It is widely accepted that bridgmanite is a primary mineral constituent of the Earth's lower mantle. Therefore, knowledge of its elastic properties at lower-mantle conditions is necessary for deriving the structure of the Earth's deep interior from seismic observations. Criniti et al. (2021) measured the acoustic velocities of single-crystal MgSiO3 bridgmanite up to 80 GPa at room temperature and constrained the variation of the elastic stiffness coefficients with pressure. While this study provided a reliable high-pressure reference for the seismic signature of this important mineral at lower-mantle pressures, the results differ significantly from those obtained from measurements on polycrystalline samples. The causes for these discrepancies are still a matter of debate.

Here, we present Brillouin spectroscopy and X-ray diffraction measurements on polycrystalline MgSiO3 bridgmanite samples with various average grain sizes. The samples were synthesized using a 1200-ton multi-anvil (MA) press at 24 GPa and at different temperatures to control the grain growth of bridgmanite. The recovered samples were polished into thin platelets (~15 μm thick) and further cut into circular disks using a focused ion beam (FIB). High-pressure X-ray diffraction (XRD) and Brillouin spectroscopy measurements were conducted so far up to 35 GPa at room temperature using a diamond anvil cell (DAC) and helium as pressure-transmitting medium. Our results show that the measured acoustic wave velocities depend strongly both on the grain size of the polycrystalline sample and on the orientational distribution. Based on a comparison with the expected velocity range calculated for up to 5000 randomly oriented grains of MgSiO3, we conclude that the measured acoustic wave velocities of polycrystalline samples are likely to be affected by the effects of grain boundaries and preferred orientation on the Brillouin scattering signal.

Criniti et al. (2021) J. Geophys. Res. Solid Earth, 126, e2020JB020967.