Structural Fe(III) reduction in smectites

Manipulation of the oxidation state of structural Fe in clay minerals is a potential method for altering important physical–chemical properties of the clay and several studies have focussed on this phenomenon. This paper summarizes current knowledge on reduction of structural Fe(III) in iron-rich dioctahedral smectites and partial stabilization of Fe(II) in reduced SWa-1 ferruginous smectite via fixation of Li⁺ cations upon heating. Fe(III) in Fe-rich dioctahedral smectites was completely reduced in citrate–bicarbonate buffer using sodium dithionite. Progress in the reduction or reoxidation process was followed by monitoring the Fe(II)–O–Fe(III) intervalence electron transfer transition using visible spectroscopy at 730 nm. Reduction proceeds from basal surfaces rather than from particle edges. One study found that trioctahedral domains and vacancies may occur within the structure of reduced minerals, but another study indicated less radical structural changes. Fully reoxidized minerals contain less OH groups. About 20% of total Fe can be stabilized as Fe(II) in reduced SWa-1 via Li⁺-saturation and heating the Li-form of a highly reduced mineral in N₂ atmosphere at 260 °C for 24 h. Part of the Li⁺ is trapped in previously vacant octahedral sites, forming trioctahedral AlFe(II)LiOH or Fe(III)Fe(II)LiOH groupings.