KINETIC AND MICROSTRUCTURAL STUDIES OF THE CRYSTALLIZATION OF COESITEFROM QUARTZ AT HIGH-PRESSURE

The kinetics of the alpha-quartz to coesite transition have been studi ed in the pressure range of 3.2 GPa to 5.2 GPa and in the temperature range 950 K-1175 K by in situ X-ray diffraction using a MAX80 cubic an vil high pressure apparatus at the Hamburger Synchrotronstrahlungslabo r (HASYLAB). During the phase transition, X-ray diffraction patterns w ere collected at intervals of 60 s. The transformed volume fraction ha s been calculated from the diffracted intensities of the respective ph ases as a function of time. By fitting a fundamental rate equation for grain boundary nucleation and interface-controlled growth to the tran sformation-time data, rates of nucleation and growth have been calcula ted. A discrepancy between the experimental determined and theoretical calculated phase boundary is discussed. After quenching the samples, the reaction products could be investigated by TEM images. A specific feature of the synthesised coesite crystals was a concentrated microtw inning. Its influence on the crystal structure and thus on the thermod ynamic behaviour of the alpha-quartz-coesite phase transition is illus trated.