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

Abstract

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.

Structural Studies of Nontronites with Different Iron Contents by 57Fe Mössbauer Spectroscopy

“…The spectra for untreated and Ca-saturated SPO nontronite ( Figure 1) were similar to those presented earlier for nontronite from Garfield, Washington (GAR) (Johnston and Cardile, 1985a). The x 2 value of 3567 for the 8-peak fit defining two octahedral, one tetrahedral, and one interlayer Fe 3+ doublets was above the upper confidence level and was indicative of disorder in the structure of this SPO nontronite although, as discussed below, some partial magnetic relaxation was also present.…”

“…In general, the computer-fitted data obtained from the spectra of all the clay samples showed two prominent Fe 3+ doublets with ~ and A values of approximately 0.38, 0.25 (inner doublet) and 0.38, 0.6 (outer doublet) mm/sec, respectively, assigned to F 3+ in the two distinct cis-FeO4(OH)2 octahedral sites (Table 2 and Figures 1, 2, 3, and 5) (Johnston and Cardile, 1985a). The precise ~ and h values, however, and the widths and areas of these respective doublets differed between samples.…”

“…The 1% and 99% confidence limits for the x 2 value which is used as a measure of the goodness of a computer-fit are 570 and 424 for a 6-peak fit and 566 and 420 for an 8-peak fit. In order for this x 2 value to be used as an indicator of both the validity of the model used and the extent of disorder within the structure, all spectra were recorded to a suitably high background count of 7-8 x 106 (Johnston and Cardile, 1985a). The experimental and computer-fitted spectra are presented in Figures 1, 2, 3, and 5; the computer-fitted parameters are given in Table 2.…”

“…Inasmuch as the electron diffraction work of Mering and Oberlin (1967), now confirmed by Besson et al (1983a), showed the transsites to be vacant, Goodman (1978) suggested that the presence of Fe 3+ in the tetrahedral sites could generate two distinct environments for the cis-octahedral sites, consistent with the computer-fitted M6ssbauer spectrum. In addition, Johnston and Cardile (1985a) showed that for untreated, Ca-and K-saturated nontronite from Garfield, Washington (GAR), the parameter values for these two computer-fitted octahedral Fe 3 § doublets displayed a marked dependence on the nature of the interlayer cation. They suggested that the electrostatic interactions from both the interlayer cations and nearby tetrahedral Fe 3+ ions could give rise to these two distinct electric-field gradients within neighboring cisFeO4(OH)2 sites.…”

Synthesis of 1:1 And 2:1 Iron Phyllosilicates and Characterization of Their Iron State by Mössbauer Spectroscopy

“…The lattice constants are 1.0-1.8% smaller than those oforthorhombic greenalite, a = 5.598 /k, b = 9.696 ,~, c = 7.213 ,~. This difference may be attributed to the fact that some of the iron of product (1985); Johnston and Cardile (1985); Cardile et al (1986); Luca and Cardile (1989). The Mrssbauer spectra of product 1 indicates that 60% of the iron exists in the ferrous state.…”