Weathering of smectite and illite-smectite under temperate climatic conditions

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