Detection of Tetrahedral Fe³⁺ Sites in Nontronite and Vermiculite by Mössbauer Spectroscopy

Mössbauer spectra were obtained for five Ca-exchanged nontronites and one Ca-exchanged vermiculite as the 2-layer hydrates following dehydration at 200°C. Exchange of the samples with Ca 2+ and subsequent dehydration resulted in the appearance of a shoulder at about −0.50 mm/s in the Mössbauer spectra of some of the samples. The appearance of these shoulders necessitated the inclusion of doublets with Mössbauer parameters corresponding to tetrahedrally-coordinated Fe 3+ ( IV Fe 3+ ) in the model used to fit the Mössbauer spectra. That the IV Fe 3+ sites detected in these samples were those in the nontronite structure was confirmed using samples whose IV Fe 3+ contents have been previously determined from chemical analysis. It appears that this sample preparation method allowed IV Fe 3+ contents to be determined to within 40%. On this basis, the IV Fe 3+ content appeared to be unrelated to the total Fe content for the samples studied.