Speciation and thermodynamic properties of manganese(II) chloride complexes in hydrothermal fluids: In situ XAS study

Abstract

The speciation of Mn(II) in acidic brines under a wide range of conditions (30–550°C, 600bar, 0.100–10.344m chloride and 0.110–2.125m bromide) was investigated using in situ X-ray Absorption Spectroscopy (XAS). Increasing temperature and/or salinity results in a structural change of the Mn(II) complexes from octahedral to (distorted) tetrahedral. Octahedral species predominate at room temperature within the whole salinity range and persist up to ∼400°C in low salinity solutions (mCl<1m), and tetrahedral species become significant above 300°C. A combination of EXAFS refinements, Density Functional Theory calculations and ab initio XANES simulations shows that at temperatures ⩾400°C, the highest order chlorocomplex predominating in high salinity solutions (mCl>3m, Cl:Mn ratio>53) is MnCl3(H2O)−, and that a lower order chlorocomplex, MnCl2(H2O)2(aq), is the predominant species in low salinity solutions (mCl<0.5m, Cl:Mn ratio<10). A similar result was also found in Mn bromide solutions: MnBr3(H2O)− and MnBr2(H2O)2(aq) are the dominant species at 500°C in high salinity solutions (e.g., 2.125m, Br:Mn ratio=33.73) and in low salinity solutions (e.g., 0.110m, Br:Mn ratio=2.04), respectively. XANES spectra of Mn(II) chloride solutions were used to retrieve formation constants of MnCl2(H2O)2(aq) and MnCl3(H2O)− at 600bar. The speciation and thermodynamic model of this study are consistent with previous solubility and UV–Vis spectroscopic studies.