled mass entirely
at will. It was even possible
to cool the melted charge
quietly down to the tempera-
ture of the room and remove
it from the furnace as a clear
glass, then, on a subsequent
day, to reheat to some point in
this sensitive zone and pound
judiciously, when crystalliza-
tion would at once begin,
marked by the release of the
latent heat of the previous
fusion as before (fig. 6, a, b).
The accompanying curves
show the situation clearly.
Curves aa' and bb', fig. 7, were
obtained from charges of crys-
talline and vitreous borax, re-
spectively, of exactly equal
weight, which were cooled and
reheated in the same electric
furnace under like conditions.
The radiation from the furnace
for like temperature conditions
is practically the same, so
that the more rapid rate of
cooling and of reheating in the
crystalline charge indicates a
much smaller specific heat than
for the vitreous form.
From the point of view of the
usual definition of the solidify-
ing point of a substance, a diffi-
culty confronts us here: (1) We were able to vary the beginning of
solidification (crystallization) at will over a range of 2500, and (2) the
temperature to which the charge rose after the undercooled liquid
had begun to crystallize did not reach the melting point, although
6000
(680°)
O
>
o
L.
O
E
C 5500
o
t_
3
e
a.
e
V
I-
5000
4500
4000
(490°)
/
---
J
/
^
/
/
/
/
a
/
1
/■'
.
/
/
1
4
/
1
1
//
1
//
//
b
,
/
s
/
1
/
/
/
Time - I division = 10 minutes
Fig. 6. — Curves showing the release of the
heat of fusion at widely different tem-
peratures.
BORAX.
33
6500
(7251
6000
once crystallization was induced only io° below it in a furnace of
constant temperature. The
rapidity with which the crys-
tallization and the accompany-
ing release of the latent heat go
on depends in part upon the
rate of cooling and the char-
acter of the disturbance which
has been applied, i. c, upon
accidental rather than char-
acteristic conditions. It thus
happens that the amount of
the heat of fusion and its slow
rate of liberation in the case of
liquids which can be greatly
undercooled and become very
viscous may be such as to
deprive it of its usual signifi-
cance as defining a solidifying
point. It is, of course, a con-
sequence of the phase rule that
the solidifying temperature of
an undercooled liquid is estab-
lished, if only equilibrium be-
tween solid and liquid (and
vapor) is reached before com-
plete solidification is accom-
plished, but equilibrium is not
necessarily attained during so-
lidification, and the devices
usually employed (sowing with
crystals, agitating) are often
totally inadequate to effect it.
The temperature to which a
crystallizing liquid rises after
undercooling is not necessarily
constant or in any way related
to the melting point and is,
therefore, not, in general, enti-
tled to be regarded as a physi-
cal constant.
We then endeavored to ascertain whether the unstable domain had
a lower limit also. For this purpose we mixed a quantity of the
5500
o
>
o
l_
o
E
c 5000
u
L.
3
0>
a.
E
u
4500
4000
3500
3200
/
'
/
/
/
/
1
/
1
1
1
■*
\
\
1
1
A
1
1
a
\
b
a
1
b'
\
I
v
1
\
\
\
1
\\
I
\\
I
\
\
I
\
\
1
-
\
\
1
\
\
I
\\
\
\
\
\
\
\
A
/
V
J
(400°) 0
20
40 60 80
Time (minutes)
100
Fig. 7. — Curves showing difference in spe-
cific heat between crystalline (ad) and
vitreous (bb) borax under like condi-
tions of cooling and reheating.
34 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
crystals with the glass and powdered them together to about the fine-
ness represented by a 150-mesh sieve and heated them very slowly.
In this condition the glass proved to be very unstable and crystallized
readily with a rapid release of its latent heat at about 4900. Very
slow heating (10 minutes per 1 degree) gave a temperature a few de-
grees lower, but such variations as could be applied within the period
of a working day did not suffice, under the most favorable conditions,
to change this temperature materially. The first evidence of molec-
ular mobility in borax glass, shown in the sticking together of the
finest particles (sintering), and the first traces of crystallization and
release of latent heat, appeared consistently at about 4900 to 5000.
Still a third phenomenon attracted our attention to this temperature.
On every occasion when borax glass was heated rapidly, either pow-
dered or in the solid block, a slight but persistent absorption of heat
appeared in this same region and continued over some 200, after which
the original rate of heating returned. We were entirely unable to
explain an absorption of heat in an amorphous substance under these
conditions except by assuming an actual change of state to exist
between amorphous glass and its melt, in which case the absorbed
heat would reappear somewhere upon the corresponding cooling curve,
which it failed to do. We then reasoned that any assumed change
in the molecular structure which would account for an absorption of
heat would also be likely to cause an interruption in the continuity
of the curve of electrical conductivity, and the relative conductivity
was determined throughout this region, but no such interruption
appeared.
Finally the matter was abandoned. The evidence did not appear
sufficient to establish any discontinuity in the cooling curve of the
glass, so long as no crystallization took place.
When these relations had been clearly established, we turned again
to the feldspars.
It became clear very early in the investigation that only artificially
prepared and chemically pure specimens would be adequate for our
purpose. Each of the end members of the series, anorthite and albite,
as found in nature, is always intermixed with some quantity of the
other, while the intermediate members generally contain iron and
potash, and all are liable to inclusions.
There was nothing new in this plan. Fouque and Levy* had
demonstrated the possibility of making pure feldspars by chemical
synthesis and had studied their optical properties some years ago. We
undertook to prepare much larger quantities than they (200 grams)
* Synthase des Mineraux et des Roches.
ARTIFICIAL FELDSPARS.
35
and to make a careful study of their heating and cooling curves under
atmospheric pressure — the conditions under which anorthite and the
plagioclases crystallize, the relations between the amorphous and crys-
talline forms, the sintering of crystalline and vitreous powders, in
short, their entire thermal behavior, as we had done with the borax.
At the same time it was our purpose to make careful determina-
tions of the specific gravities of both the vitreous and the crystalline
products, analyses of such portions as might be of special interest,
and also to prepare microscopic sections wherever they were likely
to throw light on the relations involved. The latter, after preliminary
examination, were very thoroughly studied by Prof. J. P. Iddings of
the University of Chicago, whose large petrographic experience with
mineral crystallites makes his judgment of very exceptional value.
His analyses (see Part II) of the slides form an important part of this
discussion. We are also indebted to Mr. W. Lindgren of the United
States Geological Survey for valuable assistance in the microscopical
study of our products.
Analyses of Artificial Feldspars.
An.
Ab,An5.
AbiAn2.
Ab2An,.
Found.
Calcu-
lated.
Found.
Calcu-
lated.
Found.
Calcu-
lated.
F'ound
Calcu-
lated.
Si03
A1.0S
Fe203
CaO
43-33
36 . 2 I
.29
20.06
. I I
43.28
36.63
20.09
47.IO
34-23
•15
17 .00
1.74
47.18
34.OO
16.93
I.87
51.06
31-5°
. 22
I3-65
3-68
5I-30
31.21
13.68
3-79
60.OI
24-95
.29
7.09
7-79
59-8i
25-47
6.98
7-73
Na30
IOO.OO
100. 22
100. 1 1
1 00 . 1 3
The constituents used in our syntheses were precipitated calcium
carbonate, anhydrous sodium carbonate, powdered quartz (selected
crystals), and alumina prepared by the decomposition of ammonium
alum. None of these contained more than traces of impurities, if we
except the quartz, in which 0.25 per cent of residue, chiefly oxide of
iron, was found after treatment with hydrofluoric and sulphuric acids.
All but the calcium carbonate were carefully calcined and cooled in a
desiccator before weighing. To obtain a homogeneous product, the
weighed constituents were mixed as thoroughly as possible mechani-
cally and heated in large covered platinum crucibles (ioocc. capacity)
in a Fletcher gas furnace.* After some hours' heating, during which
the temperature usually reached 15000 or more, the product was
removed from the furnace, cracked out of the crucibles, powdered,
* Buffalo Dental Company, No. 41 A. A Fletcher furnace of this type, with
ordinary city gas pressure and a small blast motor, will melt all of the feldspars.
36 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
passed through a "ioo-mesh" sieve, and then melted again. This
process probably gives a fairly homogeneous mixture, though a third
fusion in the resistance furnace was generally made before determining
the constants.
We prepared in this way albite (Ab), anorthite (An), and the follow-
ing mixtures of the two: AbiAn5, AbiAn2, AbiAnlt Ab2Ani, Ab3Ani,
Ab4Ani. All of these were obtained in wholly or partially crystalline
form, by crystallization from the melt, except albite. The syntheses
were controlled by analyses of a number of the products, the results
of which are shown in the table on p. 35.
Anorthite (Plate I).
Of the whole series of feldspars, anorthite is in many respects
the simplest to deal with. It is of relatively low viscosity when
melted, and crystallizes easily, very rapidly, and always in large, well-
developed crystals. A 100-gram charge crystallized completely in ten
minutes. Sudden chilling gave a beautiful clear glass entirely free
from bubbles, somewhat slower cooling usually resulted in a partial
crystallization from few nuclei, the crystals always being large. In
appearance it resembles the natural mineral in every respect. Its
hardness is also equal to that of natural anorthite. Thin sections
show good cleavage, and twinning according to the albite law is fre-
quent. The extinction and other microscopic characteristics are as
well marked as in natural specimens.
The heating curve of crystalline anorthite is perfectly smooth except
for the single break which marks the melting point. No trace of a
second crystalline form (Umwandlung) appeared in this or any other
of the feldspars within the temperature range of the observations
(3000 to 1600°). Some undercooling always occurs in solidification
even if the rate of cooling is slow, but it is less, under like conditions,
with anorthite than with any other member of the series. The heat-
ing curve of the glass shows a strong evolution of heat which may
occur as low as 7000, when crystallization takes place. The melting
point of crystalline anorthite was determined by three different
thermoelements upon two different mineral preparations. It will be
seen from the table on p. 37 that the determinations agree remarkably
well. This is of considerable significance with reference to the method
of temperature measurement employed. It will be remembered that
the established temperature scale ends at 1 1 500 and that temperatures
beyond that point are extrapolated with the help of some trust-
worthy phenomenon which varies with the temperature. We chose
for this purpose the thermo-electric force developed between pure
ANORTHITE.
37
platinum and platinum alloyed with 10 per cent of rhodium. Now
the constants of such thermo-elements will usually differ among
themselves and require to be determined for each element by calibra-
tion with the gas thermometer or with the melting points of the
metals. It therefore offers an excellent test of the value of the extra-
polation if some sharp melting point can be found in the extrapolated
range to serve as a point of reference. The melting point of crystal-
line anorthite serves this purpose exceedingly well, and separate
determinations of it with three separate thermo-electric systems,
gave identical values within the limits of error of observation. Our
Anorthite.
first preparation.
Date.
Element.
Electromotive
force in MV.
Tempera-
ture.
Remarks.
1
Oct. 7, 1903
A
15.939
1534°
Solid charge, open crucible
Do.
A
15,914
1532
Do.
Oct. 10, 1903
A
15,878
I530
Covered crucible.
Do.
No. 3
16,074
1533
Do.
Do.
No. 3
16,058
1532
Do.
Do.
No. 3
16,068
1532
Do.
Do.
No. 2
16,095
1532
Do.
Mean 15320
SECOND PREPARATION.
Jan. 16, 1904
A
15,860
1 532°
Covered crucible.
Do.
A
15,864
1532
Do.
Jan. 20,1 904
No. 3
15,960
1533
Do.
Do.
No. 2
l6,I02
1532
Do.
Do.
No. 2
16,092
1532
Do.
Mch. 31, 1904
No. 3
15,932
i53i
First and second prepara-
tions together.
Mean 15320
Melting temperature, 15320.
confidence that the extrapolation for these 3750 is reasonably correct
would, therefore, appear to be justified. Until the gas scale can be
extended over this range, the melting point of pure anorthite (15320)
determined in this way will serve as a useful point in thermometry.
Ab,An3 (PeatesII, III, IV, V).
This feldspar decidedly resembles anorthite in its relatively low
viscosity, the readiness with which it crystallizes, the well-marked
break in the heating curve at the melting point, and in its tendency to
form comparatively large crystals. In general, we may say that all
these characteristics are somewhat less marked than in anorthite.
Our determinations of the melting temperature follow.
38
ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
ABjANj.
FIRST PREPARATION.
Date.
Element.
Electromotive
force in MV.
Tempera-
ture.
Remarks.
Dec
9.
1903
A
15,501
I5040
Slow heating.
Dec
ii.
1903
A
15.363
!493
Rapid heating.
Do.
No. 3
15.507
1498
Do.
Dec.
12,
1903
No. 3
15,599
1505
Do.
Do.
No. 3
15,594
1505
Do.
Do.
No. 3
15,604
1506
Slow heating.
Do.
A
15,518
1505
Do.
Mean 15020
SECOND PREPARATION.
Apr
9,
1904
No. 3
15,520
1 4990
Slow heating.
Do.
No. 2
15,637
H97
Do.
Mean 149 8°
Melting temperature, 15000.
In one instance, while cooling the molten mass at a rapid rate, we
obtained a result which has a most important
bearing on the relation of the feldspars to one
another, which will be referred to again in the
concluding discussion of the experimental data.
When the charge had cooled, it was found to
consist of a compact mass of rather large crystals,
radial in structure, at the bottom of the crucible
(fig. 8), and a beautiful, transparent glass above.
It was easy to separate the crystalline portion
from the glass and to analyze the two separately.
The composition of the two portions is practically
identical, save for a slightly higher percentage of
-iron in the crystals. (A small quantity of iron
was contained in the quariz used in preparing
the feldspars.) In harmony with this latter
circumstance the color of the crystals was a
decided amethyst brown, while the glass was but slightly tinted.
The analyses follow:
Ab!An5.
Fig. 8.
Glass
residue.
Crystalline
cake.
SiO.,
AL,Os
Fe,Os
CaO
Na„0
47.46
33 • 56
a29
1699
1.87
47-34
33-51
•47
16.84
1.89
■
100. 17
100.05
INTERMEDIATE FELDSPARS.
39
It is at once clear from these determinations that the solid phase
has the same composition as the liquid phase, so far as it is within the
power of chemical analysis to establish it.
A-BiANj (Plates VI, VII, VIII, IX, X, XI).
In this feldspar we observe the same characteristics as in the two
preceding, but they are still less sharply marked. The viscosity is
greater, both solidification and melting take place more slowly, and
the undercooling is so persistent that the furnace must be cooled
slowly or the charge will come out wholly or partly vitreous.
Ab,Anj,.
first preparation.
Date.
Element.
Electromotive
force in MV.
Tempera-
ture.
Remarks.
Oct. 1 6, 1903
A
14,895
1459°
Rapid heating.
Do.
No. 3
15.14-
1460
Slow heating.
Oct. 21, 1903
No. 3
15,101
1457
Rapid heating.
Do.
No. 3
15,220
1466
Extremely slow heating.
Oct. 22, 1903
No. 3
15,204
1465
Rapid heating.
Do.
No. 3
15,160
1462
Dec. 15, 1903
No. 3
15,116
1467
Powdered charge, open
crucible.
Do.
No. 3
15,103
1466
Powdered charge, slower.
Dec. 16, 1903
No. 3
15,109
1467
Solid cake, covered.
Do.
No. 3
15,044
1462
Very fast.
Do.
No. 3
15,040
1462
Same, slower.
Do.
A
15,035
1467
Me
an 1463°
SECOND PR]
JPARATION.
Feb. 19, 1904
A
14,945
1460°
Covered, slow.
Feb. 20, 1904
No. 3
15,096
1466
Covered, faster.
Feb. 25, 1904
No. 2
15,239
1467
Fast.
Mec
in 1 464°
Melting temperature, 1463°.
