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 Na 2 O-K 2 OAl 2 O 3 -SiO 2 -HF-H 2 O and Na 2 O-K 2 O-Al 2 O 3 -SiO 2 -HCl-H 2 O, 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.

An assessment of F-C1-OH partitioning between natural apatite and biotite in a variety of rocks was used to evaluate reciprocal (Mg, Fe 2+ , Al VI ) (F, Cl, OH) mixing properties for biotite according to the reciprocal salt solution model of Wood and Nicholls (1978). Ideal mixing of F-C1-OH and Fe-Mg-Al VI in the hydroxyl and octahedral sites is assumed for biotites with dilute Cl concentrations. The reciprocal interaction parameters, in terms of Gibbs free energies, for the reactions KMg 3 [ AlSi 3 O 10 ]( OH ) 2 + KFe 3 [ AlSi 3 O 10 ]( F ) 2 = KMg 3 [ AlSi 3 O 10 ]( F ) 2 + KFe 3 [ AlSi 3 O 10 ]( OH ) 2 Phl Fann Fphl Ann and KMg 3 [ AlSi 3 O 10 ]( Cl ) 2 + KFe 3 [ AlSi 3 O 10 ]( OH ) 2 = KMg 3 [ AlSi 3 O 10 ]( OH ) 2 + KFe 3 [ AlSi 3 O 10 ] ( Cl ) 2 Clphl Ann Phl Clann are about −10 kcal/mol and −4.5 kcal/mol, respectively. These mixing properties are consistent with standard state thermodynamic properties for F and Cl endmember phases from Zhu and Sverjensky (1991). The approach of studying F-C1-OH partitioning between biotite and apatite permits distinguishing the reciprocal effects from the effects of temperature, pressure, and fluid composition. The resultant mixing properties are consistent with observations both in hydrothermal experiments and in natural mineral assemblages. The mixing properties presented in this study enable us now to predict F and Cl concentrations of hydrothermal fluids from the measured F and Cl concentrations in biotite with variable Fe-Mg-Al VI proportions. A case study of the Santa Rita porphyry copper deposits, New Mexico, shows that hydrothermal fluids responsible for the phyllic alteration had a salinity about 3 molal Cl − , in agreement with fluid inclusion studies. Our internally consistent standard thermodynamic properties and solid solution models also lead to recalibration of the apatite-biotite geothermometer. The revised geothermometer predicts temperatures that agree with those estimated from other independent geothermometers. The large reciprocal effects in biotite also point out the need to revise other geothermometers using biotite and to revise the ideal mixing model for biotite in petrologic studies.