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A B S T R A C T:A method is given for quantitative analysis of IR spectra of dioctahedral micas in the OH-stretching region, which involves spectral decomposition and correlation of octahedral cation contents with integrated optical densities of the corresponding bands. It provides a basis for the study of order-disorder in these minerals and has allowed revision of the crystallOchemical formulae of some glauconites and celadonites.The IR spectra of dioctahedral micaceous minerals and, in particular, glauconites have been studied by a number of workers (Vlasova et al., 1976;Osherovitch & Nikitina, 1975;Pliusnina, 1982;Slonimskaya et al., 1978; Yukhnevitch, 1970;Farmer, 1974;Rouxhet, 1970; Saksena, 1964;Vedder, 1964;Vedder & Wilkins 1969). However, these have not established any reliable correlation between the nearest cationic environment of the OH groups and the OH stretching frequencies. The interpretation of Vedder (1969), which is still used by some workers (Osherovitch & Nikitina, 1975;Rouxhet, 1970), does not always correspond to the relationship between the effective charge and mass of ions on the one hand, and their vibration frequencies on the other. Farmer (1974), when interpreting IR spectra of glauconites, considered only the R2+R 3+ groups. Nevertheless, IR spectroscopy may prove extremely useful in structural studies of glauconites, with respect to composition and the distribution of the isomorphic cations.The present investigation was carried out with the intention of developing a method for the analysis of IR spectra of celadonite and glauconite in the region of OH stretching frequencies. It is known that the OH groups in the structure of dioctahedral mica are coordinated by two octahedral cations. Thus the individual OH stretching frequencies depend on the type of the cations nearest to the OH group. The integrated optical density of each band is determined by the number of absorption centres of the given type (i.e., of the OH groups with the given cationic environment) and by the absorption coefficient. Therefore if the experimental values of the integrated optical densities for each IR band can be determined, and also which of the cation arrangements the latter corresponds to, this would allow analysis of the composition and the distribution of the octahedral cations. It is evident that the basis for the interpretation of the OH stretching frequencies of celadonites and glauconites should be the study of IR spectra of samples with reliably known cationic compositions of their octahedral sheets. At the same time the majority of papers on the 9 1986 The Mineralogical Society

A B S T R A C T :The general approach to the problem of the real structure of smectites requires an analysis based on the relationship between structural characteristics and diffraction patterns. This paper, which considers only the models corresponding to dioctahedral smectites saturated by K-cations and collapsed, includes: 1. Successive consideration of all models which are crystallochemically possible. These models may differ in (i) the structure and chemical composition of layers and interlayer spaces; (ii) the azimuthal orientations, translations and the mode of alternation of the layers; (iii) independent parameters which describe quantitatively the models (e.g. abundance of each type of layer, probability parameters defining the succession of layers .... ). 2. Calculation, in all accessible domains of reciprocal space, of the distribution of intensities and profile variations, obtained by changing only one parameter at a time, that defines one type of structural feature (e.g. cation distribution in individual layers, stacking of the layers, nature of stacking faults, . . .). 3. A systematical analysis of the calculated diffraction patterns to establish the diffraction criteria which will help to interpret the experimental data explicitly.In dioctahedral smectites only two of the three possible octahedral sites in the half unit cell are occupied by cations. These smectites are classified into montmorillonites, beideUites and nontronites, according to the composition of tetrahedral and octahedral sheets and the magnitude of the negative charge on the layer. To describe the crystal structure of smectites requires knowledge of (i) the composition and structure of the layers and interlayer spaces and (ii) the orientation and succession of these structural elements. For natural samples determination of both (i) and (ii) is usually difficult due to the small size of the particles and poor structural ordering. Normally, diffraction patterns of smectites contain only two types of reflections: basal 00l reflections and practically unmodulated (hk) bands corresponding to a two-dimensional diffraction. Obviously such data are not enough for a detailed determination of crystallochemical and structural characteristics of smectites. Determination of the internal structure of the 2:1 layers can be aided by a better ordering of layer stacking. For example, Mamy & Gaultier (1976) pointed out that montmorillonite shows a better ordering of layer stacking after K-saturation and several wetting and drying cycles (WD cycles), the resulting X-ray patterns contain strong reflections which have been indexed in a monoclinic one-layer unit cell. Based on selected-area electron diffraction results these authors supposed that after WD treatment 9 1984 The Mineralogical Society

The aim of this work is the study of the order‐disorder distribution of the Fe3+ cations in a dioctahedral smectite: the Garfield nontronite. The structural characteristics have been determined by selected‐area electron diffraction, oblique texture electron diffraction and X‐ray diffraction. These methods lead to the conclusion that the octahedral vacancies are in trans positions and that part of the Fe3 + ions is occupying the tetrahedral sites. The Mössbauer spectra of the nontronite were decomposed into four doublets. According to the determined structural characteristics, these doublets are interpreted in terms of trans vacant octahedral positions.