Phase transitions between high- and low-temperature orthopyroxene in the Mg2Si2O6-Fe2Si2O6 system

We observed isosymmetric phase transitions of orthopyroxene in the Mg2Si2O6-Fe2Si2O6 system during high-temperature in situ X-ray powder diffraction experiments with a multiple-detector system and a high-temperature strip heater chamber in an atmosphere of Ar plus 1% H-2. The transition temperatures we determined for natural orthopyroxenes were 1113-1147, 1120-1139, and around 1200 degrees C for Fs(10), Fs(14), and Fs(37), respectively, and those for synthetic orthopyroxenes were 1048-1075, 961-1048, and 1037-1148 degrees C for Fs(20), Fs(30), and Fs(46), respectively. Our experiments showed that the transition from low- to high-temperature orthopyroxene in the Mg2Si2O6-Fe2Si2O6 system occurred at about 1000-1200 degrees C. We concluded that the stability field of low-temperature orthopyroxene was below 1000 degrees C and that of high-temperature orthopyroxene was above 1200 degrees C.