Experiments were performed on two natural tremolite samples and compared with unpublished data of D. M. Jenkins, T J.B. Holland, and A. K. Clare on synthetic tremolite to determine what, if any, differences exist between the upper-thermal and upper-pressure stability limits of synthetic and natural tremolite. The upper-thermal stability of natural tremolite, which was investigated with experimental reversals of the reaction tremolite = 2 diopside + 3 enstatite + quartz + H20 in the range of 1.5-7 kbar, is about 40 ± 20 °C higher than that of synthetic tremolite. Similarly, the upper-pressure stability of natural tremolite, investigated with the reaction tremolite = 2 diopside + talc, is about 1 kbar higher than that of synthetic tremolite. A thermodynamic analysis of the results for these two reactions indicates that one natural tremolite (TREM 12) has a Gibbs free energy that is 2.9 kJ/mol more negative than that of synthetic tremolite, whereas the other natural tremolite (TREM 8) is 0.4 kJ/mol more negative on a constant-entropy basis. Both values are much less than the 9.2-kJ/mol difference observed by Skippen and McKinstry (1985). Of the factors that can be quantified with some degree of accuracy at present, i.e., H20 fugacity, grain size, and composition, it appears that the compositional variation, in particular the F content, is sufficient to account for the higher stability and lower Gibbs free energy of natural tremolite. One need not invoke the presence of a high density of structural defects in synthetic tremolite to explain the observed differences. At least for hydrothermal processes, synthetic calcic amphiboles model closely the behavior of natural calcic amphiboles if differences in their compositions are considered.

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