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This article in SSSAJ

  1. Vol. 35 No. 1, p. 140-145
    Received: June 8, 1970
    Accepted: Oct 12, 1970

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Stability of Montmorillonites: I. Belle Fourche and Clay Spur Montmorillonites1

  1. J. A. Kittrick2



Seven samples of montmorillonite from Belle Fourche, South Dakota and one from Clay Spur, Wyoming were equilibrated with various solutions for 3 to 4 years. Most samples did not exhibit a static equilibrium, yet solution compositions moved along or toward definite solubility lines in relation to pH-⅓ pAl3+ vs. pH4SiO4 axes. This behavior was not altered by a wide variety of sample treatments, including two particle-size fractions. The approach from both undersaturation and supersaturation indicated that true equilibria were involved.

Periodic analyses of the equilibrating liquid gave a range in pAl3+ of 2.44 to 4.26, in pH4SiO4 of 2.48 to 3.47, and in pH of 2.74 to 3.60. Analyses were also made for Mg, Na, and Cl-.

A montmorillonite-kaolinite mixture established the montmorillonite-koalinite join and demonstrated montmorillonite-kaolinite equilibria. The solution composition of several other samples helped define an experimental Belle Fourche-Clay Spur montmorillonite solubility line. The slope and intercept of this line on pH-⅓pAl3+ vs. pH4SiO4 axes agreed with a calculated slope and intercept that were based upon the premise that the solubility of montmorillonite is independent of the exchangeable ion.

A ΔG of −2468.5 ± 0.3 kcal per mole was calculated for Belle Fourche and Clay Spur montmorillonites, based upon the assumption that Fe3+ in solution was controlled by hematite.

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