The influence of structural Fe, Al and Mg on the infrared OH bands in spectra of dioctahedral smectites

Visible to near-infrared (NIR) reflectance spectra and mid-IR transmittance spectra are presented here for a collection of dioctahedral smectites. Analysis of the structural OH vibrations is performed by comparing the NIR combination and overtone bands with fundamental stretching and bending absorption features in the mid-IR region. Second derivatives are used to determine the actual band centres, which are often shifted slightly by a spectral continuum in the reflectance or transmittance spectra. New bands have been identified near 4170 and 4000 cm –1 in the NIR spectra of nontronite with tetrahedral substitution. A related band is observed near 4100 cm –1 for montmorillonites with substantial tetrahedral and/or octahedral substitution. These bands are correlated with the mid-IR bands near 680 cm –1 for nontronite and near 630 cm –1 for montmorillonite. Comparison of the OH overtone and combination bands with the fundamental stretching and bending vibrations gives consistent results.