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This article in SSSAJ

  1. Vol. 47 No. 1, p. 158-163
     
    Received: Apr 29, 1982


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doi:10.2136/sssaj1983.03615995004700010031x

Transformation of Smectite to Kaolinite in Naturally Acid Soil Systems: Structural and Thermodynamic Considerations1

  1. A. D. Karathanasis and
  2. B. F. Hajek2

Abstract

Abstract

Four acid montmorillonitic soil profiles were sampled to study thermodynamic-stability relationships and chemical weathering of soil minerals. Solid- and solution-phase characterization indicated smectite to be the metastable precursor of kaolinite with the highest rate of transformation occurring in topsoil horizons. Structures of Al-saturated smectites were found to be nonideal with respect to excess structural OH- water and below normal (≈550°C) dehydroxylation temperatures. Structural and thermochemical data suggested the weathering sequence was montmorillonite → beidellite → kaolinite. The smectite → kaolinite transformation appears to be a combination of dissolution-precipitation and structural reorganization processes. Excess structural OH- in the beidellitic structure seems to have played a key role in the transformations. Montmorillonite → beidellite and beidellite → kaolinite transformations were favored in the more weathered topsoil horizons where solutions were undersaturated with respect to amorphous SiO2. Kaolinite precipitating from supersaturated solutions appeared to have a range in crystallinity and standard free energy of formation (ΔGof), approaching those of halloysite at high solution silica activities.

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