The solubility of a low-Fe clinochlore between 25 and 175 degrees C and P-v=P-H2O

Solid mixtures of kaolinite-chlorite (+/- quartz) and/or chlorite-gibbsite have been equilibrated in aqueous 1.0 M NaCl, 0.001 and 0.005 hi hlgCl(2) s olutions at 25, 125 and 175 degrees C and saturated vapor pressures. Revers ibility of fluid-mineral equilibria was demonstrated by approaching final s olution compositions from high and low log a(Mg)(1/2)(2+)/a(H)(+) and from silica under- and super-saturation. At the end of the experiments, the solu tions were separated using immiscible displacement techniques. The chlorite -kaolinite equilibrium suggests a temperature-dependent reaction mechanism which may partly reflect changes in speciation of aqueous ions as well as v ariations in reaction kinetics. Log K for the chlorite-kaolinite reactions have been calculated to be 52.43 +/- 7.50, 62.33 +/- 6.89 and 79.06 +/- 2.6 0 at 25, 125 and 175 degrees C, respectively. A comparison of the measured Mg/H activity ratios at 25 degrees C with values predicted for (end-member) clinochlore-kaolinite equilibria indicates that the experimentally measure d assemblage is more stable; this suggests a large value of Gibbs energy of mixing (Delta G degrees(max)) for chlorite solid solutions.