Symmetry relationships of sodalite (SOD) – type crystal structures

Abstract

The mineral sodalite with the ideal composition Na₈[Al₆Si₆O₂₄]Cl₂ is the prototype for numerous related crystal structures based on the same topology of the underlying net of strong bonds. This net has been assigned the code SOD by the Structure Commission of the International Zeolite Association. Minerals and synthetic compounds with a SOD-type framework represent the most comprehensive family of zeolite-type compounds with the highest number of published crystal structures. Among these are aluminosilicates (e.g., sodalite, haüyne, helvine, lazurite, nosean, tsaregorodtsevite), beryllosilicates (e.g., danalite, genthelvite, tugtupite), chlorides, sulfides (e.g., tetrahedrite, binnite, freibergite, galkhaite, goldfieldite, tennantite), borates (rhodizite), synthetic phosphates, aluminates, phosphides, nitrides and clathrate hydrates which have been shown by crystal structure analyses to adopt the SOD-type framework. More than 900 SOD-type crystal structures, which represent over 18% of the total number of published zeolite structures, are known. The complete symmetry relationships of SOD-type crystal structures comprising 27 space groups are listed in a Bärnighausen-tree together with the type and index of symmetry reduction. No other zeolite type displays such a rich harvest of symmetry as the sodalite-type. Seven additional space groups reported for SOD-types have not been considered here because they were not well supported by experimental evidence. Of the 26 low-symmetry derivatives six are due to an ordering of tetrahedrally coordinated framework cations. In two cases the lowering of symmetry is caused by some of the framework cations assuming 5- or 6-coordinations instead of the usual 4-coordination. This means that in 18 cases the lower space group symmetries are achieved by the influence of pore-filling matter.