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

Abstract

Abstract--M6ssbauer spectra were obtained for five Ca-exchanged nontronites and one Ca-exchanged vermiculite as the 2-layer hydrates following dehydration at 200~ Exchange of the samples with Ca z § and subsequent dehydration resulted in the appearance of a shoulder at about -0.50 mm/s in the M6ssbauer spectra of some of the samples. The appearance of these shoulders necessitated the inclusion of doublets with M6ssbauer parameters corresponding to tetrahedrally-coordinated Fe 3+ (IVFe3+) in the model used to fit the Mfssbauer spectra. That the IVFe3+ sites detected in these samples were those in the nontronite structure was confirmed using samples whose lVFe3 § contents have been previously determined from chemical analysis. It appears that this sample preparation method allowed IVFe3 § contents to be determined to within 40%. On this basis, the IVFe3+ content appeared to be unrelated to the total Fe content for the samples studied.

57Fe Mössbauer Spectroscopic Study of Structural Changes during Dehydration of Nontronite: Effect of Different Exchangeable Cations

“…They are, therefore, probably due to Fe 3+ in the tetrahedral sheet. A similar proposal was made by Luca (1991) for the appearance of similar doublets in the M6ssbauer spectra of dehydrated Ca 2+-exchanged nontronite samples. The following main arguments were given for this assignment: (1) the appearance of the shoulder is dependent on the type of exchangeable cation, and cation-exchange capacity is a property ofnontronite; (2) ostensibly pure nontronite samples known to contain considerable tetrahedral Fe 3+ produce a similar shoulder on Ca2+-exchange and dehydration.…”

“…Less dramatic changes were also noted in the Mrssbauer parameters of the doublets assigned to WFe3+ sites on dehydration. The hypothesis that the WFe3+ doublet fitted to the Mrssbauer spectrum of the dehydrated Ca2+-nontronite is Fe 3+ in the tetrahedral sheet of the nontronite was supported by subsequent work (Luca, 1991).…”

Crystallochemical Study of a Population of Particles in Smectites from a Lateritic Weathering Profile

“…This result is consistent with IR data. All these data suggest that the studied sample contains no, or very small amounts of wVe3 § However, tetrahedral Fe is difficult to detect as is shown by the conflicting results obtained recently with the Garfield nontronite (Cardile, 1988;Sherman and Vergo, 1988;Luca, 1991). From Goodman (1978), Besson et al (1983b), and Daynyak and Drits (1987), the outer doublet may be attributed to octahedral Fe 3 § cations seeing the 3Si + A1 combination for tetrahedral occupation.…”

“…In most of the cases, improvement in the fit can be made by a Fe 3 § tetrahedral contribution . The WFe3+ Mrssbauer resonance lines can also be more easily evidenced when smectite samples are Ca-exchanged and dehydrated (Luca and Cardile, 1989;Luca, 1991). Figure 7 shows the M6ssbauer spectrum of the A MAG-O.025 sample, which was previously deferrated.…”

Reduction and Reoxidation of Nontronite: Questions of Reversibility

“…The dominant octahedral cation in nontronites and ferruginous smectite is Fe; and in some nontronites Fe occupies a fraction of the tetrahedral sites (Goodman et al 1976, Cardile 1987, 1989, Cardile and Johnston 1985, 1986, Luca 1991, Luca and Cardile 1989. It is the only major element in the smectite structure that potentially may exist in two relatively stable oxidation states, Fe(II) and Fe(III), and a change in its oxidation state in situ alters the physical and chemical properties of the clay (Stucki 1988, and references therein, Komade1 et al 1990, Stucki and Tessier 1991, Khaled and Stucki 1991, Gates et al 1993.…”