Mineralogical and chemical characteristics of five nontronites and Fe-rich smectites

Five Fe-bearing dioctahedral smectites (three nontronites and two Fe-rich smectites) were purified using a variety of physical and chemical procedures. The structural formulae indicate one nontronite and one Fe-rich smectite to be montmorillonitic, whereas the other three smectites are beidellitic. Mossbauer spectra showed Fe to be exclusively trivalent and were fitted with three doublets, two of which had quadrupole splittings characteristic of Fe³⁺ in octahedral coordination, whereas the third had a distinctly lower quadrupole splitting. Although the position of the Si-O stretching band in the infrared spectra could reflect tetrahedral Fe³⁺, the lack of distinctive features prevented a definitive attribution of this component to tetrahedral Fe³⁺. The ¹⁸O/¹⁶O data suggest that fractionation of nontronite-water at ambient temperatures (1000 lnα = 23 ± 2‰) is lower than that of Fe-rich smectite (1000 lnα = 27 ± 2‰). The estimated formation temperatures of the samples are below 70°C.