P. Zinn et al., KINETIC AND MICROSTRUCTURAL STUDIES OF THE CRYSTALLIZATION OF COESITEFROM QUARTZ AT HIGH-PRESSURE, Zeitschrift fur Kristallographie, 212(10), 1997, pp. 691-698
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.