Dissolution and precipitation rates (mol/m2/sec) of kaolinite were determined as a function of solution saturation state (or deviation from equilibrium) at 80-degrees-C and pH 3 in dilute aqueous solutions. Steady-state rates were measured using a flow-through reaction vessel. Congruent dissolution was observed only in solutions that were greatly undersaturated with respect to kaolinite and quartz and calculated to be undersaturated with respect to aluminous phases. Incongruent dissolution occurred in solutions closer to equilibrium with respect to kaolinite and calculated to be near saturation or supersaturated with respect to gibbsite. Incongruency was characterized by steady-state output solution compositions with 1 < Si/Al < 3 and was attributed to precipitation of an aluminous phase. The dependence of the dissolution rate on the Gibbs Free Energy of reaction, DELTA-G, to DELTA-G almost-equal-to -4.6 kcal/mol (1.5 x 10(-3) less-than-or-equal-to IAP/K(eq) less-than-or-equal-to 0) can be expressed equally well by Rate(diss) = -1.24 +/- 0.120 x 10(-12) (1 - exp (DELTA-G/RT))0.897 +/- 0.217 or by Rate(diss) = -1.24 +/- 0.120 x 10(-12) (1 - exp (0.850 +/- 0.242 DELTA-G/RT)). Within the experimental uncertainty, Rate(diss) is directly proportional to DELTA-G near equilibrium (for -0.7 kcal/mol less-than-or-equal-to DELTA-G less-than-or-equal-to 0 kcal/mol). Although the precipitation rates were not measured over as wide a range of saturation states, they can be fit to the linear relation Rate(ppt) = 3.72 +/- 1.29 x 10(-13) (DELTA-G/RT) up to DELTA-G almost-equal-to 0.8 kcal/mol (0 less-than-or-equal-to IAP/K(eq) less-than-or-equal-to 2.2). The calculated ion activity products for the solutions in which the slowest dissolution and precipitation rates were measured were used to determine an internally consistent thermodynamic equilibrium solubility product. At 80-degrees-C the measured solubility product for Twiggs County, Georgia kaolinite is given by log (K(eq)) = 3.750(-0.21)+0.16 according to the reaction Al2Si2O5(OH)4 + 6H+ = 2Al3+ + 2H4SiO4 + H2O. All reported values of DELTA-G were referenced to this solubility.
