Characterization of Structural Iron in Smectites - An Ab Initio Based X-ray Absorption Spectroscopy Study

Area of Science:

• Geochemistry
• Mineralogy
• Materials Science

Background:

• Fe-bearing clay minerals in argillaceous rocks are crucial due to redox-active structural iron controlling sorption of sensitive elements.
• The iron oxidation state (Fe2+/Fe3+ ratio) and structural distribution within clay mineral TOT-layers dictate redox reaction efficiency.
• Isomorphic substitution patterns, like individual iron or Fe-Fe pairs, can vary even in smectites from different locations.

Purpose of the Study:

• To determine the proportion of different iron distributions in Milos-, Wyoming-, and Texas-montmorillonite.
• To investigate the influence of iron arrangement on the sorption of redox-sensitive elements.
• To combine experimental spectroscopy with theoretical calculations for detailed structural analysis.

Main Methods:

• Utilized X-ray absorption spectroscopy (XAS) to analyze iron in clay minerals.
• Employed ab initio calculations based on relaxed atomic structures of smectite models.
• Modeled different arrangements of individual Fe atoms and Fe-Fe/Fe-Mg clusters to simulate XAS spectra.

Main Results:

• Fe K-edge XAS spectra analysis indicated iron is present as Fe3+ in the octahedral sheet of all studied montmorillonites.
• Texas-montmorillonite showed a tendency for Fe3+ to form clusters.
• No definitive conclusions could be drawn regarding Fe-Fe and Fe-Mg pair clustering or avoidance in Milos- and Wyoming-montmorillonite.

Conclusions:

• The structural arrangement of iron, particularly clustering, varies among different montmorillonite samples.
• Understanding iron distribution is key to predicting the behavior of redox-sensitive elements in clay-rich environments.
• The combined XAS and ab initio approach provides a powerful tool for characterizing iron speciation and distribution in minerals.