Thermodynamic Properties of Spinels and Garnets

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The project will determine activity-composition relations and cation site distributions in MgAl2O4-FeCr2O4-Fe3O4 spinels using a combination of solid electrolyte, phase equilibrium and electrical measurements. The enthalpy of almandine garnet obtained calorimetrically conflicts with values derived from precise phase equilibrium measurements. There are also discrepancies between the measured stability fields of almandine and hercynite spinel. The free energies of almandine and hercynite will be determined as a function of temperature using the ZrO2 electrolyte technique. Integration of thermodynamic data into an internally-consistent data base provides the means to calculate phase diagrams at very high pressures and to make petrologic input into problems of earth structure. This method will be used to investigate the reasons for observed reflections and conversions of seismic waves at the interfaces between subducting slabs and overlying mantle.