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Ion Selectivity of Micas as Influenced by Degree of Potassium Depletion¹

ABSTRACT

The Rb/Sr selectivity coefficient of muscovite and biotite reached a maximum at an intermediate degree of K-depletion of these micas. The Rb/Sr ratio of ions absorbed from a solution, initially containing equivalent amounts of Rb and Sr, was determined for the two micas altered to different degrees to Mg-vermiculite. X-ray diffraction of the Rb-Sr treated vermiculites gave nonintegral peaks for the basal reflections, indicating random interstratification of Rb and Sr + Mg saturated layers. The results support previous proposals that selective sites for K are probably situated at wedge-shaped zones occurring at frayed edges and deeper within the altered mica particles. As K is removed from mica, the number of such wedges should first increase and then decrease. Selectivity of K and other large monovalent ions should also follow this same trend.

¹ Contribution of the Agronomy Department, Virginia Polytechnic Institute, Blacksburg. Support by NSF Grants GP-5074 and GA-1373 is gratefully acknowledged.

² Research Associate and Professor of Agronomy, Virginia Polytechnic Institute, respectively. Present address of the senior author: Department of Soil Science, University of Natal, Pietermaritzburg, South Africa.

³ The authors wish to thank John L. Brown, Engineering Experiment Station, Georgia Institute of Technology, Atlanta, for the preparation of the electron micrographs.

Received for publication November 25, 1968. Accepted for publication June 9, 1969.