A simple preparation process of Na-4-mica (ideal brittle mica composition of Na4Mg6Al4Si4O20F4. xH(2)O) from kaolinite yielded a nearly pure and very fine Na-4-mica with a crystallite size of about 0.2 um. Strontium-sodium ion exchange in the hydrated interlayers of the Na-4-mica was demonstrated with this fine mica. The forward ion exchange of Sr2+ --> 2Na(+) (Sr-ion adsorption) was determined after equilibration for 4 weeks, and the Sr exchange capacity reached a value of about 200 mequiv (100 g)(-1). The parabolic diffusion or modified Freundlich model described the forward exchange process well. Only a small reverse exchange 2Na(+) --> Sr2+ (Sr-ion desorption) occurred from the hydrated Sr2+-saturated sample in 4 weeks. The irreversible sodium-strontium ion-exchange isotherm was obtained at room temperature after 4 weeks of equilibration. This mica showed much higher selectivities for Sr and Ba than for Mg and Ca, as expected. This selective Sr-ion exchanger with a large ion-exchange capacity and extremely low Sr leachability is expected to be useful for Sr-90 removal followed by its immobilization at room temperature.