Here again we made an attempt to discover a possible difference in
composition in the first portions to crystallize out of the melt, this
time by optical means. We first cooled the charge so rapidly that
only a relatively small portion crystallized out in fine, reddish-brown
spherulites at the surface and near the wall of the crucible. Without
disturbing these, the crucible was then replaced in the furnace and
slowly reheated (about five hours) until the remaining vitreous mate-
rial had also become completely crystallized. Upon removing from
the furnace, the charge presented a singular appearance. The red-
dish-brown stars remained undisturbed, while the later crystals were
perfectly white. But though so different in appearance, the micro-
4o
ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
scopic examination of slides cut from the different portions showed
the two to be optically identical.
We have here another instance of the tendency of the iron to con-
centrate in the crystals which first form, a tendency which was often
noticed throughout our work.* It also appeared to matter little
whether the first crystals formed at the surface or at the bottom of the
charge. This phenomenon may have significance in ore deposition,
but we have not thus far been able to give it adequate attention.
Ab,An! (Plates XII, XIII).
With this member of the feldspar group a difficulty in effecting
crystallization in the molten mass becomes noticeable. Undercooling
will continue until the vitreous melt becomes rigid, unless the cooling
is slow or some special effort in the way of mechanical disturbance or
the introduction of nuclei is applied. Furthermore, when once pre-
cipitated, crystal formation goes on slowly, even when the charge is
finelv powdered, and the crystals are always small. Of the feldspars
at least it is possible to say that the size of individual crystals varied
chiefly with the viscosity; the thinner, calcic feldspars always gave
large individuals, while AbiAni, Ab2Ani, AbaAiii and Ab4Ani crystal-
lized in closely interwoven, increasingly smaller fibers, which gave
much difficulty in microscopic study. In comparison with this appar-
ent effect of the viscosity, the rate of cooling was altogether insignifi-
cant in determining the size of individual crystals.
Several days were required to complete the crystallization of ioo
grams of AbiAm under the most favorable conditions which we were
able to bring to bear upon it. The melting temperature of the crys-
talline feldspar was still fairly well marked, however, and crystalliza-
tion began in the powdered vitreous material as low as 7000.
The melting point of this feldspar is:
AbjAnj.
Date.
Element.
Electromotive
force in MV.
Tempera-
ture.
Remarks.
Feb. 9, 1904
Do.
Feb. 10, 1904
Feb. 12, 1904
Feb. 27, 1904
A
A
No. 3
No. 2
No. 2
14,402
14,400
14.529
14.572
14,709
14160
1416
1421
i4J5
1426
Covered charge, heating
rapid.
Very rapid.
Very small charge.
Mean 14190
Melting temperature, i4igc
* See also J. P Iddings, Bull. Phil. Soc. Wash., Vol. XI, p. 97, 1888-1891
INTERMEDIATE FELDSPARS.
41
AboANj.
To effect the complete crystallization of this substance, it is best to
reduce it to a fine powder and heat very slowly, holding the temper-
ature for many days at ioo° to 2000 below the melting point. When
thoroughly crystallized, it has a melting temperature which is deter-
minable with reasonable certainty, but neither this nor any of its
thermal phenomena approach the more calcic feldspars in sharpness.
For this reason a considerably greater variation will be noticed in the
melting points tabulated below:
Ab2An!.
first preparation.
Date.
Dec. io, 1903
Dec. 15, 1903
Dec. 16, 1903
Do.
Jan. 18, 1904
Feb. 29, 1904
Do.
Element.
A
A
A
A
No. 3
No. 3
No. 3
Electromotive
force in MV.
13,726
13,887
13,969
13,728
13,967
13,812
13,854
Tempera-
ature.
I362c
1374
1381
1362
1376
1363
1366
Remarks.
Very rapid heating.
Poor.
Covered.
Do.
Do.
Mean 1 369°
SECOND PREPARATION.
Feb. 5, 1904
No. 2
13,990
13690
Covered.
THIRD PREPARATION.
Mch. 25, 1904
Mch. 29, 1904
Apr. 5, 1904
No. 3
No. 2
No. 3
13,752
13-995
13,756
1 358°
1370
1358
Mean 13620
Melting temperature, 13670.
From here on to the albite end of the series, viscosity becomes very
troublesome in restraining crystallization. The breaks which mark
the melting temperature on the heating curve of Ab3Ani are so slight
as to make the determination difficult and somewhat uncertain. It is
not that temperature measurement is less accurate here than else-
where, for these temperatures are more accessible than the melting
point of anorthite to which reference has been made in this connection.
These ultra-viscous materials do not melt at a constant temperature
but over a considerable range of temperature, as we shall undertake
to show in some detail, with illustrations from photographs, in the
discussion of albite. A glance at a series of curves (fig. 9) plotted
from our observations upon metallic silver and the feldspars An,
AbiAn5, AbiAn2 and AbiAni, in such a way as to bring their melting
points together, will show clearly the nature of this difficulty. The
melting point of the metal is sharp, but with anorthite a change in the
42
ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
character of the phenomenon is noticeable. Its poor conductivity for
heat and its viscosity, which, though small compared with the other
feldspars, are very great compared with silver, have rounded off the
corners until a really constant temperature for a period of a minute or
more during the melting is nowhere to be found. The nearest ap-
<
in
C
<
J
cl 1
I
c
j5
<
<
<
l
1
/
J
/
/
J
B
<
>>
XX)
o
L.
II
/
>
5
/
o
If)
/
c
o
/
10
>
/
/
1
/
/
L.
3
/
/
IU
L.
n
/
/
/
E
1
/
'
/
/
1
/
/
/
/
Time - I division = 5 minutes
Fig. 9. — Melting point curves of various feldspars compared with silver.
proach to a melting point is where the rise in temperature is slowest,
and this will occur when the portion nearest to the thermo-element
(see fig. 3) melts.
A series of melting-point curves containing a typical one for each of
the observed feldspars is reproduced on page 43 exactly as observed :
intermediate; feldspars.
43
Time Curves.
(In microvolts as observed).
An.
MV. AF
5050
5206
534i
544i
5526
5594
5650
5697
5738
5773
5802
5829
5853
5875
5891
5906
5920
5933
5945
5956
5965
5974
5983
5992
5998
6004
6012
6020
602S
6033
6040
6048
6056
6064
6071
6078
6087
6099
6110
6122
6i34
6149
6163
6181
6197
6213
6232
156
135
100
85
68
56
47
4i
35
29
27
24
22
16
15
14
13
12
1 1
9
9
9
9
6
6
8
8
8
5
7
7
7
9
12
1 1
12
12
15
14
18
16
16
19
At^An.,.
MV. AV
13530
13830
14130
14370
14560
I47I3
14838
14942
15028
151OI
15164
IS2I8
15264
15303
15339
I537I
15398
15423
15447
1546S
15488
15504
I552I
15537
15552
15567
i558i
15594
15608
15622
15637
15654
15672
15695
15733
15796
300
300
240
190
153
125
104
86
73
63
54
46
39
36
32
27
25
24
21
20
16
17
16
15
15
14
13
14
14
15
17
18
23
38
63
AbiAn..
MV. AF
1 1700
I22IO
12655
I296O
I3236
1 344 1
13595
13722
13832
13932
14022
14107
14186
14256
14323
14393
14456
I45I4
I457I
14620
14670
I47I4
14759
14797
14839
14877
14912
14947
14982
1 501 3
15046
15082
15118
15160
15211
15306
I54I9
5io
445
305
276
205
154
127
1 10
100
90
85
79
70
67
70
63
58
57
49
50
44
45
38
42
38
35
35
35
3i
33
36
36
42
5i
95
113
AbjAri!.
MV. AI/
134OO
13489
13573
13647
I37I5
13778
13836
13891
13942
I3991
14038
14083
14*25
14166
14206
14245
14284
14323
14363
14402
14444
14488
14538
14605
14676
89
84
74
68
63
58
55
5i
49
47
45
42
4i
40
39
39
39
40
39
42
44
50
67
7i
Ab^Ari!
MV. A]/
2480
2533
2590
2648
2701
2752
2797
2840
2881
2917
2952
2987
3020
3053
3088
3121
3154
3184
3215
3248
3283
3318
3355
3388
342i
345i
3483
35i6
3547
3576
3602
3627
3648
3664
3675
3696
3724
3758
3794
3833
3873
3914
3954
3998
4050
53
57
58
53
5i
45
43
4i
36
35
35
33
33
35
33
33
30
3i
33
35
35
37
33
33
30
32
533
3i
29
26
25
21
18
1 1
21
28
34
36
39
40
4i
40
44
52
Ab3An,.
MV. A V.
2754
2793
2834
2877
2916
2954
2989
3022
3053
3082
3109
3134
3160
3184
3207
3229
3250
3270
3288
3306
3321
3337
3354
3373
3393
34!5
3439
3466
3493
3520
3548
3576
39
4i
43
39
38
35
33
3i
29
27
25
26
24
23
22
21
20
18
18
15
16
17
19
20
22
24
27
27
27
28
28
The numbers represent the electromotive force of the thermo-
elements at intervals of one minute, together with a column of dif-
ferences at the right of each record. The B. M. F. will be seen to
approach a minimum as melting progresses and to increase again
when it is complete. This minimum rise in the temperature, of course,
indicates the maximum absorption of heat. For purposes of rough
orientation 10 MV may be considered equivalent to one degree.
44 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
There is no circulation in these viscous melts and nothing to assist
in distributing the heat uniformly. The melting point is, therefore,
not marked by a constant temperature but by the point of greatest
inclination of the tangent to the curve, with a limit of error which
increases with increasing viscosity. With Ab3Ani it was barely dis-
cernible, and with Ab4Anx all trace of the heat of fusion was lost.*
Slow heating or rapid heating merely acts to change the general incli-
nation of the curve but not to emphasize the absorption of heat.
By wav of conveying a concrete impression it may be added that
Ab3Ani just above its melting temperature resists the introduction of
a stout platinum wire (1.5 mm. diameter) unless the cold wire is thrust
in very quickly and vigorously. If the wire is first allowed to become
hot in the furnace, it will give way itself instead. No acceleration of
the melting process tending to sharpen the break in the curve appears
to be possible without the introduction of new substances or new con-
ditions (water vapor under pressure for example) which would take
the experiment outside the definition of a "dry melt." We have
undertaken some preliminary experiments in these directions, but
they belong to another phase of the subject.
A number of efforts were made to locate the melting temperature of
Ab3Ani, which are given in the list below. Although two days were
required to crystallize each charge of the material sufficiently for a
determination, the recorded numbers possess but little significance, as
will be clear from the foregoing.
Ab3Anx (Plates XIV, XV, XVI).
FIRST PREPARATION.
Date.
Element.
Electromotive
force in MV.
Tempera-
ture.
Nov. 23, 1903
Nov. 25, 1903
Nov. 28, 1903
Dec. 26, 1903
Jan. 14, 1904
A
A
A
A
No. 3
1 3.4 1 5
13.698
13,319
13,893
I3250
1336
1359
1328
I370
Mean 13440
SECOND PREPARATION.
Mch. 11, 1904
Mch. 14, 1904
A
No. 3
13,218
13.469
13200
1335
Mean 13290
Approximate melting temperature, 13400.
* Only a small portion of the charge could be crystallized. The relatively small
heat of fusion of the crystallized portion was, therefore, superposed upon the
larger specific heat of the glass. This, together with the effect of the viscosity,
destroyed all record of the melting.
ALBITE. 45
AbjAnj. (Plate XVII).
With Ab4Ani a third proof of the identity of composition of the first
crystals to separate and the vitreous residue was obtained. The
optical identification of this feldspar is absolute. If we could obtain
crystals at all in a melt of this chemical composition, therefore, it
would offer a crucial test of the relation of the solid and liquid phases
in a part of the curve where no melting point or specific gravity deter-
mination upon crystals was possible. After some days of nearly con-
tinuous heating at a temperature somewhat below its assumed melting
point, a number of crystals of Ab4Ani were obtained and identified.
Albite.
From the experiments upon natural albite and orthoclase, which
have been described, and after observing the effect of the increasing
viscosity as we approached the albite end of the artificial plagioclase
series, we had no expectation of finding a melting point for either in
the ordinary sense. Nor did we in fact succeed in locating a point of any
real significance in this connection. The various trials which were made
were simply calculated to throw all the light possible upon the char-
acter of the change from (crystalline) solid to liquid in such extremely
viscous substances. The return change or recrystallization of such
substances from the melt (solidifying point) without the introduction
of modifying conditions has never been accomplished. The time
required to do it is certainly very great, probably much greater than
the demonstration is worth at the present stage of experimentation in
this field.
Crystalline albite has been produced under exceptional conditions
several times — by Hautefeuille,* by heating a very alkaline alumino-
silicate with sodium tungstate for 30 days at 9000 to iooo0 ; by Friedel
and Sarasinf, using an atmosphere of water-vapor under very high
pressure and a moderately high temperature (an aqueo-igneous fusion) ;
by J. Lenarcict, at ordinary pressure and high temperature by crys-
tallization out of a mixture of melted albite and magnetite (1 part
magnetite, 2 parts albite by weight), and by others. It may be noted
in passing that, entirely apart from the solution relations, the last-
mentioned process reduces the viscosity to an entirely different order
of magnitude from that of pure albite ; magnetite melts to form a thin
liquid almost of the consistency of water and even in 1 : 10 solution
with albite forms a fairly mobile liquid. We endeavored to repeat
* Hautefeuille, Annales de l'Ecole Normale Superieure, 2d sen, 9, p. 363, 1880.
t Friedel & Sarasin, Bull. Min., p. 158, 1879; p. 71, 1881.
I J. Lenarcic, Centralblatt f. Min., 23, p. 705, 1903.
46 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
portions of the work of Hautefeuille and Lenarcic, but were obliged
to postpone a systematic inquiry into the conditions of crystallization,
which involved the addition of other components or extraordinary
pressures, until our plant could be somewhat extended.
Hautefeuille describes his successful preparation as a "solution" of
the alkaline alumino-silicate in sodium tungstate, out of which the
albite slowly crystallizes after long heating, but he remarks that the
crystallization does not take place if the mixture is heated sufficiently
to melt the components of the charge into a homogeneous glass. In
that case he obtained only a vitreous white enamel. His case does
not appear, therefore, to be one of simple solution, out of which the
same solid phase always reappears upon reproducing given conditions
of temperature and concentration. On the contrary, as Hautefeuille
describes the experiment, the components of the albite remain as inde-
pendent solid phases, which are then assembled in some manner
through the intermediary action of the melted tungstate.
Notwithstanding the fact that our interest was confined for the
moment to the mere production of a small quantity of chemically pure
crystalline albite, we ventured to proceed along the lines of Haute-
feuille's unsuccessful trial. We first prepared a chemically pure
albite glass, i. e., we melted the components into a homogeneous mass
before adding tungstate. This glass was then finely powdered, thor-
oughly mixed with an excess of powdered sodium tungstate, and
maintained continuously for 8 days at noo°. Upon removing from
the furnace at the close of the heating, both albite and tungstate were
found to have been completely melted and to have separated into two
distinct layers according to their specific gravities, the albite glass
being above, and showing no trace of crystallization. A second charge
was then prepared with equal parts of tungstate and albite, powdered
and mechanically mixed as before, and maintained at a temperature
of 9000 for 1 7 days. This time we were successful. After the sodium
tungstate had been dissolved away with water, the albite appeared as
a powder of about the fineness to which it had originally been pulver-
ized, except that the fragments were now crystalline and apparently
homogeneous albite. In thin section, under the microscope, to our
considerable surprise, it appeared that the original glass fragments
were unchanged in form. The bounding surfaces were all conehoidal
fractures, as they came from the hammer, and evidently had not been
in solution with the tungstate at all. Its optical properties showed it
to be undoubted albite and the specific gravity was 2.620.
