RIETVELD REFINEMENT OF THE CRYSTAL-STRUCTURE OF THE SYNTHETIC POROUS ZINCOSILICATE VPI-7

The crystal structure of the zincosilicate VPI-7 with the idealized ch emical formula Na-32[Zn16Si56O144].40 H2O has been refined using the R ietveld technique (R(wp) = 16.2%). The analysis confirms the proposed framework topology. The true symmetry of VPI-7, however, is orthorhomb ic with space group Fdd2 and a = 39.88(1) A, b = 10.326(1) A, and c = 10.219(1) A rather than the tetragonal symmetry (SG/4 ($) over bar m2) of the structure model proposed by Annen et al. The zincosilicate fra mework has an intersecting 3-dimensional channel system of 8- and 9-me mbered rings. The most characteristic feature of the framework structu re is the ''spiro-5'' unit of two interconnected 3MR with zinc complet ely ordered in each 3MR. The zincosilicate framework is one simplest e nd-member structure of an OD series of which the beryllosilicate Lovda rite is the other end member.