Experimental study of the join tremolite-tschermakite: A reinvestigation

Abstract

The inability to synthesize pure tschermakite over the range of 6–15 kbar and 700–850° C, the optimal conditions reported by Oba (1978), prompted a reinvestigation of the join tremolite-tschermakite which was originally investigated by Jasmund and Schäfer (1972). The results of syntheses performed in 10 mol% increments along this join at 12 kbar and 850° C indicate that solid solution exists only in the composition range or TS₀TR₁₀₀ to TS₅₀ TR₅₀. This was confirmed by direct chemical analyses of the synthetic amphiboles and by monitoring the shift in the d₃₁₀. Syntheses along this same join at 3 kbar and 850° C reveal an even more limited range of solid solution, namely from TS₀TR₁₀₀ to TS₁₀TR₉₀, which was also confirmed by direct analyses and variation in the d₃₁₀ values. Comparison of the results at 3 kbar and 12 kbar indicates a pressure dependency of Al solubility in tremolite corresponding to 0.17 Al (total)/kbar on the basis of 23 oxygens. An attempt to re-equilibrate tremolite and an Al-rich amphibole at 6 kbar and 850° C to an intermediate composition resulted in the Al-rich amphibole displaying the best evidence of re-equilibration, which was to a lower Al content. This observation supports the results obtained from the synthesis experiments by showing that decreased pressure decreases the Al solubility. It is emphasized that the Al content of natural calcic amphiboles, whose compositions are approximately modeled by the join tremolite-pargasite, is probably a function of both pressure and temperature and must ultimately be calibrated in a fashion analogous to the pyroxenes.