Strontium complexation in aqueous solutions and silicate glasses: Insights from high energy-resolution fluorescence detection X-ray spectroscopy and ab-initio modeling

Abstract

Although fluid–melt partitioning of trace elements like Sr, Ba, La, and Y is known to be strongly influenced by the fluid and melt chemical composition, their speciation in silicate-saturated fluids is studied insufficiently at high temperatures and pressures. Here, high energy-resolution fluorescence detection–X-ray absorption spectroscopy (HERFD–XAS) has been applied to investigate the local environment of strontium in crystalline model compounds, silicate glasses, and aqueous solutions. Acquisition of Sr K-edge HERFD–XAS spectra of aqueous solutions of SrCl2 and Sr(OH)2, and three aqueous fluids with dissolved silicate components was done in situ at temperatures to 780°C and pressures to ∼800MPa using hydrothermal diamond-anvil cells.Experiments were complemented by theoretical spectroscopy calculations using the finite difference method near edge structure (FDMNES) code. This approach was validated for a number of crystalline model compounds. For the silicate glasses and aqueous solutions (SrCl2 and Sr(OH)2), small clusters were examined. Either symmetric or distorted SrO6 clusters were found to describe Sr complexation in peraluminous or peralkaline glasses. However, small ‘static’ clusters seem not to be fully suited to account for the dynamically changing atomic arrangements in aqueous solutions at high temperature. Therefore, ab-initio molecular dynamics simulations were performed and used as input for modeling of X-ray absorption spectra. Analyses of these simulations indicated [SrCl(H2O)6]+ and Sr(OH)2(H2O)4 as the most likely complexes in the chloride and hydroxide solutions, respectively.Analysis of the spectra of the silicate-rich fluids shows that both melt and fluid composition strongly influence Sr complexation. For the silicate-rich fluids, formation of Sr–Cl complexes occurs at low (Na+K)/Cl and (Si+Al)/(Na+K) ratios in the fluid, whereas Sr hydroxide and possibly silicate complexes (similar to those in the silicate glass) are favored at higher ratios. Our X-ray spectroscopic results offer an explanation for the dependence of fluid–melt partitioning of Sr on melt composition measured in previous ex situ studies, and highlight the importance of components other than chloride (silicate and aluminosilicate) in controlling metal speciation in fluid–melt systems at high temperatures and pressures.