214 PRECIOUS STONES.
316. Albite. Albite
is usually colourless or white when unweathered, but may show tints of
pink, grey, red, or green. It is transparent to sub-translucent. The
specific gravity is 2'62 to 2'65, with a fracture and cleavages very
similar to those of other members of the group. The hardness is 6 to
6-1/2. It is a triclinic Felspar and is usually twinned repeatedly in
fine lamella?, and this gives rise to a striped appearance often seen
in the triclinic Felspars. As an original constituent of many granites
it is very widely distributed ; in these cases it is usually in
crystalline masses that have been formed simultaneously with other
minerals, and as a consequence the crystal faces are not developed.
Again, it may frequently be found in cavities in the granites, usually
in the outer part of the mass, associated with Beryl, Topaz, Smoky
Quartz and other minerals. In chemical composition it is an aluminium
sodium silicate, Na20, ALO.„ 6 SiO2. An Aventurine variety
sometimes occurs, but more important is the purer form Peristerite or
Albite Moonstone, which shows the chatoyant light very well in good
specimens. Other specimens show more play of colour like Labradorite,
but less intense. Peristerite occurs at Bathurst and Burleigh in
Ontario and at several other places in North America.
317. Oligoclase.
This
species is chiefly of interest because to it belong most of the
Sunstones met with in use as precious stones. Its composition varies
somewhat; it is essentially an aluminium-sodium-calcium silicate. It is
usually white, grey, greenish, or reddish in colour, transparent to sub-
