Iron Sites in Nontronite and the Effect of Interlayer Cations from Mössbauer Spectra

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Abstract
The 57Fe Mössbauer spectra of untreated, Ca- and K-saturated nontronite from Garfield, Washington, were measured. The spectrum of the untreated sample was computer-fitted to 8 peaks defining two octahedral, a tetrahedral, and an interlayer Fe3+-quadrupole-split doublets. In the Ca- and K-saturated samples interlayer Fe was absent. Spectra of the untreated sample were recorded at increasing increments of background counts from 2.8 × 105 to 9.2 × 106. An evaluation of the initial 4- and 6-peak models and the acceptable 8-peak model, computer-fitted to each spectrum, shows that if the χ2 value is used as a measure of the goodness of the fit, the spectra should be recorded to a background count greater than 3 × 106. The resulting χ2 value then reflects both the validity of the model used and the extent of disorder within the structure. The χ2 value depends linearly on the background counts obtained.A comparison of the spectra of the Ca- and K-saturated samples with that of the untreated sample shows that the interlayer cations exert a considerable influence on the individual component resonances, particularly the outer octahedral doublet. Hence, it is likely that electrostatic interactions of the nearby tetrahedral Fe3+ and the interlayer cations give rise to two distinct electric field gradients within neighboring cis-[FeO4(OH)2] sites, and hence two octahedral Fe3+ doublets in the Mössbauer spectrum. These results are consistent with earlier electron diffraction data in the literature.