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Aluminum in Magnesium Silicate Perovskite: Synthesis and Energetics of Defect Solid Solutions

Published online by Cambridge University Press:  01 February 2011

Alexandra Navrotsky ,
Mirko Schoenitz ,
Hiroshi Kojitani ,
Hongwu Xu ,
Jianzhong Zhang ,
Donald J. Weidner ,
Masaki Akaogi  and
Raymond Jeanloz
Alexandra Navrotsky
Affiliation:
Thermochemistry Facility and Center For High Pressure Research, Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, CA 95616, USA
Mirko Schoenitz
Affiliation:
Thermochemistry Facility and Center For High Pressure Research, Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, CA 95616, USA
Hiroshi Kojitani
Affiliation:
Thermochemistry Facility and Center For High Pressure Research, Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, CA 95616, USA Department of Chemistry, Gakushuin University, Tokyo, Japan
Hongwu Xu
Affiliation:
Thermochemistry Facility and Center For High Pressure Research, Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, CA 95616, USA
Jianzhong Zhang
Affiliation:
Center for High Pressure Research and Department of Geosciences, State University of New York at Stony Brook, Stony Brook, NY 11794, USA
Donald J. Weidner
Affiliation:
Center for High Pressure Research and Department of Geosciences, State University of New York at Stony Brook, Stony Brook, NY 11794, USA
Masaki Akaogi
Affiliation:
Department of Chemistry, Gakushuin University, Tokyo, Japan
Raymond Jeanloz
Affiliation:
Department of Geology and Geophysics, University of California at Berkeley, Berkeley, CA 94720, USA
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Abstract

MgSiO3 - rich perovskite is expected to dominate the 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 VO. Phase synthesis studies show that both substitutions do occur, and the nonstoichiometric or defect substitution is prevalent along the MgSiO3 - MgAlO2.5 join. 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) = Mg0.95Al0.10Si0.95O3 (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) = MgSi0.9Al0.1O2.95 (perovskite) has a small positive enthalpy of 8.5±4.6 kJ/mol. 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 the Earth.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 718: Symposium D – Perovskite Materials , 2002 , D2.2
Copyright
Copyright © Materials Research Society 2002

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