Microstructures of synthetic polycrystalline coesite aggregates. The effect of pressure, temperature, and time

The preservation of coesite-bearing ultrahigh-pressure metamorphic rocks in orogenic belts indicates that continental crust can be buried to a depth exceeding its present thickness. The exhumation of these rocks requires large scale flow patterns not yet understood. By analogy to the use of flow laws for quartz for the higher crustal levels, rheological data and a flow law for coesite are required for the reconstruction and modelling of these important deep processes. The first aim of the present study was the synthesis of ‘coesitite’ samples appropriate for deformation experiments. Synthesis of coesite from silica glass follows the Ostwald step rule, with an interface-controlled slow second step from metastable α-quartz tocoesite. Normal grain growth of coesite crystals is not possible at temperatures up to 1100°C, because of the dependence of interfacial energy on crystallographic orientation. This energy becomes independent of orientation at 1170°C and a foam microstructure tends to develop, allowing for normal grain growth. This temperature dependence should also hold true for natural rocks. However, normal grain growth was not measurably observed in an experiment with 1080 min duration at a temperature of 1170°C. Consequently, the desired grain size of synthetic ‘coesitite’ samples had to be achieved by controlling the nucleation rate, which was successfully done by choosing appropriate p—T—t paths in the synthesis experiments.