Chaplin, Tracey Diane; (1998) The lattice dynamics of garnets. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The garnet group of minerals is of geological importance, forming 13-15% of the Earth's upper mantle, with Al-deficient majorite garnet forming 40-60% of the transition zone. Thus, it is important to understand the properties of this group. The thermodynamic properties of crystals are governed by the motions of their constituent atoms and the processes operating at the atomistic level may be described within the framework of lattice dynamics. We have performed lattice dynamical simulations of pyrope garnet, Mg3Al2Si3O12, using the PARAPOCS code (Parker and Price, 1989); although the simulated structural and thermodynamic properties of pyrope show good agreement with the experimental values, the elastic moduli are overestimated by 15- 30%. By performing symmetry analysis of the calculated eigenvectors of pyrope, we have determined the infrared- and Raman-activity of the 240 vibrational frequencies of this phase. These show excellent agreement with modes observed using infrared and Raman spectroscopies. Analysis of the eigenvectors and the frequency shifts associated with simulated 26Mg and 30Si substitution, has enabled assignment of the vibrational modes of pyrope to specific site or atomic motions. Our assignments show that the high-frequency modes are attributable to the V1-V4 motions of the SiO4 tetrahedra, whereas the low-frequency vibrations show substantial mode-mixing. Using inelastic neutron scattering, we have measured the vibrational density of states of pyrope and grossular garnet, Ca3Al2Si3O12, with the experimental spectra successfully interpreted using lattice dynamical simulations. Using Raman spectroscopy, we have studied the intrinsic anharmonic behaviour of a germanate analogue for tetragonal majorite, CaGeO3, from the observed pressure- and temperature-induced shifts of the Raman-active modes. By comparison with the anharmonic behaviour observed for grossular and pyrope (Gillet et al., 1992), we suggest that MgSiO3 tetragonal majorite will display greater anharmonicity than our germanate analogue. This has important implications for modelling of the Earth's interior which must take into account this anharmonic behaviour.

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