Phys Chem Minerals (1994) 21:285 293 PHYSlCS [ CHEMISTRY NMIliERALS 9 Springer-Verlag 1994 A Quantum-Mechanical Study of the Relative Stability under Pressure of MgSiO3-Ilmenite , MgSiO3-Perovskite , and MgO-Periclase + SiO z- Stishovite Assemblage Ph. D'Arco 1, G. Sandrone 2, R. Dovesi 2, E. April 3,,, V.R. Saunders 3 i Laboratoire de G~ologie de l'Ecole Normale Sup~rieure (URA1316), 24 rue Lhomond, F-75231 Paris C6dex 05, France 2 Department of Inorganic, Physical and Materials Chemistry, University of Torino, via P. Giuria 5, 1-10125 Torino, Italy 3Daresbury Laboratory, Science and Engineering Research Council, Daresbury, Warrington, Cheshire WA4 AD4, UK Received January 24, 1994/Revised, accepted May 30, 1994 Abstract. The relative stability of MgSiO3-ilmenite , MgSiO3-perovskite and (periclase+stishovite) assem- blage 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 struc- ture of MgSiO3-ilmenite is fully optimized. Basis set ef- fects are explored. It turns out that relatively small basis sets reproduce correctly experimental geometries. How- ever, larger basis sets (" triple zeta" quality, plus polariza- tion d functions) are needed to yield significant thermo- chemical 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 pres- sure (0 < P < 60 GPa) the mineralogical assemblage peri- clase+stishovite has higher enthalpy than MgSiO3-il- menite or perovskite, and that ilmenite transforms to or- thorhombic perovskite around to 29.4 GPa in good agreement with experimental data extrapolated down to OK. Introduction The mantle composition is considered to be mainly peri- dotitic, therefore magnesium-silicates Mg2SiO4 and MgSiO3 play a major role in mantle mineralogy. Since the pioneering work of Ringwood (1962), extensive ex- perimental studies of Mg2SiO~ and MgSiO3 (or analo- gous) systems in the geophysical P-T domain have been carried out (see for example, Ringwood and Major 1967; Ringwood and Major 1971 ; Liu 1976; Ito and Yamada 1982; Ito and Navrotsky 1985; Ito and Takahashi 1989). The phase diagram of the MgSiO3 system shows numer- ous single mineral stability fields: pyroxenes (protoen- statite, orthoenstatite, clinoenstatite), garnet, ilmenite and perovskite, but also two-minerals domains: /~-spi- nel + stishovite and 7-spinel § stishovite. The isothermal * Present address: Pacific Northwest Laboratory, Molecular Re- search Center, K1-96, Richland, WA 99352, USA Correspondence to: Ph. D'Arco pressure evolution strongly depends upon temperature. At low temperature, pyroxene successively transforms to fl-spinel + stishovite, to 7-spinel + stishovite, to ilmen- ite, and to perovskite. At high temperature (above 1800 K), pyroxene transforms to garnet, then to ilmen- ite, and to perovskite. As temperature increases, the pressure interval of stability of ilmenite reduces rapidly. The study of these phases is challenging the experimen- talist because of their importance in analyzing chemical and physical properties of the Earth's mantle. However, experimental difficulties (high temperatures and pres- sures) emphasize the importance of theoretical ap- proaches which are expected to provide not only good calculated geometries, but also consistent sets of thermo- chemical data. Until recently, computational work on these phases was restricted for both theoretical and computational reasons to molecular dynamics (MD) simulations based on empirical model potentials (Matsui and Price 1992; Winkler and Dove 1992). Calculations on MgSiOa per- ovskites were performed using the electron gas model (Hemley et al. 1987; Cohen et al. 1989) and two-body potentials derived from the modified electron gas model (Wolf and Jeanloz 1985, Wolf and Bukowinsky 1985 and Bukowinski and Wolf 1988). Few fully ab initio calcula- tions of MgSiO3 perovskite have been published yet. The oldest one which is based on the LDA and uses a LAPW scheme (Cohen et al. 1989), refers to the cubic (hypothetical) phase. Recently, Wentzcovitch etal. (1993) investigated the stability of the orthorhombic phase with respect to the cubic one with a plane-waves- Car-Parinello scheme. Stixrude and Cohen (1993), using the LAPW method within the LDA have calculated the energetics of the rotations of the SiO6 octahedra respon- sible for the cubic to orthorhombic phase transition, and the EOS of the orthorhombic modification assuming iso- tropic compression and no internal relaxation; they con- cluded that the orthorhombic phase is stable throughout the lower mantle. More recently, D'Arco et al. (1993) published a periodic LCAO-HF study of cubic, tetra- gonal and orthorhombic perovskite, which shows that