Abstract

Minerals of the epidote group in Tanganyika are commonly produced during cataclasis of lime-bearing rocks; invariable accession of the hydroxyl radical during dislocation assists their formation. Epidote minerals are not produced from plagioclase by the action of water, even at high temperatures and pressures; shearing stress or the presence of other material is required. Saussuritization follows, and is often a result of, uralitization. Metasomatism, progressive metamorphism, and granitization of lime-bearing rocks can give rise to epidote minerals. Epidote and clinozoisite are stable in the greenschist facies and the albite-epidote amphibolite facies, but are unstable in the almandine-diopside amphibolite subfacies of the amphibolite facies. Zoisite is unstable in the greenschist facies, but is stable in the albite-epidote amphibolite facies and throughout the amphibolite facies. Lime metasomatism, which is responsible for epidotization, is largely a result of displacement by the alkalies of lime from lime-bearing minerals. Epidotization often accompanies ferruginization and sulfide mineralization. Much epidote in the anatectic granites is the result of contamination; primary magmatic epidote is unlikely to occur. The epidote minerals form between 300°C and 500°C. High hydrostatic pressures favor the formation of zoisite. Under conditions of shearing stress monoclinic epidotes are formed. The epidotes are not stress minerals; under metasomatic conditions they are frequently formed without stress.

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