360 University of California Publications. [Geology
here
and there, probably as a result of the alteration of the chalcocite,
but also as individual grains 4-6 mm. across in the granular natrolite
matrix. These may represent original chalcocite grains that have been
replaced by solutions percolating through the natrolite and thus
becoming charged with silica, but from the freshness of the surrounding
natrolite and neighboring chalcocite it may be that some of the copper
in the original solutions was held as silicate and separated out
primarily as chrysocolla.
Both of these copper minerals occur side by side with the titano-silicates.
OTHER MINERALS.
Amphiboles.
Some
of the drusy surfaces are coated with green or bluish green amphibole
needles, and the natrolite and albite crystals often carry them in
sufficient quantity to give a greenish or bluish tint. These amphiboles
vary from place to place and even the same needle may show different
optical properties along its length.
A
common type is actinolite. This has been observed in thin needles
growing out into open spaces as a newly formed mineral in the wall rock
and in veins. Occasionally it develops as asbestos films or exceedingly
fine hair-like bunches. It shows pleochroism in pale green and yellow,
the C sometimes having a bluish tint. It gives a distinct sodium
reaction in the blowpipe flame.
Another
common amphibole is bluish green in color and shows under the
microscope ο greenish yellow, ΰ gray violet, C bluish green. The axial
plane is transverse to the plane of symmetry and the dispersion is so
great that it shows no extinction in plane (010) with white light. The
extinction for red light is about 25°-30°, for violet some 5° greater.
Its properties would indicate a soda amphibole with considerable iron
in the molecule —perhaps intermediate between crossite and crocidolite,
but nearer the crossite type.
A member of the glaucophane group is sometimes present. c blue, 6 violet, a yellow;
about 8°.
