2003/07/01 by Alexandra Navrotsky, Mirko Schoenitz, Hiroshi Kojitani +4
Earth and Planetary Sciences · Materials Science · #Crystal Structures and Properties #Geological and Geochemical Analysis #High-pressure geophysics and materials
paper · doi:10.1029/2002jb002055
openalex publication_date 2003/07/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/29
MgSiO 3 ‐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, M 3+ , 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 MgSiO 3 ‐MgAlO 2.5 join. Lattice parameters associated with the first substitution (stoichiometric) show more rapid increases with increasing Al 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 MgSiO 3 (perovskite) + 0.05 Al 2 O 3 (corundum) = Mg 0.95 Al 0.10 Si 0.95 O 3 (perovskite), has an enthalpy of −0.8 ± 2.2 kJ/mol. The nonstoichiometric reaction, 0.90 MgSiO 3 (perovskite) + 0.10 MgO (rocksalt) + 0.05 Al 2 O 3 (corundum) = MgSi 0.9 Al 0.1 O 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, rheology, and water retention of silicate perovskite in Earth.