The mechanism of dissolution of the feldspars: Part I. Dissolution at conditions far from equilibrium

The dissolution of the feldspars, one of the most common of the rock-forming minerals, is important in fields as diverse as hydrometallurgy, environment sciences and geochemistry. The rate of dissolution of feldspars is dependent on the pH of the solution. The data from published sources on the rate of dissolution have been reassessed. This analysis indicates that there are three regions of interest in the pH range between 1 and 12. At low values of pH, below 3, the order of reaction with respect to H+ is close to 0.5. In the pH region between 3 and 7, the analysis of the experimental data shows that the order of reaction with respect to H+ changes to 0.25. In the alkaline region above a pH of 7, the rate of reaction is dependent on the concentration of the OH− ions. The order of reaction in this alkaline region is 0.5 with respect to OH−. A novel theory of dissolution is proposed to account for these orders of reaction, which are derived from first principles with no adjustable parameters. The mechanism posits that the removal of aluminium and silicate ions from the surface occurs in parallel and that these parallel reactions are dependent on the potential difference across the Helmholtz layer at the surface. The relationship of the proposed mechanism with other models is discussed.