MICROSTRUCTURES OF SYNTHETIC POLYCRYSTALLINE COESITE AGGREGATES - THEEFFECT OF PRESSURE, TEMPERATURE, AND TIME

The preservation of coesite-bearing ultrahigh-pressure metamorphic roc ks in orogenic belts indicates that continental crust can be buried to a depth exceeding its present thickness. The exhumation of these rock s requires large scale flow patterns not yet understood. By analogy to the use of flow laws for quartz for the higher crustal levels, rheolo gical data and a flow law for coesite are required for the reconstruct ion 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 f ollows the Ostwald step rule, with an interface-controlled slow second step from metastable cr-quartz to coesite. Normal grain growth of coe site crystals is not possible at temperatures up to 1100 degrees C, be cause of the dependence of interfacial energy on crystallographic orie ntation. This energy becomes independent of orientation at 1170 degree s C and a foam microstructure tends to develop, allowing for normal gr ain growth. This temperature dependence should also hold true for natu ral rocks. However, normal grain growth was not measurably observed ic an experiment with 1080 min duration at a temperature of 1170 degrees C. Consequently, the desired grain size of synthetic 'coesitite' samp les had to be achieved by controlling the nucleation rate, which was s uccessfully done by choosing appropriate p-T-t paths in the synthesis experiments. (C) 1997 Elsevier Science B.V.