The mineralogy of the clay fractions of Vertisol samples from Karnataka, in the Deccan plateau of India was studied using X-ray diffraction (XRD) analysis of samples variously treated. Results of XRD of K +-saturated and heated clay fractions confirmed the presence of a well formed, stable hydroxy interlayered beidellite associated with chlorite in the coarse (2–0.2 μm) and montmorillonite in the fine clay ( <0.2 μm) fractions. Further characterization of the Mg 2+-saturated coarse clay by solvating with glycerol vapour demonstrated the presence of beidellite. This beidellite has high tetrahedral charge as revealed by the expanded 18 Å peak for LiNO 3-300°C-Mg 2+ solvated with glycerol treated samples and is similar to nontronite with isomorphous substitution in the tetrahedral layer. The alkylammonium clay complex shows two 2:1 clay mineral phases: a high-charge beidellite/vermiculite concentrated in the coarse clay, and a low charge montmorillonite in the fine clay fractions. Structural formulae calculated for soil clay minerals are similar in all the thirteen soils and is similar to the formulae for beidellite in Vertisols of Africa, Turkey and Morocco which also have a high charge and are iron-rich. The presence of this iron-rich high-charge beidellite explains the K + selectivity in Vertisols of the Deccan plateau of India and elsewhere where such soils exist.