High-pressure phase transitions of clinoenstatite

Clinoenstatite (Mg₂Si₂O₆) undergoes a well-known phase transition from a low-pressure form (LPCEN, space group P2₁/c) to a high-pressure form (HPCEN, space group C2/c) at ~6 GPa. High-pressure structure refinements of HPCEN were carried out based on single-crystal X-ray diffraction experiments between 9.5 and 35.5 GPa to determine its P-V equation of state and structural evolution over an expanded pressure range relevant to pyroxene metastability. The best-fit isothermal equation of state to our data combined with the five data points between 5.34 and 7.93 GPa from Angel and Hugh-Jones (1994) yields a second-order Birch-Murnaghan equation with K_T0 = 121(2) GPa and V₀ = 403.9(5) ų (with K′_T0 = 4 implied). Further reduction of misfit upon fitting a third-order Birch-Murnaghan equation is not significant at the 90% confidence level. At ~45 GPa, a transition from HPCEN to a P2₁/c-structured polymorph (HPCEN2) was observed, which is isostructural to the P2₁/c phase recently observed in diopside (CaMgSi₂O₆) at 50 GPa (Plonka et al. 2012) and in clinoferrosilite (Fe₂Si₂O₆) at 30-36 GPa (Pakhomova et al. 2017). Observation of HPCEN2 in Mg₂Si₂O₆ completes the third apex of the pyroxene quadrilateral wherein HPCEN2 is found, facilitating a broader view of clinopyroxene crystal chemistry at conditions relevant to metastability in the Earth's mantle along cold subduction geotherms.