Calorimetry of phase transition and melting in silicates

Abstract

High temperature reaction calorimetry is used to study phase transitions and melting reactions in silicates. Two types of calorimeters are used in our laboratory: a custom built Calvet-type twin microcalorimeter for solution, drop-solution, and transposed-temperature-drop calorimetry near 700 °C and a commercial Setaram HT-1500 calorimeter for transposed-temperature-drop and step-scan measurements at 600–1500 °C. The two types of calorimeters are complementary in their capabilities and together can be used to tailor a calorimetric experiment to the exact conditions needed for a specific reaction. Metastable phases quenched from ultra-high pressure or prepared as glasses, thin films, or gels present special challenges in determining their energetics. A new differential drop-solution calorimetric method has been developed. A sample contained in a capsule made of the lead borate glass used as calorimetric solvent is dropped into one side of a twin microcalorimeter at the same time as an empty capsule is dropped on the other side. The increased sensitivity and accuracy of this method has allowed the determination of the enthalpy of formation of the high pressure perovskite polymorph of MgSiO 3 using a single 5 mg sample. The enthalpy of the reaction MgSiO 3 (orthopyroxene) = MgSiO 3 (perovskite) is 110 ± 5 kJ/mol; that of the ilmenite-perovskite transition is 59 ± 8 kJ/mol. These values support phase equilibrium studies which indicate that reactions forming MgSiO 3-rich perovskite at the 670 km discontinuity in the earth's mantle generally have negative P-T slopes. The energy systematics of pyroxene, garnet, ilmenite, and perovskite structures in silicates and germanates have been determined. The step-scan and transposed-temperature-drop methods also are giving new information on nonquenchable phase transitions, including those involving rapid order-disorder reactions, at 600–1500 °C. The spinels MgAl 2O 4 and NiAl 2O 4, and the pseudobrookite MgTi 2O 5 each show complex cation positional order-disorder. The orientation transition in CaCO 3 (calcite) near 987 °C has been characterized by in situ calorimetry, analyzed in terms of Landau-type order parameters, and applied to understanding apparent curvature in measured calcite-aragonite phase equilibria. Melting and crystallization reactions in silicates at 1100–1500 °C have been measured directly by using the Setaram HT-1500 calorimeter in step scanning mode. The heat capacities of silicate liquids have also been measured. Results for CaMgSi 2O 6 (diopside) agree well with previous thermochemical data. Crystallization runs provide kinetic as well as thermodynamic information. The ternary system CaMgSi 2O 6-NaAlSi 3O 8-CaAl 2Si 2O 8 (diopside-albite-anorthite), as a model for igneous rocks, is being studied in detail by this method.