Fe, Mg and Al distribution in the octahedral sheet of montmorillonites. Aninfrared study in the OH-bending region

Ten montmorillonites of different origins with variable Fe contents were an alysed using transmission IR spectroscopy. Special attention was devoted to the OH-bending region to obtain information about the distribution of octa hedral cations. For low to medium Fe contents (less than or equal to0.56 pe r Si-8 formula unit), a linear relationship between the position of the del ta (AlFeOH) band and Fe content was observed. Such correlation might be exp lained by changes in the cis-trans occupancy of Fe in the octahedral sheet. Deconvolution of the OH-bending domain allows us to discriminate three Com ponents (delta (AlAlOH), delta (AlMgOH) and delta (AlFeOH)) which are corre lated with cation abundances derived from chemical analysis. The relative a rea of each band can then be compared with theoretical areas calculated ass uming a fully random distribution of cations in the octahedral sheet. Using such treatment, eight of the 10 montmorillonites studied presented a nearl y randomized octahedral distribution. The two samples from Wyoming were cle arly different as they exhibited a strong ordering tendency.