Surface and intercalate chemistry of layered silicates. Part II. An iron-57 Mössbauer study of the role of lattice-substituted iron in the benzidine blue reaction of montmorillonite

Direct 57Fe-Mössbauer-spectroscopic evidence is presented for the involvement of lattice-subsituted iron(III) as an electron-accepting site, within montmorillonite, following intercalation of the clay by benzidine which results in the production of benzidine radical cations. The subsequent oxidation of the resulting iron(II), by dehydration of the benzidine blue–montmorillonite intercalate, is explained in terms of two simple equilibria which are set up at the clay surfaces. New Lewis-acid sites [involving iron (III)], created by oxidation of the original (ironII) ions in the clay, have been found to be active even under conditions where it is expected that benzidine species are present only at external surfaces of the montmorillonite.