This paper reviews synthesis, characterization and cation exchange properties of a novel swelleing mica "Na-4-mica" and their synthetic analogues with a high layer charge denisty. Na-4-mica (Na4Mg6Al4Si4O20F4) of a synthetic brittle mica has a very high Al(III) content and but exhibits unusual swelling behavior and selective cation exchange properties potentially useful in hazardous cation separations from solutions. Although normal brittle micas do not swell in water at all, this synthetic mica can readily become hydrated upon contact with water or even in moist air. This mica has a theoretical cation-exchange capacity of 468 milli-equivalents per 100 g on an anhydrous basis. The present authors found a simple and cost-effective preparation process of micro- or nano-crystallites of this mica from naturally occurring clay kaolinite. The fine mica is a selective exchanger for Sr, Ba, Ra, Pb, Cu, or Zn. The modified micas, such as "Na-3-mica" and "Na-2-mica" were also synthesized by the similar syntehtic processes. Na-3-mica improved the cation exchange kinetics and capacity for Sr. These micas also exhibited extremely low cation leachability once dehydrated at room temperature or moderate temperatures, and, hence, are expected to be useful for radioactive strontium or radium removal followed by its immobilization for safe disposal.