COMPARISON OF THE CRYSTAL-CHEMISTRY OF SELECTED MSI6O15-BASED SILICATES

The structures of four A(3)MSi(6)O(15)(. nH(2)O) silicates (A = Na or K, M = Nd or Y) recently determined by the authors are compared with o ne another and with the structures of related silicates. The 2:5 Si:O ratio in these compounds (silicate tetrahedra linked by sharing three O atoms per tetrahedron plus one unshared O atom) permits layer, doubl e-chain, or double-ring configurations. In alpha-K3NdSi6O15. 2H(2)O, b eta-K3NdSi6O15, and Na3NdSi6O15. 2H(2)O, the linkage is found to resul t in a corrugated layered structure, in beta-Na3YSi6O15 a double-chain structure, and in alpha-Na3YSi6O15 a unique double-ring structure. Al though the factors that govern the stabilities of ring vs. chain vs. l ayered structures remain to be completely elucidated, it is apparent i n the layered structures that larger M cations (such as Nd and Ce) pro duce a greater degree of corrugation than do Zr and Ti cations. The mo re open structures of neodymium and cerium silicates contain large cha nnels that may serve as pathways for fast alkali ion transport.