Crystal structure of (K1.5Na0.5)Ca3Si3O10.

Abstract

Single crystals of (K1.52Na0.48)Ca3Si3O10 [idealized (K1.5Na0.5)Ca3Si3O10, di(pot-assium, sodium) tricalcium tris-ilicate], were obtained from the crystallization of a glass with a molar oxide ratio of K2O:Na2O:CaO:SiO2 = 1.5:0.5:6:6 that was annealed at 1273 K. The crystal structure can be characterized as a mixed-anion oxidosilicate with isolated [SiO4] tetra-hedra as well as [Si4O12] vierer single rings. The insular and cyclic silicate anions occur in the ratio 2:1, resulting in the idealized crystallochemical formula K3NaCa6[SiO4]2[Si4O12]. Charge compensation is provided by K+, Na+ (occupying the same sites) and Ca2+ cations. The two unique Ca2+ cations are coordinated by six O atoms, forming distorted octa-hedra. By sharing common corners, edges and faces, these [CaO6] polyhedra build up octa-hedral layer-like motifs parallel to (010). (K/Na)+ ions reside in voids between the the silicate anions and the calcium centered octa-hedra and are coordinated by eight to nine oxygen ligands. (K1.5Na0.5)Ca3Si3O10 is isotypic with K2Ca3Si3O10 indicating the existence of a solid-solution series K2-x Na x Ca3Si3O10.