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
