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Chemical and Mineralogical Properties of Glauconitic Soil as Related to Potassium Depletion¹

ABSTRACT

The mineral glauconite has been identified in parent materials of some Libyan soils in the Barce Plateau region. Three soils, Elabiar clay loam, Elmarj silty clay, and Hamama clay, were selected to represent a weathering sequence. The cation exchange capacities (CEC) of these soils were found to increase with increased weathering intensity. This study dealt with the effects of potassium (K) depletion from clay and silt fractions on chemical and mineralogical properties of the soil glauconite. A relationship between equilibrium K concentration in the extracting solution and weathering degree was established.

Although it existed in all size fractions, glauconite (10.0, 5.0Å) was found in higher concentrations in fine silt and coarse clay fractions. Elabiar clay loam contained more glauconite and less K-depleted glauconite than Hamama clay. Total K values were proportional to the amount of glauconite in each of the soil separates.

The results of equilibrating the clay and silt fractions with 1N NaCl indicated that more than 90% of total K was exchangeable with an equivalent increase in CEC. When the replaced K was allowed to accumulate in the extracting solution, the exchange process continued until K concentration reached a certain equilibrium level. The equilibrium K concentration was a function of K depletion regardless of particle size, solution density, or both. Because of their larger surface areas and the shorter diffusion paths, the finer fractions had shorter equilibration times and more rapid exchange rates.

The K depletion resulted in expanding the 10Å glauconite to 14 to 15Å when Mg-saturated. Glycolation of the Mg-treated samples did not effect the 14 to 15Å K-depleted mineral. The results confirm those of previous research with glauconite specimens that, upon weathering, the soil glauconite produced a vermiculite- rather than a smectite-type mineral.

¹ Contribution from the Dep. of Soil and Environ. Sciences, Univ. of Calif., Riverside, CA 92521.

² First Author is Postdoctoral Associate, Physical Sciences Division, SRI International, Menlo Park, CA 94025; Second Author is Professor of Soil Science, Univ. of California, Riverside; and the Third Author is Professor of Soil Science, Univ. of Alfateh, Tripoli, Libya.

Received for publication April 6, 1981. Accepted for publication July 15, 1981.