P-T-X data on P21/c-clinopyroxenes and their displacive phase transitions
The P21/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 P21/c to high-temperature (HT) C2/c transition ranges from 425 °C in endmember MnMgSi2O6 to 125 °C in natural samples with an 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 P21/c to HPC2/c was reversed at 5.8 GPa at 417 °C. The high-temperature phase transition from P21/c to HTC2/c was rather indistinct and occurred at approximately 530 °C and 0.76 GPa. In a separate experiment, the HT transition was observed optically in a hydrothermal DAC between 0.0 and 0.4 GPa. The in-situP-T data of both experiments yielded an increase in transition temperature with increasing pressure (approx. 149 °C/GPa) and suggest a change in character of the transition from first order to continuous with increasing pressure. The data indicate that the HTC2/c and HPC2/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 Å) M1 and M2 cation sizes. The petrologic implications of these conclusions for the occurrence of “clinoenstatite” in the Alpe Arami peridotite are discussed.
KeywordsDiopside Contrib Mineral Petrol Cation Size Clinoenstatite Displacive Phase Transition
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