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

Abstract

The 57Fe M~Sssbauer 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 x l0 s to 9.2 x 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 X 2 value is used as a measure of the goodness of the fit, the spectra should be recorded to a background count greater than 3 x 106. The resulting x 2 value then reflects both the validity of the model used and the extent of disorder within the structure. The x 2 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 Fe 3 + and the interlayer cations give rise to two distinct electric field gradients within neighboring cis-(FeO4(OH)2) sites, and hence two octahedral Fe 3+ doublets in the MiSssbauer spectrum. These results are consistent with earlier electron diffraction data in the literature.