Kinetics of the Thermal Dehydration of Hydrous Silicates

J. Birch Holt*, Ivan B. Cutler and Milton E. Wadsworth
* Lawrence Radiation Laboratory, Livermore, California.
University of Utah, Salt Lake City, Utah.

Abstract: Generally, the rate of dehydroxylation of clay minerals may be controlled by reaction at the interface between the decomposed and undecomposed mineral, diffusion through the decomposed phase and/or nucleation of the decomposed phase. Only in rare instances are mineral decomposition reactions homogenous. Some examples are presented to illustrate these principles.

For macrocrystalllne muscovite the initial sample weight rather than the external surface area was found to be a parameter of the rate of weight loss. Upon grinding the muscovite to particles smaller than —65 mesh, an internal surface area factor strongly influenced the rate. This surface area was related to dislocations shown to be present in muscovite.

A rate equation is presented that explains the observed weight loss versus logarithm of time dependence based on a model involving a systematic relief of strain during dehydration.

The rate equation for pyrophyllite was exactly the same as that formulated for muscovite. As with muscovite there was a change in free energy of activation throughout the dehydration process. In contradiction to muscovite, the contribution to the free energy of activation was at least partly enthalpy while with muscovite it was an entropy of activation contribution.

The dehydration of kaolinite was carried out in vacuum. At pressures of one micron the reaction was found to be diffusion controlled with hydroxyl ions as the likely diffusing species.

Clays and Clay Minerals; 1963 v. 12; no. 1; p. 55-67; DOI: 10.1346/CCMN.1963.0120109
© 1963, The Clay Minerals Society
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