Fig. 3. Activity diagrams illustrating the activities of Al³⁺(aq) and Fe³⁺(aq) as a function of pH relative to the stability of (a) gibbsite and (b) goethite at ambient temperature (20–22°C). Metal cation activities were computed using GEOCHEM-PC and the metal–citrate (cit) binding constants listed in Table 2 (closed symbols, T2) or Table 3 (open symbols, T3). The solid lines represent the equilibrium solubility of gibbsite and goethite at 25°C (Table 1).