Structural studies of montmorillonites by 57Fe Mössbauer spectroscopy

Five montmorillonites were analysed by 57Fe Mössbauer spectroscopy and their Fe3+ occupation in tetrahedral and octahedral sites assigned. The montmorillonites, with a range of IVFe3+ contents, were used in an attempt to correlate IVFe3+ content with VIFe3+cis- and trans-OH site occupation to verify the assertion that increasing IVFe3+ content directs VIFe3+ to cis-OH sites. However, VIFe3+ in montmorillonite appears to be located essentially in the trans-OH sites. The two doublets computer-fitted to the VIFe3+ Mössbauer resonance for montmorillonite appear to represent a distribution of a continuum of slightly different Fe3+ resonances, which arise from the variable atomic environment surrounding the VIFe3+ sites, rather than discrete cis- and trans-OH sites. Therefore Mössbauer spectroscopy cannot be used to demonstrate whether an increasing IVFe3+ content directs VIFe3+ substitution into particular sites for montmorillonites.