ALFLARSENITE: STRUCTURE AND RELATIONSHIP TO OTHER Be-Si AND ZEOLITE FRAMEWORK STRUCTURES

Abstract

Alflarsenite, ideally NaCa 2 Be 3 Si 4 O 13 (OH)·2H 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 P 2 1 , with unit-cell parameters refined from single-crystal data: a 7.1222(4), b 19.8378(11), c 9.8071(5) A, β 111.287(1)°, V 1291.1(2) A 3 and Z = 4. The crystal structure, a zeolite framework, was refined to R 1 = 0.046 for 3283 observed reflections. The Be–Si framework of tetrahedra has cages containing the Na and Ca atoms and H 2 O groups. The framework has Cmcm space-group symmetry; the addition of Na and Ca cations reduces the symmetry to space group Cmc 2 1 , whereas taking into consideration the H 2 O groups, the structure reduces in symmetry to monoclinic P 2 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.