Characterization of the Michaľany-Lastovce bentonite-like deposit, east Slovakia

Abstract

The shortage of analytical data on bentonite from the Michaľany-Lastovce deposit was the motivation for the current study. The main goal was to characterize the bentonite deposit by providing new data that could shed light on the geological conditions that contributed to the formation of similar bentonites. Twenty-two samples from the surface and boreholes were analyzed. Most of the samples were composed of Ca-montmorillonite and opaline polymorphs (mainly opal-CT). Feldspars, quartz, micas, kaolinite, and clinoptilolite were also present. The cation exchange capacity (CEC) values for bulk and clay fraction of bentonites were 35–58 and 45–80 meq per 100 g, respectively. The BET specific surface areas for bulk and clay fraction samples were 50–73 and 69–101 m ² g ^–1 , respectively. Most samples exhibited the presence of both cis - and trans -vacant varieties of montmorillonite. The observed distribution of rare earth elements was almost in agreement with silicic explosive volcaniclastic units in the Hungarian Tokaj Mts. The results of the Bertaut-Warren-Averbach method along with microscopic and electron probe microanalysis studies of thin sections, pyrite sulfur isotope, and X-ray micro-computed tomography suggested that the Michaľany-Lastovce bentonite-like deposit probably originated from acidic pyroclastic material during diagenesis. Occasional increases in Fe content observed in whole-rock chemical analyses and in the montmorillonite structure might suggest a partial influence of andesitic rocks from the surrounding area. Despite the relatively small smectite content (<40 wt.%), the Michaľany-Lastovce deposit remains commercially exploited and its potential application as supplementary cementitious material needs to be re-evaluated.