Redox reactions involving structural Fe in smectite clay minerals contribute significantly to the chemistry of soils and sediments. Changes in oxidation state from Fe (III) to Fe (II) alters the clay structure, which in turn modifies the chemical nature of the smectite basal surfaces. These changes in structure are readily observed by infrared spectroscopy. Structural OH groups are progressively lost as the level of Fe (II) increases, and large shifts occur in the Si-0 stretching and bending modes of the tetrahedral sheet. The communication between octahedral and tetrahedral sheets alters the configuration of basal oxygens, as well as changing the charge, pH, and reduction potential of the surface. Infrared spectra also provide evidence for the formation of trioctahedral domains as a result of extensive Fe (III) reduction.