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Mineralogy of Soil Separates and Alkali-Ion Exchange-Sorption¹

ABSTRACT

X-ray and petrographic examination of a number of California soils derived from granite-diorite sedimentary material showed the presence of biotite weathering products which appeared to include slightly altered biotite, vermiculite, and a regularly interstratified biotite-vermiculite. Magnetic and specific gravity separations of fine sand- and silt-size material from a Hanford soil yielded relatively pure minerals giving rational series of X-ray reflections from 25, 14.48, or 10Å. spacings when Mg-saturated; 23.5, 12.25, or 10Å, when Na-saturated; and 10Å. when K-saturated. Cation-exchange capacities of vermiculite, hydrobiotite, and biotite separated from 100- to 200-mesh material were 0.95, 0.40, and 0.15 me. per g., respectively. Tagged rubidium sorbed on exchange sites was displaced by 0.1N NaCl to the extent of 10% from vermiculite, 20% from hydrobiotite, and 42% from biotite fractions. Soil vermiculite and hydrobiotite had smaller cation-exchange capacities than similar separates from the Libby, Montana, vermiculite deposit, and the reversibility of the Rb-sorption reaction was less for the soil minerals.

Vermiculite-hydrobiotite-biotite appeared to be the predominant clay mineral in all size fractions from the Hanford soil except the < 0.5µ clay. Cation-exchange capacities of coarse clay and silt were relatively large (0.35 to 0.0067 me. per g.) and sorbed Rb was only around 50% exchanged by 0.1N NaCl. Rubidium could be almost completely displaced from < 0.5µ clay. It is concluded that vermiculite and hydrobiotite in the coarse clay, silt, and fine sand fractions account for around 40% of the cation-exchange capacity of the soil and for essentially all of its capacity to sorb Rb or K irreversibly.

¹ Paper No. 1442, University of California Citrus Research Center and Agr. Exp. Sta., Riverside. Presented before Div. II–IV, Soil Science Society of America, Ithaca, New York, Aug. 20, 1962.

² Graduate Assistant, visiting Soil Scientist on leave from Soil Survey Laboratory, SCS, USDA, Beltsville, Md., and Professor Soils and Plant Nutrition, respectively.

Received for publication December 3, 1962. Accepted for publication March 12, 1962.