Single‐crystal elasticity of natural Fe‐bearing orthoenstatite across a high‐pressure phase transition

Sound velocities and elastic moduli have been measured on Fe‐bearing orthoenstatite (OEN) single crystals up to 12 GPa by Brillouin spectroscopy. The ambient adiabatic bulk modulus ( K s 0 ) and shear modulus ( G 0 ) are determined to be 113(1) GPa and 75.9(7) GPa, respectively. A fourth‐order finite strain fit to the data yields pressure derivatives of K s 0 ′ = 8.8(1), K s0 ″ = −0.68(6), G 0 ′ = 2.9(1), and G 0 ″ = −0.40(2). These values are significantly higher than those for other major mantle minerals up to 10.5 GPa but lower than some previous measurements on OEN. A pronounced increase of shear anisotropy was observed at a pressure of 12.06(9) GPa, coinciding with a phase transition from orthoenstatite to a recently discovered high‐pressure phase with space group P2 1 /c. A high‐pressure phase transition in OEN is unlikely to be the cause of the X discontinuity in the 250–325 km depth range. Rather, a change in seismic anisotropy would be expected to accompany the orthoenstatite‐to‐P2 1 /c phase transition in the upper mantle at greater depths.