Topotactic route to synthesis of novel hydroxylated phases: I. Trioctahedral micas.

K-depleted phlogopite mica was used as a topotactic precursor, and treated with alkali (Li⁺, K⁺, 036NH⁺ ₄, Rb⁺, Cs⁺), alkaline-earth (Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) and trivalent (Al³⁺) cations under hydrothermal conditions of 200oC and 30 MPa pressure. K-, NH₄-, Rb- and Cs-aluminosilicate micas were synthesized at 200oC in one day. The synthesis of Cs-aluminosilicate mica, with potential applications in the management of nuclear wastes, has been achieved for the first time by this approach. Ion exchange by Li⁺, Na⁺ and alkaline-earth cations under hydrothermal conditions did not produce anhydrous mica phases, but resulted in hydrous phases with one or two layers of water molecules between the clay layers. The formation of hydrous phases may be attributed to the high hydration energy of the above cations, compared with K⁺, 036NH⁺ ₄, Rb⁺ and Cs⁺. Ion exchange with Al³⁺ produced a chlorite-like phase because of the hydrolysis of Al³⁺ under these hydrothermal conditions. These studies are of relevance in the immobilization of wastes, where hazardous ions can be fixed in highly stable insoluble phases like mica or chlorite. (Authors' abstract) -D.J.M.