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% H2. The transition temperatures we determined for natural orthopyroxenes were 1113–1147, 1120–1139, and around 1200 °C for Fs10, Fs14, and Fs37, respectively, and those for synthetic orthopyroxenes were 1048–1075, 961–1048, and 1037–1148 °C for Fs20, Fs30, and Fs46, respectively. Our experiments showed that the transition from low- to high-temperature orthopyroxene in the Mg2Si2O6-Fe2Si2O6 system occurred at about 1000–1200 °C. We concluded that the stability field of low-temperature orthopyroxene was below 1000 °C and that of high-temperature orthopyroxene was above 1200 °C.

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