Abstract
Chemical analyses of 300 solutions produced by the artificial weathering of eight different feldspars in fourteen experiments of up to 1200hr duration were used to study the evolution of water during weathering. The range of pH was between 4 and 5·5. Within 4 hr of dissolution, the activity of Al was controlled by the pH and the solubility of microcrystalline gibbsite. After 100 hr of dissolution, the pH and microcrystalline halloysite controlled the activities of Al and silicic acid in all of the solutions. Microcrystalline halloysite was the only phase identified in the weathering of plagioclases in distilled water and 1 atm CO 2 partial pressure. Montmorillonites, halloysite and other clay minerals were produced from oligoclase in aqueous solutions containing high initial concentrations of Ca. Mg, K and SiO 2. The experimentally determined log solubility product of microcrystalline gibbsite was −32·78 ± 0·04 and log K for the hydrolysis of microcrystalline halloysite was 11.58 ± 0·05. The results suggest that very poorly crystalline metastable phases may control the initial compositions of some waters in contact with rocks containing feldspar minerals.
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