Effect of aluminium on the compressibility of silicate perovskite

Volume measurements for aluminous MgSiO(3) perovskite containing 5 mol% Al(2)O(3) were carried out up to pressures of 40 GPa at ambient temperature, using monochromatic synchrotron X-ray diffraction. A least-squares refinement of the data to the Birch-Murnaghan equation of state yields the following parameters V(0) = 163.234(8) Angstrom(3), K(T0) = 251.5(13) GPa, K(0)' = 4. Within uncertainties, the presence of 5 mol% Al(2)O(3) in MgSiO(3) perovskite induces a decrease of the bulk modulus in the range of 0% to 1.8%. Thus, K(T) of perovskite is affected little if at all by the presence of Al(3+). This result is in excellent agreement with the values deduced from sound velocity measurements on the same sample [Jackson et al., 2004]. We discuss the possible origin of discrepancies among the different bulk moduli reported to date for aluminous perovskite. In light of recent calculations, our results are consistent with aluminium being dissolved in MgSiO(3) perovskite through a coupled substitution mechanism involving the replacement of both Mg(2+) and Si(4+) in the dodecahedral and octahedral sites by 2 Al(3+). Moreover, any slight reduction in the bulk modulus of MgSiO(3) perovskite induced by the dissolution of 5 mol% Al(2)O(3), indicates that the relative proportions of the minerals characteristic of the lower mantle, as inferred from seismological models, should not be significantly altered by the introduction of Al in the system.