Polarized ATR-FTIR investigation of Fe reduction in the uley nontronites

Reduction of structural Fe in smectites affects the surface chemical behavior of the clay, but the underlying mechanism and changes in clay structure are still in need of investigation, particularly with respect to changes in the tetrahedral sheet. The purpose of this study was to probe changes in the tetrahedral sheet that occur when structural Fe is reduced in the Uley nontronites, NAu-1 and NAu-2, using polarized attenuated total internal reflection Fourier-transform infrared spectroscopy. Despite the differences in their structures NAu-2 has tetrahedral Fe³⁺ while NAu-1 does not the changes observed in the Si-O stretching region were quite similar. Reduction results in a shift of the in-plane Si-O stretching modes to lower frequencies, while the out-of-plane Si-O stretch shifts to higher frequencies. The magnitude of these shifts is greater in NAu-2 than in NAu-1, but the crystallinity of the tetrahedral silicate sheet of NAu-2 is preserved upon reduction. In both nontronites, the orientation of the out-of-plane Si-O bond changes and becomes completely perpendicular to the basal (001) surface of the clay, indicating the formation of trioctahedral domains wherein the individual tetrahedra reorient relative to the plane of the clay layer.