CALCULATIONS OF PRESSURE-INDUCED PHASE-TRANSITIONS IN MANTLE MINERALS

The crystal structures and energies of SiO2 stishovite, MgO periclase, Mg2SiO4 spinel, and MgSiO3 perovskite were calculated as a function o f pressure with the polarization-included electron gas (PEG) model. Th e calculated pressures of the spinel to perovskite phase transitions i n the Mg2SiO4 and MgSiO3 systems are 26.0 GPa and 27.0 GPa, respective ly, compared to the experimental zero temperature extrapolations of 27 .4 GPa and 27.7 GPa. The two oxide phases are found to be the most sta ble form in the pressure range 24.5 GPa to 31.5 GPa, compared to the e xperimental zero temperature extrapolation of 26.7 GPa to 28.0 GPa. Th e volume changes associated with the phase transitions are in good agr eement with experiment. The transition pressures calculated with the P EG model, which allows the ions to distort from spherical symmetry, ar e in much better agreement with experiment than those calculated with the modified electron gas (MEG) model, which constrains the ions to be spherical.