The preparation of albite which we had synthesized by heating
with an equal weight of sodium tungstate was first purified by thor-
ALB1TE.
47
ough washing with warm water, but this was not sufficient to remove
all the tungstate. A determination of tungstic acid showed 0.62 per
cent still present, which is equivalent to 0.78 per cent of sodium tungs-
tate. After removing the water by heating carefully to a dull redness,
the product was submitted to a microscopic examination, which
showed it to be entirely crystalline and apparently homogeneous.
Determinations of the specific gravity gave 2.620 (see table, p. 58).
If this is corrected for 0.78 per cent of sodium tungstate of specific
gravity 4.2, we obtain 2.607.
A portion of the preparation was then purified further by fusing
for a few minutes with acid sodium sulphate (Hautefeuille) at as low
a temperature as practicable, after which the excess of sulphate was
extracted with water and the product dried (the temperature was
raised to a dull red heat to remove all water) and analyzed.
Found.
Calculated
SiO
68.74
19.56
"■73
.02
.16
68.68
19.49
11.83
Al.,6, and Fe,03 . . .
Na~,6
SO,
wo3
100. 21
The specific gravity of it was 2.604, which may be corrected as
before for the remaining trace of tungstic acid assumed to be in the
form of the sodium salt. The value then falls to 2.601.
A second portion of the same albite was purified by another process.
Instead of fusing with acid sodium sulphate, the powdered sample
was first digested for a short time with dilute hydrochloric acid (1:1),
which set free tungstic acid. The excess of hydrochloric acid was
removed with water, the tungstic acid with ammonia, and finally the
excess of reagent and the ammonium tungstate by further washing
with water. When dried at a low red heat, the preparation had the
following composition :
Found.
Calculated.
SiOs
Al.O,
Fe,Os
Na.O
W03
H,0
68.91
18.9s)
11-59
. 22
• 13
68.68
19.49
11.83
99.98
48
ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
The specific gravity determination gave 2.615, which, when cor-
rected for the small quantity of sodium tungstate becomes 2.612. If,
as is possible after the above treatment, the tungstic acid is present
as the anhydride, sp. gr. 7.1, the correction would lower the value
to 2.605, m excellent agreement with the other determinations.
The products of both methods of purification were carefully scrutin-
ized by the microscope, but no conclusion could be reached as
to which was the purer. Neither the sodium sulphate fusion, nor
the digestion with acid and ammonia appeared to have changed the
particles in the slightest degree. Diligent search was made for opaque
or amorphous matter on the surface of the grains, or any other indica-
tion of decomposition, but none was found. While the chemical anal-
ysis indicates a rather higher purity for the first product, purified by
fusion, the differences are nearly within the limits of error and, there-
fore, hardly conclusive. Both powders were ground finer than usual
for the specific gravity determinations to avoid errors introduced by a
spongy structure.
Reverting now to Hautefeuille's directions, it is clear that glass of
albite composition crystallizes homogeneously under substantially the
conditions which he obtained, as well or better than the mechanically
mixed component parts ; but the part played by the tungstate requires
some further experimental study before a conclusion can be reached .
Except for the specific gravity, the experiments upon crystalline
albite and orthoclase which follow were made upon natural specimens
from well-known localities (a fragment of the Mitchell County albite is
shown in plate XVIII), for which we are indebted to Dr. G. P. Merrill of
the United States National Museum and Dr. Joseph Hyde Pratt, State
Mineralogist of North Carolina. The specimens were selected with
great care, but like all natural specimens, they contained other feld-
spars and inclusions. The analyses follow :
Albite, Amelia Co.,Va.
Nat. Mils.
Albite, Mitchell Co., N.C
(Pratt).
Orthoclase, Mitchell Co..
N. C, Nat. Mus.
Found.
Calculated to
anhydrous
composition.
Found.
Calculated to
anhydrous
composition.
Found.
Calculated to
anhydrous
composition.
SiOs
A1203
Fe,03
CaO
Na.,0
k2o
H20
68.22
19.06
•15
.40
11.47
. 20
.69
68
19
1 1
7i
20
15
40
53
20
66 . 03
20.91
.18
2.00
9-97
.70
•59
66.42
21.03
.18
2.00
1 0 . 03
.70
65
17
2
12
49
98
36
42
29
95
ST
65
18
2
13
83
07
36
42
30
02
IOO. 19
100.38
IOO.OO
ALBITK.
49
It will be remembered that in the preliminary experiments (p. 28
et scq.) the heating curve of these natural feldspars did not show an
absorption of heat which we were able to detect ; our first step was,
therefore, to find out what manner of process it was by which a charge
of crystalline albitc or orthoclase became amorphous without leaving
a thermal record behind.
We prepared a charge of albite glass from a previous melt powdered
to " 100-mesh." In this glass powder a small crystal fragment (per-
haps 2 x 5 x 10 mm.) from the same original specimen and, therefore,
of the same chemical composition, was embedded beside the thermo-
10.
1 1.
Fig. 10. — Albite crystal embedded in charge of powdered albite glass.
Fig. 11. — Same after heating.
element as indicated in fig. 10. This charge was heated slowly to
exactly 12000, slowly cooled again, and several thin sections prepared
from the crystal fragment and its immediate neighborhood. What
the microscope showed can best be seen from the accompanying
illustrations (Plate XX) — groups of crystal fragments of microscopic
size, preserving their original orientation (extinction) perfectlv, but
with narrow lanes of glass where cleavage and other cracks had been,
forming a perfect network without a trace of disarrangement. Con-
siderable melting had taken place but no flow. Neither had the
charge as a wrhole made any movement to take the form of the con-
taining vessel after sintering together (fig. 11).
50 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS
Surmising that we had accidentally hit upon the approximate melt-
ing temperature, a fresh charge of like material was prepared and the
same experiment carefully repeated, except that the temperature was
carried up to 12060 and maintained there for 30 minutes. Instead
of showing the melting to be complete, the slides (Plate XXI) looked
precisely like the first, save that the lanes of glass were somewhat wider
and the crystal fragments relatively smaller than before. Further
trials under precisely the same conditions, with the temperature in-
creased to 12250 (Plate XXII) and 12500 (Plate XXIII), respectively,
for like periods of time, showed only more advanced stages in the same
process. In the latter case the remaining crystal fragments were rela-
tively very small compared with the separating lanes of glass, but the
orientation of the tiny particles still remained perfectly undisturbed.
The evidence contained in this series of slides shows plainly that we
have here an unfamiliar condition — a case of a crystalline compound
persisting for a long time above its melting temperature for a given
pressure. Albite or orthoclase glass sinters tightly at 8oo°. At the
temperature where melting began, therefore (below 12000), the charge
consisted of crystal fragments of microscopic size embedded in a large
vitreous mass of the same composition and known temperature.
These fragments melted so slowly over the 500 included between the
first slide and the last, with the rate of heating slow (i° in 2 minutes)
and the upper temperature continued for 30 minutes, as to leave con-
siderable portions unmelted at the close. Furthermore, the extreme
viscosity, of which further evidence will be given directly, and the
absence of any disturbance in the orientation of the particles indicat-
ing flow, assured us that the lanes of glass represented actual melting
and not an inflow of glass from without. Finally, the perfectly homo-
geneous character of the glass and the unchanged appearance of the
crystals as heating progressed gave no hint of any chemical decom-
position.
In the hope of obtaining a point of value for comparison with the
melting points of the other feldspars, some time and patience were
expended in trying to locate the lowest temperature at which certain
evidence of melting appeared. We did not extend any single trial
beyond a single day, so that our results can not pretend to establish
the lowest point at which albite melts. Such an effort with a natural
specimen known to contain impurities would yield nothing of value.
Mitchell County albite showed signs of melting after four hours at
1 ioo°. Under a high power the crystal edges appeared weathered or
toothed — strongly resembling the incipient melting of the ice on a
frosted window pane. These extremely fine teeth could be followed
ALBITE. 51
through the slide on exposed edges. At 11250 (Plate XIX X 600) a
four hours' heating gave unmistakable glass in tiny pockets and lanes.
The above experiments with the Cloudland albite were completed
before we obtained the Amelia County material, but the latter proved
to be so much nearer to the type of pure soda feldspar that nearly all
the experiments were repeated with it, except that the crystal blocks
were embedded in powdered crystals. We did not develop any new
fact, however; the effects noted above reappeared in the same order,
except perhaps that melting went on a little faster in the Amelia
County specimen. As much melting was found after one-half hour
at 1 2000 with the Amelia County sample as the Cloudland (Mitchell
County) albite showed in the same time at 12250, which is readily
enough explained by the relatively large quantity of lime (anorthite)
in the latter.
Since both time and temperature enter into the delimitation of the
metastable region, further trials at temperatures above 12500 did not
seem likely to add anything to the knowledge already obtained. And
if the heating were very rapid, the temperature differences within the
charge would be considerable. A few isolated crystalline fragments
were found in a microcline melt which had been heated as high as
14000 for another purpose. Another which had reached nearly
15000 showed no microcline, but one or two minute quartz inclusions
still remained undissolved.
We made a rough attempt to get a more tangible idea of the viscos-
ity of these feldspars at their melting temperature in the following
way: A long, slender sliver (perhaps 30 X 2 X 1 mm.) of albite and
one of microcline were chipped from larger portions, spanned across
small empty platinum crucibles, and placed side by side in the furnace.
These exposed crystals were heated to 12250 for three hours. When
removed they were completely amorphous (melted), but retained
their position with hardly a trace of sagging.
After this a number of similar slivers were prepared, mounted in the
same way, and heated to temperatures of from 12000 to 13000 for a
few moments. At their highest temperature a platinum rod was in-
serted through a hole in the top of the furnace and allowed to rest as a
load upon the middle of the crystal bridges. Under this load the
partially melted slivers gradually gave way and were taken from the
furnace in the various forms shown in the illustrations. Slides cut
from these showed no squeezing out of the melted portion between the
crystal fragments on the side toward the center of curvature, or open
cracks on the outer side (Plates XXIV, XXV, and XXVI) . It will be
noticed that the melting began on the convex surface, where the
52 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS
strain was greatest. On the other hand, a variable extinction angle
in an unbroken crystal fragment frequently gave unmistakable evi-
dence of the bending of the crystal as well as the vitreous portion.
From these qualitative experiments it seems possible to assert with
confidence that the order of magnitude of the viscosity of the molten
portion (glass) is the same as that of the rigidity of the crystals at
these temperatures. Plate XXIV shows a piece of Mitchell County
albite heated to 12000 under load. The sagging is indicated by the
curved cleavage cracks. A sliver of microcline, similarly treated, is
reproduced in Plate XXV. The displacement is shown by the curva-
ture of the crystal edges and the cleavage cracks ; the black portions
are glass. It is interesting to observe that while the crystal has
melted completely across, there has been no displacement of the
cleavage plane (indicated by a clotted line).
Plate XV is from a charge of composition Ab3Ani which had been
heated to 13750 and completely melted. It was then allowed to cool
slowly in the furnace. On the following clay it was reheated to about
12500 for most of the day. The slide was made from this mass. The
dark portions of the slide are glass in which the crystals were induced
by the subsequent reheating. At first sight it would seem that crys-
tallization ought to be complete after the mass had been allowed to
cool in the furnace and had been reheated for six hours at a tempera-
ture within 125 degrees of its melting point, but the slide plainly
shows that equilibrium is reached very slowly in melts of this extreme
viscosity, even after nuclei have formed.
The preceding experiments gave a clear idea of the phenomena
attending the melting of albite and orthoclase, and convinced us that
the absorption of heat accompanying fusion, which we had searched
for in vain upon the heating curves in the earlier experiments, had
eluded us merely because it was extended over so long a stretch of the
curve as not to be noticeable. Some very exact measurements of the
temperature change from minute to minute were therefore made in
the hope that a more intelligent search might be more successful.
Separate charges of glass and of crystals of the same composition and
of equal weight were prepared and successively heated in the same
furnace with the same current. The specific heat is, of course, not
identical in the two cases, but the curves were comparable in form.
Above 11000 we felt sure that one of the curves must contain an
absorption of heat which would be absent from the other. Such a
pair of curves (I), taken from the microcline measurements, is repro-
duced in the adjoining figure (fig. 12), and appears to show such an
absorption clearly, extending from 11350 to 12750. The dotted line
ATJBITK.
53
13000
12500
12 000
C '1500
a)
3
a.
E
11000
("35°)
10500
10000
9500
(1000°)
/
/
1
/
'
fn
/
/
I
I
/
/
j
f
/
/
/
V
/
//
1
1
y
/
/-
/
1
//
1
1
1
A
'/
III
1
1
1
/
//
1
If
/
//
II
/
1 ,
1
//
/
//
/
//
1
/
'/
/
/
/
1
/
/
/ /
1
1
1
/
/
/
/
/
/
/
^
1
/
/
/
I
1
\
I
Time - I division =10 minutes
Fig. i2. — Curves showing the absorption of heat in melting orthoelase.
shows the course of the curve without the absorption, as inferred from
the glass curve. The same figure contains two other curves (II, III),
similarly obtained, which were made upon fresh charges of the same
54 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
material but with different rates of heating. It will be noticed that
the absorption begins to be noticeable at a slightly lower temperature
if the heating is slower.
This peculiar behavior shown by compounds which melt to form
hyperviscous liquids seems not to have been observed before and to
contain features of more than ordinary interest. Here are evidently
crystalline substances which not only can exist for considerable peri-
ods of time at temperatures far above their melting temperatures,
but which melt with extreme slowness in the lower portion of this
range of instability. It would certainly be no exaggeration to say
that the albite with which we worked would require some weeks to
reach the amorphous state if maintained at a constant temperature
of 11250.
An interesting question arises here as to the state of the crystalline
material at temperatures above its melting point. It is easily con-
ceivable that the crystals are merely superheated without loss of any
of their properties as solids, and that they thus present an analogy to
superheated liquids. In the transformation (Umwandlung) of a solid
crystalline substance into another crystal form such superheating has
long been known. The change is dependent upon temperature and
pressure like ordinary fusion, but it is possible to pass the transforma-
tion temperature in either direction. This must be due to the unfa-
vorable opportunity for molecular motion which solids afford, and the
latter should differ in no essential particular from ultraviscosity.
On the other hand, it does not seem a violation of any known prin-
ciple to conceive cases of unstable equilibrium in which the molecules
of a liquid are oriented as in a crystal. Maxwell's demons might
arrange them much like a school of fish, and there is no apparent reason
why the fluidity should be destroyed thereby. Were such an arrange-
ment one of minimum potential, the mass would be a liquid crystal.
In the supposed case such a substance would possess a melting point
dependent upon the temperature and pressure above which Maxwell's
definition* of a true solid — that its viscosity be infinite — would no
longer obtain, although deorientation might not become apparent, in
the face of extreme viscosity, for a considerable time afterward. Such
a melting point would be determinable only with the greatest diffi-
culty, for all the functions — mechanical, thermal, or electrical — which
usually become suddenly discontinuous at the melting point would
be equally powerless to define a change of state in the face of such
extreme molecular inertia.
* Maxwell's Scientific Papers, vol. 2, p. 620
SPECIFIC GRAVITY. 55
In substances like these, which we found to be still viscous at the
temperature of the electric arc, the sharpness of a minimum due to
heat absorption, for example, is not dependent upon the magnitude
of that absorption entirely, but also upon the rapidity with which the
change which involves it proceeds. In albite and orthoclase the
velocity of this change is very small.
SPECIFIC GRAVITY.
