Hayward, Christopher Leon; (1993) The vibrational spectroscopy of borate minerals. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis is concerned with a study of the vibrational spectroscopy of two boron minerals, sinhalite (MgAl(BO4)) and danburite (CaB2Si2O8), which are the analogues of the geologically important phases forsterite (Mg2SiO4) and anorthite (CaAl2Si2O8) respectively. The work has been undertaken because these minerals are interesting from a crystal chemical point of view as they are analogues; to make a contribution to the understanding of the dynamic characteristics of the olivine and feldspar structures; and to provide the first thermodynamic data for these minerals. They occur in medium to high grade metamorphic environments, but little is known about the behaviour of boron or phase relations between the boron minerals in these environments, (or indeed many other environments), and such thermodynamic data will help towards an understanding of the behaviour of boron under these conditions. High quality polarised spectra of these two minerals have been collected using single crystal polarised reflectance infrared spectroscopy in the range 5000-100 cm-1 and single crystal polarised Raman spectroscopy in the region 1300-0 cm-1. The excellent results produced by these techniques are far superior to those possible using powdered samples. In the sinhalite spectra, 31 of the 35 predicted infrared modes and 32 of the 36 predicted Raman modes have been observed and the majority of these have been assigned. Raman bands occur in the 750-700 cm-1 region. In the infrared spectra bands in this region are assigned to AlO6 modes, but this assignment is not possible for the Raman bands because Raman-active modes involving aluminium displacement are forbidden in sinhalite by symmetry analysis because aluminium occupies a centre of symmetry. An X-ray structure determination of sinhalite was carried out to check the previously published structure and the results are consistent with space group Pbnm, with aluminium lying on the centre of symmetry. The cell parameters were found to be a = 9.8819(4)Å, b = 5.6813(3)Å, c = 4.3320(3)Å with V = 243.21(3)Å3. A detailed crystal chemical study has been carried out on four sinhalite samples, and the data indicates that two substitution mechanisms involving the octahedral cations occur. These are 3Mg ≈ 2A1 + □ which is the dominant mechanism, and Mg + Fe3+ ≈ A1 + Fe2+. The polarised danburite spectra represent the first such spectra for any feldspar in which most of the optically active modes are observed, and hence are a significant advance over previous vibrational data. In the spectra 53 infrared and 63 Raman bands are observed out of the 57 and 78 modes respectively predicted to be active, and most of these bands have been assigned. The heat capacity and entropy of sinhalite and danburite have been calculated from the polarised spectroscopic data using in the lattice dynamics-based Kieffer model. The results are the first such data for these minerals. For danburite Cp and Sp have been calculated, but because no experimental thermal expansion or bulk modulus data exist for sinhalite, the values of CP and SP produced are considered to be the best predictions. The values of heat capacity and entropy of sinhalite and danburite produced by the Kieffer model are likely to be very reasonable predictions over the geologically relevent temperature range of 300-1000K. Atomistic simulations of sinhalite have been run using the recently developed boron-oxygen interatomic potential BTH1. The results permit further assignments of experimentally observed infrared and Raman modes that cannot be assigned from analysis of the spectra alone, and provide considerable insight into the mixing between the various vibrational modes in the structure. The simulated heat capacity data (Cv) for sinhalite is in very good agreement with that calculated by the Kieffer model, which provides further support for this data.

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