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Published in Soil Sci Soc Am J 47:158-163 (1983)
© 1983 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Transformation of Smectite to Kaolinite in Naturally Acid Soil Systems: Structural and Thermodynamic Considerations1

A. D. Karathanasis and B. F. Hajek2

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 ({approx}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 ({Delta}Gof), approaching those of halloysite at high solution silica activities.

NOTES

1 Contribution from the Department of Agronomy and Soils, Alabama Agricultural Experiment Station, Auburn University, AL 36849.

2 Research Associate and Professor of Soils, respectively.

Received for publication April 29, 1982. Accepted for publication September 5, 1982.






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