3 5

ILLINOIS STATE

LABORATORY
OF NATURAL HISTORY

LIBRARY

•e+4+4+4+444+4+4+t+4+44

X I E> HAR.Y

OF THE

U N I VERSITY

Of ILLINOIS

^URAL HISTORY- SURVE

550.5

FI
v.3,cop.CL

REMOTE gf L

LIBRARY
UNIVERSITY OF ILLINOIS

URBANA

Field Columbian Museum.

Publication 112.

Geological Series. Vol. Ill, No. 4.

ZOISITE FROM
LOWER CALIFORNIA

BY

Oliver Cummings Farrington, Ph. D.
Curator, Department of Geology.

Chicago, U. S. A.
May 15, 1906.

ZOiSITE FROM LOWER CALIFORNIA

BY OLIVER CUMMINGS FARRINGTON

While at San Diego, California, in the spring of 1905, the writer
obtained from Mr. Ernest Riall of that city several specimens of a
radiated mineral, collected by Mr. Riall at the Trace mine, in the
Juarez District of Lower California. This locality, according to Mr.
Riall, is situated sixty miles south of the international boundary. The
accompanying cut shows the appearance of a typical specimen of the
mineral. It occurs as divergent groups of long, prismatic crystals irreg-
ularly penetrating a matrix of a white, granular mineral. The length
of the crystal groups varies from one to three inches. Their form
is essentially conical, the angle of the cone being about io°. In coloring
the cones are pink peripherally, pass interiorly into nearly colorless
and at the center are brownish-gray. Their constitution of numerous
individual crystals is shown by elongated brilliant surfaces into which
they readily separate longitudinally, but transversely the cones break
as units. The cones as a whole are translucent, but small fragments
are transparent. No terminal planes can be observed on any of the
crystals. The longitudinal fragments show roughly prismatic bound-
aries, but it was found impossible, with a reflecting goniometer, to ob-
tain satisfactory measurements of the prismatic angles, since numer-
ous longitudinal striations produce long series of reflections. Besides
the striated planes, others not striated appear to be cleavage planes
to the brachypinacoid. The longitudinal fragments are colorless and
transparent and show in polarized light extinction parallel to the long
axis. No pleochroism is observable. The character of the double
refraction is positive. In convergent light the emergence of an optic
axis may be seen on such fragments. Sections perpendicular to the axis
of the cone are colorless and show no pleochroism. Numerous cleavage
cracks making angles of 530 with each other penetrate such sections.
Between crossed nicols a polysynthetic twinning structure is seen to
characterize the whole, the field being filled with lamellae in parallel
position. These lamellae divide into two groups as regards width,
the broader being from .1 to .07 mm. and the narrower from .025 to
.012 mm. The broad and narrow lamellae alternate. The direction

55

56 Field Columbian Museum — Geology, Vol. III.

of the lamellae is such as to bisect the above-noted cleavage angle
of 530. Single cleavage cracks || with the lamellae are also occa-
sionally seen.

The interpretation of this structure is difficult, but the following
may be suggested: The cleavage cracks at angles of 53 ° are those of
prisms of which the crystal groups are composed. These prisms have
the symbol 540, corresponding to an angle of 520 44'. This is a new
form for zoisite. The twins of which these prisms are made up are
formed on c as the twinning axis and the twinning plane is some
highly inclined brachy-dome such as e (061).

The lustre of fragments of the mineral is vitreous and the frac-
ture sub-conchoidal. Hardness 6.5 and specific gravity, determined
with a chemical balance, 3.32. The mineral fuses B. B. at 3 with in-
tumescence, to a brownish enamel and is only slightly attacked by
hydrochloric acid. Qualitative tests showed it to be essentially a
hydrous calcium aluminum silicate, from which the water could be
driven off only by strong ignition. Quantitative analysis by Mr. H.
W. Nichols gave the following result:

Ratio

SiO, 38.15 3.02

ALP, 29.50 \

FeA 4.60 f I51

MuO 0.55

CaO 22.71 /

MgO 0.63 \ 2°5

' H20 3.76 1. 00

K*° I tr

Na,0 f tn

99.90

These ratios lead to the formula H4 Ca4 Al6 Si6 Oa,, which is
that usually accepted for zoisite with the addition of one molecule
of water. For the determination of the water both of Penfield's
methods* were employed. By the first method, that of heating in a
blast lamp, 1.81% of water was obtained. The mineral did not fuse.
By the second method, which consists in heating the tube containing the
assay in an oven of fire-brick lined with charcoal, an additional percent-
age of water amounting to 1.95% was obtained. Under this treatment
the mineral fused completely. The close similarity between the per-
centages of water obtained by the two methods, each corresponding to
one molecule, suggests that the molecules may be differently com-
bined. Thus one may be united with aluminum and the other with

* Amer. Jour. Sci. 1894, 3rd ser. Vol. XLVIII, pp. 30-37

May, 1906. Lower California Zoisite — Farrington. 57

calcium. No further investigation was made of this point, however.
Although the formula of zoisite is usually considered to be Hs Ca4 Al4
Si„ 0„, other analysts have obtained percentages which indicate
that an additional molecule of water is present. This is true, for
instance, of the analyses of zoisite from Fuschthal and Traversella
quoted by Dana.* The high temperature required to drive off the water
from the Lower California mineral seems to preclude the possibility
of its being present as the result of alteration, as might otherwise
be assumed. The amount of iron in the zoisite shown by the analysis
is high for this mineral and approximates that afforded by epidote.

The mineral with which the zoisite is associated is, as stated,
white and granular. In cavities it exhibits minute imperfect crystals
which have a distinct, pearly luster when fractured. The blowpipe
and other characters of this mineral indicate it to be prehnite and a com-
parison with fragments kindly furnished by Dr. W. T. Schaller leaves
little doubt that it is the same mineral analyzed f by him and found
to be prehnite. Its association with zoisite is of interest owing to the
similarity in composition of the two minerals. The prehnite seems
generally to furnish a matrix which the zoisite penetrates, but occa-
sionally it coats the zoisite groups in such a way as to suggest that it
is an alteration product of the latter. The unusual features of the
zoisite seem to be therefore, its radiating habit, its high content of
water and iron and its association with prehnite.

Through the kindness of Prof. L. P. Gratacap of the American
Museum of Natural History, ths writer was permitted to study two
specimens of zoisite in the collection of that institution which were
undoubtedly from the same locality as the above. They have the
more usual ash-gray color of zoisite and the grouping of the crystals into
cones is only partial. For the most part the crystals occur in hemi-
spherical cavities which were, in the specimens studied, about three
inches in diameter. The crystals interlace these cavities with great
variations of size and direction. Many of the crystals are quite
minute. All are from acicular to bladed in habit. Although some
crystals have free terminations, no end faces could be discerned.
These specimens show that grouping into cones is not constant for the
zoisite from this locality but its occurrence at all is noteworthy .

* System of Mineralogy, 6th ed., p. 514.
tBull. U.S.Geol. Survey No. 262, p. 128

s
•2

O
3

Oh

.S

£J

B