epidota

Epidote is a mineral species in the epidote group of sorosilicates. Its chemical formula is Ca2(Al,Fe)3(SiO4)3(OH), and it commonly occurs as elongated prismatic crystals that are usually pistachio-green to olive-green. It crystallizes in the monoclinic system and typically shows a vitreous to resinous luster. The hardness on the Mohs scale is about 6 to 7, and its specific gravity is around 3.3 to 3.5. Cleavage is imperfect and fracture is uneven.

Epidote forms through metamorphic and hydrothermal processes. It is common in regional and contact metamorphic rocks, especially in aluminous rocks such as pelites and marbles, and in hydrothermal veins. It often occurs together with quartz, calcite, chlorite, albite, garnet, amphibole, zoisite, and other epidote-group minerals. Epidote commonly appears in rocks ranging from greenschist to amphibolite facies and in metasomatically altered rocks.

The mineral is an important indicator mineral in metamorphic geology because its presence can reflect metamorphic grade and alteration conditions. The name is used in several languages as epidote or its local variant. There are iron-rich varieties commonly referred to as ferro-epidote, reflecting solid-solution with iron.

The color range of epidote includes green to yellow-green, sometimes brownish. It is transparent to translucent, and crystals may be cut for collectors, though the mineral’s imperfect cleavage makes large, high-quality faceting difficult. Gem-quality material is sometimes used as cabochons in jewelry, and its distinctive color makes it a recognizable specimen in mineral collections.

Notable localities include the Alps, Norway, Austria, Canada, and parts of the United States, Asia, and Africa, among others. Epidote remains of interest to geologists as both a mineral specimen and a practical indicator mineral in metamorphic petrography.