Aluminum in Magnesium Silicate Perovskite: Synthesis and Energetics of Defect Solid Solutions
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Aluminum in Magnesium Silicate Perovskite: Synthesis and Energetics of Defect Solid Solutions Alexandra Navrotskya, Mirko Schoenitza, Hiroshi Kojitania,b, Hongwu Xua, Jianzhong Zhangc, Donald J. Weidnerc, Masaki Akaogib, Raymond Jeanlozd a
b c
d
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 Center for High Pressure Research and Department of Geosciences, State University of New York at Stony Brook, Stony Brook, NY 11794, USA Department of Geology and Geophysics, University of California at Berkeley, Berkeley, CA 94720, USA
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. INTRODUCTION Magnesium silicate perovskite, MgSiO3, as the major phase of the Earth’s lower mantle, has been the subject of numerous theoretical and experimental studies. Crystal structure [1] equation of state [2], elastic moduli [3,4] and thermodynamic properties [5,6] of the pure compound have been studied and documented extensively. Starting from the pure end-member MgSiO3, solid solution series containing divalent cations (calcium, ferrous iron) and trivalent cations (aluminum, ferric iron) have been investigated [7-12]. A summary has been given by Fei [13]. At least two distinct mechanisms for the substitution of trivalent cations into silicate perovskite are conceivable (Fig. 1). The coupled substitution along the MgSiO3–Al2O3 join 2+
MgA + Si B = Al A + Al B 4+
3+
3+
(1)
D2.2.1 Downloaded from https://www.cambridge.org/core. The Librarian-Seeley Historical Library, on 09 Jan 2020 at 22:11:04, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-718-D2.2
Figure 1. Compositions in the MgO – Al2O3 – SiO2 system used for phase synthesis studies. replaces cations in both the octahedral “B” and large
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