KINETICS OF DEHYDRATION OF Ca-MONTMORILLONITE: AN IN SITU SYNCHROTRON X-RAY STUDY. Helen J Bray, Simon A T Redfern, Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, United Kingdom

The dehydration of Ca-montmorillonite has been studied using time-resolved high-temperature energy-dispersive X-ray diffraction at the Daresbury Synchrotron Radiation Source. Upon heating, water loss from the interlayer region causes changes in the basal spacing of smectites. We have used the d-spacing of the 001 reflection as a measure of the progress of the dehydration reaction. Non-isothermal and isothermal kinetics have been investigated using an ambient pressure heating cell. The position and width of the 001 reflection reflect the average reaction coordinate and spatial heterogeneity of the dehydration process, respectively. We have used these data, in conjunction with in-situ infrared and thermogravimetric studies, to develop a kinetic model for the reversible dehydration of montmorillonite at temperatures up to 450 K. The results point to a change in dehydration mechanism from low to high temperature, as well as demonstrating the variation in equilibrium water content with temperature up to 450 K.