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Title: The Textures and Microstructures of the Calcium Sulphate Minerals Anhydrite Gypsum and Bassanite
Author: Hildyard, Rebecca Christine
ISNI:       0000 0004 2698 3471
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2009
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Evaporitic rocks containing minerals such as anhydrite, gypsum and bassanite, are of central importance in a number of geological scenarios. They play an important role in localizing deformation, acting as detachment horizons in many foreland fold and thrust belts, implying that their strength is generally lower than other rocks. Characterizing the microstructure of evaporites is therefore important for interpreting the larger scale deformation history of these rocks. Dehydration reactions of high grade metamorphic rocks, e.g. serpentinite, in subduction zones, are thought to be one mechanism that can trigger seismicity. Similar dehydration reactions of hydrous gypsum to bassanite or anhydrite can be used as analogues for these deeper scenarios. This thesis studies the microstructures of naturally deformed, coarse grained and fine grained anhydrite rocks from the Triassic Evaporites of the northern Apennines, using electron backscatter diffraction (EBSD). The technique was used to measure crystallographic preferred orientation (CPO) and misorientations both within grains and between grains. CPO and grain distortions within the coarse grained anhydrite rocks are found to be consistent with activity on the known (100)[001] easy slip system. It is found that there is little evidence of recovery although some dynamic recrystallization by grain boundary bulging is observed. CPO, grains distortions and misorientation distributions within the fine grained anhydrite rocks indicate activity of the (100)[010] and (001)[100] slip systems. Dynamic recrystallization by grain migration and sub-grain migration is inferred and evidence is found of a possible grain boundary sliding mechanism at the finest grain sizes. A suite of experimentally deformed and dehydrated Volterra gypsum samples are also analysed using EBSD. CPO in post-experiment bassanite is found to mimic the original gypsum c-axis orientation. Bassanite grain size is shown to relate to the degree of reaction overstep. In some samples two distinct morphologies of bassanite grains are found; large (0.5-2 mm), euhedral crystals and small «50 urn), acicular crystals, representing two different bassanite polymorphs.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available