The study of the specific gravities yielded one interesting result
which was not anticipated. The artificial feldspars, being chemically
pure and homogeneous, gave a perfectly definite specific gravity which
could be determined with great accuracy if the specimen was com-
pletely crystallized. If vitreous inclusions were still present, the
results were of course variable and were all too low. It was antici-
pated that the specific gravity of pure glasses, even when transparent
and free from bubbles, as they were in the more calcic members of the
series, might yield values varying more or less with the rate of cooling,
or after annealing, but this did not prove to be the case. Our results
did not vary more than two units in the third decimal place in the same
preparation, even with the more calcic feldspars, which required to be
very rapidly chilled in order to cool the melt without crystallization.
The determination of specific gravities is a trite subject, but we have
found the common methods liable to such grave errors that we ven-
ture to give some useful details. The error due to the evaporation of
water about the stopper of the picnometer is very much less with
finely ground stoppers than with coarse grinding, and if the stopper
is slightly vaselined just before the final weighing the error from this
cause will hardly affect the third decimal place with 25 cc. picnometers.
The simplest form of flask with a small capillary opening in the
stopper is, in our judgment, far superior to one carrying a ther-
mometer. The temperature should be made sure by the use of the
thermostat.
For removing the air from a powdered charge, we used the device of
G. E. Moore,* slightly modified, as indicated in the accompanying
sketch (fig. 13). The bulb A contains boiled water. When the appa-
ratus is exhausted, the water is allowed to flow back into the picnom-
eter containing the charge, then by tapping and warming with water
at 400 to 500 to produce boiling within, the air is effectively removed.
The material projected from the flask, if the boiling is violent, is then
washed back from the tube B with boiled water, and any small particles
* G. E. Moore, Journ. prakt. Chem., 2, 319, 1870.
M°
■■ :
56 ISOMORPHISM AND THERMAL PROPERTIES OP FELDSPARS.
remaining are washed into a tared dish and finally weighed. It is
very important that not the smallest grain of material should get into
the ground joint between the neck and the stopper of the picnometer.
To obviate this, wipe out the neck with filter paper before stoppering
and burn the paper in the tared dish. If the powder is very fine, it is
advisable to allow the filled picnometer to stand for some hours in the
thermostat in order that suspended material may settle. With a
25 cc. picnometer and 5 to 10 grams of material, this method usually
yields concordant results to the third decimal place, and the error from
all causes should never be greater than 2 units ( ± 1) in the third place.
Aspirator
Picnometer
with charge
Fig. 13. — Apparatus for specific gravity determination.
A determination of this accuracy is of course subject to a correction
for buoyancy, and all the numbers which follow have been thus
corrected.
There is another error to which accurate specific gravity determina-
tions upon powdered minerals will be subject unless suitable precau-
tion is taken. The exposure to the air during the period of grinding
the samples gives opportunity for the condensation of sufficient atmos-
pheric moisture upon the grains to affect the weight in air. The
amount varies measurably with the size of the grains, as will be seen
from the accompanying data, and probably with the degree of satura-
tion of the atmosphere and the time of exposure.
SPECIFIC GRAVITY.
57
Determination of Moisture in i gram of Powdered Mineral
upon Exposure to the Air.
Mineral.
Fineness (mesh).
Moisture.
Orthoclase (natural glass)
Ab[ An5 (artificial glass)
AbjArij (artificial crystal) ....
AbjArij (artificial glass)
AbiAn, (artificial crystal) ....
Ab (natural crystal)
Do
<I5Q
Selected, coarse
<C ioo > I 20
< 100 > 120
< 100 >■ 120
Coarse
< 150
Gram.
0 . 006 1
.OOOO
.OOIO
.0007
.OOIO
.0006
.0069
Orthoclase (natural crystal).. . .
Do. (same sample)
Do. (same sample)
< 120 > 150
< 150
Still finer.
.OOI I
.0031
.0059
Orthoclase (artificial glass) ....
Do. (portion of same.) . .
Everything <C 100
> 150
.0065
.0022
<^ = finer than. > = coarser than.
In the last two groups, note that the moisture in graded portions of
the same sample varies with the fineness.
We also verified the conclusion of Bunsen* that this adsorbed mois-
ture is not removed at temperatures only slightly above ioo°, but
requires 6oo° to 8oo° — equivalent to a low red heat. Several samples
for which the moisture had been determined were laid away in corked
test-tubes for a number of weeks, after which redetermination gave
exactly the former value.
It is worth noting in this connection that these measured quantities
of adsorbed water are of the same order of magnitude as those usually
obtained for the water content in feldspar analyses,! where again, of
course, the finer the sample is ground for the analysis the greater the
possible error from this cause. It may be that a part and occasionally
all of the moisture usually found in these analyses is adsorbed and the
significance of its presence there mistaken.
The number of feldspars of which specific-gravity determinations
could be made was limited only by the possibility of obtaining com-
plete crystallization within a reasonable time. Thus Ab2An! was
reheated many times before a constant value was reached. Ab3Ani
required 17 days and Ab4An! was not completely crystallized in any of
our attempts. Crystalline albite was produced under other condi-
tions.
* Wied. Ann., 24, p. 327, 1885.
| Dana, System of Mineralogy, 6th ed., pp. 314 et seq.
58
ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
The specific gravities of the glasses and of so many of the crystalline
mixtures as we could obtain are tabulated below:
Specific Gravities of Artificial Crystalline Feldspars.
[Determinations in duplicate are braced together.]
An.
AbiAn5
(2.764 (2.734
(2. 765 (2.734
0,2 . 767
Ab,An.,.
AbjAni.
2.710 J2.6SO
2.-08 : (2.680
a_f2.732 , J2.7IO
'2. 732 '2.710
A2.734
Mean 2.765 2.733
10
b\2p9
(2.677
2.679
Ab,An,
(2 .660
2.660
2. 660
U2.659
( 2 . 660
2.660
AbaAnj.
(2 .650
"< 2 . 648
.649
Ab.
First de-
termina-
tion.
' 2 . 620
1 2.620
.(2.614 2.601
(2.61.S
'5
e2 . 604
Corrected,
2.607
I 2.612
g 2 . 605
2 ■ 605
Specific Gravities of Feldspar Glasses.
(2.700 (2.647
"( 2 . 700 { 2 . 649
(2.648
"(2.647
(2.648
1 2 . 649
(2.647
(2.647
(2
"(2
(2
12
< 2
"(2
593
594
59i
59i
590
588
Mean 2.700
64S
2-591
(2.533
(2-534
(2.482
"(2.482
<Z2
485
533
2-483
(2.458
(2.459
2-383
2.382
2.458 2.382
a Another preparation.
b Same material reheated for several days at temperatures about 150° below the melting
point.
c Contained about 0.8 per cent of sodium tungstate.
d Purified by warming with dilute hydrochloric acid, then with water, and afterwards with
ammonia.
e Purified by fusion with acid sodium sulphate.
/ Assuming the residual tungsten to be present as Na.W04.
g Assuming the residual tungsten to be present as WO,.
SINTERING.
Incidental to this work upon the relation between the feldspars, we
made a great many observations upon the sintering of powdered min-
erals, both crystalline and vitreous, of natural and artificial composi-
tion. While the results have not enabled us to offer positive conclu-
sions of importance, they are worth a note in passing. Powdered
glasses sinter slowly or rapidly several hundred degrees below the melt-
ing temperature of crystals of the same composition. When the vis-
SINTKRING. 59
cosity is relatively small (anorthite) crystallization begins at a low
temperature and proceeds very rapidly, the sintering probably being
due to the interweaving of the crystal fibers during their formation.
In viscous glasses (albite) sintering also begins at very low tempera-
tures— the finer the powder and the slower the heating, the earlier the
first traces appear. Long-continued heating, even at comparatively
low temperatures, yields a perfectly continuous cake (except for the
included bubbles) the surface area of which constantly tends toward a
minimum. There is no doubt that the sintering of powdered glasses is
due to flow in the undercooled liquid and is a phenomenon in viscositv
and surface tension. All the feldspar glasses sintered readilv between
7000 and 9000, depending on the fineness of the powder and the time.
Powdered crystalline feldspars do not sinter readily below their
melting temperature. Indeed, we were at first inclined to the view
that when only pure, dry, stable crystals are present they do not
sinter at all, however finely they may be powdered. We observed the
phenomenon in natural albite at 10000, but the crystals were not
wholly free from inclusions which may have caused chemical reactions
resulting in cementation. Crystalline fluorite also sinters 3000 below its
melting temperature, but here we were able to establish a decomposi-
tion; acid fumes were evolved during the experiment, and the sintered
product contained 1 per cent of free lime. Our final experiments with
long-continued heating for specific-gravity determinations, however,
showed that the purest feldspars which we could prepare, even after
they had reached their maximum density, still sinter very slowly. Thus
AbiAn5 powder, which was shown by a determination of its specific
gravity to be holocrystalline, formed a compact chalky mass in four
hours at a temperature about 1 500 below its melting point ; in three
davs the cake was as hard as porcelain. Other feldspars showed the
same behavior. It is hardly possible that inhomogeneities sufficient to
produce diffusion between portions of different concentration could
have existed in these charges. There is considerable indication that
some of the crystalline nuclei grow at the expense of others — perhaps
through exceedingly slow sublimation — which may account for it.
We made repeated attempts to locate some fixed sintering point
which should be characteristic of a particular material by means of
continuous measurements of the electrical conductivity, but thev
all indicated that no such point exists. The conductivity of a
dry powder increases enormously after sintering begins and would,
therefore, seem to offer a most sensitive test, but the phenomenon is
altogether gradual, even with a crystalline feldspar containing only a
small percentage of glass. We purpose to extend these observation?
to other substances.
6o
ISOMORPHISM AND THERMAL PROPERTIES OP FELDSPARS.
CONCLUSIONS.
It now remains for us to gather the results together and to draw-
such conclusions as they appear to justify.
(i) If the melting points are now plotted in a system of which they
form the ordinates, while the percentage compositions of the different
feldspars form the abscissas (fig. 14), we discover, within the limits of
accuracy of possible measurement at these temperatures, a nearly
linear relation ; the melting point varies very closely with the compo-
sition. We have no maximum, no minimum, no branching of the
curve, but from each fusion there separates a solid phase of the same
1600
xa 1400
4J 1300
£ 1200
I-
1100
, _^___ , ^. ^
An
Ab,An5
An 100
84.1
Ab 0
15.9
Ab, An.
Ab2An,
Ab3An,
51.5
34.7
26.1
0 An
48.5
65.3
73.9
100 Ah
Ab,An2
68.0
32.0
Percentage composition
Fig. 14. — Curve of melting temperatures of the soda-lime feldspars.
composition as the vitreous matrix. In Abi An5 it will be remembered
that this was proved by the separation and analysis of the two phases ;
in Ab!An2 partial crystallization was accomplished in the first cooling
and the remainder in a subsequent reheating and cooling, the two
groups of crystals proving optically identical; a small quantity of
Ab4Ani, which admits of absolute identification optically, was crystal-
lized out of a melt of that composition and readily recognized. More-
over, evidence to show that the same phase always separated was
likewise presented.
Stated in this way, the relation appears to be a simple additive one
in which liquid and solid phases of like composition are stable in all
proportions of the components and behave like a series of separate
feldspars. But as soon as we consider it with reference to the laws of
solution and the phase rule, it can not be explained in this simple way.
CONCLUSIONS. 6 1
First of all, the phase rule tells us at once that we can have no true
compound here between the components albite and anorthite, for such
a compound would mean one more component and an additional phase
in every solution before equilibrium could be established. Moreover,
if the mixture had been eutectic in character, the component (albite or
anorthite) which happened to be in excess would have crystallized out
in each case, causing a continual change in the composition of the re-
maining glass until the eutectic proportion was reached and the result-
ing charge would have contained only crystals of one (or, in case of
hysteresis, both) of the components and the eutectic. Our curve is
continuous and the resulting charges homogeneous for all proportions
of the components. Lane's suggestion* that the triclinic feldspars
form a eutectic series in which the eutectic proportion is at or near
Ab2An3 is, therefore, not borne out by our experiments.
Laying aside the eutectic mixture, and passing over to solutions of
components which are miscible in many or all proportions, we find a
small number of examples, chiefly organic compounds, which have
been studied as types by Roozeboom, Kuster, Bodlander, Garelli,
Bruni, Van Eyk, and others, among which our series appears to fall.
APPLICATION OF THE LAWS OF SOLUTIONS.
From the physico-chemical standpoint, the case we now have in
hand closely resembles Kuster's problem of 1891.! His measurements
were made upon mixtures of organic compounds of low melting point,
while ours reached a maximum temperature of 15320, but we have,
between albite and anorthite, an exactly similar series of solid solutions
the melting pointsj of which change in nearly linear relation to the
percentage of the two compounds which enter into their composition.
This simple linear relation was called by Kuster perfect isomor-
phism, and he formulated the ' ' Rule " which has since borne his name,
that the solidifying point of an isomorphous mixture lies on a straight
line joining the melting points of the components and can be calcu-
lated from the percentage composition of the mixture. If this line
proved to be slightly concave or convex, as it did in most cases,
imperfect isomorphism was assigned as the cause. To this rule an
* Lane, Journal of Geology, xu, 2, p. 83, 1904.
f F. W. Kuster, Zeitsehr. fiir Phys. Chem., 8, p. 577, 1891.
X Kuster measured solidifying points, but we have pointed out above that such
measurements lead to no positive result in liquids of such viscosity as the feld-
spars, in which equilibrium is not established during solidification. Undercooling
rarely appeared at all in Kuster's cases.
62
ISOMORPHISM AND THERMAL PROPERTIES OP FELDSPARS.
objection was raised by Garelli* and elaborated by Bodlanderf — if the
solid solution behaves like other solutions, a small quantity of com-
ponent B added to component A can only lower the solidifying point
of A when the solid phase is richer in A than the liquid phase. The
reasoning is this (Bodlander): Let X\ (fig. 15) be the vapor-tension
curve of component A in the liquid state, y\ the solidifying point (t{)
of A, and z\ the vapor-tension curve of solid A . Now, if a small quan-
tity of B is added and the solid phase which crystallizes out contains
the same proportions of A and B as the liquid mixture in which it
formed, the vapor tensions of the liquid and solid phases must have
been lowered equally and the solidifying point will fall at y2 with the
same temperature as the pure solvent. (Equality of vapor tension
100 A •« — Composition
Fig. 16.
100 B
in the solid and liquid phases determines the temperature of change of
state.) If A crystallizes alone from A +B, the vapor-tension curve
will continue on to z2 and the temperature of solidification fall to t2 ',
while if the solid phase contains both components but is richer in A
than the liquid phase, solidification will occur at an intermediate point.
Fig. 16 will serve to show the crucial character of the issue raised.
The ordinates represent temperatures and the abscissas percentages
of A and B. Kuster finds his solid and liquid phases identical in
composition within the limits of experimental error and the solidify-
ing temperature on the line A B at a point which can be determined
from the proportions of the components — at d for example. But the
laws of dilute solutions tell us that if the phases are identical in com-
position the solidifying point of A -+- B must fall at c, i. c, must
remain the same as for pure A .
The temperatures at which Krister's observations were made and
their painstaking character leave no doubt as to the validity of the
*F. Garelli, La Gazzetta Chimica Italiana, xxvi, p. 263, 1894.
t Bodlander, Neues Jahrb. f. Min., Beilage, Bd. xn, p. 52, 1899.
CONCLUSIONS. 63
experimental fact. Neither can it be objected that Krister's solutions
were not sufficiently dilute to reveal the true relation, for the observa-
tions upon naphthaline and ,5-naphthol have been repeated by
Bruni* with very dilute solutions of one of the components in the
other, and completely verified.
Now, the laws of solutions hold for solid solutions even for moder-
ately high concentrations (Bodlander) when the components are not
isomorphous, and on the other hand, even liquid crystals, when iso-
morphous, follow Kuster's rule more nearly than the law of solutions.
