270. CALCITE.----LABRADORITE.
This mineral,
so important in general mineralogy, is of but small consequence in so
far as its use as a precious stone goes. A great deal of the material
known to Theophrastus, Pliny, and other ancient writers, belonged to
this species. A complete mineralogical description is here uncalled
for, as the crystallised varieties are practically beyond the range of
our subject, only some of the many massive varieties being used.
When
pure it is colourless, but, like so many other colourless minerals, it
is subject to much variation in colour from impurities, even when these
are in very small quantity. Thus it may be cream, pink, grey, green,
blue, violet, yellow, brown, etc. The lustre of pure specimens is
vitreous; impure forms may be earthy. Similarly all gradations from
transparent to opaque are seen.
One
of its outstanding physical properties is its remarkably pronounced
double refraction, and this property has given the name Doubly
Refracting Spar to one of the varieties. The refractive indices for the
D line of the spectrum are, for the ordinary ray l-658, and
for the extraordinary ray 1'486. On heating it decrepitates, and turns
opaque white ; coloured varieties may change colour. The specific
gravity is 2-71 to 2-72 in pure varieties, but may be up to 2'84 in impure forms. The hardness is 3. It
