Abstract
Laboratory experiments have been conducted with natural minerals to determine the relation of \(f_{{\text{o}}_{\text{2}} } \) to epidote stability, and to determine stability curves for clinozoisite and epidote. Under oxidizing conditions Fe-epidote decomposes to grandite, anorthite, hematite, and quartz. Under more reducing conditions corundum becomes a stable product instead of quartz, and magnetite, and finally hercynite replace hematite. As conditions change from oxidizing to reducing the temperature of epidote breakdown decreases, epidote becomes more aluminous and the grandite produced increases in grossularite component and, to a lesser extent, in almandine.
At 3000 bars under oxidizing conditions epidote is stable up to 694° C, epidote-corundum is stable to 692° C, clinozoisite is stable to 658° C, and clinozoisite-quartz is stable to 628° C. Approximate curves for the fractional decomposition of Al-Fe epidote have been determined as a function of Fe content under oxidizing conditions. Extrapolation of clinozoisite results to an Fe-free composition, and comparison with zoisite stability results suggest that at elevated pressures clinozoisite inverts with increasing temperature to zoisite along a nearly vertical phase boundary at 635±75° C.
The stability relations provide an upper limit for epidote mineral stability mainly applicable to calcareous rocks. The epidote composition present in any given rock must be a function largely of bulk composition and \(f_{{\text{o}}_{\text{2}} } \). Zoisite replaces Al-clinozoisite in rocks of medium grade and high pressure.
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Holdaway, M.J. Thermal stability of Al-Fe epidote as a function of \(f_{{\text{o}}_{\text{2}} } \) and Fe content. Contr. Mineral. and Petrol. 37, 307–340 (1972). https://doi.org/10.1007/BF00371011
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DOI: https://doi.org/10.1007/BF00371011