Abstract:
The speciation and thermodynamic properties of ferric chloride complexes in hydrothermal solutions and hypersaline brines are still poorly understood, despite the importance of this element as a micronutrient and ore-component. Available experimental data are limited to room temperature and relatively low chloride concentrations. This paper reports results of UV-Vis spectrophotometric and synchrotron XAFS experiments of ferric chloride complexes in chloride concentrations up to 15 m and at temperatures of 25-90 °C. Qualitative interpretation of the UV-Vis spectra shows that FeCl2+, FeCl2+, FeCl3(aq) and FeCl4- were present in the experimental solutions. As chloride concentrations increase, higher ligand number complexes become important with FeCl4- predominating in solutions containing more than 10 m at 25 °C. The predominance fields of FeCl3(aq) and FeCl4- expand to lower Cl concentrations with increasing T. Both XANES and UV-Vis spectra reveal a major change in the geometry of the complex between FeCl2+ and FeCl3(aq). EXAFS data confirm that the number of chloride ligands increases with increasing chloride concentration and show that Fe3+, FeCl2+ and FeCl2+ share an octahedral geometry. FeCl3(aq) could be either tetrahedral or trigonal dipyramidal, while FeCl4- is expected to be tetrahedral. EXAFS data support a tetrahedral geometry for FeCl4-, especially at 90 °C, but do not allow to distinguish between a tetrahedral or trigonal dipyramidal geometry for FeCl3(aq) because of similar Fe-Cl distances. At room temperature, EXAFS data suggest that FeCl3(aq) may be a mixture of octahedral and tetrahedral or trigonal dipyramidal forms. The room temperature formation constants for three ferric chloride complexes (FeCl2+, FeCl3(aq) and FeCl4-) determined from the UV data are generally in good agreement with previous studies. Calculations based on the properties extrapolated to 300 °C show that hematite solubility is much higher than previously estimated, and that the high orders complexes FeCl3(aq) and FeCl4- are important at high temperatures even in solutions with low chloride concentrations. The accuracy of these properties is limited by a poor understanding of activity-composition relationships in concentrated electrolytes, and by limitations in the available experimental techniques and extrapolation algorithms; however, the inclusion of higher order complexes in numerical models of ore transport and deposition allows for a more accurate qualitative prediction of Fe behaviour in hydrothermal and hypersaline systems. © 2006 Elsevier B.V. All rights reserved.
