P-T-X data on P21/c-clinopyroxenes and their displacive phase transitions

The P2(1)/c clinopyroxene kanoite (ideally MnMgSi2O6) was studied as a function of pressure and temperature using powder X-ray diffraction, differential scanning calorimetry (DSC) and optical methods. The temperature of the P2(1)/c to high-temperature (HT) C2/c transition ranges from 425 degrees C in endmember MnMgSi2O6 to 125 degrees C in natural samples with arn aegirine component. Compiling pigeonite and clinoenstatite-clinoferrosilite literature data, the temperature of the transformation was found to decrease linearly with M2 cation size. A synchrotron powder diffraction study in a heated diamond-anvil cell (DAC) yielded compression and thermal expansion data for low kanoite of composition Mn1.2Mg0.4Fe0.4Si2O6 The high-pressure (HP) phase transition from P2(1)/c to HP C2/c was reversed at 5.8 GPa at 417 degrees C. The high-temperature phase transition from P2(1)/c to HT C2/c was rather indistinct and occurred at approximately 530 degrees C and 0.76 Cpa. In a separate experiment, the HT transition was observed optically in a hydrothermal DAC between 0.0 and 0.4 GPa. The in-situ P-T data of both experiments yielded an increase in transition temperature with increasing pressure (approx. 149 degrees C/GPa) and suggest a change in character of the transition from first order to continuous with increasing pressure. The data indicate that the HT C2/c and HP C2/c polymorphs are distinct phases with different stability fields. Since the high-temperature and the high-pressure polymorphs of kanoite were shown to be isotypic with other low-Ca clinopyroxenes such as the (Mg,Fe)SiO3 series, the conclusions we draw from this study are valid for all clinopyroxenes with small (< 0.88 Angstrom) M1 and M2 cation sizes. The petrologic implications of these conclusions for the occurrence of "clinoenstatite" in the Alpe Arami peridotite are discussed.