A QUANTUM-MECHANICAL STUDY OF THE RELATIVE STABILITY UNDER PRESSURE OF MGSIO3-ILMENITE, MGSIO3-PEROVSKITE, AND MGO-PERICLASE-STISHOVITE ASSEMBLAGE(SIO2)

The relative stability of MgSiO3-ilmenite, MgSiO3-perovskite and (peri clase + stishovite) assemblage phases as a function of the pressure is investigated with the periodic quantum mechanical ab initio Hartree-F ock program CRYSTAL. For the first time, the structure of MgSiO3-ilmen ite is fully optimized. Basis set effects are explored. It turns out t hat relatively small basis sets reproduce correctly experimental geome tries. However, larger basis sets (''triple zeta'' quality, plus polar ization d functions) are needed to yield significant thermochemical re sults. All contributions to the OK enthalpy are discussed. On the basi s of the present highest level calculations, it appears that in the ex plored range of pressure (0<P<60 CPa) the mineralogical assemblage per iclase-stishovite has higher enthalpy than MgSiO3-ilmenite or perovski te, and that ilmenite transforms to orthorhombic perovskite around to 29.4G Pa in good agreement with experimental data extrapolated down to O K.