Crystal Shape, Rotation and Preferred Orientation in Rocks
Abstract:Recently, we have shown that a significant crystallographic preferred orientation (CPO) of forsterite develops during Newtonian flow of the forsterite aggregate (Miyazaki et al., 2013 Nature). Since the aggregate also exhibits (i) superplasticity (>>100 % tensile strain) (Hiraga, 2010 Nature), (ii) the same phase aggregation at the direction of compression (Hiraga et al. 2013 Geology) and (iii) essentially no change in grain shape before and after the deformation, we concluded that grain boundary sliding (GBS) should have accommodated a majority of the sample strain. One of the distinct natures of the observed CPO was that the preexisting grain shape, which is controlled by crystallography of forsterite, controls CPO development and its pattern. Based on these results, we concluded that the preferential GBS at the boundary parallel to the specific crystallographic plane (i.e., low-index plane grain boundary) resulted in CPO. The development of CPO requires a grain rotation toward the specific direction in the sample geometry. Such rotation was well identified by the shape change of line markers imposed on the sample surface prior to the sample deformation. Further, scanning probe microscopy on the sample surface reveals the anisotropic grain rotation, that is, a significant rotation around the axis perpendicular to the compression axis whereas essentially zero rotation around the axis parallel to the compression axis.
We will demonstrate that such CPO, which is originated from crystallography-controlled GBS, is not limited to forsterite system but it is a common process in various mineral systems. CPO in rocks has been considered as a consequence of dislocation creep. Here we show an alternative model of CPO development in the earth’s interior.