Potassium-smectites with various layer-charge densities and layer-charge locations were subjected to as many as 100 wetting and drying cycles, thereby producing randomly interstratified illite/smectite that contained illite layers stable against exchange by 0.1 Ν SrCl 2 or 1 Ν NaCl. The percentage of illite layers formed by this process in montmorillonite was proportional to layer-charge and, based upon very limited data, their stability with respect to sub sequent exchange was inversely proportional to alpha, the angle of tetrahedral rotation. Most of the illite layers were produced during the first 20 cycles. Sodium-smectites treated in wetting and drying experi ments that contained potassium-minerals (e.g. feldspar) formed illite layers by fixing potassium released from dissolution of the potassium-minerals. The presence of NaCl, KCl, and HCl in the experimental solutions had little effect on reaction rates, but CaCl 2 decreased and KOH increased the rate of illitization. The reaction with KOH increased layer charge, whereas, in all of the other experiments, layer charge remained constant. Oxygen isotope data confirmed the conclusion drawn from chemical data that the reaction mechanism for illite formation at high pH (chemical reaction of 2:1 layers) differed from that found at more acid pH (mechanical rearrangement of 2:1 layers around potassium).The wetting and drying process may be responsible for producing mixed-layer illite/ smectite from smectite at surface temperatures, and for accelerated dissolution of sparingly soluble potassium-minerals in sediments and soils.One method for studying reactions that occur at the mineral-water interface is to concentrate the interface by drying a clay mineral slurry.Certain reactions are promoted by this process, for example, reactions that respond to Bronsted acidity (!_, 2). Repeated
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