An isosymmetric phase transition of orthopyroxene found by high-temperature X-ray diffraction

High-temperature synchrotron X-ray powder diffraction experiments for the composition of (Ca_0.06Mg_1.94)Si₂O₆ have been carried out in the present study to clarify whether orthopyroxene has a transition between low- and high-temperature phases. Our results show that discontinuous changes of unit-cell dimensions and volume occur at 1170 °C during both heating and cooling processes and that the space group of Pbca does not change during this reversible phase transition. These facts indicate a first-order and isosymmetric phase transition. This high-temperature phase is thermodynamically distinct from the low-temperature phase, i.e., orthoenstatite in the Mg-rich portion of Mg₂Si₂O₆-CaMgSi₂O₆ phase diagram, although they have the same space group.