Abstract
New experimental data for the breakdown of synthetic tremolite to the assemblage diopsidic clinopyroxene + enstatitic orthopyroxene + β-quartz + H2O in the pressure range of 1.5–7 kbar and to the assemblage diopside + talc in the range of 650–760 °C were combined with existing experimenlal data for these reactions and all pertinent experimenlal phase equilibria in order to obtain a refined value for the enthalpy of formation of tremolite. Using the least-squares regression procedure and data base of Holland and Powell (1990), the derived for tremolite is −12302.90 ± 7.05 (1σ) kJ/mol for fixed values of entropy, volume, heat capacity, expansivity, and compressibility for tremolite. This analysis was performed with the assumption that all phases were pure and had unit activity. This agrees with the value of −12300.47 ± 14.81 kJ/mol determined for natural stoichiometric tremolite by Welch and Pawley (in preparation). Assuming that Ca and Mg mix ideally on the M4 site in Mg-enriched, synthetic tremolite and that the compositions of the pyroxenes in equilibrium with synthetic tremolite are known, the will be approximately 1 kJ/mol less negative or approximately −12302 kJ/mol for stoichiometric tremolite. The dP/dT slope of the reaction tremolite = 2 diopside + talc is 8.8 bar/K. Extrapolation of this univariant boundary to low temperatures (200 °C) shows that tremolite is stable to at least 20 kbar and is not limited at low temperatures by this reaction at virtually any depth in the Earth's crust. The calculated decomposition temperature of tremolite at 1 bar is 448 °C, which is about 350 °C lower than the observed dehydration temperature of tremolite. It is suggested here that the dissolution of silica, rather than the loss of hydroxyls, may control the decomposition of tremolite in hydrothermal environments.
