Weathering profiles developed on the top surface of a bentonite (containing Al-Mg montmorillonite) and a K-bentonite (containing mixed-layer illite-smectite (I-S)) under Central European temperate conditions were studied by XRD, HRTEM, FTIR, K-Ar and chemical analyses. Weathering of montmorillonite results in the decrease of cation exchange capacity (CEC), total surface area and Mg content. The process is interpreted as montmorillonite dissolution and precipitation of amorphous SiO2. Weathering of I-S produces an increase in CEC and total surface area. The XRD data suggest dissolution of I-S and appearance of smectite as a separate phase at intermediate depths. The fixation of ammonium is documented in the topmost sample. In both profiles, abundant aeolian contaminants, including mica, were identified and their migration was traced using K-Ar dating.

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