Theoretical and experimental study of iron substitution in montmorillonite

Abstract

Clays are among the most abundant and widely distributed mineral groups in the Earth’s crust. Within this group, smectite, particularly montmorillonite (MMT), are characterized by a 2:1 phyllosilicate structure and a high cation exchange capacity, which make them suitable for numerous industrial and environmental applications. Isomorphic substitutions in the MMT crystal lattice, especially those involving iron (Fe), strongly influence its structural and electronic properties. In this work, a combined theoretical and experimental study was carried out to investigate substitutional Fe in the octahedral and tetrahedral sites of sodium montmorillonite (Na-MMT), considering different Fe concentrations. Using ab initio calculations based on Density Functional Theory (DFT), structural parameters such as basal spacing and formation energies were determined, together with hyperfine parameters associated with the various Fe environments. The Mössbauer spectra of natural MMT samples from Argentina and Turkey were fitted using the hyperfine parameters obtained from the calculations. In addition, the electronic density of states was analyzed to understand how Fe affects the electronic properties of MMT and how these results are modified by the inclusion of the Hubbard correction. The results show that Fe incorporation into tetrahedral sites is energetically unfavorable, whereas substitution at octahedral Mg sites is preferred. The calculated basal spacings for Fe substituted octahedral configurations show good agreement with experimental data. Theoretical hyperfine parameters enabled a new fitting of the Mössbauer spectra without the need to include a paramagnetic relaxation site, confirming the coexistence of different Fe environments in the samples.