Kinetic and microstructural studies of the crystallisation of coesite from quartz at high pressure

Abstract

The kinetics of the α-quartz to coesite transition have been studied 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 anvil high-pressure apparatus at the Hamburger Synchrotronstrah-lungslabor (HASYLAB). During the phase transition, X-ray diffraction patterns were collected at intervals of 60 s. The transformed volume fraction has been calculated from the diffracted intensities of the respective phases as a function of time. By fitting a fundamental rate equation for grain boundary nucleation and interface-controlled growth to the transformation-time data, rates of nucleation and growth have been calculated. 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 microtwinning. Its influence on the crystal structure and thus on the thermodynamic behaviour of the α-quartz-coesite phase transition is illustrated.