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

Alexandra Navrotsky, Mirko Schoenitz, Hiroshi Kojitani, Hongwu Xu, Jianzhong Zhang, Donald J. Weidner, Raymond Jeanloz

Research output: Contribution to journalArticlepeer-review

47 Scopus citations

Abstract

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: MgA + SiB = MA + MB and SiB = AlB + 0.5 (vacancy)O. Phase synthesis studies show that both substitutions do occur and the nonstoichiometric or defect substitution is prevalent along the MgSiO3-MgAlO2.5 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) = Mg0.95A10.10Si0.95O3 (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) = MgSi0.9A10.1O2.95 (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.

Original languageEnglish (US)
Pages (from-to)ECV 2-1 - 2-11
JournalJournal of Geophysical Research: Solid Earth
Volume108
Issue number7
StatePublished - Jul 10 2003

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science

Keywords

  • Crystal chemistry
  • Defect
  • Energetics
  • Experimental mineralogy and petrology
  • Physical thermodynamics
  • Silicate perovskite

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