A mineral wadeite in glass, occurrence and crystal structure

Abstract

Crystals with a hexagonal lath-like morphology were found in glass products for cathode ray tubes. These crystals are preserved at the interfaces between glass and ZrO2 ceramics in a glass melting tank. An electron microprobe analysis gives SiO2: 45.6, ZrO2: 31.5, K2O: 19.0, Na2O: 1.92, BaO: 1.46, SrO: 0.33, in wt%. These values and X-ray analyses indicated the crystals to be precipitated with the same structure as mineral wadeite, K2ZrSi3O9, whose K atom sites are partially substituted by Ba, Sr or Na atoms. The structure is hexagonal with the space group P63/m, and the unit cell dimensions of a=6.918(2) and c=10.151(3)A. The structure was refined using X-ray diffraction intensities from a single crystal. The bond distances and angles for Si3O9 rings are well comparable with those of benitoite, BaTiSi3O9, which is another example mineral including Si3O9 rings, with slight but significant difference in non-bridging Si-O bond distances. The mean square displacements of atoms in Si3O9 rings show that the tetrahedron is rigid for Si-O bond but strongly distorted for the O-O edges of non-bridging O atoms during thermal vibrations, coordinating to Zr and K. The thermal vibrational behaviors for the non-bridging oxygen are significantly different from those in benitoite.