Crystal structure of (K(1.5)Na(0.5))Ca(3)Si(3)O(10)

Single crystals of (K(1.52)Na(0.48))Ca(3)Si(3)O(10) [idealized (K(1.5)Na(0.5))Ca(3)Si(3)O(10), di(pot­assium, sodium) tricalcium tris­ilicate], were obtained from the crystallization of a glass with a molar oxide ratio of K(2)O:Na(2)O:CaO:SiO(2) = 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 [SiO(4)] tetra­hedra as well as [Si(4)O(12)] vierer single rings. The insular and cyclic silicate anions occur in the ratio 2:1, resulting in the idealized crystallochemical formula K(3)NaCa(6)[SiO(4)](2)[Si(4)O(12)]. Charge compensation is provided by K(+), Na(+) (occupying the same sites) and Ca(2+) cations. The two unique Ca(2+) cations are coordinated by six O atoms, forming distorted octa­hedra. By sharing common corners, edges and faces, these [CaO(6)] 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. (K(1.5)Na(0.5))Ca(3)Si(3)O(10) is isotypic with K(2)Ca(3)Si(3)O(10) indicating the existence of a solid-solution series K(2–x )Na( x )Ca(3)Si(3)O(10.)