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This article in SSSAJ

  1. Vol. 47 No. 6, p. 1247-1251
    Received: Mar 22, 1983
    Accepted: July 11, 1983

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Stability Relationships in Kaolinite, Gibbsite, and Al-hydroxyinterlayered Vermiculite Soil Systems1

  1. A. D. Karathanasis,
  2. Fred Adams and
  3. B. F. Hajek2



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 pH4SiO4) of soil solutions plotted on a stability diagram along with solubility lines of soil kaolinite, gibbsite, and HIV with the formula K0.24Ca0.08(Si3.24Al0.76)(Al1.56Fe0.24Mg0.20)[Al1.45(OH)3.79]O10(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 pk, 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.

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