The solubility of chlorite solid solutions in 3.2 wt% NaCl fluids from 300-400°C, 500 bars

Details The solubility of chlorite solid solutions in 3.2 wt% NaCl fluids from 300-400°C, 500 bars The solubility of chlorite solid solutions in 3.2 wt% NaCl fluids from 300-400°C, 500 bars

Jan 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58..567s&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 58, Issue 2, pp.567-585

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Scientific paper

Solubility equilibria for the assemblage chlorite-albite-paragonite-quartz in 3.2 wt% NaCl fluids has been investigated as a function of chlorite composition from 300-400°C, 500 bars. Natural samples of a low Fe-clinochlore and high Mg-chamosite were employed as reactants representing a range of Fe 2+ / (Fe 2+ + Mg) ratios from 0.09 to 0.52, respectively. At each temperature, reaction of low Fe-clinochlorealbite-paragonite-quartz in NaCl solutions resulted in relatively Mg-rich and Fe-poor fluids compared to reaction of the high Mg-chamosite bearing assemblage. In both cases, Mg concentrations decrease from 300 to 350°C and then increase to 400°C. Iron concentrations increase sharply with increasing temperature. Thus, the aqueous Fe/Mg concentration ratio of NaCl solutions coexisting with chlorite increase with temperature and mole fraction chamosite. Activity-composition models for chlorite solid solution (both ideal and regular) have been evaluated in the context of our solubility data. For this analysis, the enthalpy interaction parameter ( W H ) was assumed to be independent of temperature and the excess free energy ( G ex ) was symmetric with respect to composition. The results show that the measured dependence of chlorite solubility on composition is consistent with G ex values from 0 (ideal) to 200 cal/mol. A larger departure from ideality ( 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 high temperature-pressure phase equilibria experiments are consistent with the aqueous Fe / Mg ratios measured as a function of temperature and chlorite composition. Systematic discrepancies were noted, however, between measured and predicted Fe and Mg concentrations in equilibrium with chlorite-albite-paragonite-quartz assemblages. It is suggested that these discrepancies result from uncertainties in the thermodynamic data for albite and paragonite. Requisite adjustments to the G o f and H o f of the Na-Al silicates are derived that bring into agreement measured and predicted chlorite solubility. The results of this study are relevant to a wide variety of hydrothermal environments where chlorite solid solution is a common phase and should better constrain the chemical consequences of water-rock interaction at temperatures previously unsupported by experimental data.

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