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

Abstract

[1] 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 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.INDEX TERMS: 3620 Mineralogy, Petrology, and Mineral Physics: Crystal chemistry; 3630 Mineralogy, Petrology, and Mineral Physics: Experimental mineralogy and petrology; 3924 Mineral Physics: High-pressure behavior; 3939 Mineral Physics: Physical thermodynamics; 3954 Mineral Physics: X ray, neutron, and electron spectroscopy and diffraction; KEYWORDS: silicate perovskite, defect, energetics, crystal chemistry, experimental mineralogy and petrology, physical thermodynamics Citation: Navrotsky, A., M. Schoenitz, H. Kojitani, H. Xu, J. Zhang, D. J. Weidner, and R. Jeanloz, Aluminum in magnesium silicate perovskite: Formation, structure, and energetics of magnesium-rich defect solid solutions,

Rapid decrease of MgAlO2.5 component in bridgmanite with pressure

“…At 27 GPa, the number of oxygens in bridgmanite for the En 90 Brm 10 composition is 2.973 ± 0.004, which is slightly lower than those in Navrotsky et al (2003) and Kojitani et al (2007) and substantially lower than that of En 95 Cor 5 bridgmanite. This value shows clearly the presence of oxygen vacancies as consistent with 27 Al NMR observations by Stebbins et al (2003).…”

“…The oxygen vacancy substitution, one of the most likely mechanisms, in aluminous bridgmanite may provide suitable storage sites to incorporate water in the shallower part of the lower mantle (e.g., Navrotsky, 1999;Brodholt, 2000) according to the following reaction: 2MgAlO 2.5 (Brg) + H 2 O (fluid) = 2MgHAlO 3 (Brg) Eq. 2…”

“…The other mechanism is the oxygen-vacancy substitution (2Si 4+ + O 2-= 2Al 3+ + V O , where V O is an oxygen vacancy; components along the MgSiO 3 -MgAlO 2.5 binary system), where two Al cations replace two Si in the B sites and one oxygen vacancy forms to compensate for the excess negative charges. The oxygen vacancies in bridgmanite have special importance for understanding lower mantle processes because oxygen vacancies may induce water (V o + O 2-+ H 2 O = 2OH -1 ; Navrotsky, 1999) and noble gases into bridgmanite (Shcheka and Keppler, 2012).…”

Subsolidus phase relations and perovskite compressibility in the system MgO–AlO1.5–SiO2 with implications for Earth's lower mantle

“…1). Navrotsky et al [17] reported the possible incorporation of O-defects in perovskite in a composition with ∼ 10 mol% AlO1.5 along the MgSiO3-MgAlO2.5 join. According to theoretical calculations this could reduce the bulk modulus by at least 10% [9].…”

“…Andrault et al [8] and Walter et al [14] have measured the compressibilities of a series of aluminous perovskites along the charge-coupled join, and observed only a mild effect on the bulk modulus generally consistent with theoretical calculations. Navrotsky et al [17] synthesized perovskites along the O-defect join, as well as other MgO-saturated perovskite compositions.…”