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Dehydration isotherms of five smectites and one vermiculite saturated with Na, K, Mg, and Ca were used to calculate the hydration energies of clay minerals in different water-activity and cation-exchange conditions. As the water activity decreased, differences in hydration energies appeared to be dependent on the nature of the exchangeable cation and on the type of clay mineral involved. These differences in hydration energies were used to evaluate the variations of the apparent cation-exchange constants as a function of the water activity. As the water activity decreased, the less hydrated cations (i.e., K+ for all samples and Na+ or Mg2+ for some) were strongly preferred to the more hydrated cations (i.e., Ca2+ for all samples and Na+ or Mg2+ for most). As the water activity decreased, nontronite showed a significant selectivity for K+, and hectorite had a strong preference for Mg2+. From these data, a model for the transformation of nontronite into glauconite, montmorillonite into illite, and stevensite into corrensite or chlorite during sediment compaction and burial diagenesis has been developed.

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