An extended discussion of existing data from this standpoint would
involve us in unnecessary detail ; but there can be no question that
Kuster's rule represents the data which have been gathered upon
isomorphous mixtures — at least approximately — while the laws of
dilute solutions appear to fail of application there. On the other
side, the rule admits of no independent theoretical derivation. Van't
Hofff suggests that judgment be suspended pending the accumula-
tion of further data and intimates that the close similarity of chemi-
cal composition and molecular structure in compounds which form
isomorphous mixtures gives them an unusually close inter-relation,
and their influence one upon the other may render a simple theo-
retical treatment very difficult.
Our case is especially interesting when considered from this stand-
point, but it distinctly emphasizes the difficulty rather than helps
toward its solution: (1) Although the chemical reactions of albite
and anorthite are not of such a character as to prove or disprove a
close analogy between them, a comparison of their formulas certainly
does not suggest an isomorphous relation. If their formula weights
represent true molecules, they possess the same number of atoms to
the molecule (NaAl S13O8, CaAl2 Si208) and the group Si208 in com-
mon, but the remaining atoms taken separately are not mutually re-
placeable. (2) The melting points of the components in the feldspar
series are very far apart — more than 3000 — while Kuster's organic
mixtures were all included within a narrow temperature interval
(20 to 560). For reasons which will appear presently, both GarelliJ
and Roozeboom have pointed out that the farther apart the melting
points of the components the less probable is the linear relation.
(3) The homogeneity of the solid phase is established within 1 per
cent by the optical examination of the slides. Moreover, separate
chemical analyses of the solid and liquid phases of the mixture Abi An5
*G. Bruni, Atti della reale Accademia dei Lined, 5, vn, p. 138, 1898.
f Van't Hoff, Vorlesungen ub. Theoret. u. Phys. Chem. (Braunschweig, 1901).
Part II, p. 64.
\ F. Garelli, loc. cit.
64
ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
in an exceptionally favorable case showed still closer identity of
composition.
It appears altogether improbable that the laws of solutions can
apply in the face of so extreme a controverting case.
If it has proved difficult to bring the isomorphous mixture within
the general laws of solutions, a most satisfactory theoretical deriva-
tion of the conditions of equilibrium in such mixtures has been
developed by Roozeboom. No other principle is required than the
second law of thermodynamics as applied to solutions by Gibbs:
A system of substances will be in equilibrium for a particular pres-
sure when the thermodynamic potential (C-function) of the system
is a minimum. The scheme of representation is the graphical one
proposed by Van Ryn Van Alkemade,* and is itself a powerful
instrument of analysis in this field.
P, T constant
*Zeitschr. f. Phys. Chem., n, p. 289, 1893.
Except for the suggestions of Vogt to which reference has been made, this
method seems not to have been utilized for the study of mineral solutions before.
A brief outline of it will, therefore, be given here.
In a system of rectilinear coordinates (fig. 17) the ordinates may represent the
potential of a particular system — (Gibbs' ^-function, not directly measurable) and
the abscissas the number of gram-mole-
cules of solvent (water for example)
supposed to contain 1 gr. mol. of solute.
In other words, every point of the curve
represents a solution of which the x
coordinate is concentration and the y
coordinate the potential. The condi-
tions of pressure and temperature are
assumed constant for a particular dia-
gram.
Every such curve for substances solu-
ble in all proportions will be convex
downward, otherwise there would be some particular point on the curve which
would not represent a minimum potential for a particular composition and the
solution would tend to separate into two, the mean potential of which would be
lower.
The condition for equilibrium between such a solution and its solid phase (pure
salt) may now be readily found. Lay off on the ^-axis a distance equal to the
potential of the solid salt and from the point so obtained draw a tangent to the
curve. This tangent is the locus of minimum potential (stable systems) for any
composition. At the point a, for example, we have a saturated solution contain-
ing the number of gr. mol. of solvent indicated by the corresponding abscissa and
etc
the proportion -7 of salt, the balance of the salt remaining in solid phase. At b
we have the saturated solution with all the salt included ; to the left of b upon the
curve, supersaturated solution; and to the right unsaturated solution. With
increase of temperature the form of the curve changes and c approaches d, the
melting point of the salt.
Concentration
Fig. 17.
CONCLUSIONS. 65
Roozeboom distinguishes three general classes of isomorphous
mixtures :
(1) The components are miscible in all proportions from o to 100
per cent in both solid and liquid phases.
(2) Miscibility is limited to certain concentrations.
(3) More than one type of crystal occurs.
In the feldspars we are concerned with the first class only, but here
also Roozeboom distinguishes three possible types:
Type I. — Melting (or solidifying) points of the mixtures lie on a con-
tinuous curve joining the melting points of the components and con-
taining neither maximum nor minimum.
Type II. — The curve contains a maximum.
Type III. — -The curve contains a minimum.
These types are for the moment purely hypothetical and are a prod-
uct of the method of analysis, though they are being rapidly identi-
fied for various isomorphous pairs by pupils of Roozeboom and br-
others.
The method of reasoning which yields these three possible types
will be briefly described with the help of the Van Alkemade graphical
analysis :
If we indicate the potential (~) of a particular mixture by the length
of the ordinate (fig. 18), and the number of molecules of .4 and B by
subdividing the horizontal axis (A -\- B = 100) in the proper propor-
tion, assuming atmospheric pressure and constant temperature for
each diagram, then every point within the coordinates represents a
particular phase of known composition and potential. Suppose, now
(Roozeboom), a temperature is assumed above the melting point of
the higher-melting component; clearly, whatever the composition,
only the liquid phase can have a stable existence. If potential differ-
ence represents the measure of the tendency to change and the
tendency of all change is toward the minimum potential, for this tem-
perature all change will be toward the liquid ; and the potential of a
solid, if one existed there, would be greater than that of the liquid for
all compositions — hence the curve 5 (solid) above the curve L (liquid)
throughout.
Suppose the potential to be lowered to a point where crystallization
can begin. The tendency to melt no longer obtains for all composi-
tions ; the two curves will be displaced relatively and, being of different
form, will intersect. Draw a common tangent to the curves and apply
Van Alkemade's reasoning above noted. The trend of the potential
of both phases between the points of tangency, i. e., of all mixtures
between these limits of composition, is toward the minimum repre-
66
ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
sented by this tangent. Crystallization will then begin at a (fig. 18,
II), with the mixture richest in the higher melting component, crys-
tals of composition a will be in equilibrium with the liquid phase b in
all proportions, and solidification (or melting) will not take place at a
single temperature, but through a range of temperature. If we now
100 A
100 B
Fig. 19.
plot the length of the abscissa cor-
responding to ah in a separate dia-
gram with the observed tempera-
ture range of solidification, adding
all the other possible cases which
will arise from the continued dis-
placement of the C-curves, we ar-
rive at the accompanying diagram
(fig. 19) of Roozeboom's Type I.
Types II and III appear in the same
way when the form of the C-curves
changes as indicated in figs. 20
and 21.
The physical side of the system of
reasoning is readily inferred from
the figures. If we start with a mix-
ture of the composition indicated
by m (fig. 22) and temperature
above the melting point, crystalliza-
tion will begin at a, the separating
crystals will have the composition
b, while that of the remaining melt approaches d. Upon cooling to e,
solidification ends with crystals of this composition. Melting is ex-
actly the reverse operation. Whether these first crystals of compo-
sition b remain stable as such or undergo solid transformation or wholly
or partly redissolve appears to remain undetermined in any general
way by Roozeboom's theory, and may be radically influenced by
00 B
Fig. 18.
CONCLUSIONS.
67
100 A
100 A a
100 A b
IV
100 B
I
a b 100 G
100 B
100 A
100 B
100 A
100 B
100 A a b
Fig. 20.
100 B
accompanying phenomena like viscosity and undercooling. If a liquid
mixture of composition a undercools to e before crystallization begins,
crystals of composition e will appear and no others (provided the re-
68
ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
£
100 A
Composition
Fig. 22.
100 B
lease of latent heat does not raise the temperature above c again).
Such a situation is certainly unavoidable in viscous mixtures like the
feldspars and accounts very well for the homogeneous solidification ob-
served by us. This would classify the feldspars with Type I of Rooze-
boom's series. A comparison of our
melting-point curve with figs. 19,
20, and 2 1 shows this to be the only
type under which it could possibly
fall. There is no trace of a maxi-
mum or minimum in the feldspar
curve. Vogt's expectation that they
would fall under Type III, therefore,
fails of fulfilment from our experi-
ments.
That our curve so closely resem-
bles one branch of Roozeboom's
typical curve is remarkable. The
difficulties of observation in those
portions of the curve where the
viscosity becomes so disturbing are
too great to enable stress to be laid
upon the form which our curve happens to take there, but near the
anorthite end of the series its slight convexity is unquestionably real.
It should be added that Professor Iddings has found slight traces of
inhomogeneity (less than 1 per cent) in the slides of several of our
intermediate feldspars. Crystals have been found which were evi-
dently of the earliest formation, and with one exception were more
calcic than the body of the charge, as Roozeboom's theory would lead
us to expect. The exception was an occurrence of tiny plates of
Ab4Aiii discovered in a charge of Abi An5. The extremely small quan-
tity of the optically different feldspar, the fact that it could not be
found in all the slides of this composition, and that in one case a less
calcic feldspar appeared, suggest that the inhomogeneity may have
been of other origin — perhaps merely a consequence of the tremendous
difficulty in mixing a homogeneous charge where ultraviscosity pre-
cludes stirring, for example. The chemical analysis of the solid and
liquid phases, it will be remembered, showed identical composition
within the limits of experimental error.
It is clear that if Roozeboom's theory is valid, the line of the melt-
ing points can not become perfectly straight unless the --curves for
the solid and the liquid phases can be superposed point for point
throughout, i. e., are identical. This would mean that the energy
LITHOLOGICAL APPLICATIONS. 69
content per gr. mol. of solid and liquid phase was the same for all com-
positions, i. e., that all mixtures and the components separately should
have the same melting point — a case which is known (Roozeboom,
d- and I- camphor oxime) , but is certainly confined to optical antipodes.
Another reason for supposing the case to be much less simple than
a mere linear relation with equilibrium between solid and liquid phases
of identical composition appears at once from a direct application of
the phase rule. A necessary condition for equilibrium in any mixture
is that the number of phases exceed the number of components by
two. If the solid and liquid phases are homogeneous, the number of
phases (counting vapor) is only three, and equilibrium can not obtain
there.
LITHOLOGICAL APPLICATIONS.
Supposing the case for the feldspars to be established, by this line
of reasoning, as falling under Type I of Roozeboom's classification,
important light is thrown on the significance of zonal structure in
feldspars and also on the meaning of its absence. A very considera-
ble proportion of the feldspars found in thin sections of rocks show
zonal structure, though it is more frequent in effusive lavas than in
the granular massive rocks.
Furthermore, with rare exceptions, the outer zones are more sodic
than those which they inclose. The width and definition of the zones
vary greatly; they are sometimes sharply separated; not infre-
quently they show transitions at the edges of the zones, and occasion-
ally the gradation is a continuous one, so that the extinction during
a rotation of the slide resembles a shadow moving at a uniform rate.
This last case is immediately explicable by Roozeboom's theory. If
a feldspar magma of any particular composition were to solidify with-
out undercooling, the composition would change continuously during
solidification in a perfectly definite manner, within limited ranges of
temperature and composition, as has been indicated in the discussion
of the theory above, the center being always more calcic than the
periphery.
Homogeneous crystals are also readily explained. If undercooling
occurs the magma does not begin to crystallize until it has passed
below the range of temperature at which the change in concentra-
tion can take place.
Sharply emphasized zones, or zones showing transitions only at
their edges, point to changes in physical conditions during crystalliza-
tion. Now abrupt changes of pressure are not likely to be frequent
excepting during the act of intrusion or extrusion, but in complex
JO ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
magmas there inevitably must be local variations in temperature in
consequence of the liberation of energy during the crystallization
of the feldspars and of the accompanying mineral constituents,
especially the ferromagnesian silicates. When the crystallization
goes on slowly and smoothly, the magma may be expected to cool
gradually over the range between the two curves (fig. 22), and a
uniform zonal structure result, the extreme viscosity of the liquid
operating to prevent any considerable diffusion, resorption, or other
modifying phenomena. Any sudden or irregular release of heat
which tends to prevent uniform cooling, if it occurs within the range
of possible zonal formation, may be expected to result in some varia-
tion in the bands; constant temperature for a considerable interval
will tend to produce broad bands of uniform composition through the
resorption of more calcic crystals already formed, and sharp demarka-
ations. In fact, any considerable disturbance, either mechanical or
thermal, would probably result in sharp demarkations between bands.
Again, if the temperature change should carry the crystals below this
critical region, only homogeneous crystals would form unless a near-by
release of heat could raise it again. It is easily conceivable that this
latter case might produce a partial reversal of the order of the bands.
In a word, if feldspar crystals begin to form within the range where
a change in concentration can occur, zonal structure will probably
result, and every change in the temperature will have its effect upon
the arrangement of the zones. Long-continued freedom from ther-
mal disturbance will produce broad zones, and rapid variation, either
continuous or irregular, will produce narrow or sharply bounded ones.
Inversely, it would appear that whenever a set of thin sections
shows traces of zonal structure, and there are few hand specimens in
which this structure can not be detected, solidification of the feldspars
has taken place within well-defined limits of temperature. When, as
in the granites, water vapor or its components have entered into the
composition of the magma, it is probable that this range of tempera-
ture is a different one, a point to be determined by further researches,
but it is evidently practicable to determine for granites as well as for
the nearly anhydrous lavas at what temperature the feldspars have
solidified, wherever zonal structure can be found.
SUMMARY OF CONCLUSIONS.
71
SUMMARY OF CONCLUSIONS.
Reviewing this discussion briefly : (1) The triclinic feldspars are solid
solutions and form together an isomorphous series. It is a sufficient
condition for the latter that the curve of melting points is continuous
(Bruni, loc. tit.). Like Kuster's curves for organic compounds, the
curve of melting points does not follow Van't Hoff's law of dilute solid
solutions and does approximate closely to a straight line joining the
melting points of the components. The case appears to fall under
Type I of Roozeboom's theoretical classification of isomorphous mix-
2.900
2800
2700
> 2.600
2.500
2.400
2.300
2.200
-
Cr)
stal:
Gla
5S
An
An 100
Ab 0
Ab,An5
84.1
15.9
Ab,AnE Ab,An,
68.0 51.5
32.0 48.5
Percentage composition
AbjAn, Ab3An,
34.7 26.1
65.3 73.9
Ab
0
100
Fig. 23. — Curves of specific gravity of the feldspars and feldspar glasses.
tures, in which case the line can not become exactly straight unless
the melting points of the components are nearly or quite identical, nor
the solidification absolutely homogeneous without reducing the num-
ber of phases to three and destroying the equilibrium. The theory
also accounts for an absence of sharpness in the intermediate melting
points of the feldspars, but the fact that this lack of sharpness culmi-
nated in albite instead of terminating there shows that the viscosity
was the chief factor in our difficulties from this cause. Albite was
clearly shown to melt through a variable range of 1 500 or more, while
the intermediate feldspar bytownite (AbiAn5) melted almost as
sharply[as anorthite, as one would expect it to do in view of the flat-
72
ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
ness of the melting-point curve (p. 60). The fact that practically no
differences of composition could be detected in our melts we attribute
to the effect of viscosity and consequent undercooling, which resulted
in crystallization invariably resulting at much too low a temperature
for equilibrium to become established between the solid and liquid
phases at any stage of the crystallization process.
(2) When the specific gravities are plotted, like the melting points,
as a function of the composition (fig. 23), the isomorphism of the feld-
spars is strongly confirmed.
