Stability Relationships in Kaolinite, Gibbsite, and Al‐hydroxyinterlayered Vermiculite Soil Systems

Abstract The stability of common soil minerals found in Ultisols was studied relative to their natural soil solution composition. The mineralogy of the clay fraction (<2 µm) ranged from 22 to 60% in kaolinite, 0 to 20% in gibbsite, 17 to 57% in hydroxyinterlayered vermiculite (HIV), and 1 to 12% in quartz. Ion activity functions (pH‐⅓pAl and pH 4 SiO 4 ) of soil solutions plotted on a stability diagram along with solubility lines of soil kaolinite, gibbsite, and HIV with the formula K 0.24 Ca 0.08 (Si 3.24 Al 0.76 )(Al 1.56 Fe 0.24 Mg 0.20 )[Al 1.45 (OH) 3.79 ]O 10 (OH) 2 indicated all solutions to be supersaturated with respect to reference kaolinite. Only a few surface soils containing gibbsite had solutions oversaturated relative to gibbsite. Solubility products of HIV were very close to its estimated p k , indicating a near‐equilibrium condition with this mineral, especially in eluviated and transitional subsoil horizons (E and EB). Although the HIV appeared to be metastable relative to reference kaolinite, the data indicated a possible equilibrium coexistence of the two minerals with an HIV stability field below or above that of soil kaolinite, depending on the degree of Al‐hydroxyinterlayering.