The Gibbs free energy (Delta G 0 (sub (1)) ) of the reaction kaolinite <--> dickite was generated from solubility measurements of natural kaolinite and dickite performed in acid solutions at temperatures ranging from 150 to 300 degrees C under vapor-saturated conditions. The Delta G 0 (sub (1)) values increase from -0.620+ or -0.150 to -0.218+ or -0.210 kcal/mol with increasing temperature from 150 to 300 degrees C. Regression of these data yields a value of -0.90+ or -0.10 kcal/mol for Delta G 0 (sub (1)) at 25 degrees C. The standard Gibbs free energy of formation (Delta G 0 (sub f.298) ) of dickite deduced from Delta G 0 (sub (1)) and the Delta G 0 (sub f.298) of kaolinite (Zotov et al., in preparation) is -908.36+ or -0.40 kcal/mol. The results obtained in this study indicate that kaolinite is metastable relative to dickite at temperatures to at least 350 degrees C. It follows that the timing of observed kaolinite to dickite transformations in diagenetic and many hydrothermal systems is controlled by the kinetics of this reaction rather than thermodynamic equilibria.

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