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This article in SSSAJ

  1. Vol. 35 No. 5, p. 815-820
    Received: Jan 11, 1971
    Accepted: June 2, 1971

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Montmorillonite Equilibria and the Weathering Environment1

  1. J. A. Kittrick2



Stability equations were derived for Belle Fourche and Aberdeen montmorillonites. Both the composition of structural ions and the cation exchange capacity (CEC) were considered to be essential features of montmorillonite stability. However, montmorillonite stability was considered to be independent of the solution activity of exchangeable ions.

Simultaneous solution of the stability equations for Belle Fourche and Aberdeen montmorillonites indicated the two minerals could be at equilibrium with one another, but only under solution conditions quite different from those of their original stability determinations. The two montmorillonites (with hematite to control pH — 1/3pFe3+, kaolinite to control H4SiO4 activity, and a constant high Mg2+ activity) were then equilibrated from both low pH (undersaturation) and from high pH (supersaturation). The agreement between experimental and predicted equilibria was well within the experimental error of the original montmorillonite stability determinations. The good agreement between calculated and experimental equilibria under solution conditions far different from those employed for the original stability determinations (about 4 pH units higher, for example) confirms the accuracy of the original stability determinations.

Agreement between predicted and experimental equilibria also supports the view that structural ions such as Mg2+, Fe3+, Al3+, and Si4+, as well as the CEC, are essential features of the montmorillonite structure. The relationship between montmorillonite stability and the activities of constituent ions in solution permits the construction of several diagrams showing the variation in stability of Belle Fourche and Aberdeen montmorillonites with change in composition of the soil solution. From these diagrams, it is concluded that the various montmorillonite compositions encountered in nature are primarily a reflection of diverse solution environments rather than a chance incorporation of ions during the crystal growth process.

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