Hackwell, Timothy Patrick; (1994) The structural behaviour of aluminosilicate frameworks at high pressures and temperatures. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Framework silicates are the most abundant constituents of the Earth's crust and upper mantle. Feldspars are quantitatively the most important of the rock-forming silicates, making up to 60% of the Earth's crust. In order to understand the complex igneous and metamorphic processes which take place within the earth it is vital that we understand the response of major rock-forming minerals, such as feldspars, to pressure and temperature. The crystal structures of feldspars are reviewed, starting with the ideal formula, MT4O8. The topology of the structure is then discussed, followed by the perturbation to the structure caused by the Mail cation and distribution of the T atoms. This is followed by an introduction to Landau theory, and the means by which a phase transition in a material can be approached macroscopically. The last twenty-five years has seen rapid and widespread development of DACs suitable for single crystal X-ray diffraction experiments. The various types of DACs are reviewed and the relative advantages and disadvantages of each cell discussed. This is followed by a description of the construction and operation of each cell. The second half of this chapter concentrates on the development of a new DAC suitable for use at high pressure and temperature. Again the construction and operation of the cell is described. The compressibilities of a number of alkali and plagioclase feldspars have been determined between 0 GPa and 5.2 GPa by single crystal X-ray diffraction, using a Merrill-Bassett diamond anvil cell. The unit-cell parameter data from these experiments plus the use of previously published high pressure unit-cell parameters has enabled the Mumaghan equation of state to be fitted to the volume-pressure data, and a bulk modulus obtained for each crystal structure. Changes in the unit-cell parameters, and principal strain directions at high pressure, allow a number of conclusions to be made on the structural response of feldspars to high pressure. The P1-I1 phase transition in end-member anorthite has been reversed in-situ at high pressures and temperatures, using a single crystal sample and an externally heated DAC. At high pressures the phase boundary is linear and nearly isobaric, and marked by a first order step in volume. At high temperatures the boundary is linear and isothermal and there is no detectable discontinuities in unit-cell parameters. The compressibilities of sanidine (Or82Ab18), amazonite microcline (Or87) and Norwegian microcline (Or85) have been determined between 0 GPa and 5.2 GPa by single crystal X-ray diffraction, using a Merrill-Bassett DAC. The sanidine appears to exhibit straightforward compressional behaviour, with no evidence of the previously reported phase transition at 1.8 GPa. High pressure unit-cell parameter data for microcline reveals a change in the compression mechanism at 3.7 GPa (amazonite) and 4.2 GPa (Norwegian microcline).

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