In-situ X-ray absorption study of Iron(II) speciation in brines up to supercritical conditions

X-ray absorption spectroscopy (XAS) measurements were used to determine the coordination structure and to derive the speciation of aqueous ferrous chloride complexes in acidic chloride brines over a wide range of conditions (25–450 °C, 500 bar, 0.5–12 m chloride molality), covering the range from sedimentary brines to magmatic hydrothermal fluids. EXAFS analysis coupled with ab initio free potential XANES calculations confirmed the octahedral geometry of the different Fe chlorocomplexes at low temperature (< 200 °C) and low (< 1 m) chloride concentration ([FeClx(H2O)6 − x]2 − x, x = 0–2), and attest the stability of a high-order tetrahedral Fe(II)-chloride complex at high-temperature (> 300 °C) and high (> 2 m) chloride molality ([FeCly]2 − y; y = 4 or y = 3; Fe–Cl distance = 2.31 ± 0.01 Å). These spectroscopic results contrast with the interpretation of most recent high-temperature studies of Fe(II) speciation in brines, which assumed that [FeCl2]0 is the predominant species in brines at high temperature. A reinterpretation of the experimental Fe solubilities measured by Fein et al. [Fein, J.B., Hemley, J.J., D'Angelo, W.M., Komninou, A., Sverjensky, D.A., 1992. Experimental study of iron-chloride complexing in hydrothermal fluids. Geochim. Cosmochim. Acta 56, 3179–3190.] for the magnetite–pyrite–pyrrhotite–quartz–muscovite–K-feldspar assemblage in KCl solutions at 300 °C/500 bar and 400 °C/500 bar shows that these solubility data can be explained using the high-order [FeCl4]2− complex. This study illustrates the complementarity between solubility and spectroscopic studies, and provides further evidence of the importance of high-order chlorocomplexes for the transport of transition metals (e.g., Zn, Ni) in high-temperature and/or supercritical fluids.