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 (Ks0) and shear modulus (G0) 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 Ks0′ = 8.8(1), Ks0″ = −0.68(6), G0′ = 2.9(1), and G0″ = −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 P21/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‐P21/c phase transition in the upper mantle at greater depths. Key Points Ks0′ and G0′ of Fe‐bearing orthoenstatite (OEN) were measured to be 8.8(1) and 2.9(1) by single‐crystal Brillouin spectroscopy The phase transition from OEN to HPCEN2 is characterized by a pronounced increase of anisotropy occurring between 10.50 and 12.06 GPa High‐pressure Pbca‐P21/c phase transition of enstatite is unlikely to be the cause of the X discontinuity