407. Epidote.
Epidote,
like Cyanite and Andalusite, is of wide distribution, but is rarely
found of a kind suitable for use as a gem. The colour is
characteristically pistachio-green, merging to a yellow-green and clear
yellow, or to a brownish green or green-black; rarely colourless or
red. It has a vitreous lustre, and when cut as a gem is very brilliant.
The gem variety is transparent, but the common varieties are found
quite opaque sometimes. It is strongly refracting, and the double
refraction is very marked too, the greatest and least indices for red
light being 1.768 and 1.730. Pleochroism is unusually well marked,
showing tints of green, brown and yellow in three different directions.
The specific gravity is 3-35 to 3-5. The mineral
is brittle, and has an uneven fracture; it shows two cleavages, one
perfect and the other imperfect. It crystallises in monosymmetric
forms, usually prismatic, sometimes acicular. It is often found in
attached crystals in cavities, usually in thermo-metamorphic rocks. Its
common associates are Calcite, Apatite, Feldspar, and Asbestos. In
composition it is a silicate of calcium and aluminium, with iron,
containing some water, H20, 4 CaO, 3 (Al, Fe)208, 6 Si02;
the water is driven off on strongly heating. The chief locality is
Knappenwand, in the Untersulzbachthal in Salzburg, where it occurs in
an epidote-scbist. It is also found in the United States at Haddam in
Connecticut; at Roseville in Sussex County, New Jersey ; and in
Georgia. Some Epidote of gem quality has been found in Brazil, with
green Tourmaline in Minas Geraes.
It is distinguished from most similarly coloured minerals
