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Cation Release from Sodium- and Calcium-Saturated Clay-Sized Soil Fractions¹

ABSTRACT

The cation release of sodium- and calcium-saturated clay fractions of two montmorillonitic and one vermiculitic aridland soils was compared with that of specimen Ca- and Na-saturated montmorillonites. The clays were reacted with distilled water (0.25, 0.5, 1.0, and 2.5% wt/v) at 25°C. Hydrolysis rates of soil clays were roughly comparable to those of the specimen montmorillonites. Sodium hydrolysis exceeded calcium hydrolysis for soil clay fractions, whereas Na and Ca hydrolysis rates were about equal for the specimen montmorillonites, due to the dissolution of other silicates in the soil clays. Sodium kaolinite hydrolyzed so rapidly that it could not be prepared free of Cl without almost total loss of exchangeable Na. The suspensions were filtered after about 500 hours, and the solution compositions were used to determine ion activity products for the various montmorillonites and other silicate phases. Solutions were slightly supersaturated with respect to crystalline gibbsite. Most soil-clay solutions were also supersaturated with respect to kaolinite, montmorillonite, K mica, and illite. Although the major cation chemistry of the Ca-saturated soil clay suspensions was predominantly controlled by the surface hydrolysis reactions, the Na-saturated suspensions were affected by dissolution of non-smectite minerals. The weathering of less stable minerals also resulted in additional silica and aluminum release. This is consistent with feldspar undersaturation and montmorillonite supersaturation in the soil-clay suspensions. The pure montmorillonites gave activity product values consistent with published montmorillonite stabilities.

¹ Contribution from the U.S. Salinity Laboratory, 4500 Glenwood Ave., Riverside, CA 92501.

² Geochemist and Visiting Soil Chemist, respectively. The permanent address of Haim Frenkel is Inst. of Soils & Water, ARO, The Volcani Center, P.O. Box 6, Bet Dagan, Israel.

Received for publication January 9, 1980. Accepted for publication April 10, 1981.

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