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Abstract:The crystal structures of co-existing monoclinic 2M 1 and trigonal 3T polytypes of phengitic micas synthesized at 11 GPa and 900 ºC have been refined at ambient conditions. The compositions of both . The molar volumes of the two polytypes are identical within error (approximately one part in 4000). The structures show closely similar distortions consistent with the nearly pure silicate tetrahedral layer. The tetrahedral rotation angles, α, are both about 2.4º and thus the smallest yet reported for dioctahedral micas. There is no indication of tetrahedral ordering of Al and Si. The 3T polytype contains two distinct octahedral sites that appear to be distinctly different in size indicating possible ordering of Mg and Al. The unit cell parameters of the 2M 1 sample have been measured at several pressures up to 7.5 GPa and those of the 3T sample to 4.0 GPa. Fitting compression data to a third-order Birch-Murnaghan equation of state gives a K 0 of 57 ± 3 GPa with K' of 9.2 ± 1.7 for the 2M 1 and a K 0 of 62 ± 2 GPa with a fixed K' of 9 for the 3T. These are statistically identical and represent the largest bulk modulus yet measured for any mica. As with other micas, compressional anisotropy is large with compression normal to the layers being about seven times that within the layers.

The P2 1 /c clinopyroxene kanoite (ideally MnMgSi 2 O 6 ) was studied as a function of pressure and temperature using powder X-ray diraction, dierential scanning calorimetry (DSC) and optical methods. The temperature of the P2 1 /c to high-temperature (HT) C2/c transition ranges from 425°C in endmember MnMgSi 2 O 6 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 diraction study in a heated diamond-anvil cell (DAC) yielded compression and thermal expansion data for low kanoite of composition Mn 1.2 Mg 0.4 Fe 0.4 Si 2 O 6 . The high-pressure (HP) phase transition from P2 1 /c to HP C2/c was reversed at 5.8 GPa at 417°C. The high-temperature phase transition from P2 1 /c to HT C2/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-situ P-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 ®rst order to continuous with increasing pressure. The data indicate that the HT C2/c and HP C2/c polymorphs are distinct phases with dierent stability ®elds. Since the hightemperature and the high-pressure polymorphs of kanoite were shown to be isotypic with other low-Ca clinopyroxenes such as the (Mg,Fe)SiO 3 series, the conclusions we draw from this study are valid for all clinopyroxenes with small (<0.88 A Ê ) M1 and M2 cation sizes. The petrologic implications of these conclusions for the occurrence of``clinoenstatite'' in the Alpe Arami peridotite are discussed.