2017 年 46 巻 4 号 p. 135-144
New phase diagrams in Mg2Si2O6-CaMgSi2O6 system and Mg2Si2O6-Fe2Si2O6 system and stability field of high-temperature orthopyroxene in these systems are proposed.
The enthalpy and volume change of the phase transition between low- and high-temperature orthopyroxenes with composition (Ca0.06Mg1.94)Si2O6 at atmospheric pressure were obtained to be 6.2 kJ/mol and 10.25 Å3/unit cell, respectively, and the slope was calculated to be 0.0056 GPa/°C at transition temperature, 1170 °C. From these values, the phase boundary of these phases was defined as P(GPa) = 0.0056T(°C)-6.55. Using this relationships and data of synthetic experiments in previous studies, the phase diagrams and the stability fields of high-temperature orthopyroxene in Mg2Si2O6-CaMgSi2O6 system from atmospheric pressure to 1.0 GPa were determined.
The transitions between low- and high-temperature orthopyroxenes with the compositions in Mg2Si2O6-Fe2Si2O6 system were observed at about 1000-1200 °C by using 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 stability field of high-temperature orthopyroxene was above 1200 °C and new phase diagram in Mg2Si2O6-CaMgSi2O6 system at atmospheric pressure was proposed.