Mineralogical and chemical characteristics of five nontronites and Fe-richsmectites

Five Fe-bearing dioctahedral smectites (three nontronites and two Fe-rich s mectites) 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 wit h three doublets, two of which had quadrupole splittings characteristic of Fe3+ in octahedral coordination, whereas the third had a distinctly lower q uadrupole splitting. Although the position of the Si-O stretching band in t he infrared spectra could reflect tetrahedral Fe3+ the lack of distinctive features Fe3+. The O-18/O-16 data suggest that prevented a definitive attri bution of this component to tetrahedral Fe fractionation of nontronite-wate r at ambient temperatures (1000 ln alpha = 23 +/- 2 parts per thousand) is lower than that of Fe-rich smectite (1000 ln alpha = 27 +/- 2 parts per tho usand). The estimated formation temperatures of the samples are below 70 de grees C.