Elastic properties of stishovite and the CaCl2-type silica at the mantle temperature and pressure: An ab initio investigation

- Increasing temperature shifts the shear instability of stishovite to higher pressure.
- Large and unusual positive dln⁡VS/dT of stishovite around the phase boundary.
- Stishovite to CaCl2 silica transition accompanied by a jump of 1 km/s in VS and 0.45 km/s in VP along geotherm.
- Both stishovite and CaCl2-type silica show strong anisotropy around phase boundary.

The elastic constant tensors of stishovite and the CaCl2-type silica at the Earth's mantle temperature and pressure were determined using first-principles calculations with local density approximation. The elastic properties of stishovite show not only strong pressure dependence but also temperature dependence. By increasing temperature, the shear instability of stishovite is shifted to an elevated pressure with a slope of ∼5.4±1.4 MPa/K. The softening of the shear modulus and the positive Clapeyron slope result in crossing of the sound velocities at different temperatures, which leads to the unusual positive temperature dependence of the sound velocities around the phase boundary. The transition from stishovite to the CaCl2-type silica at the lower mantle's temperature occurs at a depth far deeper than 1200 km and is accompanied by a velocity jump of ∼0.98±0.08 km/s in S wave velocity (VS) and ∼0.45±0.15 km/s in P wave velocity (VP). This transition is likely related to the seismic discontinuity at the depth of ∼1670 km in the vicinity of Mariana Island. The unusual positive temperature dependence of VS of stishovite and strong anisotropy of stishovite and the CaCl2-type silica around the phase boundary provide potential ways to identify the origin of the seismic discontinuity.