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Chemical analyses were carried out on individual particles in <2-μm size fractions to study smectite-palygorskite and smectite-sepiolite assemblages extracted from calcareous and dolomitic continental formations in Provence, southeastern France. A plot of the octahedral composition of 145 individual particles of each clay mineral species considered on a (Al + Fe3+) vs. Mg diagram shows a great variability for smectites and fibrous clays. The octahedral composition fields of the smectites and fibrous clays partly overlap. The sepiolite field is clearly in the trioctahedral domain, whereas the paly-gorskite field is both in the dioctahedral domain as well as between the dioctahedral and trioctahedral domains of smectites. Similar to smectites, the structure of palygorskite appears to be accommodating for octahedral substitutions; however, this characteristic cannot be inferred from chemical analyses of whole <2-μm fractions, which represent the average composition of several particles.

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