Al2O3 incorporation in MgSiO3 perovskite and ilmenite

First-principles calculations in the MgSiO3-Al2O3 system predict that MgSiO3-perovskite dissolves about 15 mol% Al2O3 at the top of the lower mantle, limited by coexistence with Al-rich ilmenite (corundum). The solubility increases with pressure so that the lower mantle is likely undersaturated in alumina in all but the coldest parts of the uppermost lower mantle. The akimotoite-corundum solid solution is highly non-ideal with a symmetric regular solution parameter W = 66 kJ/mol per formula unit at the top of the lower mantle. The critical temperature for exsolution of ilmenite is predicted to be 2000 K. The MgSiO3 solution is also significantly non-ideal; assuming only Tschermak substitutions, the value of W = 12 kJ/mol per formula unit. Total energy calculations over a range of compositions and several configurations for each composition are based on density functional theory in the local density approximation. We find that the entropy of solution of both structures is nearly ideal on the basis of random sampling of simulations using semi-empirical interatomic potentials. Results also confirm previous experimental and computational results showing that aluminum has little effect on the elasticity of MgSiO3 perovskite.