New enthalpy and entropy data have been obtained for a natural end-member tremolite from phase equilibrium experiments on the high-temperature dehydration reaction tremolite = 2 diopside + 1.5 orthoenstatite = β-quartz + H2O. Six reversed experimental brackets were obtained at 0.25 kbar (996–1033 K), 0.6 kbar (1038–1058 K), 1 kbar (1075–1104 K), 1.5 kbar (1101–1125 K), 2 kbar (lll5–1128 K), and 8 kbar (1179–1209 K), from which the enthalpy of formation and the third-law entropy of tremolite have been calculated using the internally consistent thermodynamic dataset Thermocalc of Holland and Powell (1990). The values obtained are ΔHf0(Tr)=12299.48±14.84 kJ/mol and S0(Tr) = 0.5525 ± 0.0100 kJ/mol·K, which are in excellent agreement with the values given in Thermocalc and agree well with the values of Berman et al. (1985). We use the new thermodynamic data to calculate revised limits of tremolite stability in the system CaO-MgO-SiO2-H2O (CMSH). We also discuss the apparent major discrepancy between values for ΔHf0(Tr) obtained from solution calorimetry (Kiseleva and Ogorodova, 1984) and the phase equilibrium values. The difference between enthalpies for end-member tremolite determined from phase equilibrium and calorimetric studies (36 kJ) is comparable to that observed for talc (Kiseleva and Ogorodova, 1984), perhaps suggesting anomalous dissolution behavior of the talc-like elements of tremolite.

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