Infrared spectroscopy and the chemistry of dioctahedral smectites

Analysis of the infrared spectra of a suite of smectites with a wide range of Fe 3+ contents (and thus a wide range of Al 3+ and Mg 2+ ), confirms that a set of rules, developed specifically for the OH stretching bands of micaceous minerals (Besson and Drits 1997a, 1997b), applies generally to the assignment of OH - sharing octahedral cations to bending, stretching and combination bands in the mid and near infrared spectra of dioctahedral smectites. More generally, the position of various bands depends on how isomorphic substitution influences the following factors: (a) The identity (mass and valence sum) of the OH - sharing octahedral cation pair responsible for the absorption, (b) The bond strength of the O ap to which the OH group H-bonds, (c) The bond strength of the OH group, (d) The amount of proton - octahedral cation repulsion, and (e) The relative angle of the OH vector in relation to the octahedral sheet. The primary effect of isomorphic substitution is to change the reduced mass and/or the valence sum of the OH - sharing cation pair. The secondary effects of isomorphic substitution are on the bond character of the O ap , and thus how it influences the bond strength and orientation of the OH by affecting H-bonding as well as the amount of proton - cation repulsion that occurs. Decomposition of the NIR region associated with OH - sharing octahedral cation pairs allowed for more definitive assignments of combination bands. Detailed analysis of the OH bending region in a suite of smectites with differing Fe 3+ contents allowed a band near 840-845 cm -1 in nontronites to be positively assigned to a secondary Fe 3+ 2 -OH band in sites adjacent to tetrahedral Fe 3+ substitution. While analysis of the OH bending region in smectites may provide accurate determination of spectral decompositions, probable missassignments of bands, especially in the region between 740 to 800 cm -1 , should be noted. It is therefore, necessary to check the entire IR spectra to judge whether diagnostic bands for mineral impurities may impact the results. The IR spectra of smectites can be used to obtain r efinements on the numbers of octahedral Fe 3+ , Al 3+ or Mg 2+ atoms within the structure of dioctahedral smectites. Further, it is possible to discern the effects of tetrahedral charge in the Al 3+ 2 -OH bending bands of montmorillonites, Fe 3+ substitutions for Si 4+ in the tetrahedral sheets of nontronites and Mg 2+ for Al 3+ substitutions in ferruginous smectites. While DRIFT spectra generally provide excellent spectral qua lities, analysis of the OH stretching region is problematic unless precautions are made to remove the excessive IR absorption by sorbed water. Therefore, unless spectral decompositions in this region are performed on IR spectra of desiccated samples, extreme care should be taken when interpreting this region.