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Using a novel seven-circle X-ray diffractometer with an area sensitive detector, the three-dimensional shape and the intensity of a pair of satellite reflections from an incommensurate sample (An65) was followed as a function of temperature. A large displacive component was found, in contrast to the widely accepted hypothesis that the incommensurate structure is a consequence solely of competing Al/Si ordering schemes. The presence of diffuse satellites in precession photographs of a highly disordered volcanic sample supports this result. On a mesoscopic length scale, domains of perfect periodic antiphase boundaries are seen in the TEM, delimited by rings of defects. The domain size is strongly composition dependent, rising to several thousand Å at An71, and falling to zero at ~An30, possibly indicating the compositional limit of the <i>e</i>₂ structure. Infrared spectroscopy was used to probe the microstructural variations in natural and heat-treated plagioclase series. In comparing the macroscopic strain (cosg from lattice refinements) with a measure of local strain, w, the two incommensurate phases show different behaviour, with the <i>e</i>₁ phase extrapolating to anorthite, and <i>e</i>₂ to disordered albite. On this length scale, the behaviour of the <i>e</i>₁ samples is indistinguishable from the <i>I</i>⁻¹ phase. Order parameter values for <i>I</i>⁻¹ ordering are proposed from the <i>e</i>₁structure as a result of this work and extrapolate to zero between An40 and An50. The autocorrelation function is applied to the spectra and used to generate a local order parameter which is found to scale well with previous measurements of enthalpy of solution. Experiments were also performed on the similar incommensurate structure found in synthetic anorthites, in which high degrees of short range order were found even in the macroscopically disordered state. The two sets of data form single trends as functions of degree of order.

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Electron backscatter diffraction (EBSD) has been used to analyse 25 quartz rich rocks. The rock samples represent a range ofcommon microstructures which exhibit variations in the defonnation conditions such as changes in temperature and the amount of strain accumulated. As natural sample's defonnation conditions are poorly constrained, five out of the 25 samples were experimentally defonned samples. EBSD has been used to measure the full crystallographic orientation of all the grains contained within the mapped area The mapped microstructures have been separated out in to original 'parent' grains and recrystallized 'daughter' grains. Neighbour-daughter grains are recrystallized grains which are still in contact with a parent grain (although not necessarily its own). The samples exhibit between 10% and 95% recrystallized microstructures. The samples can be separated into two main groups based upon their microstructural and statistical characteristics. The first group represents samples which have an average subgrain size which is similar in size to the neighbour-daughters. The parent grains show a systematic increase in misorientation from the centre ofthe grain to the edges. These data are consistent with subgrain rotation (SGR) as being the controlling nucleation and recrystallization mechanism The second group ofsamples show an average subgrain size which is much larger than the size of the neighbour-daughter grains. The internal defonnation of the parent grains is randomly arranged and does not gradually increase. These data are inconsistent with SGR The recrystallization was facilitated by bulging at low temperatures or during strain-induced grain boundary migration (SIGBM). All samples studied exhibited angles between the parent and neighbourdaughter grains which had increased after nucleation and recrystallization had taken place. Each sample analysed had at least 50% of the grain boundaries with misorientation angles ofgreater than 30°. Other processes have increased the misorientation angles. The distnbutions of the neighbour-daughter grains have also been redistnbuted from being in contact with the parent they are theorized to have recrystallized from to being located either next to another parent or in the matrix. The microstructures have been modified. Grain boundary sliding (GBS) is interpreted as the controlling modification mechanism which caused the neighbour-switching and further rotations of the recrystallized grains to cause the increased misorientation angles observed.

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No description available.