THE SOLUBILITY OF CHLORITE SOLID-SOLUTIONS IN 3.2 WT-PERCENT NACL FLUIDS FROM 300-400-DEGREES-C, 500 BARS

Solubility equilibria for the assemblage chlorite-albite-paragonite-qu artz in 3.2 wt% NaCl fluids has been investigated as a function of chl orite composition from 300-400 degrees C, 500 bars. Natural samples of a low Fe-clinochlore and high Mg-chamosite were employed as reactants representing a range of Fe2+/(Fe2+ + Mg) ratios from 0.09 to 0.52, re spectively. At each temperature, reaction of low Fe-clinochlore-albite -paragonite-quartz in NaCl solutions resulted in relatively Mg-rich an d Fe-poor fluids compared to reaction of the high Mg-chamosite bearing assemblage. In both cases, Mg concentrations decrease from 300 to 350 degrees C and then increase to 400 degrees C. Iron concentrations inc rease sharply with increasing temperature. Thus, the aqueous Fe/Mg con centration ratio of NaCl solutions coexisting with chlorite increase w ith temperature and mole fraction chamosite. Activity-composition mode ls for chlorite solid solution (both ideal and regular) have been eval uated in the context of our solubility data. For this analysis, the en thalpy interaction parameter (W-H) was assumed to be independent of te mperature and the excess free energy (Delta G(ex)) was symmetric with respect to composition. The results show that the measured dependence of chlorite solubility on composition is consistent with Delta G(ex) v alues from 0 (ideal) to 200 cal/mol. A larger departure from ideality (Delta G(ex) > 200 cal/mol) is inconsistent with the solubility data. An analysis of fluid-mineral equilibria in this chemical system shows that thermodynamic data for clinochlore and chamosite retrieved from h igh temperature-pressure phase equilibria experiments are consistent w ith the aqueous Fe/Mg ratios measured as a function of temperature and chlorite composition. Systematic discrepancies were noted, however, b etween measured and predicted Fe and Mg concentrations in equilibrium with chlorite-albite-paragonite-quartz assemblages. It is suggested th at these discrepancies result from uncertainties in the thermodynamic data for albite and paragonite. Requisite adjustments to the Delta G(f )(o) and Delta H-f(o) of the Na-Al silicates are derived that bring in to agreement measured and predicted chlorite solubility. The results o f this study are relevant to a wide variety of hydrothermal environmen ts where chlorite solid solution is a common phase and should better c onstrain the chemical consequences of water-rock interaction at temper atures previously unsupported by experimental data.