CALCULATED ELASTIC-CONSTANTS AND ANISOTROPY OF MG2SIO4 SPINEL AT HIGH-PRESSURE

We calculated the elastic properties of Mg2SiO4 spinel, using the plan e-wave pseudopotential method. The athermal elastic constants were cal culated directly from the stress-strain relations up to 30 GPa, which encompasses the experimentally observed stability field of spinel. The calculated elastic constants are in very good agreement with Brilloui n scattering data at zero pressure. We calculated the isotropically av eraged elastic wave velocities and the anisotropy from our single crys tal elastic constants. We find that the elastic anisotropy is weak (az imuthal and polarization anisotropy of S-waves: 5%, azimuthal P-wave a nisotropy: 2.5%, at zero pressure) compared to other silicates and oxi des. The anisotropy decreases initially with increasing pressure, chan ging sign at 17GPa before increasing in magnitude at higher pressures. At typical pressures of the earth's transition zone (20-25 GPa), the elastic anisotropy is 1% and 2% for P- and S-waves respectively.