No abstract

No abstract

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.

We have carried out ab initio hybrid Hartree-Fock/Density Functional Theory simulations to determine the structure and vibrational modes of zircon, ZrSiO 4 , as a function of different applied strains. The changes in phonon-mode wavenumbers are approximately linear in the unit-cell strains, and have been fitted to determine the components of the phonon-mode Grüneisen tensors of zircon which reproduce the change in measured Raman shifts with pressure. They can therefore be used to convert Raman shifts measured from zircon inclusions in metamorphic rocks into strains that in turn can be used to determine the metamorphic conditions at the time that the inclusion was trapped. Due to the strong anisotropy in the thermal pressure of zircon, the phonon-mode Grüneisen tensor is not able to reproduce the temperature-induced changes in Raman shifts. Because zircon inclusions are normally measured at room conditions this does not prevent the calculation of their entrapment conditions.