The frequencies of Si-O stretching, νSi-O, in layers of montmorillonite and of H-O-H bending, ν2, in the interlayer water were investigated by infrared spectroscopy. Oriented gels of Li-and Na-montmorillonite equilibrated at different water contents, mw/mc, were obtained by using a miniature pressure membrane apparatus. One part of each gel was used for the gravimetric determination of mw/mc. The other part was transferred to an FTIR spectrometer where the spectrum of the gel was measured by attenuated total reflectance. Thus, the dependence of both νSi-O and ν2 on mw/mc was determined in the same samples. The absorption bands for Si-O stretching were resolved by using a curve-fitting technique to determine the frequency, area, and height of the component peaks. Two prominent peaks, labeled peak III and peak IV, were selected for analysis. It was found that ν Si-O for each peak and the ratio of the areas (or heights) of the two peaks were all exponential functions of mc/mw. Moreover, a linear relation was found between ln(νSi-O) and ln(ν2). These results were interpreted to mean that the Si-O stretching vibrations in the montmorillonite layers and the H-O-H bending vibrations in the interlayer water are coupled and that both change as the layers come closer together.

The reduction of structural Fe affects many surface chemical properties of smectite clay minerals, including their hydration and interfacial forces. To expand our knowledge of these processes, Fourier transform infrared (FTIR) spectroscopy was used to investigate the effects of structural Fe oxidation state and hydration on layer Si−O stretching vibrations in montmorillonite. Aqueous gels of unaltered, reduced, and reoxidized smectites were equilibrated at different water contents, m w/m c, and swelling pressures, Π, using a miniature pressure membrane apparatus. One part of each gel was used for the gravimetric determination of m w/m c. The other part was transferred to an attenuated total reflectance cell in the FTIR spectrometer where the spectrum of the gel was measured. The frequencies of four component peaks of Si−O stretching, νSi - O, in montmorillonite layers and of the H−O−H bending, ν2, in the interlayer water were determined by using a curve-fitting technique. Reduction of structural Fe shifted the out-of-plane Si−O vibration to higher frequency and the in-plane Si−O vibration to lower frequency. A linear relation was found between νSi - O and ν2 for each oxidation state of the montmorillonite. These observations were interpreted to mean that the structural Fe oxidation state is a primary factor in the control of interfacial interactions and processes of the aqueous clay colloid system.

Oxidation-reduction processes within natural systems greatly influence the properties of sediments, soils and clays. The objective of this experimental study was to gather new evidence for the effects of changes in redox conditions (reduction and reoxidation) on structural properties of ferruginous smectite and to understand better the mechanisms involved. The <2 µm fraction of a ferruginous smectite (sample SWa-1), which contains 17.3 wt.% of total structural Fe, was studied by infrared (IR) spectroscopy. The pure Na-saturated clay was reduced by Na dithionite for 10 to 240 min to obtain various Fe(II):(total Fe) ratios ranging from 0 to 1.0. Selected reduced samples were then reoxidized completely by bubbling O2 gas through the suspensions for up to 12 h. Infrared spectra of the initially unaltered, reduced and reduced-reoxidized samples were collected. Reduction generated changes in the three studied spectral regions (O-H stretching, M-O-H deformation, and Si-O stretching), indicating that major modifications occurred within the clay crystal beyond merely a change in Fe oxidation state. partial dehydroxylation and redistribution of Fe, and perhaps Al, cations occurred upon reduction of SWa-1, changing the structural properties of its tetrahedral and octahedral sheets. Water molecules, probably generated by dehydroxylation within the octahedral sheet upon reduction, were tightly bound to the clay surface and were possibly trapped within the clay structure. Except for dehydroxylation and the Fe oxidation state, all these modifications were largely irreversible. The tightly bound water was not completely removed upon reoxidation and the cationic rearrangements generated during reduction were not reversed: either they were preserved as in the reduced state or cations were redistributed into a different configuration from the unreduced clay.