.4200
»
.4000
Gla
ss
E
3 .3900
0
>
0
i*-
"5 .3800
V
a.
Cry:
;tals
—-s"
.3700 <
T"^
,
3600
.3500
An
An 100
Ab 0
Ab,AnB
84.1 "
15.9
AbiAnj Ab,An, Ab2An.
68.0 51.5 34.7
32.0 48.5 65.3
Percentage composition
Ab3An,
26.1
73.9
Ab
0
100
Fig. 24. — Curves of specific volume of the feldspars and feldspar glasses.
The curve indicates a perfectly continuous relation which the suc-
cessful preparation of chemically pure albite enabled us to follow
through to the end. The order of accuracy is also extraordinarily
high throughout by reason of the chemical purity of all the prepara-
tions and the consistent effort made to obtain complete crystalliza-
tion, even with the more viscous feldspars. Several of the charges
were heated for two weeks or more consecutively, then removed for a
determination, then replaced in the furnace for another week in order
that we might assure ourselves, from the consistent reappearance of
the same value, that a maximum, and, therefore, holocrystallization,
had been reached. It is of some practical importance to note in pass-
SUMMARY OF CONCLUSIONS. 73
ing that preparations which appeared completely crystalline in the
slides frequently proved not to have reached their maximum specific
gravity. It is very difficult to detect the last traces of glass with the
microscope.
If our confidence in these determinations is justified, the form of
the specific-gravity curve is very significant. It was pointed out by
Retgers* that if the isomorphous mixture is merely a "mechanical
aggregate" the volume of which remains exactly equal to the sum of
the volumes of the components, then the specific-volume curve of the
mixtures for percentages by weight of the two components must be a
straight line. He also offers a number of isomorphous pairs for which
he finds the specific-volume curves to be straight lines, in support of
his hypothesis that this relation is general. Our values when plotted
in this way (fig. 24) also give a straight line with maximum varia-
tions amounting to 0.005, which is probably not greater than the
aggregate error in the syntheses and in the determinations of the
specific gravity.
In spite of this apparent corroboration, it does not seem to us that
Retgers was quite justified in assuming that this relation is entirely
without limitation. The temperature at which the specific gravity
is determined is so far below the temperature of solidification (in our
case more than 10000) that the density at 250 will depend, to a con-
siderable degree, upon the coefficient of expansion of the material as
well as upon composition and molecular structure. The coefficient
of expansion will, in general, differ for different compositions, and is not,
in general, a linear function of the temperature. Considering Retgers's
generalization in the light of these facts, the relation of the specific
gravities at 250 would be necessarily continuous, but not necessarily
linear.
The specific gravities of the glasses are also plotted (fig. 23) to show
the divergence from the line of the crystals toward the albite end of
the series, i. e., as the percentage of albite increases the density of
the glass is diminished more than that of the crystals.
There is nothing new in the conception of isomorphism in the feld-
spars, but the positive character of our experimental results makes
them of more than ordinary interest by reason of the fact that so
good authority on the subject as Fouque and Levy has passed upon
it adversely on the basis of optical evidence derived from artificial
preparations. More recently Violaf has declared that the optical
evidence is insufficient to prove isomorphism in the natural feldspars
* J. W. Retgers, Zeitschr. fiir. Phys. Chem., 3, p. 507, 1889.
\Loc.cit.
74
ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
The melting points and specific gravities plotted above are brought
together in a convenient table here.
Feldspar.
An ... .
At^Aris
Ab,An3
AbjAri!
Ab.Aiii
AbaArii
Ab . . . .
Melting
temperature
(degrees).
1532
15OO
H63
1419
1367
J340
Specific gravity.
Crystals.
2.765
2-733
2.710
2.679
2 .660
2.649
2.605
Glass.
1 . 700
:.648
'■591
:-533
■•483
1.458
!.382
(3) In the melting of albite and microcline we appear to have sub-
stantial evidence of a phenomenon which is unfamiliar both to physics
and to mineralogy. Microscopic crystals of a homogeneous com-
pound, when slowly heated, were shown to persist for 1500 or more
above where melting began, the amorphous melt remaining of the
same order of viscosity as the rigidity of the crystals. By careful
observation, curves were also obtained showing that the absorbed
heat of fusion was distributed over this interval.
From the experimental standpoint a substance of this kind can
hardly be said to have a melting point, but passes gradually from
crystalline to amorphous at temperatures which can be considerably
varied by merely changing the rate of heating. In moderate charges
of albite or orthoclase at atmospheric pressure this melting began so
slowly that it was not possible to locate even approximately a lowest
temperature for the beginning of the change of state. As a matter of
definition, this minimum temperature above which melting will con-
tinue (for a given pressure) more or less rapidly, according to the
conditions, is the "melting point," whether it can be located or not,
so far as the equilibrium of the system is concerned; and crystals
which continue to exist unmelted at higher temperatures appear
to form a metastable phase, perhaps comparable to that of a crys-
talline solid when heated above the "Umwandlungstemperatur"
without immediate change of crystal form. It is also possible that
the mass is fluid when heated above the melting point, but that
deorientation of the molecules is delayed by viscosity. This meta-
stable stage can easily extend over 1 500 in albite and orthoclase and
would persist for days in the lower portion of this range.
(4) We also found that viscous and poorly-conducting melts which
solidify only after considerable undercooling do not give constant
solidifying points. The solidifying point must not be used, therefore,
without great caution as a physical constant; it bears no relation
SUMMARY OF CONCLUSIONS. 75
whatever to the melting point unless equilibrium is reestablished
before solidification is complete — a condition which rarely obtains and
often can not be produced in viscous mineral melts. Especial attention
is directed to this because of the importance of the lowering of the
solidifying point in the study of solutions, and the possibility of its
application to mineral solutions recently suggested by Vogt.*
(5) Incidental to the experimental work upon the feldspars we were
able to establish the fact that there are no differences of density in
the feldspar glasses due to the rate of cooling which are greate r than
our errors of observation (± 0.001). Also that powdered crystalline
feldspars which are free from inclusions and from glass, even when
very fine, do not sinter until melting begins ; powdered glasses of like
composition sinter readily at relatively low temperatures (7000 to
goo°), depending primarily upon the degree of pulverization. Again,
that powdered feldspars when exposed to the atmosphere adsorb
moisture in quantities of an order of magnitude equal to those usually
quoted in analyses. (Dana's System of Mineralogy, /. c). It is,
therefore, altogether possible that the significance of this moisture has
sometimes been mistaken.
*J. H. L. Vogt, loc. cit.
Part II.
The Isomorphism and Thermal Properties
of the Feldspars.
OPTICAL STUDY.
BY
J. P. 1DDINGS.
LIME-SODA FELDSPARS CRYSTALLIZED IN OPEN
CRUCIBLES FROM FUSED CONSTITUENTS.
INTRODUCTION.
The results of these synthetical experiments agree closely in some
respects while differing in others. They agree in general in the habit
and arrangement of the crystals of the different feldspars produced,
while differing in the size of the crystals of the various feldspars
according to their composition. These results have an important
bearing on the problem of texture and granularity in igneous rocks.
First, as to the habit of the feldspar crystals produced from solu-
tion of the feldspar constituents without admixture of other material.
So far as can be determined by microscopical study of the sections, the
crystals are in most cases blade-like in form ; that is, they are elongated
plates. They vary, however, from one extreme to another, being in
some cases equidimensional plates of extreme thinness, in other cases
prisms, elongated in one direction with the other two dimensions
equal. The development of these forms takes place in feldspars of
various compositions, and appears to be chiefly a function of the rate
of crystallization and not of the chemical composition of the feld-
spar, except as this modifies the viscosity of the solution. It is not
possible to recognize any fixed relation between the habit of the
crystals and the composition of the feldspar. This is, of course, in
accord with the well-known isomorphism of the feldspar group.
The common mode of crystallization in these preparations is that
of spherulitic aggregations, more or less completely developed in
spherical forms.
The elements of the spherulites are bundle- or sheaf-like aggrega-
tions of long, thin blades, which blades lie nearly parallel to one
another in the middle or narrower part of the bundle, and diverge at
the ends into fan-like or plumose forms. Several of these bundles or
blades cross one another at the middle, and when there are a sufficient
number of bundles, or when they diverge sufficiently, a completely
spherulitic aggregation results.
In some cases a spherulite consists of bundles or prisms that extend
uninterruptedly from the center to the outer margin, the rays of the
spherulite being nearly straight. In other cases the spherulite is a com-
79
8o ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
posite of divergent bundles shorter than the radius, which have been
added to one another as though new plumes had started from the ends
of earlier ones.
In most cases the middle portion of the feldspar bundles consists of
stouter crystals than the outer parts. It also appears that the middle
portion is more prismatic, in certain cases somewhat cuboidal, the
outer parts becoming delicately tabular. This, with the divergence in
position, explains the spread of the outer part of the sphere. There is
a great increase in the number of individual crystals in the outer
portion of the spherulite, and in some cases the crystals also increase
in size in the outer part.
The shapes of the crystals are due to the flattening of the crystal
parallel to the second pinacoid (oio), and its elongation parallel to
the crystal axis a. The outlines of the plates appear to conform to
traces of several pinacoids in the zone of the b axis, (ooi), (201),
(101), (201), (304), (203), not all of these occurring together. It is
quite probable that pinacoids in the zone of the c axis also may be
developed, but they were not recognized.
Bladed forms in some cases prove to be aggregates of thin plates
not strictly parallel to one another in the plane of flattening, so that
the blade is curved and not straight in the direction of its longest axis.
In some spherulites the component crystals are prisms throughout,
with no tabular flattening. The number of crystal prisms increases
from the center of the spherulite outward by the development of new
prisms at slightly divergent angles, in arborescent arrangement.
The most complex arrangements are produced by twinning and
divergence combined, resulting in feather-like aggregates. Long,
narrow, tapering blades in albite twins form a shaft, elongated parallel
to the crystal axis a, on two sides of which diverge at a slight angle a
double set of thin blades, like barbs. These consist of branched
smaller blades or prisms, like barbules, the branch prisms having
approximately the direction of the crystal axis c. The two sets in
each "barb" are apparently related to one another as the halves of a
manebach twin. The small prisms are composed of many subpar-
allel plates flattened in the plane of the second pinacoid (010). These
correspond to barbicels in a feather.
With respect to the size of the crystals it is extremely significant
that pure anorthite (An) develops in comparatively large plates, 5 mm.
thick and 20 to 30 mm. long, in a few hours, whereas the more sodic
the feldspar the smaller the individual crystals formed under almost
the same conditions of cooling. Thus with oligoclase (AhtAni) the
individual crystals composing a bundle of blades are considerably
ANORTHITE (AN). 8 1
less than o.oi mm. thick, probably about o.ooi mm., a difference in
thickness when compared with anorthite of about 5,000 to 1. This
as shown elsewhere is due to the greater viscosity of the liquid
feldspars near their solidifying point as they approach the albitc end
of the series.
Any comparison of the grain of rocks, that is, the size of the con-
stituent crystals, with a view to determining the physical conditions
attending the solidification of the magma, must be based in the first
instance on a knowledge of the behavior of the various rock-making
minerals under similar physical conditions, both separately and in
combination, that is, in solution with one another. The granu-
larity of rocks is clearly a function of the chemical composition.
With respect to the homogeneity of the crystals separating from
the liquid, it is observed that the great part of each crystal aggregation
appears to be of one composition, but that in some cases a small
proportion, probably less than 1 per cent, is different from the bulk of
the feldspar, both in composition and habit. In one instance this
small variant differed in composition but not in habit from the main
mass of crystals.
In the first case it appears that crystallization began with feldspar
richer in the anorthite molecule than the solution and developed
cuboidal forms. These were prolonged into prismatic bundles, the
prisms having the composition of the main mass of crystals.
In the second case the small variant crystallized toward the end of
the crystallization and contained more albite molecules than the main
mass of feldspar crystals. It had the same habit as the other more
calcic portion, and appears to have crystallized at the same time with
it, the crystals with different optical properties being by the side of
one another and not in zonal relation. Neither of the feldspars
represents the end member of the series, An or Ab.
The detailed description of the thin sections of these laboratory
preparations of lime-soda feldspars follows:
Anorthite (An).
(19). This aggregation consists of tabular crystals 3 to 5 mm.
thick in somewhat radial arrangement, and between these are smaller
tabular crystals in similar radial clusters. The clusters are twinned
according to the albite law in lamellae, o. 1 5 mm. thick and less. The
thinnest lamellae are not always continuous throughout the length of
a crystal.
The optical orientation is uniform throughout the length of each
82 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
lamellar section without evidence of zonal structure, proving that the
crystals are chemically homogeneous.
Fracture lines cross the crystal irregularly and follow possible cleav-
age planes to only a limited extent. In some cases lamellae have
broken apart along the composition plane, which is also the second
cleavage plane (oio).
In several places the albite twins have been cut at right angles to
the twinning plane and also at right angles to one of the optic axes of
the crystal. In one lamella it is almost exactly normal to the plane
of section, in the other very slightly inclined. The plane of the optic
axes in one lamella stands at 630 to the trace of the twinning
plane (010); in the other lamella it is 620 30' approximately. One
of the optic axes of the crystal lies almost parallel to the pinacoid
(010). This is the position given it in Michel-Levy's diagram* for
anorthite (An).
The crystals contain numerous inclusions of colorless, apparently
amorphous substance, with a refraction higher than anorthite. It
appears to be isotropic. The outline is very irregular and rounded.
The shapes are curved and elongated. They contain gas bubbles.
These inclusions are distributed in planes, lines, and swarms, having
various directions with respect to the feldspar crystals. The arrange-
ment in some cases suggests skeleton forms. In some places the
inclusions are mostly gas. But the suggested feather structure bears
no fixed relation to the crystal orientation or the lamellar structure
of the anorthite. It appears like the structure of something obliter-
ated by the crystallization of the anorthite, or in some cases as though
the skeleton form were completely filled up by anorthite in perfect
orientation. The distribution of the inclusions in some instances is
such as to suggest changes in the rate of crystallization of different
parts of the crystal. In some places the crowding of minute inclusions
suggests very rapid crystallization. It appears as faint cross-banding
in the crystal shown in Plate I, from another preparation of anor-
thite (An).
The glass is probably composed of material in excess of the propor-
tions necessary for the anorthite (CaAl2 Si208 = An). It is not Si02
alone, for this would have an index of refraction lower than anorthite.
It may be a silicate of Al or Ca.
(5°a~b)- This preparation is similar to (19). The crystals are
tabular parallel to the second pinacoid (010). The larger plates are
1.5 mm. thick. There is multiple twinning according to the albite
* iltude sur la Determination des Feldspaths, etc. Paris, 1894. Plate vn.
BYTOWNITE (ABiANj. 83
law, and no evidence of variation in optical orientation or zonal struc-
ture in any one crystal. The crystals are homogeneous. In sections
cut at right angles to an optic axis the plane of the optic axes makes
an angle of 650 with the trace of the second pinacoid (010). In some
sections there is a remarkable appearance of the twinned lamellae.
They appear to be faulted in bands across the tabular crystal, as
shown in Plate I. But there is no evidence of dislocation in the out-
line of the crystal plate ; in fact, there may be continuous lamellae on
both sides of the apparently faulted belt. Close inspection of twinned
lamellae shows that the several series of discordant belts do not cor-
respond in number or in width of the lamellae composing them, so
that they are not displaced, faulted sections of a large multiple twin
of feldspar, but independent crystallizations in parallel position.
