Zeolites and clathrasils ― two distinct classes of framework silicates

Abstract Framework silicates are divided into three classes according to the size of the cavities in the tetrahedral framework and the free diameter of the windows between these cavities. (i) Pyknolites with interstices just large enough to house monatomic ions and single H 2 O molecules. (ii) Clathrasils with larger polyhedral cavities but windows too small to let the encaged polyatomic ions and/or molecules pass. (iii) Zeolites containing tunnels or larger polyhedral cavities interconnected by windows large enough to allow ready diffusion of the polyatomic guest species through the crystal. With regard to the replacement of Si by other tetrahedral cations the framework silicates are treated as solid solutions assigning the Si-free and the pure-Si end members to the same class as the mixed crystals.