The crystal structures of co-existing monoclinic 2M1 and trigonal 3T polytypes of phengitic micas synthesized at 11 GPa and 900°C have been refined at ambient conditions. The compositions of both crystals are approximately K(Al1.21Mg0.75Fe0.04)(Al0.19Si3.81) O22(OH1.2 F0.8). The unit cell parameters for the 2M1 sample are a = 5.2046(8) Å; b = 9.0368(16) Å; c = 19.886(4) Å, β = 95.615(14)°; vol. = 930.8(2) Å3; and for the 3T: a = 5.2110(4) Å; c = 29.689(5) Å; vol. = 698.08(13) Å3. 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 2.4° 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 2M1 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 K0 of 57 ± 3 GPa with K’ of 9.2 ± 1.7 for the 2M1 and a K0 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.

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