The Effect of Al on the Compressibility of Silicate Perovskite

Experimental data on compressibility of aluminous silicate perovskite show widely disparate results. Several studies show that Al causes a dramatic increase in compressibility^1-3, while another study indicates a mild decrease in compressibility⁴. Here we report new results for the effect of Al on the room-temperature compressibility of perovskite using in situ X-ray diffraction in the diamond anvil cell from 30 to 100 GPa. We studied compressibility of perovskite in the system MgSiO₃-Al₂O₃ in compositions with 0 to 25 mol% Al. Perovskite was synthesized from starting glasses using laser-heating in the DAC, with KBr as a pressure medium. Diffraction patterns were obtained using monochromatic radiation and an imaging plate detector at beamline BL10XU, SPring8, Japan. Addition of Al into the perovskite structure causes systematic increases in orthorhombic distortion and unit cell volume at ambient conditions (V₀). Compression of the perovskite unit cell is anisotropic, with the a axis about 25% and 3% more compressive than the b and c axes, respectively. The magnitude of orthorhombic distortion increases with pressure, but aluminous perovskite remains stable to at least 100 GPa. Our results show that Al causes only a mild increase in compressibility, with the bulk modulus (K₀) decreasing at a rate of 0.7 GPa/0.01 X_Al. This increase in compressibility is consistent with recent ab initio calculations if Al mixes into both the 6- and 8-coordinated sites by coupled substitution⁵, where 2 Al³⁺ = Mg²⁺ + Si⁴⁺. Our results together with those of [4] indicate that this substitution mechanism predominates throughout the lower mantle. Previous mineralogic models indicating the upper and lower mantle are compositionally similar in terms of major elements remain effectively unchanged because solution of 5 mol% Al into perovskite has a minor effect on density. 1. Zhang & Weidner (1999). Science 284, 782-784. 2. Kubo et al. (2000) Proc. Jap. Acad. 76B, 103-107. 3. Daniel et al. (2001). Geophy. Res. Lett. 28, 3789-3792. 4. Andrault et al. (2001). Earth Planet. Sci. Lett. 193, 501-508. 5. Brodholt (2000). Nature 407, 620-622.