Stability of mantle minerals from lattice calculations and shock wave data

Abstract

Shock wave and static high pressure data for mantle minerals have indicated that at high pressures a series of denser polymorphs form whose crystal structures can at present only be inferred from calculated densities and crystal chemical arguments. In order to determine the admissibility of some of these proposed structures theoretical Madelung lattice energies are calculated for several oxides (FeO, Al 2O 3, Cr 2O 3, Fe 2O 3, SiO 2, TiO 2) spinels (Al 2MgO 4, Mg 2SiO 4, Fe 2SiO 4, Ni 2SiO 4, FeCr 2O 4, Fe 2TiO 4, Fe 3O 4) and perovskites (CaTiO 3, SrTiO 3, MgSiO 3, Fe 2 3+O 3, Fe 2+Fe 4+O 3). Comparison of calculated enthalpies of formation with measured values yield approximate values for the effects of covalency on enthalpies of formation for AlO 6, TiO 6, SiO 4, SiO 6, Fe 3+O 6, Cr 3+O 6, Fe 3+O 4 and Fe 2+O 4. This effect is seen to be very similar for the same ion pair in the same coordination but in different compounds. The calculations indicate that enstatite (MgSiO 3) can not enter a perovskite with a density greater than about 3.9 g/cm 3 and that the high pressure phase of Fe 2O 3 can be a perovskite only if the Fe 3+ disproportionates into Fe 2+ and Fe 4+ and the 3d electrons in the latter are spin paired.