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P-T-X data on P21/c-clinopyroxenes and their displacive phase transitions |
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| Authors: | Thilo Arlt Martin Kunz Jano Stolz Thomas Armbruster Ross J Angel |
| Institution: | Bayerisches Geoinstitut, Universit?t Bayreuth, D-95440 Bayreuth, Germany, DE Labor für Kristallographie, ETH Zentrum, Sonneggstr. 5, CH-8092 Zürich, Switzerland, CH Laboratorium für chemische und mineralogische Kristallographie, Universit?t Bern, Freiestr. 3, CH-3012 Bern, Switzerland, CH |
| Abstract: | 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. |
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