Epidote
(Pistacite)
Epidote often possesses a peculiar shade of yellowish green, similar to that of the pistachio-nut—hence the origin of its alternative name—which is unique among minerals, though scarcely pleasing enough to recommend it to general taste. Its ready cleavage renders it liable to flaws; nevertheless, it is occasionally faceted. The name epidote, from ἐπίδοσις, increase, was given to it by Haüy, but not on very precise crystallographical grounds.
In composition this species is a silicate of calcium and aluminium, with some ferric oxide in place of alumina, corresponding to the complex formula, Ca2(Al,Fe)2[(Al,Fe)OH](SiO4)3. It occurs in monoclinic, prismatic crystals richly endowed with natural faces. The colour deepens with increase in the percentage amount of iron, and the stones become almost opaque. The double refraction is large in amount, 0·031, biaxial in character, and negative in sign. The dichroism is conspicuous in transparent stones, the twin-tints corresponding to the principal optical directions being green, brown, and yellow. The values of the least and greatest of the refractive indices given by transparent stones are 1·735 and 1·766 respectively; the specific gravity varies from 3·25 to 3·50, and the hardness from 6 to 7 on Mohs’s scale.
Transparent crystals have come from Knappenwand, Untersulzbachtal, Salzburg, Austria; Traversella, Piedmont, Italy; and Arendal, Nedenäs, Norway. Magnificent, but very dark, crystals were discovered about ten years ago on Prince of Wales Island, Alaska.