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Evaluation of Aluminum‐Smectite Stability Equilibria in Naturally Acid Scoils
Author(s) -
Karathanasis A.D.,
Hajek B. F.
Publication year - 1984
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1984.03615995004800020039x
Subject(s) - clay minerals , albite , dissolution , kaolinite , chemistry , soil water , gibbsite , solubility , weathering , aluminosilicate , saponite , hydroxide , mineralogy , inorganic chemistry , geology , quartz , soil science , geochemistry , paleontology , biochemistry , catalysis
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 (p k ) for each smectite were calculated from δG 0 t 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 p k s 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 (δ G o t for Al‐smectites, computed from estimated p k values, ranged from −10 161 ± 1021 to −10 345 ± 1040 kJ/mol. The highest δ o f values were encountered in smectites with the most structural OH ‐ .