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.

An Fe-rich smectite from Ölberg (Germany) was characterized using X-ray diffraction, infrared spectroscopy and analytical electron microscopy. Progressive reduction of the octahedral charge was performed through the Hofmann & Klemen effect at increasing temperatures. The sample was heterogeneous, consisting of two smectite populations. One population, which comprises a minor portion of the sample, is an ‘Fe3+-montmorillonite’ with little or no tetrahedral charge and Fe3+ as the major octahedral cation. The other population, a major constituent of the sample, contains less Mg and more Al than the first one and exhibits some tetrahedral charge. This fraction may be considered as an inter-grade between nontronite (dominant tetrahedral charge) and Fe3+-montmorillonite (dominant octahedral charge). These two populations may occur as separate particles but also as interstratified layers.

ZusammenfassungDie kristallchemischen Strukturformeln von 2:1-Schichtsilikaten werden unter Voraussetzung der idealen Struktur mit 22 negativen Ladungen von 10 Sauerstoffen und 2 Hydroxylgruppen berechnet. Für die Verteilung der 22 Kationenladungen auf Tetraeder-, Oktaeder- und Zwischenschichtpositionen werden unter Berücksichtigung der direkt gemessenen Zwischenschichtladungen (Lagaly & Weiss 1971) bzw. Kationenumtauschkapazitäten Voraussetzungen und Berechnungsregeln angegeben. Als Beispiele werden die Formeln eines Glaukonits und eines Nontronits berechnet.Die Verteilung der Kationenladungen in der Kristallstruktur von 2:1-Schichtsilikaten wird am besten in einem Dreiecksdiagramm mit den Ecken Pyrophyllit-Seladonit-Muskowit dargestellt. Jeder Punkt im Dreieck stellt die gesamte Kationenladung in der Formeleinheit gleich 22 dar. Es ist die Summe der Ladungen aus Tetraeder-, Oktaeder- und Zwischenschichtkationen.

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Mineralogical and chemical heterogeneity within three standard clay mineral samples have been identified by X-ray diffraction and chemical analysis of various size-fractions. This heterogeneity is partly attributed to accessory minerals, but mostly to structural and compositional variations in the 2:1 layer minerals of different particle size in the same specimen. MATERIALS AND METHODSSamples of illite-smectite from the Mancos shale, Montana (CMS special clay ISMt-I), illite from Silver Hill, Montana (CMS source clay IMt-1), and K-bentonite from Strasbourg, Virginia (standard mineral no. 37, API research project 49) were prepared by dissolving carbonates with EDTA solutions and dissolving iron oxides by reduction with sodium dithionite. The adsorbed cations on the clay surfaces were exchanged against Na § ions during the processing of the clays. Then the samples were dispersed in distilled water and fractionated by sieving, settling and centrifugation (KOster, 1982).The bulk clays and the fine size-fractions <0.2, 0.2-0.6, and 0.6-2 I~m d~ were analysed chemically (KOster, 1979), the limits of precision (three times the standard deviation) for SIO2, A1203, Fe203, MgO and K20 being 1.5%, 0.9%, 0.1%, 0.1% and 0.2%, respectively. Quantitative data on trace element concentration should exceed at least ten times the standard deviation (SB) which is valid for the detection value of the analysed trace element; the values 10 x SB for the analysed trace elements (Tables 2-4) RESULTSThere is only a relatively small percentage of the size-fraction <0.2 ~tm ff particularly for the illitesmectite sample ISMt-1. The particle size distribution of the clay samples proves that by using the method described for processing, hardly any primary particles of these clays were split (Table 1), because the maxima of particle sizes in the size-fractions 0.2-0.6 and 0.6-2 lam ~b, respectively, are identical with the dominant particle sizes of the clay mineral samples as found by electron microscopy (Vali & KOster, 1986).The size fractions 20-63 and 63-200 l~m ~b of the clay samples consist almost completely of quartz. All size-fractions <20 Ixm ~b were analysed by X-ray diffraction (XRD). The size-fraction 2-20 Ixm d~ consists of various amounts of mica minerals and quartz, whereas the clay minerals are concentrated in the size-fractions <6.3 ~tm dp, particularly <2 Itm ~b.