Epidote (redirected from Pistacite)

epidote

Any of a group of colourless to green or yellow-green silicate minerals with the general chemical formula A₂B₃(SiO₄)(Si₂O₇)O(OH), in which A is usually calcium (Ca) and B is generally aluminum (Al), although other elements are sometimes substituted. The epidote minerals occur as reaction products of other (earlier) minerals in low-grade (formed under relatively low-temperature, low-pressure conditions), regionally metamorphosed rocks, where their occurrence is used as an indicator of metamorphic grade.

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epidote [′ep·ə‚dōt]

(mineralogy)

A pistachio-green to blackish-green calcium aluminum sorosilicate mineral that crystallizes in the monoclinic system; the luster is vitreous, hardness is 6½ on Mohs scale, and specific gravity is 3.35-3.45.

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Epidote 

a rock-forming mineral of the silicate class, with chemical composition Ca₂[Al, Fe, Mn]₃[SiO₄][Si₂O₇]O(OH). Epidote crystallizes in the monoclinic system, often forming well-faceted, elongated prismatic crystals. It also occurs in the form of granular, divergent, or parallel columnar aggregates. It has a perfect cleavage and a vitreous luster. Its hardness on Mohs’ scale is 6–7. Its density increases from 3,300 to 3,450 kg/m³ with increasing iron and manganese content. The color of epidote is related to inclusions of ions of transition metals in the structure and varies from colorless or bluish gray (for clinozoisites, which do not contain Fe and Mn) to dark green (for large concentrations of Fe). Epidote with a high Mn content, called piedmontite, has a pink or red-brown color.

Epidote is a typical mineral of limestone skarns, as well as of low- and medium-temperature metamorphic rocks rich in Ca. Experimental study of certain important chemical reactions between epidote and other silicates (garnet and anorthite) has made it possible to use epidote as an indicator of the physicochemical conditions of the formation of the rocks in which it is found.

REFERENCE

Kepezhinskas, K. B., and V. V. Khlestov. Statisticheskii analiz mine-ralov gruppy epidota i ikhparageneticheskie tipy: Moscow, 1971.

L. IA. ARANOVICH