Charge properties of mixtures of minerals with variable and constant surface charge

Surface charge of mixtures of lepidocrocite and illite determined by adsorption of potential-determining and electrolyte ions is studied as a function of pH and NaCl concentration. Deviations between results obtained by both methods were a function of the illite content of the mixtures, and can be mainly attributed to coatings of hydroxyaluminium complexes on the constant charge surfaces of the clay mineral, which can react with H⁺ ions or release exchange sites by precipitation of well-formed hydroxides. These coatings of aluminium complexes can cause H⁺ consumptions similar to those due to lepidocrocite. Some ideas are put forward for modelling charge properties of mixed systems.