AN EXPERIMENTAL-STUDY OF KAOLINITE AND DICKITE RELATIVE STABILITY AT 150-300 DEGREES-C AND THE THERMODYNAMIC PROPERTIES OF DICKITE

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