Gibbs free energy of formation of kaolinite from solubility measurement in basic solution between 60 and 170 degrees C

The solubility of a natural hydrothermal kaolinite was measured in buffered alkaline solutions at temperatures ranging from 60 to 170 degrees C. At temperatures below 110 degrees C equilibrium was obtained from undersaturation, while above 110 degrees C two different steady states were obtained, one from undersaturation and another from supersaturation. Raman spectroscopy studies of the solid phases recovered at the end of each run, demonstrated that the ''undersaturated'' experiments attained equilibrium with respect to kaolinite, while the ''supersaturated'' experiments attained equilibrium with an illite-like mineral. Regression of kaolinite solubility products yields a Gibbs free energy of formation of kaolinite (Al2Si2O5(OH)(4)) at 25 degrees C of -907.68 +/- 0.2 kcal/mol. This value is more negative than most literature values, but similar to that generated from calorimetric data by Barany and Kelley (1961). Furthermore, calculated thermodynamic parameters generated from kaolinite solubility obtained in this study are in close agreement with those deduced from boehmite-kaolinite equilibrium data at temperatures higher than 200 degrees C.