Reversals of the orthoenstatite‐clinoenstatite transition at high pressures and high temperatures

The MgSiO 3 orthoenstatite‐clinoenstatite (OEn‐CEn) phase boundary has been reversed between pressures of 70 and 110 kbar and temperatures of 900°C and 1700°C with a split‐sphere anvil apparatus (USSA‐2000). Starting materials contained PbO‐PbF 2 (1∶1) flux to promote equilibration of the charges and eliminate a potential effect of deviatoric stresses on the phase boundary. The phase boundary separating orthoenstatite from clinoenstatite at high pressures and temperatures can be described by the equation P (kbar) = 0.031 T (°C)+50. Our results are consistent with previous unreversed determinations of the boundary at these high‐pressure, high‐temperature conditions. However, the dP / dT slope determined in the present study is much smaller than that implied by earlier experimental studies of the orthoenstatite/low clinoenstatite boundary at low pressures and temperatures. We propose that the clinoenstatite observed at high pressures and temperatures is a new high‐pressure clinoenstatite phase which is possibly an analogue of MgGeO 3 clinopyroxene.