STANDARD THERMODYNAMIC PROPERTIES OF ALMANDINE

The standard entropy and high-temperature heat capacity of synthetic a lmandine, Fe3Al2Si3O12, were calculated from experimental high-T, high -P data on the equilibrium: almandine + rutile = ilmenite + sillimanit e + quartz (GRAIL reaction). The calculated standard entropy of almand ine of 340.28 J/mol agrees almost exactly with the value measured by a diabatic calorimetry. Calculated heat capacity coefficients yield entr opy and heat content values in the range 600 - 1200 K, which are consi derably larger than those of the major self-consistent datasets. Howev er, the predicted heat content increment from 965 K (H965 - H298.15 = 306.32 kJ) is in substantial agreement with a drop-calorimetry measure ment of 305.52 +/- 1.88 kJ, performed in 1972 in this laboratory. The standard enthalpy of formation of almandine was calculated from recent high-pressure experimental data on the reaction: almandine + O2 = mag netite + kyanite + quartz, buffered at hematite-magnetite oxygen fugac ity, making use of the derived heat capacity coefficients of almandine . The resulting value of DELTAH-degrees(f), -5266.76 kJ/mol at 298.15 K, is close to those of the self-consistent datasets, but 10 kJ less n egative than that given by high-temperature solution calorimetry. The enthalpy of formation of ilmenite, FeTiO3, that is consistent with the present calculations, is 4 kJ more negative than given by acid soluti on calorimetry, but supports the revision of Anovitz et al. (1985) bas ed on reinterpretation of the available measurements of oxygen fugacit y on reactions responsible for the formation of ilmenite.