Isosymmetric Phase Transition in Orthopyroxene at High Temperature

Abstract

An isosymmetric phase transition from orthopyroxene to a new high-temperature orthorhombic phase was observed by molecular dynamics simulations for the Mg end-member composition of pyroxene (enstatite : Mg₂Si₂O₆) and by high-temperature synchrotron X-ray powder diffraction experiments for the composition of (Ca_0.06Mg_1.94)Si₂O₆. This new phase has the same space group as orthoenstatite, Pbca. The discontinuous changes of the cell volume and cell parameters during the transition indicate a first-order transition. The transition is characterized by the switching of bonds between Mg atoms at the M2 sites and the coordinated O3 atoms. This high-temperature phase is thermodynamically distinct from the low-temperature phase, i.e., orthoenstatite in the Mg-rich portion of Mg₂Si₂O₆ - CaMgSi₂O₆(diopside) phase diagram.