The process of smectite to illite transformation (smectite illitization) in low-temperature environments is known to produce a range of interstratifications with varying extent of ordering (Reichweite). Reichweite values of R = 2 are rarely described in contrast to R values of 0, 1, or 3. In the present study a clay dominated by an R2 interstratification composed of illitic and expandable layers from the Tokaj Mountain hydrothermal area (Hungary) is presented. The interstratification was highly enriched in the <0.1 μm fraction with minor impurities of illite and kaolinite. The sample was characterized by a multimethod approach using X-ray diffraction (XRD) and Rietveld refinement, X-ray fluorescence, Fe2+/3+ determination, simultaneous thermal analyses, cation exchange capacity determination of both untreated and Li-saturated specimens, and transmission electron microscopy. The parameters refined by the Rietveld method were comparable to measured or calculated data. The expandable layers were characterized as dioctahedral with an average layer charge per formula unit of 0.63 mol(+)/mol exceeding slightly the range defined for smectite. Hence, this interstratification should be denoted as R2 illite–dioctahedral vermiculite. However, an R2 illite–smectite Rietveld structural model fitted the measured XRD pattern of the R2 illite–dioctahedral vermiculite in different states very well and can be used for quantification purposes in natural or artificial mixtures.

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