Thermal And Elastic Properties Of Stishovite And The CaCl₂-type Silica:an Ab Initio Investigation

Polymorphs of silica are found to be portions of subducted slab in considerable amounts and serve as prototypes of silica minerals at high pressure and temperature. More than20percent of silicate polymorphs making up of the subducted earth crust in the mantle excessive melt and the lower mantle in the natural mid-oceanic ridge basalt (MORB) and have a significant impact on the thermodynamic and the elastic properties of the lower mantle. Under the lower mantle pressure and temperature, the phases of the silica polymorphs are stishovite and the CaCl2-type silica. The transition from stishovite to the CaCl2-type silica is accompanied by large velocity and anisotropy jumps and these jumps are very important to understand the velocity and anisotropy discontinuity of the subducted zone in the lower mantle. In this study, the research is mainly about the thermodynamic and elastic properties of stishovite and the CaCl2-type silica. The main work includes:1. The stable structures of stishovite and the CaCl2-type silica are established first using ab initio method. After the calculation the the lattice vibrational properties of the two phases, the energy of the phonons can be obtained to study the thermodynamic properties of stishovite and the CaCl2-type silica.2. The static elastic constants are calculated using the ab initio method and a new method to calculate the elastic constants at high temperature and pressure is used. The velocity and anisotropy jumps of stishovite and the CaCl2-type silica at the phase transition boundary are established. With the elastic constants obtained over a wide range of pressure and temperature, we generate the velocity and the anisotropy properties along the geothrm and the depth of these discontinuities.