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

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 SiO 2 . Kaolinite precipitating from supersaturated solutions appeared to have a range in crystallinity and standard free energy of formation (Δ G o f ), approaching those of halloysite at high solution silica activities.