Reduction and Oxidation of Fe3+ in Dioctahedral Smectites — 2: Reduction with Sodium Sulphide Solutions

Abstract Reaction with Na 2 S solutions at high pH led to almost complete, reversible reduction of iron in montmorillonite, whereas the structural iron of nontronite persisted in the ferric form. Concentrated Na 2 S solutions caused severe corrosion of nontronite and extracted appreciable amounts of iron, which was precipitated as sulphides. In contrast the morphology of montmorillonite was preserved and only very minor amounts of iron were extracted. These differences were attributed to the high concentration of Fe-OH-Fe groups in nontronite, which are unstable on reduction. The difference between Na 2 S and other reducing agents is discussed.