Aluminum in magnesium silicate perovskite: Formation, structure, and energetics of magnesium-rich defect solid solutions

MgSiO₃-rich perovskite is expected to dominate Earth's lower mantle (pressures >25 GPa) with iron and aluminum as significant substituents. The incorporation of trivalent ions, M³⁺, may occur by two competing mechanisms: Mg_A + Si_B = M_A + M_B and Si_B = Al_B + 0.5 (vacancy)_O. Phase synthesis studies show that both substitutions do occur and the nonstoichiometric or defect substitution is prevalent along the MgSiO₃-MgAlO_2.5 join. Lattice parameters associated with the first substitution (stoichiometric) show more rapid increases with increasing A1 content than those for the second substitution (nonstoichiometric), consistent with the differences in size of substituting ions. Oxide melt solution calorimetry has been used to compare the energetics of both substitutions. The stoichiometric substitution, represented by the reaction 0.95 MgSiO₃ (perovskite) + 0.05 Al₂O₃ (corundum) = Mg_0.95A1_0.10Si_0.95O₃ (perovskite), has an enthalpy of -0.8 ± 2.2 kJ/mol. The nonstoichiometric reaction, 0.90 MgSiO₃ (perovskite) + 0.10 MgO (rocksalt) + 0.05 Al₂O₃ (corundum) = MgSi_0.9A1_0.1O_2.95 (perovskite) has a small positive enthalpy of 8.5 ± 4.6 kJ/mol. Configurational T ΔS terms play a role in both substitutions. The defect substitution is not prohibitive in enthalpy, entropy, or volume, is favored in perovskite coexisting with magnesiowüstite and may significantly affect the elasticity, theology, and water retention of silicate perovskite in Earth.