Standard thermodynamic properties of almandine

Abstract

The standard entropy and high-temperature heat capacity of synthetic almandine, FcjAUSijO^. were calculated from experimental high-T. high-P data on the equilibrium: almandine+rulile =ilmenite +sillimanite +quartz (GRAIL reaction). The calculated standard entropy ofalmandineof340.28 J/mol agrees almost exactly with the value measuredby adiabatic calorimetry. Calculated heal capacity coefficients yield entropy and heat content values in the range 600 - 1200 K, which are considerably larger than those ofthe major self-consistent datascts. However, the predicted heat content increment from 965 K(H%5 - H2<«.i5 =306.32 kJ) is in substantial agreement with a drop-calorimetry measurement of 305.52 ± 1.88 kJ. performed in 1972 in this laboratory. The standard enthalpy of formation ofalmandine was calculated from recent high-pressure experimental data on the reaction: almandine +O, =magnetite +kyanitc +quartz, buffered at hematite-magnetite oxygen fugacity, making use of the derived heat capacity coefficients ofalmandine.The resulting value ofA//°f. -5266.76 kJ/mol at 298.15 K. isclose to those ofthe self-consistent datascts. but 10 kJ less negative than that given byhigh-temperaturesolutioncalorimetry.The enthalpyof formation ofilmenite. FeTiOj, that isconsistent with the present calculations, is4 kJ more negative than given byacid solution calorimetry. but supports the revision of Anovilz etal. (1985) based on reinterpretation ofthe available measurementsofoxygen fugacity on reactions responsible for the formation of ilmenite.