The illustration shows a cross-section of tabular feldspar cut at
right angles to one optic axis and nearly at right angles to the crys-
tallographic axis c. The crystal is tabular parallel to the pinacoid
(010) ; the belts of multiple twins, which have the appearance of being
faulted, extend at right angles to (010). Their growth appears to
have progressed from one side of the tabular crystal to the other, for
they are blended with a continuous lamella on one side and exhibit
a broken limiting line on the other side, against another continuous
lamella. They may represent a coordinated set of prismatic feldspar
crystals, elongated parallel to the crystallographic axis c, twinning
independently of one another, while thickening in the direction of
the b axis.
Bytownite (AbjAnb).
(58a_b). These sections are from spherulitic aggregations of
twinned crystals. The spherulite consists of radiating groups of
highly twinned bladed crystals of feldspar, which are nearly parallel
to one another within one group. But the different groups stand at
various angles to one another. This is shown in cross-section (58b),
Plate II. The blades are not plane-faced or parallel-faced. They
curve somewhat and wedge out abruptly. They vary in thickness
from 0.18 mm. to about 0.07 mm. and less. The breadth of the
blades varies considerably, averaging about 1 mm The groups of
subparallel blades are from 2 to 5 mm. in diameter. In length, as
shown in section (58a), Plate III, the blades are about 10 mm. long.
Upon magnifying these blades they are seen to be highly complex,
and their outline quite irregular. Cross-sections exhibit multiple
twinning according to the albite law, the lamellae being sharply defined
in some places and indistinct in others. In thin section there are
84 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
opaque lines between blades. These are very irregular and appear
to be impurities introduced into spaces between the surfaces of blades
during the grinding of the section. The surfaces of blades are com-
posed of crystal faces at various angles which are the terminations
of component lamellae. A blade so delimited in cross-section is trav-
ersed in places by distinct, straight lamellae, which extend without
interruption from one side of the blade to the other, as shown in
Plate II, On both sides of such a twinned belt the blade is composed of
shorter lamellae which appear to originate near the middle of the blade
and run outward, their cross-section being like a curved wedge whose
apex is at the middle of the blade. In other places there are parallel-
edged lamellae in two bands standing at a slight angle to each other.
These lamellae appear to blend or to be interwoven in the middle of
the blade. They are not twinned in some cases, but are twinned in
others. Each of these lamellae terminates at the surface of the blade
as an independent crystal, so that the surface consists of the angular
terminations of these crystals. In some places they terminate in
a common plane. As they do not always exhibit uniform optical
behavior, they appear to overlie one another in thin section as
inclined plates or prisms.
In longitudinal section (5Sa) parallel to the rays of the spherulite
the long shafts of feldspar exhibit very delicate feather-like structure.
This is very intricate and is often blended to such an extent that its
precise character is obscure. It appears differently according to the
position in which the groups of bladed crystals or aggregations have
been cut by the section. In some positions of the section the shaft of
a "feather" consists of long, narrow, very sharply defined stripes of
albite twins (Plate III), which are clearly longitudinal sections of the
well-defined bands of albite twins observed in cross-sections of the
blades. This shaft tapers gradually toward the apex. On both
sides of this shaft are long, straight-edged lamellae, which make an
angle of 40 or 50 with the twinned shaft, and farther out toward the
apex an angle of 70. They appear like barbs in a feather. In some
places, noticeably toward the apex of the feather, the barb-like parts
are crossed by delicate parallel lines, like barbules.
. The position of the twinned lamellae parallel to (010) and the devel-
opment of the pinacoidal cleavages parallel to (010) and (001) which
appear in cross-sections (58b) show that the feather-like blade is
elongated in the direction of the crystal axis a, and is broadened par-
allel to the basal pinacoid (001). Some longitudinal sections parallel
to the second pinacoid (010) show a feather-like arrangement of some-
what curved branches or barbs, each composed of extremely thin
BYTOWNITE (ABjAN5). 85
plates in parallel orientation. The plates are tabular in the pinacoid
(010) and are bounded by the planes (001), (100), (201), and (101).
The angle at which these barbs approach the central part of the
feather, though somewhat variable, is approximately 650 in some
sections. This suggests the manebach twinning in the portions of
the aggregated blades from which such longitudinal sections are cut.
Referring to the illustrations already mentioned, it will appear that
the aggregated blades, which in subparallel bundles form the rays of
the spherulite, consist in certain parts of long, flat, twinned lamellae,
elongated in the direction of the crystal axis a, each lamella flattened
parallel to the second pinacoid (010), the plane of twinning. On
both sides of these twinned lamellae are slightly inclined, thin, flat,
blade-like lamellae (barbs) which are in double arrangement on each
side of the shaft, as shown in cross-section (Plate II). In other parts of
the composite blades these doubled barbs appear to be compounded
of thin plates flattened parallel to (010) (barbules), the doubling
appearing to be due to manebach twinning. In the middle portion
of such aggregates there is great confusion of detail, due to wedging of
crystals and overlapping within the thin section. Crystallization
appears to have advanced from the central portion of such aggregates
outward, producing in some places wedge-shaped crystals or wedge-
shaped aggregates of parallel tabular crystals, which may behave as
a continuous crystal within the body of the aggregate, but may have
an outline or surface corresponding to a parallel aggregation of smaller
crystals.
The optical behavior of these feldspar aggregates indicates that
their substance is homogeneous throughout, except in several places
where the optical properties show that feldspar of another composi-
tion has crystallized. These portions are small in proportion to the
bulk of the feldspar crystals. They are in several instances fortu-
nately cut by the thin section, the two cleavages being almost exactly
normal to the section plane. The section is almost perpendicular to
(010) and (001). In these feldspars the acute bisectrix is almost
exactly normal to the plane of section ; the plane of the optic axes is
parallel to the basal cleavage (001). The acute bisectrix is the direc-
tion of vibration of the fastest ray, the mineral is optically negative,
and the optical properties are those of Ab4Ani, as shown in Michel-
Levy's diagram.* Apparently the feldspar compound first crystal-
lizing was a little richer in calcium than the mixture of the solution,
and feldspar of this composition continued to grow until the solution
* Op. tit., Plate 11. . ... „
86 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
in places reached the composition of Ab^Ani. But there is no evi-
dence that the crystallization of the more calcic feldspar had ceased
before that of the more sodic feldspar began. On the contrary, they
appear, judging from the shape of the aggregates, to have grown
simultaneously, but toward the end of the act of solidification of the
solution.
(30) and (31). These are thin sections from one preparation, which
is glassy below and crystalline above. (31) is from the upper part of
the preparation at right angles to the upper surface. It consists of
a minutely crystallized aggregation with more coarsely crystallized
parts composed of lath-shaped and bladed crystals, the largest being
1.8 mm. long and 0.18 mm. thick, there being all gradations in size
from the largest to the smallest (Plate IV). The crystals lie at all
angles, sometimes radiating, but spherulitic aggregates have not been
developed. In many cases the crystals have rectangular outlines; in
others they taper at the extremities, or wedge out by reason of the
interference of adjacent crystals. Twinning is common, but some
are not twinned. The albite law prevails, and in some crystals the
symmetrical extinction angles are 450. The habit of the crystals is
not definitely determinable, whether tabular plates or elongated
blades. The crystals are of different thicknesses within the thin
section, the plates or blades being thinner than the section of the
preparation. For this reason the Becke method of testing relative
refringence of adjacent crystals is not applicable, as the thicker
parts of crystals in glass or balsam always appear in higher relief
than the thinner parts.
In (30), from the middle of the preparation, just above the glassy
bottom half, there are aggregations of delicate tabular crystals in par-
allel and subparallel groups in various angular positions (PI. V). In
some places the tabular crystals in cross-section are sharply outlined
and straight-edged. In most cases the outline is indefinite and the
larger plates consist of multitudes of parallel and subparallel plates,
whose outline in the plane of flattening, however, is often sharply
defined. They appear to be plates parallel to (010), bounded by the
pinacoids (001), (201), and (I01). The angle between the traces of
(001) and (201) is about 8i°, and that between the traces of (001)
and (I01) about 520. In some cases the plates are nearly equi-
dimensional, in others they are elongated into blades parallel to (001)
or to (201). Owing to the aggregation of subparallel crystals of
extreme thinness, the optical behavior is that of aggregates, and
confused. In one plate of a thicker crystal cut parallel to (010)
LABRADORITE (aBjAN,,). 87
the extinction angle, measured from the cleavage plane (001), is
about 330. The crystals appear to be alike and homogeneous, hav-
ing the composition AbiAns.
Labradorite (Ab,Ans).
(6oa_b). The preparation consists of radiating plates or blades,
about 0.05 mm. to less in width and as much as 0.7 mm. long. Two to
five blades intersect at various angles, wedging out at the point of
intersection (Plate VI). Bach plate consists of two or more twinned
lamellae. Between the thicker plates there are more delicate crystals
composed of subparallel plates and skeleton growths of extremely
thin blades with crystal outline, probably the traces of (001) and
(201) on the second pinacoid (010). The more solid plates or blades
feather out at the ends to somewhat divergent plumes. There are
in some cases branching, feather-like forms in crystallographic posi-
tions suggesting the extension of a single crystal in directions parallel
to the a and c, and possibly the b, axes, the angles of branching being
about 64°, and in some cases about 900. Albite twins yield maximum
symmetrical extinction angles of 370. The crystals appear to be
homogeneous.
(6ia~b). The preparation is glass, with an index of refraction
higher than that of balsam, and feldspar spherulites about 10 mm.
in diameter. The spherulites are very beautiful aggregations of
somewhat divergent, plumose bundles of prismatic crystals (Plate VII)
that appear as distinct crystals at the surface of the spherulite, from
which they project at slightly different lengths into the surrounding
glass, each prism, 0.003 or 0.004 mm. in diameter, being terminated
by crystal faces nearly equally inclined to the long axis of the prism.
The component short bundles show similar plagiohedral terminations
to the individual prisms composing them. These in some cases are
flattened and blade-like, and are in subparallel aggregations, the
plates having nearly rectangular outline. In one part of section
(6ia) there are groups of albite twinned feather-like aggregates similar
to those in (58b). The groups are shorter and less parallel, and are
more curved. There are longitudinal sections of radial elements of
the spherulites (Plate VIII) with the same feather-like structure
observed in (58a). The feldspars of the spherulites appear to be
homogeneous optically, and are probably so chemically.
(23a_b). An aggregation of radiating blades and possibly prisms
somewhat spherulitic, the radii being 5 mm. long in some cases (Plate
IX) . The apparently prismatic forms may be cross-sections of blades
which are recognizable as such in other positions. They form dis-
88 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
tinctly branching, curved, radiating aggregates, in some cases exhibit-
ing albite twinning. In places these prismatic, rod-like forms broaden
out to rectangular cuboidal shapes, which extend in short prismatic
branches almost at right angles to the longitudinal direction of the
long prisms. These cuboidal crystals show to a slight extent a cross
twinning which is in the position for pericline twinning. The angle
between the traces of the cuboidal faces is nearly 90°, which indicates
that the crystals have been cut parallel to the basal plane and the
sections are bounded by the first and second pinacoids (ioo) (oio).
The sections parallel to the flat side of the blades show an intricate
structure composed of parallel and subparallel thin plates with crystal
outlines at several angles, the most frequent being nearly qo°. These
aggregates of plates form bands that branch in feather-like structures
( Plate X) . There are occasionally small rectangular but quite irregu-
larly outlined sections, whose shape is that of cuboidal crystals, with
zonal markings about the center, which have developed small pris-
matic projections parallel to one axis (?a), the prisms being located
at the four corners of the rectangular section. This corresponds to
the microlitic crystals of feldspar found in volcanic glasses, where the
projecting prisms are delicate fibers. The feldspar crystals in this
preparation appear to be homogeneous.
(22a-b-c) The preparation was first cooled rapidly from a melted
condition, then heated again to 12500 at a maximum. This has had
a very interesting result, namely, two periods of crystallization, the
first rapid, the second slower. The main mass of the preparation
consists of bundles of feldspar fibers and delicate network of crossed
fibers. The bundles of fibers are about 0.015 mm. wide and 0.165 mm.
long, and occur singly or cross one another at various angles, several
intersecting in the middle. Single bundles have two strong tapering
fibers on the outside spreading slightly. These bundles of fibers
extinguish light parallel, or at a small angle (70), to the length of the
fiber. The fastest ray vibrates nearly parallel to the fibers, which
appear to be elongated in the direction of the axis a. There is a small
amount of isotropic glass.
Within this mass are spherulites about 10 mm. in diameter, the
outer shell, 1 millimeter thick, having a somewhat different appear-
ance from the central portion. The central part consists of radiating,
branching prisms or blades, not in straight rays but in plumose aggre-
gations. The outer marginal zone is a blend of the inner spherulite
and surrounding matrix of small bundles of fibers (Plate XI), and it
is evident that the spherulitic feldspar crystallization had advanced
in the already crystalline matrix by a process of recrystallization, the
ANDESINE-I.ABRADORITE (AB,ANi). 89
small bundles of fibers losing their optical orientation and finally their
distinct outline. Their place is occupied by spherulitic feldspar rays
whose orientation is independent of the position of the former bundles
of fibers. The former position of the bundles is shown in places by
pairs of stouter fibers, the outside members of the bundles of fibers;
in other places by clusters of minute inclusions resembling air spaces
between a network of fibers. These fade out in passing from the
margin of the spherulite inward through the millimeter-thick shell.
They have entirely disappeared in the central part of the spherulite.
The feldspar of the spherulite is homogeneous and the small bundles
appear homogeneous. They must have the composition of the pre-
pared mixture. The small angle of extinction of the bundles of fibers
is difficult to account for. It behaves more like oligoclase than
labradorite.
(20). A branching spherulitic aggregation with numerous cavities
elongated in the direction of the radiating fibers. The section is too
thick to show well the microscopic structure of the parts. They
appear to be prisms and blades composed of minute subparallel
parts. The feldspar appears to be homogeneous.
Andesine-Labradorite (ABjAn,).
(26). The section of this preparation is extremely fine-grained
at one end and glassy at the other, the lower end of the crucible.
The crystals and spherulites become larger toward the glassy end of
the section (Plate XII). The glass has lower refraction than balsam.
There are many opaque white lumps, which appear to be unmelted
powder. They constitute 5 to 10 per cent of the mass.
The feldspar crystals form fibrous bundles, single and in sets, cross-
ing one another at various angles, spreading out into plumes and
spherulitic groups.
In the central portion of some fibrous bundles there are rectangu-
lar, lath-shaped, and block-like crystals with albite twinning, which
exhibit symmetrical extinction angles of 450. These parts must have
the composition of AbjAn^, at least. The thinner prisms or fibers
outside of these show lower extinction angles, about 250. If these
correspond to maxima in each case, the fibrous feldspar is about
AbiAni. The feldspars, except the central portions, which are com-
paratively few, appear to be homogeneous. Some of the plumose
aggregates which are albite twins are very beautiful.
(27). This preparation consists of glass with twinned prisms of
feldspar in radiating groups about 1 mm. long which are not properly
spherulites. They are shown in Plate XIII. They are in most cases
go ISOMORPHISM AND THERMAL, PROPERTIES OF FELDSPARS.
prisms, and not plates or blades. In places these are thin plates or
blades in twinned aggregations, as in (58a_b). The prisms are more
distinctly developed at the ends of some of the radiating aggregates,
where they are distinctly twinned according to the albite law and
yield symmetrical extinction angles of 300.
(59a-b)- This preparation is glass, with spherulitic aggregations
about 2 mm. in diameter grading into smaller radiating bundles cross-
ing one another in groups of 2, 3, 4, and more, as in other cases already
described. The middle part of the bundles consists of stout prisms
passing into extremely thin fibers. The stouter portion yields ex-
tinction angles of 300. The feldspars appear to be homogeneous
crystals.
