SUNSTONE OR AVANTURINE FELSPAR
Is
usually a variety of oligoclase, or soda lime felspar, having golden
yellow, reddish, or prismatic internal reflections, due to the presence
of minute imbedded and scattered crystals of haematite, gothite, or
mica. Some avanturine, is, however, a mixture of albite and orthoclase,
and the same name is given to quartz containing brilliant imbedded
micacious crystals. The green avanturine, called amazon-stone, is
microcline, a felspar.
ObsidiaN, or Volcanic Glass -
Is
often nothing more than fused or vitreous orthoclase—that is, potash
felspar. But obsidian frequently contains many other minerals in small
quantities, such as augite and olivine ; in fact, obsidian is a melted
lava, and contains the various minerals of the lava melted or else
associated together. Obsidian when transparent has about the specific
gravity 2.4, and is softer than crystalline felspar. Black specimens of
it resemble black garnet, spinel, and tourmaline, but are much more
translucent in thin splinters, as well as striated and full of bubbles.
Epidote.
The
various hues of olive, brownish, and pistachio green which are
presented by tourmaline occur also in great measure in epidote. The
latter mineral is, however, less dichroic than the former, although in
some green Siberian and Brazi ian specimens an emerald green image and
a yellow one may be seen in the dichroiscope. The most famous locality
is the Knappenwand, Salzburg. The hardness of epidote is about 6.5,
and its specific gravity 3.3 to 3.4. It occurs in oblique prisms, often
much elongated. Green epidote presents in 100 parts about the following
composition :
