Abstract--The 57Fe M6ssbauer spectra of several montmorillonites, measured at room temperature and 453 K, showed a considerably broadened Fe 3+ resonance which can be computer-fitted with a similarly broadened Fe 3+ doublet. In some spectra, particularly if all linewidths were constrained to be equal, this broadened Fe 3 § resonance was further resolved into overlapping inner and outer Fe 3+ doublets, also having broad linewidths. In accordance with recent electron diffraction evidence, the assignment by previous workers of the inner doublet to Fe 3+ in the octahedral sites having the cis-arrangement of OH groups and the outer doublet to the octahedral site having the trans-arrangement of OH groups is incorrect. Instead, the Fe 3+ was found to be located largely in the trans-octahedral sites. Because of the relatively low iron content of the montmorillonite examined, the next and more distant neighboring-cation environment varied considerably about the octahedral Fe 3+ ions. This variation produced a broadened experimental resonance, and the resulting two-doublet computer fits probably represent the mean extremes of a continuum of slightly different Fe 3+ resonances arising from the variable nature of the environment surrounding these such trans-sites, rather than distinct cis-and trans-sites. In addition, a small resonance indicating the substitution of Fe 3 + into the tetrahedral sites was observed. The interlayer species probably influenced the Mrssbauer resonance of Fe 3+ in the tetrahedral and octahedral sites.

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.

Abstract--The 57Fe Mrssbauer spectra of a series of untreated and Ca-saturated nontronites showed a predominant Fe a+ resonance which was computer-fitted with two Fe 3 § doublets defining iron in nonequivalent cis-FeO4(OH)2 octahedral sites. In most spectra a doublet indicating tetrahedral Fe 3. was fitted and in one untreated sample a doublet indicating interlayer Fe 3+ was identified. In a further untreated sample the interlayer iron was present as Fe 2+. Upon Ca-saturation the interlayer iron was displaced. It also appears that the interlayer iron was present in at least two different interlayer sites. From the computerfitted data it was clear that the interlayer cations have a significant effect on the MOssbauer resonances of iron in the two non-equivalent cis-octahedral and the tetrahedral sites of nontronite.

Abstract--The ~TFe M6ssbauer spectra of an iron-rich montmorillonite, an illite, and two glauconites were measured and computer-fitted with appropriate Fe 3+ and Fe 2+ doublet resonances. The broad experimental Fe 3 § resonance of montmorillonite probably arises from Fe 3 § in the octahedral sites and a trans-arrangement of OH groups; however, a large variation in the neighboring environment of these sites exists. In illite this Fe 3 § resonance is similar but shows less broadening; it arises from Fe 3+ located predominantly in trans-OH octahedral sites, with some Fe 3 § being located in cis-OH octahedral sites. Because of the increased iron content less variation exists, compared with montmorillonite, in the neighboring octahedral sites. The Fe 3 § resonance is narrower still for the glauconites and represents Fe 3 § substituting primarily into cis-OH octahedral sites, similar to that previously reported for nontronite.The tetrahedral Fe 3 § content is very low for montmorillonite and increases progressively for illite and glauconite, suggesting that a higher tetrahedral Fe 3 § content directs Fe 3 § in the octahedral layer into cis-OH sites. In montmorillonite, the Fe 2 § is located only in trans-OH sites; in illite Fe 2 § is largely in trans-OH sites and only slightly in cis-OH sites; and in glauconite, Fe 2 § is located largely in cis-OH sites and only slightly in trans-OH sites. These assignments suggest that for FC-, the doublet with the larger quadrupole interaction arises from Fe 2 § in trans-OH sites and the doublet with the smaller quadrupole interaction, from Fe 2+ in cis-OH sites.

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