KINETIC-STUDIES OF THE CRYSTALLIZATION OF COESITE USING SYNCHROTRON-RADIATION

For energy-dispersive X-ray powder diffraction under high pressure and high temperature a multi-anvil-X-ray apparatus (MAX 80) has been inst alled at HASYLAB. This instrument allows to measure diffraction data a t very short intervals and thus study time dependent phenomena. As an example we have investigated the kinetics of the alpha-quartz-coesite phase transformation. The kinetics was determined by in situ experimen ts between temperatures of 870 and 1170 K and in a pressure range of 3 0-50 kbar. The structural changes were directly imaged using synchrotr on radiation. The experimental results yield the rate constant K-c for the growth of coesite from quartz as a starting material under dry co nditions. For the transition three K-c values from 6.0 X 10(4) to 2.2 X 10(4) s(-1) for different p/t-ranges were determined.