Epidote

Epidote is a calcium aluminium iron sorosilicate mineral.

Well developed crystals of epidote, Ca₂Al₂(Fe;Al)(SiO₄)(Si₂O₇)O(OH), crystallizing in the monoclinic system, are of frequent occurrence: they are commonly prismatic in habit, the direction of elongation being perpendicular to the single plane of symmetry. The faces are often deeply striated and crystals are often twinned. Many of the characters of the mineral vary with the amount of iron present for instance, the color, the optical constants, and the specific gravity. The color is green, grey, brown or nearly black, but usually a characteristic shade of yellowish-green or pistachio-green. It displays strong pleochroism, the pleochroic colors being usually green, yellow and brown. Clinozoisite is green, white or pale rose-red group species containing very little iron, thus having the same chemical composition as the orthorhombic mineral zoisite. The name, due to Haüy, is derived from the Greek word "epidosis" (ἐπίδοσις) which means "addition" in allusion to one side of the ideal prism being longer than the other.