Powder X-ray diffraction and infrared spectroscopy were used to identify the mineralogical composition of clay obtained from a systematically sampled bentonite deposit at Lieskovec (Central Slovakia), developed from andesitic pyroclastics. The main mineral is an iron-rich montmorillonite, the admixtures present in all samples include kaolinite, quartz, cristobalite and plagioclase, while muscovite/illite and orthoclase appear in most samples. This bentonite deposit is relatively homogeneous as is demonstrated by its chemical composition and quantitative analysis using Rietveld refinement. Total Fe₂O₃ content (5–9 %) in the Lieskovec samples is higher than in most other Slovak bentonites. Mössbauer spectroscopy shows that Fe(II) covers less than 5 % of total Fe. Goethite and/or hematite are not present in all <2 mm fractions separated from the raw bentonite and they contain up to 26 % of total Fe. Low tetrahedral Al and octahedral Mg substitutions cause that the smectite is of lower charge than montmorillonite separated from the best known Slovak bentonite: Stara Kremnicka-Jelsovy potok. This is in accord with the absence of the AlMgOH bending vibration in the IR spectra of Lieskovec samples.