Crystal structures and compressibilities of synthetic 2M(1) and 3T phengite micas

The crystal structures of co-existing monoclinic 2M(1) and trigonal 3T poly types of phengitic micas synthesized at 11 GPa and 900 degrees C have been refined at ambient conditions. The compositions of both crystals are approx imately K(Al1.21Mg0.75Fe0.04)(Al0.19Si3.81) O-22(OH1.2 F-0.8) The unit cell parameters for the 2M(1) sample are a = 5.2046(8) Angstrom; b = 9.0368(16) Angstrom; c = 19.886(4) Angstrom, beta = 95.615(14)degrees; vol. = 930.8(2 ) Angstrom(3); and for the 3 T: a = 5.2110(4) Angstrom; c = 29.689(5) Angst rom; vol. = 698.08(13) Angstrom(3). 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, alpha, are both about 2 .4 degrees and thus the smallest yet reported for dioctahedral micas. There is no indication of tetrahedral ordering of Al and Si. The 3T polytype con tains 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 b ulk modulus yet measured for any mica. As with other micas, compressional a nisotropy is large with compression normal to the layers being about seven times that within the layers.