Mineral assemblage solubilities were measured in cold-seal pressure vessels as a function of pressure, temperature, and potassium chloride concentration in order to determine the nature and thermodynamic properties of iron-chloride complexes under hydrothermal conditions. The assemblage pyritepyrrhotite-magnetite was used to buffer ƒS₂ and ƒO₂, and K⁺H⁺ ratios were buffered at reasonable geologic values using the assemblage potassium feldspar-muscovite (or andalusite)-quartz. The pressure-temperature ranges were 0.5-2.0 kbar and 300–600°C, and initial fluid compositions ranged from 0.01–2.0 molal KCl. With all other factors constant, the concentration of iron in solution increases with increasing temperature, with decreasing pressure, and with increasing total potassium chloride concentration.

Changes in iron concentrations as a function of KCl concentration, in conjunction with charge balance, mass action, and mass balance constraints on the system, place constraints on the stoichiometry of the important iron-chloride complexes under each of the experimental conditions. Using least-squared linear regression fits to determine these slopes, the calculations yield values for the average ligand numbers that are in the range 1.2-1.9, with uncertainties ranging from ±0.1-0.6 at the several PT conditions considered. The slopes of the regressed fits to the data suggest that both FeCl⁺ and FeCl₂⁰ are important in the experimental fluids, with FeCl₂⁰ becoming dominant at the higher temperatures. Theoretical calculations, however, indicate that FeCl⁺ does not contribute significantly to the solubility. Because of the large uncertainties associated with some of the calculated average ligand numbers, we base our data analysis on the theoretical calculations. A statistical analysis is applied to the solubility data in order to determine the values and uncertainties of the dissociation constant for FeCl₂⁰ that best fit the data at each of the experimental pressures and temperatures. The calculated stability of FeCl₂⁰ increases with increasing temperature and total chloride concentration, and with decreasing pressure. The values of the dissociation constant of FeCl₂⁰that are calculated in this study are in moderately good agreement with FeCl₂⁰dissociation constants from other studies of iron-chloride complexing in supercritical fluids. Differences are likely due to different assumptions made concerning activity coefficients of aqueous species. Log k_d values for full dissociation of FeCl₂⁰ at 0.5 kbar—300°C—and at 1 kbar—400, 500, and 600°C, respectively—are −3.75 ± 0.40, −6.25 ± 0.10, −9.19 ± 0.44, and −13.29 ± 0.09.