PHASE RELATIONS, STRUCTURE AND CRYSTAL CHEMISTRY OF SOME ALUMINOUS SILICATE PEROVSKITES

Abstract

Solid solution of #25 mole% Al2O3 expands the compositional stability field of Mg,Fe silicate perovskite well beyond the limits encountered in the simple ternary system MgO-FeO-SiO2. Aluminous perovskites synthesised in laser-heated diamond anvil cell experiments at 55-70 GPa from starting materials on the compositional join between Mg3Al2Si3O12 and Fe3Al2Si3O12 (pyrope and almandine) can contain as much as 90 mole% of the ferrous end member. However Fe0.75 Al0.50Si0.75O3 perovskite could not be synthesised. Predictions that garnet coexists with aluminous perovskite at these pressures are unsubstantiated. These new perovskites are approximately isochemical with garnet and accommodate the full complement of Al2O3 (25 mole%) even at # 70 GPa. Some contain as much as 30 mole% Al2O3, and solid solution is probably facilitated by temperature. However, there is certainly no evidence to substantiate a recent proposal that the capacity of perovskite to accommodate Al2O3 in solid solution is progressively inhibited by pressure. Magnesian silicate perovskite should therefore have no difficulty in accommodating the mantle inventory of Al2O3 in solid solution throughout the entire lower mantle pressure regime. There is no reason to expect that a new aluminous phase would be stabilised at depth within the lower mantle. Nor would exsolution of an aluminous phase at core-mantle boundary pressures be a plausible explanation for the D'' layer. Aluminous perovskites are almost always rhombohedral R3c rather than orthorhombicPbnm , and their unit cell volumes increase by about 3% as 75 mole% of ferrous iron replaces magnesium. These new perovskites are slightly non-stoichiometric, with modest amounts of an M2(Al,Si)O5.5 (M = Mg,Fe) component in solid solution. Crystal chemistry fundamentals successfully predict the site occupancy of minor and trace elements in magnesian silicate perovskite.