ON THE ENCLATHRATION OF NAB(OH)4 IN THE BETA-CAGES OF SODALITE - CRYSTALLIZATION KINETICS AND CRYSTAL-STRUCTURE

The hydrothermal formation of sodium hydroxyborate enclathrated sodali te synthesized from an initial ternary mixture of mullite, cristobalit e and corundum has been investigated. Depending on temperature and pre ssure the compound Na7.5[AlSiO4]6[B(OH)4]1.5 . 2H2O could be synthesiz ed in form of polycrystalline powder as well as in form of single crys tals. The kinetics of the sodalite growth could be determined at 423 K and 473 K. Si-29- and Al-27 MAS NMR was shown to be the preferred met hod to describe the conversion into sodalite quantitatively. Evidence for the first formation of polycrystalline sodalite has been found aft er the total dissolution of mullite and cristobalite. Single crystal X -ray diffraction data were used for the structure determination and re finement. The position and orientation of the hydroxyborate anions wit hin the sodalite's beta-cages could only be found in the course of a ' 'rigid body'' refinement with constraints for the B(OH)4 tetrahedron. The boron atoms are located in an off-centre position within the sodal ite cages. One out of the four oxygen atoms of the hydroxyborate group is positioned near the centre of a face of the sodium tetrahedron, wh ereas the three remaining oxygen atoms are near to the middle of its e dges.