| Summary: | In the past years an increasing demand for bentonites resulted in the opening of new bentonite deposits in the Jastrabá Formation. The shortage of information, in particular analytical data, on the bentonites from the newly opened Jastrabá Fm. deposits was the motivation for the current study. Smectite is the predominant mineral in all bulk bentonites from the new deposits. Its amount varied between 43 and 90 wt. %. The bulk bentonites also contain variable amounts (10–57 wt. %) of mineral admixtures such as feldspars, mica, opal-CT, kaolinite, quartz and sometimes goethite. The smectite mineral comprising the studied bentonites was montmorillonite. The octahedral Al in the structure of montmorillonite was partially substituted by Mg, and to a lesser extent by Fe. The interlayer space of montmorillonite is occupied predominantly by divalent exchangeable cations (Ca2+ and Mg2+). The dehydroxylation temperature of smectites (>600 °C) determined on the DTG curves indicates the presence of the cis-vacant variety of montmorillonites. The mean crystallite thicknesses of smectites (TMEAN) calculated by BWA analyses ranges from 7.2 to 11.5 nm. The shape of the crystallite thickness distributions (CTDs) for smectites is lognormal in all cases. Cation exchange capacity (CEC) and total specific surface area (TSSA) increases with increasing amount of smectite. The CEC of 101 meq/100g and TSSA of 616 m2/g correspond to bulk bentonite from the Stará Kremnička III deposit containing 89 wt. % of smectite.
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