Crystal structure of (K_1.5Na_0.5)Ca₃Si₃O₁₀

Abstract

Single crystals of (K1.52Na0.48)Ca3Si3O10 [idealized (K1.5Na0.5)Ca3Si3O10, di(potassium, sodium) tricalcium trisilicate], 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] tetrahedra 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 octahedra. By sharing common corners, edges and faces, these [CaO6] polyhedra build up octahedral layer-like motifs parallel to (010). (K/Na)+ ions reside in voids between the the silicate anions and the calcium centered octahedra 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.