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 (Ca0.06Mg1.94)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.
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