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

Abstract

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. Mössbauer 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.

Mineralogical Characterization of Ni-Bearing Smectites from Niquelândia, Brazil

“…These features are characteristic of Fe-rich montmorillonites with Mg (and/or probably Ni) in the octahedral sites and indicate that Fe, Mg (and/or probably Ni) may occur in adjacent octahedral sites responsible for the heterogeneity of the samples. The IR spectra of Niquelândia dioctahedral smectite (ASL-I5, CSL-G8, AS-BNi2 À Figure 5) are very similar to the Femontmorillonite characterized in weathered ultrabasic nodules in Ö lberg, Germany (Kö ster et al, 1999;Petit et al, 2002) or in pyrite-bearing calcareous sediments in the Costa Rica margin (Gaudin et al, 2005).…”

Rock Fragments Control Size and Saturation of Organic Carbon Pools in Agricultural Topsoil

“…Th e cumulative loading weights of a PLSR prediction model display the spectral features used within the statistical prediction model (Haaland and Th omas, 1988). For the prediction of the Fe oxides signifi cant areas within the cumulative loading weights could be attributed to OH stretches of FeOOH (~3660 cm -1 , 1130 cm -1 , 890 cm -1 , 810 cm -1 ; Cornell and Schwertmann, 1996), Fe-O bending vibrations (~660-690 cm -1 , 910 cm -1 ; Cornell and Schwertmann, 1996;Koster et al, 1999), Al/Fe-OH vibrations (940 cm -1 , Madejova et al, 2002), and also to Al-OH bending vibrations at ~2300 cm -1 (Clark et al, 1990).…”

A comprehensive characterization of dioctahedral smectites

“…Pure tv smectites should have at least 1.0 Fe mol/f.u. Iron ions reduce the thermal stability of 2:1 clay minerals due to the reduced bond energy MgÀOH > AlÀOH > FeÀOH (Kö ster and Schwertmann, 1993). Five samples can be classified as tv but none is a pure tv montmorillonite.…”

“…According to Gü ven (1988), a miscibility gap exists between montmorillonite and nontronite, the existence of which has promoted some controversy in the literature: Meunier (2005) pointed out that the b-cell dimensions of beidellite and nontronite are too different to allow a continuous series. According to Köster et al (1999), no complete solid-solution is indicated between montmorillonite and beidellite. Recent studies, however, do favor a continuous series.…”

“…Montmorillonites are very abundant in nature whereas nontronites are rare. Bishop et al (2002), Brigatti (1983), Gaudin et al (2004aGaudin et al ( , 2004b, Kö ster et al (1999), Madejová et al (2000), and Mayayo et al (2000) investigated Fecontaining smectites ranging from montmorillonites to nontronites. The ternary system Al 2 , Mg 2 , and Fe 3+ 2 shows a continuous substitution of Al for Fe, or vice versa, for dioctahedral smectites as revealed by the octahedral cation distributions from the above results Figure 11.…”