Abstract: Completeness of exchange of K from muscovite by Ba2+ ions decreased with particle size below 20 µ. Accompanying K exchange at 120°C, using a repeated batch technique, was a marked loss of Si and the formation of boehmite in the finer fractions. Several possible explanations for the unexpected high K retention of fine mica fractions are discussed. The formation of a diffusion-inhibiting surface “skin” is discounted, because equilibrium was obtained more rapidly with the fine fractions than with coarser fractions. The average degree of bending of unit mica layers due to peripheral expansion is probably greater in large particles. With bending, rotation of tetrahedra and shifting of adjacent layers with respect to each other may induce greater release of K and a lower selectivity for this ion. Fault planes may induce preferential expansion of individual layers and initiate interstratification and splitting of particles. Splitting of particles may reduce bending and increase K selectivity.