Stishovite, the high pressure polymorph of silica SiO2, is of great interest in the mineralogy of the Earth’s mantle as well as for fundamental physics of phase transformations. Theoretical and experimental studies reveal that stishovite transforms to an orthorhombic CaCl2-type structure around 50 GPa. This transition may contribute to unexplained seismic structure in the Earth’s lower mantle. According to theoretical models, the phase transition of stishovite is trigged by the lattice instability of a soft transverse acoustic mode associated with the combined elastic constants (C11-C12)/2. Therefore, it is of interest to measure the full elastic constant tensor of stishovite under high-pressure to improve understanding of the phase transition mechanism and its potential effect on seismic structure in the Earth’s mantle.

We carried out detailed Brillouin scattering measurements of the complete set of elastic constants of stishovite up to 12 GPa. Brillouin experiment on stishovite at high-pressures in a diamond anvil cell is technically challenging because of the overlapping of stishovite P-wave with the diamond S-wave, we developed experimental approaches to overcome these issues. The present study is the first high-pressure measurements of full elastic constants and their pressure derivatives. The present data-set may resolve discrepancies for elastic moduli and related pressure derivatives in previous experimental studies. Upon compression to 20 GPa, velocity decrease of the slow shear wave by 11% has been directly observed. From the pressure dependence of individual elastic constants, it is seen that C12 had the fastest increase of any modulus with pressure. This leads to the decrease of the combined elastic constants (C11-C12)/2 with increase of pressure, indicating a transverse acoustic mode softening in agreement with theoretical models. Seismic wave-velocity and anisotropy are calculated. Strong anisotropy and shear wave softening of stishovite under compression may be relevant in understanding the origin of observed seismic anisotropy.