Crystal system Monoclinic
Transparency Transparent to opaque
Luster Vitreous to greasy
Fracture Uneven
Cleavage perfect basal
Specific Gravity 3.40
Hardness 6-7
Optical Character Biaxial + ; Double Refractive
Refractive index 1.729-1.768
Birefringence 0.019 to 0.045
Dispersion 0.030
Pleochroism Strong; green , brown & yellow
Chemical Formula Ca2(Al,Fe)3(SiO4)3(OH)
Streak White

Epidote (EPP-in-dote) is a complex hydrous silicate of calcium and aluminum, in which both calcium and aluminum are replaced by other metals. It is usually an alteration product, and its complexity is such that a wide variation in composition, and thus in properties, is common. The formula is usually given as Ca2(Al,Fe)3(SiO4)3(OH). In its composition, the aluminum may be replaced in part by ferric iron, manganese, or the cerium and yttrium metals. The calcium may be replaced partially by magnesium or ferrous iron. If the aluminum is replaced by cerium and yttrium, the member of the epidote group becomes allanite, or orthite, which is an important ore of these rare earth minerals; it is brown or black. Zoisite is a white, gray or brown or blue epidote with little iron. Thulite, a pink variety of zoisite, is used frequently in ornamental jewelry pieces in the form of cabochons or sometimes cameos and intaglios. With an increase in manganese, epidote passes into dark-red piemontite.

Epidote has a hardness of 6 to 7, poor toughness (Because of perfect basal cleavage), and its S.G. varies from 3.25 to about 3.50. The fracture is uneven, the streak is white to grayish, and no characteristic inclusions are evident. The luster is described as vitreous to greasy. Epidote is monoclinic and usually occurs in prismatic crystals, whereas zoisite occurs in the orthorhombic system; thus both are biaxial. The accompanying photograph of a group of epidote crystals was reproduced with the permission of the Smithsonian Institution, Washington, D.C. Zoisite is positive in sign and epidote is negative. There is a significant difference in refractive indices between the two, the green epidote showing figures of approximately 1.73 and 1.768 and zoisite approximately 1.696 - 1.702. Epidote varies from about 1.729 - 1.768 down to the varieties low in iron, with readings of 1.716 - 1.723. Thus, the birefringence varies from about .038 down to .007. The indices and birefringence of piemontite, a dark-red variety are even higher; the low index varies from 1.738 to 1.75 and the high index from approximately 1.778 to 1.83. All of the transparent forms of epidote are highly pleochroic; green material shows green, brown and yellow and piemontite shows red, violet and yellow. The dispersion is .030. Epidote is partially decomposed by hydrochloric acid and is fusible.

Sources for this mineral are Salzberg, Austria; piedmont, Italy; Norway; Prince of Wales Island, Alaska; Brazil; Madagascar; North Carolina; and Kenya. Epidote and epidote minerals occur in metamorphic silicate rocks, usually in association with calcite, apatite, feldspar and some rare minerals.

Because of its rather high birefringence at a point on the refractometer scale in the mid 1970's, the identification of epidote is usually not difficult. Varieties with lower indices could possibly be confused with idocrase, but the difference in optic character would permit a separation.

Transparent material is faceted, with crown angles of 37° and pavilion angles of 42° proving satisfactory. Linde A powder on a tin lap or cerium oxide on a lucite lap produce a good polish. Because epidote is strongly pleochroic, careful orientation is necessary. Cleavage may prove troublesome.