Lattice-preferred orientations in post-perovskite-type MgGeO 3 formed by transformations from different pre-phases

Abstract

Lattice-preferred orientations (LPOs) of post-perovskite (PPv)-type MgGeO 3 formed by transformations from two different pre-phases, orthopyroxene (OPx) and perovskite (Pv), were studied using radial and axial X-ray diffraction techniques combined with a laser-heated diamond-anvil cell. When the PPv was made from Pv, strong LPO was formed in the PPv immediately after the transformation with an alignment of the (0 0 1) or (0 1 1) plane almost perpendicular to the compression axis. This present result contrasts markedly from that observed when the PPv was formed from OPx and those reported on MgGeO 3-PPv and (Mg,Fe)SiO 3-PPv transformed from OPx, in which the (1 0 0) or (1 1 0) plane was aligned to the compression axis. It was clarified that when the PPv-type MgGeO 3 is formed in diamond-anvil cells, it has strong transformation-induced LPO from the beginning even before plastic deformation occurs, and that its characteristics depend on the pre-phase before the transformation. The formed transformation-induced LPO of PPv did not change in spite of further compression using a conventional gasket because the sample became too thin to deform. However, by using a diamond gasket technique, we were able to deform the PPv at room temperature, resulting in significant variations of the relative intensities of several diffraction peaks. The change in the axial diffraction patterns can be explained only by assuming that the (0 0 1) plane aligns almost perpendicularly to the compression axis. The present results suggest that the (0 0 1) is a dominant slip plane in MgGeO 3-PPv at room temperature. If one makes the assumption that the dominant slip system(s) of silicate PPv deformed in the D″ layer is or are identical to that of germanate PPv at room temperature, then a large S-wave polarization anisotropy in the Earth's D″ layer would be explained by the small degree of deformation-induced LPO of the PPv phase that is expected to exist in this layer.