THE ALBITE-WATER SYSTEM .1. THE KINETICS OF DISSOLUTION AS A FUNCTION OF PH AT 100-DEGREES-C, 200-DEGREES-C, AND 300-DEGREES-C

Albite feldspar was hydrolyzed at 100, 200, and 300 degrees C in solutions at pH values ranging from 1.4 to 10.3. The dissolution experiments were carried out in a single-pass tubular flow system. Rates were determined both by the mass loss of the sample and from the reactor output concentration of released Si. All of the measured rates represent limiting rates since the experiments were run at conditions far from equilibrium with respect to albite saturation. The only secondary phase to form was boehmite, precipitation of which mostly occurred under acid conditions. The dissolution rate curves displayed a U-shaped relationship with pH, as evidenced by a decrease in the rates with increasing pH in the acid region, a neutral pH region marked by no pH dependence, and a basic pH region where the rates increased with increasing pH. For any given temperature, the absolute values of the slopes (n) of the log rate vs. pH curves were approximately equal in the acid and basic pH regions. The absolute values of n increased from 0.2 to 0.6 in bath the acid and basic pH regions as the temperature increased from 100 to 300 degrees C. The calculated energies of activation were 89, 69, and 85 kJ/mol in the acid, neutral, and basic pH regions, respectively. These values reflect the greater dependence of the rates on temperature in the acid and basic pH ranges in comparison to the neutral pH range. The results from this study have been compared to a few published studies in the literature concerning the hydrolysis of albite and other feldspars at 100 degrees C and higher temperatures. A comparison of the rates revealed that the differences in rates are potentially dependent on the design of the experiments. Even after taking into account experimental uncertainties, it appears that static hydrolysis experiments almost always produce measured rates that are lower than those obtained in flow systems. This may be explained by the precipitation of secondary phases and/or chemical affinity considerations.