Characterization of structural iron in smectites ― an ab initio based XAS study

Fe-bearing clay minerals are abundant in argillaceous rocks as redox-active structural iron may control the sorption mechanism of redox sensitive elements on the surface of clay minerals. The extent and efficiency of the redox reactions depend on the oxidation state (Fe2+/Fe3+ ratio) and structural distribution of the substituting cations in the TOT-layer of clay minerals. Even smectites with similar structure originating from different locations might have distinct arrangement of isomorphic substitutions (e.g. individual iron or Fe–Fe pairs). In this study, the proportion of different iron distribution in Milos–, Wyoming– and Texas–montmorillonite was determined by combining X-ray absorption spectroscopy (XAS) with ab initio calculations. The relaxed atomic structures of the smectite models with different arrangement of individual Fe atoms and Fe–Fe/Fe–Mg clusters served as the basis for the calculations of the XAS spectra. The combination of simulation results and measured Fe K-edge XAS spectra of Wyomi...