Laboratory study demonstrates peak rubidium-strontium selectivity at intermediate potassium depletion in micas, indicating wedge-shaped edge sites govern ion binding.
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.
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Roux et al. (1969) studied this question.
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