This is an unofficial adaptation of an article that appeared in an ACS publication. ACS has not endorsed the content 1 of this adaptation or the context of its use. 2 This document is the accepted manuscript version of the following article: 3 Kéri, A.; Dähn, R.; Krack, M.; Churakov, S. V. Characterization of structural iron in smectites -an ab initio 4 based XAS study. Abstract 18 Fe-bearing clay minerals are abundant in argillaceous rocks as redox-active structural iron may 19 control the

more »

... tion mechanism of redox sensitive elements on the surface of clay minerals. The 20 extent and efficiency of the redox reactions depend on the oxidation state (Fe 2+ /Fe 3+ ratio) and 21 structural distribution of the substituting cations in the TOT-layer of clay minerals. Even 22 smectites with similar structure originating from different locations might have distinct 23 arrangement of isomorphic substitutions (e.g. individual iron or Fe-Fe pairs). In this study, the 24 proportion of different iron distribution in Milos-, Wyoming-and Texas-montmorillonite was 25 determined by combining X-ray absorption spectroscopy (XAS) with ab initio calculations. The 26 relaxed atomic structures of the smectite models with different arrangement of individual Fe 27 atoms and Fe-Fe/Fe-Mg clusters served as the basis for the calculations of the XAS spectra. 28 The combination of simulation results and measured Fe K-edge XAS spectra of 29 Milos-and Texas-montmorillonites suggested that iron is present as Fe 3+ in the octahedral 30 sheet. Fe 3+ in Texas-montmorillonite has a tendency to form clusters, while no definitive 31 statement about clustering or avoidance of Fe-Fe and Fe-Mg pairs can be made for Milos-and 32 Wyoming-montmorillonite. 33