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Epidote

The Silicate Minerals

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Epidote, Ca2Al2(Fe3+,Al)(SiO4)(Si2O7)O(OH), is a common mineral in some metamorphic rocks. Typically it has a pistachio- or avocado-green color. (more below)
Hydrous calcium iron silicate
Photo (c) 2008 Andrew Alden, licensed to About.com (fair use policy)
Epidote has a Mohs hardness of 6 to 7. The color is usually enough to identify epidote. If you find good crystals, they show two strongly different colors (green and brown) as you rotate them. It might be confused with actinolite and tourmaline, but it has one good cleavage where those have two and none, respectively.

Epidote often represents alteration of the dark mafic minerals in igneous rocks such as olivine, pyroxene, amphiboles and plagioclase. It indicates a level of metamorphism between greenschist and amphibolite, particularly at low temperatures. Epidote thus is well known in subducted seafloor rocks. Epidote also occurs in metamorphosed limestones.

The molecular structure of epidote is hard to describe: chains of aluminum oxide octahedra alternate with hydroxylated versions, and silica units both single (SiO4) and doubled (Si2O7) bind the chains together. Rare earths may replace the calcium, and the ferric iron (Fe3+) may be replaced by trivalent (Al, Mn3+, V3+) or divalent (Fe2+ or Mg) species.

Other Metamorphic Minerals

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