A quantum-mechanical study of the relative stability under pressure of MgSiO3-ilmenite, MgSiO3-perovskite, and MgO-periclase+SiO2-stishovite assemblage
✍ Scribed by Ph. D'Arco; G. Sandrone; R. Dovesi; E. Aprà; V. R. Saunders
- Publisher
- Springer-Verlag
- Year
- 1994
- Tongue
- English
- Weight
- 924 KB
- Volume
- 21
- Category
- Article
- ISSN
- 0342-1791
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✦ Synopsis
The relative stability of MgSiO3-ilmenite , MgSiO3-perovskite and (periclase+stishovite) assemblage phases as a function of the pressure is investigated with the periodic quantum mechanical ab initio Hartree-Fock program CRYSTAL. For the first time, the structure of MgSiO3-ilmenite is fully optimized. Basis set effects are explored. It turns out that relatively small basis sets reproduce correctly experimental geometries. However, larger basis sets (" triple zeta" quality, plus polarization d functions) are needed to yield significant thermochemical results. All contributions to the 0 K enthalpy are discussed. On the basis of the present highest level calculations, it appears that in the explored range of pressure (0 < P < 60 GPa) the mineralogical assemblage peri-clase+stishovite has higher enthalpy than MgSiO3-ilmenite or perovskite, and that ilmenite transforms to orthorhombic perovskite around to 29.4 GPa in good agreement with experimental data extrapolated down to OK.