Elasticity of MgSiO3 orthoenstatite

The single-crystal elastic moduli of the MgSiO3 orthoenstatite end-member h ave been measured by Brillouin spectroscopy at ambient conditions. The aggr egate elastic moduli at 1 atm are K-s = 107.6(15) GPa and mu = 76.8(7) GPa, for the adiabatic bulk modulus and shear modulus, respectively. These valu es are in excellent agreement with the results of previous acoustic studies of synthetic MgSiO3. Comparison of our results with pressure-volume (P-V) measurements do not support the presence of a change in the P-V trajectory of orthoenstatite at similar to 4 GPa, as suggested previously on the basis of X-ray measurements to 8.5 Cpa. The effects of chemical composition on t he elasticity of orthopyroxenes are well represented by the variation of co mpressional and shear velocities with density, which exhibit well defined t rends for both synthetic and natural samples.