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Phengite micas are thought to be significant host minerals for H and K in the upper mantle and are stable to depths of at least 330 km. There are two crystalline forms, called polytypes, which are common. 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(Al_1.21Mg_0.75Fe_0.04)(Al_0.19Si_3.81)O₂₂(OH_1.2 F_0.8). 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, , are both about 2.4° and thus the smallest yet reported for dioctahedral micas. There is no indication of tetrahedral ordering of Al. The 3T polytype contains two distinct octahedral sites that appear to be distinctly different in size indicating possible ordering of Mg. The unit cell parameters of the 2M₁ sample have been measured at several pressures up to 7.5 GPa and 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.