The Kinetics of Dehydration in Ca-Montmorillonite: An in situ X-ray Diffraction Study

Helen J. Bray1, Simon A. T. Redfern1 and Simon M. Clark2
1 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK
2 CLRC, Daresbury Laboratory, Daresbury, Warrington, WA4 4AD, UK

Abstract: The thermal dehydration of naturally occurring Ca-montmorillonite has been studied by in situ X-ray diffraction at temperatures between 60–120°C. The time-temperature-dependence of the position of the basal (001) reflection reveals that interlayer water loss on isothermal dehydration occurs in two stages. After an initial rapid decrease in interlayer spacing (on shock heating to an isothermal soak temperature) the reaction proceeds towards equilibrium more slowly. Furthermore, the width of the (001) reflection changes with time, reflecting transformation-dependent changes in homogeneity perpendicular to (001) with a maximum in peak width at the point where the rate of the reaction appears to change. This suggests that, as the interlayer spacing collapses, a local change is induced in the structure, affecting the means of movement of the water from the interlayer.

Keywords: montmorillonite • dehydration • synchrotron radiation • smectite • kinetics

Mineralogical Magazine; October 1998 v. 62; no. 5; p. 647-656; DOI: 10.1180/002646198548034
© 1998, The Mineralogical Society
Mineralogical Society (