Compressional and shear wave velocities of a polycrystalline specimen of MgSiOb>3b> orthoenstatite have been measured by ultrasonic interferometry to 1373 K at 300 MPa in an internally heated gas-medium apparatus. The elastic wave velocities and bulk and shear moduli vary linearly with temperature to 1073 K. Below 1073 K, the temperature derivatives of the elastic moduli [(∂Kb>Sb>/∂T)b>Pb> = −28.3(7) MPa/K and (∂G/∂T)b>Pb> = −14.5(1) MPa/K, respectively] determined in this study are consistent with averages of single-crystal elastic constants measured using Brillouin spectroscopy by ref-ref-17-1" class="xref-bibr">Jackson et al. (2007). The measured temperature dependence of elastic moduli, along with pressure dependence of elastic moduli, thermal expansion and calorimetric data have been assimilated into a finite-strain equation of state of the type proposed by ref-ref-33-1" class="xref-bibr">Stixrude and Lithgow-Bertelloni (2005). This analysis suggests significant revisions to the optimal values of the zero-pressure Grüneisen parameter γb>0b> and its zero-pressure logarithmic volume derivative qb>0b>. The unusually high absolute values of (∂K/∂P)b>Tb> and (∂K/∂T)b>Pb> are related through the extrinsic part of the temperature derivative. Above 1073 K, a pronounced softening of the elastic wave velocities is observed, which is plausibly associated with a phase transformation for which there is microstructural evidence: The recovered specimen was found to have transformed to the low-pressure clinoenstatite polymorph.
