Evaluation of Aluminum-Smectite Stability Equilibria in Naturally Acid Scoils
1984
Karathanasis, A.D. | Hajek, B. F.
Stability characteristics of six Al-smectites in naturally acid (pH < 5.0) montmorillonitic soils were determined by using structural ionic constituents and in situ soil solution compositions. Approximate solubility products (pk) for each smectite were calculated from δG⁰ₜ values of hydroxide counterparts of cation structural components. Calculated ion-activity products (pQ) of soil solutions were approximately equal (within the range of analytical uncertainties) to extimated pks of smectites in three of six soils. The other three solutions were either supersaturated (C horizon) or undersaturated (Bt horizons) relative to smectite. Solutions obtained from C horizons were supersaturated with respect to amorphous silica due to dissolution of biogenic opal and albite. In these borizons, where large quantities of smectite have been preserved, smectite stability appeared to be enhanced by high solution levels of Si and Mg. Low ratios of solution Si/Al in A and B horizons, where not compensated by relatively high pH, promoted weathering of smectite to kaolinite. Standard free energies of formation (δGᵒₜ for Al-smectites, computed from estimated pk values, ranged from −10 161 ± 1021 to −10 345 ± 1040 kJ/mol. The highest δᵒf values were encountered in smectites with the most structural OH⁻.
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