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(pot-assium, sodium) tricalcium tris-ilicate], 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 ] tetra-hedra 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 octa-hedra. By sharing common corners, edges and faces, these [CaO 6 ] 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 1.5 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.