(64a_b). This preparation consists of spherulitic aggregations simi-
lar to (59), and also short rectangular prisms with almost square
cross-section. They are 0.04 to 0.07 mm. long and 0.007 to 0.010 mm.
wide. The forms are similar to the lath-shaped feldspar microlites
common in andesites. They are in some cases twinned, in others not.
They yield extinction angles of 300.
Andesine (Ab^ANj).
(54a~b). This preparation is extremely minutely crystallized. The
main mass appears to be holocrystalline, composed of flake-like
microlites of feldspar overlapping one another at all angles, so as to
produce weak double refraction. The crystals are larger in patches
and in shells about isotropic spaces, as though the crystallization was
coarse about small spaces, like the walls of cavities of geodes. The
feldspar crystals project into the spaces. But these spaces are
filled with colorless isotropic material with refraction considerably
lower than balsam, presumably glass. It amounts to several per cent
of the whole. This residual glass probably has a different composi-
tion from the feldspar mass, otherwise it should not have solidified as
glass, for the larger crystallization of the feldspar in juxtaposition with
it indicates that the controlling conditions became more favorable to
the crystallization of the feldspar.
(66a_b) . The small thin sections of this preparation are holocrystal-
line, without glass. The preparation consists of crossed bundles of
prisms and blades, without true spherulites. The bundles vary in
width from 0.0 1 mm. to less and in length from 0.5 mm. to less. Some
of the crystals are in albite twins, others not. While the prismatic
sections exhibit nearly parallel extinction in all cases, the long axis of
the prism being the direction of vibration of the fastest ray, and appear
OLIGOCUASE-ANDESINU (aB3AN,).
91
to be alike in composition, there are rectangular sections, which at
first appear to be cross-sections of square prisms with hollow centers,
but are nearly equidimensional crystals which in some cases have
prismatic prolongations. These rectangular crystals have rectangular
spaces at the center, are twinned, and exhibit symmetrical extinction
angles of about 300. As in other preparations of the more sodic
feldspars, there are comparatively few small crystals of more calcic
feldspar, approximately AbiAni, which began to form in cuboidal
shapes, but were followed by the crystallization of the bulk of the
mixture in feldspars of the average composition.
Ougoclase-AndESinE (Ab3Ani).
(21). Colorless glass, without crystals in the thin section studied
microscopically.
(32). Colorless glass, with feldspar microlites and aggregates in the
form of bundles about o. 1 mm. long and in crossed bundles and to
some extent in spherulitic arrangements. The isolated microlites
i
Fig. 25. — Microlites of oligoclase-andesine (AbgAnj).
are instructive both on account of the exhibition of the habit of the
feldspar crystals in these preparations and also as an evidence of the
changes in habit during the short period of their growth. There are
two types of microscopic crystals, one tabular, the other prismatic.
These occur near one another intermingled in the glass. They are
0.03 mm. long and smaller.
In many cases there appears to be a nucleus of feldspar within
feldspar; in some of these there are also small, irregularly shaped,
colorless grains with rather strong index of refraction whose compo-
sition is not determinable. These are extremely minute and not
abundant.
The feldspar nucleus exhibits stronger refraction than the marginal
feldspar, but the direction of extinction is the same in both parts,
proving like optical orientation and showing that the central part of
the microlite is thicker than the margin and of the same composition.
The initial crystal in these cases is thicker, that is, the crystallization
92 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
was somewhat more uniform from all sides, at the beginning, but
became more rapid, developing very thin plates or prisms outside
the nucleus.
These are illustrated in fig. 25. In one (a) several minute grains
form a nucleus of a tabular feldspar microlite, whose form indicates
that it is tabular parallel to the pinacoid (010) and is bounded by
(101) and (201). The central part of this plate is thicker than the
margin, and thins out in irregularly placed rays, like spurs and gulches
leading from a mesa to a plain. The microlite is double, consisting of
two thin plates in parallel position. A cross-section of such a double-
plated microlite is shown in (b).
In (c) the central part is a flattened prism or blade, passing at the
extremities into fibers or needle-like prisms and then into a thin plate
completely surrounding the nucleus. In (d) the thin plates at both
ends of the blade do not unite. The shape of the plate shows only a
center of symmetry; diagonally opposite corners are sharp angles a
little less than 900 (101) and (201). The other corners are rounded or
formed by two obtuse angles. The outline corresponds to the traces
of the basal pinacoid (001) and the two pinacoids of the second kind
(201) and (101) on the second pinacoid (010). The crystals are
elongated in the direction of the crystal axis, a, and flattened parallel
to the second pinacoid (010). In (e) a small rectangular prismatic
crystal is enlarged to a more elongated form with fibrous termina-
tion. In (/) a squarish form with rectangular projections is extended
in opposite directions as straight fibers. There are narrow prismatic
forms with needle-like extensions at the ends. These are isolated or
more often grouped in subparallel bundles and in almost parallel
lines, as in (g). These microscopic crystals vary in size from a length
of 0.64 mm. to less. They are so small that their optical characters
can not be used to determine their chemical composition. They
appear to be alike and homogeneous, and may be assumed to have
the composition of the preparation Ab3An,.
(28). This preparation is colorless glass, with feldspar crystals in
rods or prisms about 0.08 mm. long and 0.005 mm. wide. A few of
these are isolated ; most of them are crossed groups of two or more, or
form radiating aggregates. Relatively few crystals have the shape of
blades or plates. The prisms are frequently double, joined in the
middle like an H. The central connecting part sometimes occupies
a small portion of the length of the double prism, sometimes a large
portion. The prisms are grouped in subparallel bundles as in prepara-
tion (32). The extinction angle is almost zero in nearly all bundles
of prisms. The feldspars appear to be homogeneous, and must have
OUGOCLASE-ANDESINE (AB3AN,). 93
the same composition as the preparation, since the mass is holocrystal-
line in places.
(25s). The preparation consists of glass with spherulitic aggre-
gations. These consist of bundles about 0.4 mm. long, of prismatic
crystals, which are definite in the middle and spread out to plumose
forms at both ends, merging with others in crossed position to form
spherulites in favorable positions (Plate XIV).
In the more spherulitic groups the terminal parts are made up of
the most delicate fiber-like crystals. In other parts of the preparation
the sheaf-like bundles are composed of more distinct, thicker prisms,
which are somewhat broader in one direction than in another; that
is, they are somewhat blade-shaped. In some cases these are twinned
according to the albite law. In the central parts of some of the
bundles there is a rectangularly jointed development of the stouter
feldspar crystals which are continuous with the more prismatic
crystals forming the main part of the bundles.
The optical behavior of those rectangular parts which show albite
twinning corresponds to that of AbiAni, there being symmetrical ex-
tinction angles of 300. In the long prisms the angle of extinction is
small, and the length of the prisms is the direction of vibration of the
fastest ray. This corresponds to the optical behavior of a more sodic
feldspar with a prismatic development parallel to the crystal axis a.
In parts of the preparation the feldspars have crystallized in thin
tabular plates parallel to the second pinacoid (010).
(24). This preparation is similar to (25a). It is partly glass,
partly spherulitic aggregations of bundles of prismatic crystals,
spreading out at the ends. The central portions of some aggrega-
tions are of stout prisms and rectangular groups with symmetrical
extinction angles of 300 (Plate XV). As in preparation (25s) there
was a first crystallization of feldspar with higher content in anor-
thite molecules than the average of the mixture. This formed a
small fraction, probably less than 1 per cent of the whole. The
principal crystallization appears to be homogeneous and must have
essentially the composition of the mixture Ab3An!.
The outline of the bladed crystals in the plane (010), judging from
the optical orientation, and the elongation of the blades parallel to a,
is determined by the pinacoids of the second kind (201), (304) in
some cases furnishing an angle of about 8o°, the angle between the
two being nearly bisected by the trace of the third pinacoid (001).
In other cases the blades appear to be bounded by (201) and (203),
with the apex angle truncated by (201). Such plates are nearly
1^1$ «*•*"*" ^£>1
94 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS.
normal to the optical bisectrix y, and the plane of the optic axes is
almost parallel to the direction of elongation of the blade.
(67a-b). This preparation consists of spherulites about 2 mm. in
diameter, and some interstitial glass. The spherulites are irregular
in outline, due to mutual interference (Plate XVI). They consist of
radiating sectors made up of rather distinct prisms, which start at
the center of the spherulite as stout prisms and become innumerable,
slightly diverging prisms which terminate at different lengths as
distinct prisms terminated by a pinacoid almost at right angles to the
axis of the prism, probably (201). In other cases they are terminated
by two pinacoids making an acute angle. In places the terminations
are somewhat rounded. The crystals in these spherulites are homo-
genous and correspond to the composition of the mixture.
OUGOCXASE (Ab4ANi).
(29). The preparation was heated to a temperature of about
14000 and allowed to cool for 15 hours to about 4250. The resulting
solid is a glass with abundant crystals of feldspar in crossed bun-
dles of blades or plates, and spherulite-like radiating aggrega-
tions (Plate XVII). There are numerous rounded and subangular
grains of colorless quartz in the glass which have no definite relation
to the feldspar crystals, and appear to be undissolved fragments of
quartz used in compounding the preparation. There are also small
lumps of white aggregates, probably undissolved powder, which in
many cases form centers of spherulitic crystallization, that is, they
become points at which feldspar crystallization began.
The colorless glass has an index of refraction noticeably lower than
that of Canada balsam, 1.5393.* The length of the feldspar bundles
and diameter of the spherulitic aggregates is about 0.2 mm., the width
of the bundles about 0.0 1 mm. The narrow cross-sections of the
bundles of feldspar, which has a higher refraction than the glass,
exhibit low extinction angles, nearly parallel to the longitudinal
direction. The bundles are clearly composed of numerous parallel
or approximately parallel individuals, which spread out fan-like or
plumose at the extremities. In other positions these aggregates are
seen to be relatively broad, blade-like, or tabular, and made up of
subparallel plates of extreme thinness. This is shown by numerous
crystal edges almost parallel to one another, and also by the com-
posite character of the interference phenomena with crossed nicols,
the crystal blade being mottled instead of uniformly dark in the
* x-5393 is the index of refraction of the balsam used by the U. S. Geological
Survey, as determined by Dr. J. E. Wolff, in 1896.
OLIGOCLASE (AB4ANi). 95
position of extinction. The interference figure in convergent light
shows a bisectrix normal to the plane of the bladed crystal bundle,
the bisectrix being the direction of vibration of the ray having the
greatest index of refraction, ;-. The angle between the optic axes is
larger, and the plane of the optic axes lies in the longer diameter of the
crystal blade. This is the optical orientation of Ab4Ani* when the
feldspar plate is tabular parallel to the second pinacoid (oio), the
common case.
The angles between the crystal edges of the thin plates, the position
of the plane of optic axes, and the generally low extinction angles ex-
hibited by longitudinal cross-sections of the bundles show that the
feldspar crystals are tabular parallel to (oio) and elongated parallel
to the crystal axis a, and are probably bounded by the planes (201)
and (304) or by (201) and (203).
* Michel-Levy, A., op. cit., Plate in.
PLATE
CRYSTALS OF ANORTHITE SHOWING THE APPEARANCE OF DISLOCATION
IN THE TWINNED LAMELL/E. (50b) X 57.
I B
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PLATE
TANGENTIAL SECTION OF SPHERULITE OF BYTOWNITE SHOWING TWINNING
IN FEATHER-LIKE BLADES. ABiANs (58b) x 55.
PLATE II
RADIAL SECTION OF SPHERULITE OF BYTOWNITE. FEATHER-LIKE AGGREGATES
OF TWINNED CRYSTALS. ABiANs (58») X 50.
PLATE IV
CRYSTALS OF BYTOWNITE FROM TOP OF CRUCIBLE.
ABiAN., <31< X 50.
PLATE V
TABULAR CRYSTALS OF BYTOWNITE FROM MIDDLE OF CRUCIBLE.
ABiANs (30) X 50.
PLATE VI
TWINNED BLADED CRYSTALS OF LABRADORITE.
ABiAN-j (60) X 56.
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PLATE VII
SPHERULITE OF PLUMOSE BUNDLES OF PRISMATIC CRYSTALS OF LABRADORITE.
ABiAN, (61a' X 49.
PLATE VIII
SPHERULITE OF FEATHER-LIKE AGGREGATES OF CRYSTALS OF LABRADORITE.
ABiAN,- (61a) X 49.
PLATE IX
RADIATING AGGREGATES OF BLADES AND PRISMS OF LABRADORITE.
AB,AN? (23a) x 49
PLATE X
BLADES COMPOSED OF PARALLEL AND SUB-PARALLEL THIN PLATES OF LABRADORITE.
AB.AN? (23'J) X 49.
PLATE XI
MARGIN OF SPHERULITE OF LABRADORITE BLENDING WITH MICROSCOPIC
FIBERS OF SAME MINERAL. ABiANa (22b) X 55.
PLATE XII
FIBROUS BUNDLES AND SPHERULITES OF ANDESINE-LABRADORITE.
AB.AN, (26> X 55.
PLATE XII
ARBORESCENT AND FEATHER-LIKE AGGREGATIONS OF PRISMATIC AND BLADED
CRYSTALS OF ANDESINE-LABRADORITE. AB1AN1 (27) X 50.
PLATE XIV
SPHERULITES OF OLIGOCLASE-ANDESINE IN GLASS.
AB3AN1 f25a) X 50.
PLATE XV
SPHERULITES OF OLIGOCLASE-ANDESINE IN GLASS.
AB3ANi (24) X 70.
PLATE XVI
SPHERULITES OF OLIGOCLASE-ANDESINE.
AB3AN1 (67a) X 50.
PLATE XVII
GLASS WITH SPHERULITIC AGGREGATES OF BLADES OR PLATES OF OLIGOCLASE.
AB,AN, (29^ X 55.
\
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PLATE XVIII
A SPECIMEN OF MITCHELL COUNTY ALBITE BEFORE HEATING.
(33) X 20.
PLATE XIX
A CLEAVAGE CRACK (VERTICAL) SHOWING INCIPIENT MELTING
AFTER FOUR HOURS AT 1125°. (51») X 600.
PLATE XX
NATURAL ALBITE AFTER HEATING TO 1200°. MELTED AREAS ARE DARK.
(34) X 40.
PLATE XXI
NATURAL ALBITE AFTER THIRTY-FIVE MINUTES' EXPOSURE AT 1206°.
MELTED PORTIONS ARE DARK. (37^ X 40.
PLATE XXI
NATURAL ALBITE AFTER THIRTY MINUTES' EXPOSURE AT 1225°.
UNMELTED PORTIONS ARE BRIGHT. <39) X 40.
PLATE XXIII
NATURAL ALBITE AFTER THIRTY MINUTES' EXPOSURE AT 1247°.
UNMELTED PORTIONS ARE BRIGHT. (36) X 40.
PLATE XXIV
A CRYSTAL OF NATURAL ALBITE BENT AT 1200° UNDER LOAD.
THE DARK PORTIONS HAVE MELTED. (41) X 10.
PLATE XXV
ORTHOCLASE FRAGMENT BENT AT 1200° UNDER LOAD. A CLEAVAGE CRACK (DOTTED) HAS RETAINED
ITS ORIENTATION, ALTHOUGH THE FRAGMENT IS MELTED THROUGH. (46) X 20.
PLATE XXVI
ALBITE FRAGMENT BENT AT 1200° UNDER LOAD.
(45) X 40.
THE ISOMORPHISM AND THERMAL PROPERTIES
OF THE FELDSPARS.
PART I— THERMAL STUDY,
ARTHUR L. DAY and E. T. ALLEN.
PART II— OPTICAL STUDY, - J. P. 1DDINGS.
WITH AN INTRODUCTION BY
GEORGE F. BECKER.
Washington, D. C. :
Published by the Carnegie Institution of Washington.
1905.
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