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

Single crystals of (K_1.52Na_0.48)Ca₃Si₃O₁₀ [idealized (K_1.5Na_0.5)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. (K_1.5Na_0.5)Ca₃Si₃O₁₀ is isotypic with K₂Ca₃Si₃O₁₀ indicating the existence of a solid-solution series K_2- Na Ca₃Si₃O_10.