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Abstract The release of K from K‐Ca‐illites by repeated extraction with 0.5 N solutions of various salts was determined. From comparison between the action of NH 4 ions and Na ions it could be deduced that the rapidly exchangeable K occupies two types of sites, which were named “planar” sites and “edge‐interlattice” sites, respectively. The very high preference for K of the edge‐interlattice sites explains why, e.g., Na ions and Ca ions are very ineffective in removing K from these sites as long as the released K ions are not removed from the system. NH 4 ions are, in contrast, highly competitive for the edge‐interlattice sites, but their action is kept within bounds by the fact that the mineral lattice remains fairly well closed during the penetration of NH 4 ions. An analysis of the exchange curve against Na ions yielded an estimate of the distribution and properties of the different types of sites, viz about 500 µeq. per g. rapidly exchangeable sites, of which 96% is situated on the external planar side of the lattice (preference factor for K about 2 in comparison to Ca ions), and 4% is located on the edges (preference factor for K about 500 in comparison to Na ions). In addition to this the illite studied contained about 700 µeq. K per g. in interlattice positions, which will lead to a very slow release of K if the chemical potential of K in solution is maintained at a low level.
Bolt et al. (1963) studied this question.