Abstract
Calcic amphiboles are observed in ultramafic rocks that have equilibrated under a broad span of geological conditions and might prove to be good indicators of metamorphic grade if their stabilities could be determined as a function of their compositions. Experiments were performed on the stability of tremolite plus forsterite in the system H2O-CaO-MgO-SiO2 from 5 to 20 kbar. A univariant curve was fitted to the experimental brackets using volume, water fugacity, and heat capacity data. The results indicate that the maximum stability of tremolite in the presence of forsterite is about 825° C at 5 kbar. Addition of Al2O3 to this system increases the stability of tremolitic amphibole by only 20°–40° C and induces solubility of 5–7 wt.% Al2O3 in the amphibole, as determined from quantitative SEM analyses of individual amphibole crystals. Thus substitution of the tschermakite component (Ca2(Mg3Al2) (Si6Al2) O22(OH)2) alone cannot lead to the greatly enhanced Al2O3 contents or thermal stability of natural calcic amphiboles.
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