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Mineral formula: {Ca2}{Al2Fe3+}(Si2O7)(SiO4)O(OH)

Mineral group: member of the Epidote group

Crystal system: Monoclinic

Crystal habit: Crystals prismatic to 35 cm, also stubby, rarely tabular or pseudo-octahedral. Fibrous, coarse to finely granular, massive. Prismatic crystals may show a pseudo-hexagonal cross-section.

Cleavage: perfect on {001}, imperfect on {100}

Fracture: irregular / uneven

Color: Yellowish-green, green, brownish-green, black

Luster: vitreous

Diaphaneity: transparent, or translucent, or opaque

Moh’s scale hardness: 6

Streak: colorless

Specific gravity: 3.38-3.49

Named after: the Greek επιδοσιζ ("epidosis"), meaning "increase", in allusion to the crystal characteristic of one longer side at the base of the prism, in 1801 by Rene Just Haüy.

Type locality: Le Bourg-d'Oisans, Grenoble, Isère, Auvergne-Rhône-Alpes, France

Geological occurrence: regional and contact metamorphic rocks. Saussuritisation (alteration of plagioclase).