The crystal structure of the interrupted framework silicate K9.6Ca1.2Si12O30 determined from laboratory X-ray diffraction data

The crystal structure of a potassium calcium silicate with composition K9.6Ca1.2Si12O30 (or K8CaSi10O25) has been solved by direct methods aided by distance least squares optimization from laboratory X-ray powder diffraction data. The trigonal compound adopts the non-centrosymmetric space group image with the following basic crystallographic data: a=11.13623(5) Å, c=21.9890(2) Å, V=2361.63(2) Å3, Z=3, Dcalc=2.617 g cm−3. The crystal structure can be classified as an interrupted framework with exclusively Q3-units. It can be thought of as being built from layers parallel to (001) containing isolated six-membered tetrahedral rings in UDUDUD conformation. Corner sharing of tetrahedra belonging to adjacent sheets results in a tetrahedral framework. The framework density of the structure is 15.2 T-atoms/1000 Å3. The coordination sequences are identical for both silicon atoms in the asymmetric unit: 3-6-11-20-32-46-60-80-102-122. The vertex symbols for the two tetrahedral centers are 102·102·61. Topologically, the structure can be described as an Archimedean three-dimensional 3-connected net. It can be derived from the diamond or cristobalite net by removing 20% of the knots. Charge compensation in the structure is achieved by the incorporation of mono- and divalent M-cations (M: K, Ca). These extra-framework ions are coordinated by six to nine oxygen ligands. Ca/K distributions for the five symmetrically independent M-sites were obtained from a combination of bond distance considerations, site occupancy refinements and the bulk chemical composition. The structural characterization is completed by a detailed Raman spectroscopic study. Furthermore, possible implications of the structural chemistry of interrupted framework silicates for the field of silicate glass research are addressed.