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Stability of Montmorillonites: I. Belle Fourche and Clay Spur Montmorillonites
Author(s) -
Kittrick J. A.
Publication year - 1971
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1971.03615995003500010040x
Subject(s) - spur , geology , clay minerals , mineralogy , paleontology
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 pAl 3+ vs. pH 4 SiO 4 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 pAl 3+ of 2.44 to 4.26, in pH 4 SiO 4 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‐⅓pAl 3+ vs. pH 4 SiO 4 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 Δ G of −2468.5 ± 0.3 kcal per mole was calculated for Belle Fourche and Clay Spur montmorillonites, based upon the assumption that Fe 3+ in solution was controlled by hematite.