Mechanical instability near the stishovite-CaCl2 phase transition: implications for crystal preferred orientations and seismic properties

Abstract

Deformation experiments have been performed on polycrystalline stishovite at 14 G Pa and 1300 °C in am ulti-anvil press. The easy slip systems have been determined by Transmission Electron Microscopy. We found (001){100} and (001){110} with minor slip in the direction. Analysis of the elastic energy of these slip systems together with previously reported systems in as-grown stishovite reveals that easy glide (001){100} and (001){110} systems are unaffected by the stishovite to CaCl 2 transformation, whereas the energy of {110} system decreases to zero at the transition, which is symptomatic of a plastic instability. We have used the visco-plastic self-consistent (VPSC) model to simulate the crystal preferred orientation (CPO) of polycrystalline stishovite in simple shear. The CPO in stishovite stability field shows a strong alignment of (001) axes parallel to direction of maximum finite strain (X) and a girdle of (100) normal to X. At conditions close to the transition, simulated by making the CRSS of (110)(110) equal to easy glide (001) systems, the CPO were similar to the previous case, but slightly weaker. At the transition, simulated by making the {110} systems the only easy glide systems, the CPO of (001) axes is very weak. The anisotropic seismic properties were calculated from the CPO. In the stishovite stability field at a pressure of 21 GPa the Vp maximum is parallel to X and Vp minimum parallel to Z with an anisotropy of about 13%. The shear wave splitting has a maximum in a girdle normal to X with anisotropy of 5 to 8%. Calculations near the phase transition at a pressure of 48 GPa for CPO with easy {110} and (001) systems showed similar patterns to those at 21 GPa except that S wave anisotropy was stronger (11-18%). In conditions at the phase transition where {110} are the only easy glide systems, the seismic properties are very different, with Vp almost isotropic with anisotropy of 1.8% and a relatively low anisotropy of S waves with a maximum shear wave splitting of 4.5% in a single peak parallel to Z.