STUDY OF THE KINETICS AND MECHANISMS OF DISSOLUTION OF ALBITE AT ROOM TEMPERATURE AND PRESSURE (WEATHERING, EXCHANGE, FELDSPARS, REACTION RATES)

The dissolution kinetics of albite was studied at room temperature and pressure using the fluidized bed reactor under a broad range of experimental conditions including pH, concentration of the dissolved components and reaction time between the feldspar and the aqueous solution. The results indicate that the weathering mechanism involves three successive steps: (1) instantaneous exchange of alkali ions by hydrogen ions resulting in the formation of hydrogen feldspar, (2) rapid build-up of a Na-depleted layer enriched in Si and/or Al, and (3) slow dissolution of the residual layer at the solid-solution interface accompanied by diffusion of ions from the fresh feldspar boundary leading to a steady-state dissolution stage. The rate of this last step is controlled by the decomposition of activated complexes derived from surface complexes whose configurations depend on pH and the concentration of dissolved Al, and probably also on the Na concentration especially in alkaline solutions.;A model for the dissolution kinetics of albite at 25(DEGREES)C, based on transition state theory, is presented to permit the description of the influence of pH, dissolved Al concentration, and reaction time between the feldspar and the aqueous solution.;Two configurations of the surface complexes, resulting from the reaction of H-feldspar with the aqueous solution, are proposed to explain the dissolution kinetics. Near neutrality, the rate depends on the degree of hydration of H-feldspar resulting in the formation of the activated complex (H(,3)O)AlSi(,3)O(,8). Under acidic or alkaline conditions, the rate is controlled by the occurrence of a negatively charged surface complex, resulting from the substitution of aluminum in the lattice by protons, which may be represented by HSi(,6)O(,13)('-). The related release of Al('3+) or Al(OH)(,4)('-) explains the rate dependence in pH ranges where these two dissolved species prevail. The stoichiometry of this complex is supported by the observation that under highly acidic conditions the residual layer becomes noticeably depleted in Al. The nature of this negatively charged surface complex can be justified on the basis of coordination chemistry of oxide surfaces.