Phase transitions in orthopyroxene (En₉₀) to 49GPa from single-crystal X-ray diffraction

Synchrotron-based high-pressure single-crystal X-ray diffraction experiments were conducted on ~Mg_0.9Fe_0.1SiO₃ (En₉₀) orthopyroxene crystals at room temperature to a maximum pressure of 48.5GPa. The sample was compressed in a diamond anvil cell with a neon pressure-transmitting medium and a gold pressure calibrant. In addition to the previously described orthopyroxene to β-opx transition (designated HPCEN2 in previous studies), we observe two further phase transitions at 29.9GPa and 40.3GPa. However, we do not observe the γ-opx phase recently described in an Fe-rich orthopyroxene composition. The structures of both of the new phases were solved in space group Pca2₁. While their Mg-O layers remain pyroxene-like, their Si-O layers transform in a stepwise fashion to akimotoite-like sheets, with sites in 4-, 4+1-, or 6-fold coordination, depending on the specific structure and layer. Due to the increased Si-O coordination number, we designate the new structures α- and β-post-orthopyroxene (α-popx and β-popx). α-popx has one Si-O layer that is entirely tetrahedral, and one layer that contains both tetrahedra and 4+1-coordinated Si in distorted square pyramids. β-popx retains the mixed 4- and 4+1-coordinated Si layer found in α-popx, while the other Si layer adopts fully octahedral coordination. The α- and β-popx structures show a progressive transformation towards the arrangement of Si layers found in akimotoite, a potentially important phase in the earth's transition zone. Metastable transformations in pyroxenes are of interest for understanding possible metastability in geological environments such as subducting slabs and meteorite impacts.