epidote

Epidote is a calcium aluminum iron silicate mineral that forms the epidote group, a family of sorosilicates. The ideal composition is often written as Ca2(Al,Fe)3(SiO4)(Si2O7)OH, with substantial substitution of iron for aluminum and the inclusion of rare-earth–bearing varieties such as allanite in the group. Epidote minerals occur in a range of solid-solution series, and clinoepidote is a closely related polymorph.

Formation and occurrence

Epidote forms during regional and hydrothermal metamorphism, commonly in greenschist to amphibolite facies, and also in certain skarns and contact metamorphic rocks. It is typically found in metamorphic rocks such as schists, phyllites, and calc-silicate rocks, as well as in hydrothermal veins. Epidote-rich zones can indicate particular pressure–temperature conditions during rock transformation.

Crystal habit and properties

Epidote crystals are typically prismatic and elongated, with a vitreous luster. Fresh specimens are usually pistachio-green to olive-green, though color can fade or darken with alteration or iron content. It has a hardness around 6 to 7 on the Mohs scale and a specific gravity near 3.3 to 3.5. Epidote is pleochroic, often showing a yellow-green to green color in different crystallographic directions. It can be confused with other green minerals such as zoisite or tourmaline, but its characteristic color and habit help distinguish it.

Uses and significance

Epidote is primarily a collector mineral and may be encountered in gemstone contexts, though it is less commonly used in jewelry due to cleavage and stability considerations. The epidote group’s presence aids in interpreting metamorphic histories and the conditions of rock formation.