Characterisation of bentonite from Hliník nad Hronom deposit (Jastrabá Formation of the tiavnica stratovolcano, Western Carpathians)

The Hliník nad Hronom bentonite deposit is situated in Jastrabá Formation on the NW margin of the tiavnické vrchy Mts. Geological exploration was realised in the deposit at the end of eighties. The bentonite was described as lower quality. The deposit is very rarely operated. The purpose of presented study was characterisation of the bentonite predominately by actual X-ray diffraction techniques and infrared spectroscopy and comparison of the Hliník nad Hronom deposit with other Western Carpathian bentonites. Studied bentonite and bentonized rhyolitic tuff from Hliník nad Hronom were composed of 30–53 wt % of Al-Mg montmorillonite, 19–45 wt % of opal-C or opal-CT, mostly less than 14 wt % of volcanic glass and less than 10 wt % of K-feldspar. Also illite, biotite, kaolinite and other were determined as minor and trace mineral phases. Cation-exchange capacity (CEC) of bulk rock samples ranged from 30 to 65 meq/100g. CEC was significantly higher in clay fraction (85–95 meq/100g) that is in good correlation with presence of almost pure montmorillonite. The BWA analysis of montmorillonite 001 XRD peaks was performed to calculate the mean crystallite size and the crystallite size distribution. Surface controlled crystal growth mechanism was determined for Hliník nad Hronom′s montmorillonite according to calculated parameters of lognormal particle thickness distribution (α and β2), similarly as for other Western Carpathian smectites. The mean thickness of smectite particles from Hliník nad Hronom (about 6.78 nm) belongs to the lowest values that were calculated for Western Carpathian smectites. Based on the presented results and previous knowledge, we can assume that the deposit originated in lacustrine environment. The insufficient fluid flow rate caused the precipitation of high amount of opal-C or/and opal-CT. The crystallisation temperature of smectites was low, only 20–50°C, at the bentonitization of rhyolitic tuff of Hliník nad Hronom area.