Recrystallization microstructures and mechanisms in quartzites

Abstract

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.EThOS - Electronic Theses Online ServiceGBUnited Kingdo