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

Abstract

High-temperature synchrotron X-ray powder diffraction experiments for the composition of (Ca 0.06 Mg 1.94 )Si 2 O 6 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 Mg 2 Si 2 O 6 -CaMgSi 2 O 6 phase diagram, although they have the same space group.