Clay Minerals (1996) 31, 333-345 EFFECT OF NON-SWELLING LAYERS ON THE DISSOLUTION OF REDUCED-CHARGE MONTMORILLONITE IN HYDROCHLORIC ACID P. KOMADEL, J. BUJDAK, J. MADEJOVA, V. SUCHA* AND F. ELSASS t Institute oflnorganic Chemistry, Slovak Academy of Sciences, 842 36 Bratislava, Slovakia, *Department (?['Geology of Mineral Deposits, Comenius University, 842 15 Bratislava, Slovakia, and t Station de Science du Sol, 1NRA Versailles, France (Received 15 August 1995; revised 15 February 1996) ABSTRACT: A series of reduced-charge montmorillonites (RCM), prepared from the same parent Li-montmorillonite (Jelgov2? Potok, Slovakia) by heating at various temperatures (105-210~ for 24 h, was treated with 6 M HC1 at 95~ for periods up to 30 h. Reaction solutions obtained were analysed for A1, Fe, Mg and Li and the solid reaction products were investigated by FrlR spectroscopy. Both analyses provided evidence that the extent of dissolution decreased with increased amounts of Li fixed within the montmorillonite structure, i.e. with increased heating temperature. Differences in the acid dissolution process were reflected in the structural changes which occurred within the RCM samples, due presumably to different positions of fixed Li. The ethylene glycol monoethyl ether (EGME) surface areas, and XRD and HRTEM analyses of the RCM series revealed an increased amount of non-swelling layers in the samples prepared at higher temperatures, which caused a substantially slower decomposition of M7 and M8 in HC1. The calculated XRD patterns of M6 and M7 confirmed the presence of 20% and 45% pyrophyllite-like layers, respectively, in these samples. Mixed-layer pyrophyllite-like-smectite and pyrophyllite-like crystals, containing only non-swelling layers, were found in sample M8. The results confirmed that the amount of swelling layers in RCM significantly affects their dissolution rate in HC1. The use of acid treatments of clay minerals, which have been applied for decades to obtain information regarding mineral structure, was reviewed recently by (~f~el & Komadel (1994). Acid treatment removes tetrahedral and octahedral cations from the smectite structure at similar rates (Luca & MacLachlan, 1992; Tk~i~ et al., 1994). Readily soluble, octahedral plus tetrahedral and 'insoluble' portions of constituent atoms can be calculated from the dissolution curves, thus providing information on the distribution of atoms in the sample. Readily soluble portions include exchange- able cations and easily soluble admixtures such as goethite (Komadel et al., 1993). Common 'insoluble' phases found in the fine fractions of bentonites are kaolinite, quartz, anatase and volcanic glass ((~f~el & Komadel, 1994). Kato et al. (1966) reported that the A1 content retained in the solid reaction products after 2 h reaction with 25% HzSO4 was 24% for montmorillonite, 88% for pyrophyllite and 94% for kaolinite. Nov~ik & (~f~el (1978) found that the rate of dissolution of dioctahedral smectites increased with octahedral Fe and Mg for A1 substitution. Low octahedral substitution is one of the reasons for the observed lower dissolution rate of pyrophyllite compared to montmorillonite. However, the effect of swelling on dissolution rate is unclear. Fixation of Li and reduction of layer charge was observed after heating Li-montmorillonite at about 200-300~ (e.g. Hofmann & Klemen, 1950). Bujd~ik et al. (1991, 1992) prepared a series of reduced-charge montmorillonites (RCMs) by heating Li-montmorillonite at various temperatures (105-210~ for 1-24 h. Individual examples of this series, prepared from the same parent clay, have similar Si, A1, Fe and Mg contents but different layer charges and different amounts of non-swelling layers due to the various contents of fixed Li. 9 1996 The Mineralogical Society