Partitioning of F-Cl-OH between minerals and hydrothermal fluids

A thermodynamic analysis of F-Cl-OH partitioning between minerals and hydrothermal fluids has resulted in the retrieval of standard-state Gibbs free energies for fluormuscovite, fluorphlogopite, fluorannite, fluortremolite, fluortalc, hydroxyapatite, fluorapatite, chlorapatite, and chlorannite from hydrothermal experimental studies. Standard-state entropies, heat capacities, and volumes are either derived from experimental studies or estimated following Helgeson et al. (1978). The derived thermodynamic properties are internally consistent and consistent with the thermodynamic properties of minerals and aqueous species from Berman (1988, 1990), Sverjensky et al. (1991a), Shock and Helgeson (1988), and Shock et al. (1989), and therefore can be extrapolated over a wide range of temperatures and pressures for application to geochemistry, igneous and metamorphic petrology, and ore deposits. The derived standard-state thermodynamic properties for F and Cl endmember phases provide a basis for predicting the fluoride and chloride concentrations of former aqueous fluids from the measured F and Cl concentrations in minerals. Speciation and solubility calculations simulating F and Cl partitioning between minerals and hydrothermal fluids in the systems Na2O-K2OAl2O3-SiO2-HF-H2O and Na2O-K2O-Al2O3-SiO2-HCl-H2O, and systems containing apatites, show that the partitioning is a strong function of temperature, pressure, and fluid composition. Increase of temperature favors partitioning of F into fluids with respect to minerals, while it favors partitioning of Cl into annite. The decrease of both pressure and pH of fluids favors partitioning of Cl into annite with respect to fluids. In addition to predicting fluoride and chloride concentrations in hydrothermal fluids, the results of this study enable mass transfer calculations including both F-C1-OH partitioning and metal complexing of halides during water-rock interactions in a variety of geological systems.