STRUCTURE AND TOPOLOGY OF DUMORTIERITE AND DUMORTIERITE-LIKE MATERIALS

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Dumortierite, ca . (Al,□)Al 6 (BO 3 )Si 3 O 13 (O,OH) 2 , has a complex and unusual crystal structure that is shared by several minerals and an increasing number of synthetic materials, which we group together as dumortierite-like materials (DLMs). Dumortierite has a strongly pseudo-hexagonal orthorhombic structure based on rod-like double-chains of Al octahedra in a framework based on the {6·4·3·4} semi-regular planar tiling. The large hexagonal channels contain chains of face-sharing Al octahedra attached to the framework by rings of SiO 4 tetrahedra. Smaller triangular channels contain planar BO 3 groups, while in other materials the triangular channels are occupied by tetrahedral or pyramidal groups. Holtite and magnesiodumortierite are isostructural with dumortierite, while ellenbergerite, phosphoellenbergerite, ekatite, and the synthetic materials have very similar structures to dumortierite, but with hexagonal symmetry. These minerals and materials differ from one another in the identity of the metal cations occupying the framework and face-sharing chains, and in the tetrahedral, pyramidal, or triangular groups within the hexagonal and triangular channels. We write the following structure-generating function, which describes both topological and stoichiometric properties of DLMs: X ( M 2 Φ ) 3 ( T h Θ h 3 θ h ) 3 ( T t Θ t 3 θ t ) where X is the face-sharing hexagonal channel octahedrally coordinated site, M is the framework octahedrally coordinated site, T h and T t are three- to four-coordinate cation sites in the hexagonal and triangular channels, Φ is a four-coordinate anion site, Θ h and Θ t are three-coordinate basal anion sites, and θ h and θ t are one- to three-coordinate apical anion sites. We discuss various structural and crystal chemical properties of DLMs, illustrating their commonalities as variations on the theme described by the structure-generating function. We also discuss three closely related structures which involve modifications of different geometrical or topological aspects of the DLM structure: the structure of lyonsite and its synthetic analogues, the structure of satterlyite and holtedahlite, and the structure of the AB polytype of the cancrinite group of feldspathoids.