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

Ten montmorillonites of different origins with variable Fe contents were analysed using transmission IR spectroscopy. Special attention was devoted to the OH-bending region to obtain information about the distribution of octahedral cations. For low to medium Fe contents (≤0.56 per Si 8 formula unit), a linear relationship between the position of the δ AlFeOH band and Fe content was observed. Such correlation might be explained by changes in the cis-trans occupancy of Fe in the octahedral sheet. Deconvolution of the OH-bending domain allows us to discriminate three components (δ AlAlOH , δ AlMgOH and δ AlFeOH ) which are correlated with cation abundances derived from chemical analysis. The relative area of each band can then be compared with theoretical areas calculated assuming a fully random distribution of cations in the octahedral sheet. Using such treatment, eight of the 10 montmorillonites studied presented a nearly randomized octahedral distribution. The two samples from Wyoming were clearly different as they exhibited a strong ordering tendency.