Abstract
Phase relations for the bulk composition 2CaOX5FeO x X8SiO 2 +H 2 O at moderate temperatures and fluid pressures, and low oxygen fugacity (fO 2 ) values, indicate a field of stability for the amphibole ferrotremolite, Ca 2 Fe 5 Si 8 O 22 (OH) 2 . With fO 2 defined by the magnetite-iron buffer, ferrotremolite is stable up to 437 degrees C at 500 bars, 543 degrees C at 3,000 bars; the high-temperature assemblage of equivalent composition consists of fayalite+quartz+hedenbergitic pyroxene+fluid. With fO 2 specified by the magnetite+quartz-fayalite buffer, the amphibole high-temperature stability limit is reduced about 4 degrees C, and magnetite accompanies the other high-temperature phases. At still higher fO 2 values, represented by the Ni-NiO 2 buffer, ferrotremolite is not stable at any temperature. Reasons for restriction of ferrotremolite in nature are examined.
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