Structural Study of Nepheline Hydrate I, an Inorganic Ion Exchanger

摘要

The crystal structures of nepheline hydrates I, Na sub 3 Al sub 3 Si sub 3 O sub 12 x 2H sub 2 O, and three compounds produced by ion exchange with aqueous KCl, RbCl and CsCl at 80 degrees C, have been studied using X-ray diffraction methods. This synthetic silicate has a tetrahedral framework with a two-dimensional pore system consisting of perpendicular 8-ring and 6-ring channels. The long-range ordering of Si and Al into adjacent tetrahedra is well developed. Some aspects of the topology, geometry and bonding of the tetrahedral frame are discussed. Related framework types are derived by unit cell twinning of the idealized cristobalite structure. A limit in the ion exchange is observed when about 1/3 of the Na sup + ions have been replaced. This behaviour is explained by the restricted volume of two Na sites situated in the 6-ring channel. The readily exchangeable ions and water molecules in the 8-ring channels is an arrangement which gradually changes when the size of the alkali metal-ion increases. Most K sup + -exchanged crystals have a unit cell which is determined by the translational symmetry of the framework, while the original Na form has a two-fold superstructure and the Rb sup + -exchanged form has a five-fold superstructure. Caesium-ion-exchanged crystals have incommensurate structures. The occurrence of superstructures is related to long-range ordering of the species in the 8-ring channels. (Atomindex citation 17:046654)