Alflarsenite; structure and relationship to other Be-Si and zeolite framework structures

Alflarsenite, ideally NaCa (sub 2) Be (sub 3) Si (sub 4) O (sub 13) (OH).2H (sub 2) O, is a new zeolite species from the Tuften larvikite quarry, Tvedalen, Larvik, Vestfold, south Norway. The crystal structure has been solved in space group P2 (sub 1) , with unit-cell parameters refined from single-crystal data: a 7.1222(4), b 19.8378(11), c 9.8071(5) Aa, beta 111.287(1) degrees , V 1291.1(2) Aa (super 3) and Z=4. The crystal structure, a zeolite framework, was refined to R1=0.046 for 3283 observed reflections. The Be-Si framework of tetrahedra has cages containing the Na and Ca atoms and H (sub 2) O groups. The framework has Cmcm space-group symmetry; the addition of Na and Ca cations reduces the symmetry to space group Cmc2 (sub 1) , whereas taking into consideration the H (sub 2) O groups, the structure reduces in symmetry to monoclinic P2 (sub 1) . Topological diagrams were constructed for all framework beryllium silicates and for zeolites that have topological features similar to alflarsenite. These diagrams are used to identify the secondary building units (SBU), 5-1 and spiro-5, and to extract vertex symbols and coordination sequences for each topologically unique site having tetrahedral coordination. The SBUs and coordination-sequence numbers show structural similarities to bikitaite and the synthetic zeolites CAS and NSI. All four structures have slabs composed of alternating chains of 5- and 8-connected rings. The cross-linkage of these slabs varies in each structure.