Sound Velocities of Stishovite at Simultaneous High Pressure and High Temperature: Implication for the X-Discontinuity in the Upper Mantle

Stishovite is a high-pressure polymorph of silica and is stable above 9 GPa at 1000 oC, crystalizing into a tetragonal structure (space group P42/mnm). It has the highest hardness among all oxide minerals found in nature. Previous studies suggest that the coesite-stishovite transition may offer a plausible explanation for the seismic X-discontinuity within 250-300 km depths. However, elastic properties, especially the shear properties, of stishovite at high pressure and temperature have not been directly constrained by experimental studies.

In this work, we synthesized poly-crystalline stishovite and measured its sound velocity at simultaneous high pressure and high temperature. The poly-crystalline stishovite specimen was synthesized at 14 GPa and 1200 °C using fused SiO2 rod as starting material in the USCA-2000 multi-anvil press installed at Stony Brook University. The recovered sample has a bulk density that agrees with the theoretical density of stishovite within 0.5%, suggesting that the recovered sample is well sintered with negligible porosity. Acoustic measurements were conducted using ultrasonic interferometry techniques in the 1000-ton Kawai-type large-volume apparatus (T-25) installed at the GSECARS beamline 13-ID-D of the Advanced Photon Source, Argonne National Laboratory. By using a dual mode LiNbO3 broadband transducer, both P and S waves were measured simultaneously. At high pressure and temperature, sample length and density were determined by in-situ X-ray imaging and diffraction techniques, respectively. In this meeting, we will present the P and S wave velocity of stishovite, as well as its elastic bulk and shear moduli, up to ~14 GPa and 800 °C. In addition, implications of the new data on the X-discontinuity in the upper mantle will be discussed.