Effect of chemical composition and swelling on acid dissolution of 2: 111 clay minerals

Dissolution of fine fractions of three clays, containing as the main minerals a montmorillonite, a mixed-layer illite/smectite and an illite, in 6 mol dm^-3 HCl at 95°C was investigated. The extent of dissolution was followed by chemical analysis of the reaction liquids. The solid materials were characterised by X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy in mid- and near-IR regions and by determination of cation exchange capacities. The objective was to compare the effects of chemical composition and swelling ability of three dioctahedral clay minerals from the montmorillonite-illite series on their dissolution in HCl. Montmorillonite with swelling interlayers (Jelšový Potok, Slovakia) was completely dissolved within 18 h, whereas the residues of illite could be distinguished in both illitic samples with prevailing non-swelling interlayers treated for 36 h. Chemical composition of dioctahedral clay minerals has greater effect on the dissolution rate than swellability. Greater substitution of Mg and Fe for Al in the octahedral sheets and of Al for Si in the tetrahedral sheets in the layers of Ca-illite from Morris (Illinois, USA) makes it more easily soluble in HCl than the mixed-layer illite/smectite mineral (Dolná Ves, Slovakia) with 30% swelling interlayers. FTIR spectroscopy in both mid- and near-IR regions provides complex information on dissolution of dioctahedral clay minerals.