Elasticity of a Pseudoproper Ferroelastic Transition from Stishovite to Post-Stishovite at High Pressure

Elastic moduli (C_{ij}) of single-crystal stishovite and post-stishovite are determined using Brillouin light scattering, impulsive stimulated light scattering, and x-ray diffraction up to 70 GPa. The C_{12} of stishovite converges with the C_{11} at ∼55  GPa, where the transverse wave V_{S1} propagating along [110] also vanishes. Landau modeling of the C_{ij}, B_{1g} optic mode, and lattice parameters reveals a pseudoproper type ferroelastic post-stishovite transition. The transition would cause peculiar anomalies in V_{S} and Poisson's ratio in silica-bearing subducting slabs in the mid-lower mantle.