Stability field of the high-temperature orthorhombic phase in the enstatite-diopside system

This research investigated the phase transition between low- and high-temperature orthopyroxenes with composition (Ca(0.06)Mg(1.94))Si(2)O(6) using differential scanning calorimetry experiments and in situ high-temperature X-ray diffraction. The transition enthalpy, temperature, volume change, and slope were estimated to be 6.2 kj/mol, 1170 degrees C, 10.25 angstrom(3)/unit cell, and 0.0056 GPa/degrees C, respectively. The phase boundary between low- and high-temperature orthopyroxene was defined as P(GPa) = 0.0056T (degrees C) - 6.55. This relationship shows that the invariant point for four-phase equilibria (protoenstatite + high-temperature orthopyroxene + pigeonite + diopside) is approximately 1240-1280 degrees C and 0.1-0.2 GPa, rather than the equivalent system involving low-temperature orthopyroxene as described in previous studies. We developed phase diagrams for Mg(2)Si(2)O(6) and the Mg(2)Si(2)O(6)-CaMgSi(2)O(6) system taking into account the results of previous synthetic experiments and the phase boundary that we determined between low- and high-temperature orthopyroxene. The developed phase diagrams for Mg(2)Si(2)O(6) showed that high-temperature orthoenstatite is more stable than protoenstatite at pressure above similar to 0.8 GPa, and that the boundary between high-temperature orthoenstatite and protoenstatite has a gentle negative slope. As pressure is increased from 1 atm to about 0.2 GPa, the lower temperature limit of stability of high-temperature orthopyroxene decreases from similar to 1370 to similar to 1200 degrees C. Above 0.9 GPa, the stability field of protoenstatite disappears and high-temperature Ca-free orthopyroxene is stable. On the basis of these results, it is suggested that further high-resolution analyses of the thermodynamics of the enstatite-diopside system at high temperatures and high pressures are required.