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