High-pressure phase transitions of clinoenstatite

Abstract 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 KT0 = 121(2) GPa and V0 = 403.9(5) Å3 (with KʹT0 = 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.

摘要 斜顽辉石（Mg₂Si₂O₆）在约6 GPa时会发生从低压形式（LPCEN，空间群P2₁/c）到高压形式（HPCEN，空间群C2/c）的著名相变。基于9.5到35.5 GPa之间的单晶X射线衍射实验对HPCEN进行了高压结构精修，以确定其在与辉石亚稳性相关的扩展压力范围内的P - V状态方程和结构演化。将我们的数据与Angel和Hugh - Jones（1994年）在5.34到7.93 GPa之间的五个数据点相结合，得到最佳拟合的等温状态方程为二阶Birch - Murnaghan方程，其中KT₀ = 121(2) GPa，V₀ = 403.9(5) ų（隐含K′T₀ = 4）。在90%置信水平下，拟合三阶Birch - Murnaghan方程时，拟合度的进一步降低并不显著。在约45 GPa时，观察到从HPCEN到P2₁/c结构的多晶型物（HPCEN2）的转变，它与最近在50 GPa的透辉石（CaMgSi₂O₆）（Plonka等人，2012年）和在30 - 36 GPa的斜铁辉石（Fe₂Si₂O₆）（Pakhomova等人，2017年）中观察到的P2₁/c相具有相同结构。在Mg₂Si₂O₆中观察到HPCEN2完成了辉石四边形的第三个顶点（在此处发现了HPCEN2），有助于更全面地了解沿冷俯冲地温线在与地球地幔中亚稳性相关的条件下的单斜辉石晶体化学。