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

Solid solution of similar to 25 mole% Al2O3 expands the compositional stability field of Mg,Fe silicate perovskite well beyond the limits en countered in the simple ternary system MgO-FeO-SiO2. Aluminous perovsk ites synthesised in laser-heated diamond anvil cell experiments at 55- 70 Cpa from starting materials on the compositional join between Mg3Al 2Si3O12 and Fe3Al2Si3O12 (pyrope and almandine) can contain as much as 90 mole% of the ferrous end member. However Fe-0.75 Al0.50Si0.75O3 pe rovskite could not be synthesised. Predictions that garnet coexists wi th aluminous perovskite at these pressures are unsubstantiated. These new perovskites are approximately isochemical with garnet and accommod ate the full complement of Al2O3 (25 mole%) even at similar to 70 GPa. Some contain as much as 30 mole% Al2O3, and solid solution is probabl y facilitated by temperature. However, there is certainly no evidence to substantiate a recent proposal that the capacity of perovskite to a ccommodate Al2O3 in solid solution is progressively inhibited by press ure, Magnesian silicate perovskite should therefore have no difficulty in accommodating the mantle inventory of Al2O3 in solid solution thro ughout 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-m antle boundary pressures be a plausible explanation for the D '' layer . Aluminous perovskites are almost always rhombohedral R $($) over bar $$ 3c rather than orthorhombic Pbnm, and their unit cell volumes incre ase by about 3% as 75 mole% of ferrous iron replaces magnesium. These new perovskites are slightly non-stoichiometric, with modest amounts o f an M(2)(Al,Si)(5.5) (M = Mg,Fe) component in solid solution. Crystal chemistry fundamentals successfully predict the site occupancy of min or and trace elements in magnesian silicate perovskite.