Structural Studies of Selected Smectites

Abstract

The 57Fe Mössbauer spectra of montmorillonites, illte/ glauconites and nontronites all show distinctively different experimental absorption resonances. It is therefore possible to study the structural transition of the Fe-poor montomorillonite endmember through to the Fe-rich nontronite endmember of the 2:1 phyllosilicate series by the use of the Mössbauer spectroscopic method. The computer-fitting of the various Mössbauer spectra for the samples showed that the montmorillonites have a small tetrahedral Fe3+ component, but that most of the Fe is present as Fe2+ or Fe3+ in the octahedrally coordinated sites with the trans-arrangement of OH groups. The illite/ glauconite group of minerals were shown to have a larger Fe3+ tetrahedral component, while the bulk of the Fe was present as Fe2+ and Fe3+ in both the cis-OH and trans-OH octahedrally coordinated sites. The nontronites were shown to have tetrahedrally coordinated Fe3+ ranging from slightly more than that of the illite/ glauconite group to more substantial quantities, but only containing octahedrally coordinated Fe3+ in the cis-OH sites. Such structural concepts are at variance with the traditional model, but are in accordance with recent electron diffraction studies. It is considered likely that it is the tetrahedral Fe3+ component which directs the octahedrally coordinated Fe ion substitution within the structure. A low tetrahedral Fe3+ content directing Fe ions to the trans-OH sites, an intermediate tetrahedral Fe3+ content allowing occupation of both cis- and trans- OH sites. While a significant Fe3+ tetrahedral component excludes Fe2+ from the octahedral sites and directs the Fe3+ octahedral component to the cis-sites only.