[Open access]

Acta Cryst. (2012). E68, i86-i87
[ doi:10.1107/S160053681204384X ]

Redetermination of durangite, NaAl(AsO4)F

G. W. Downs, B. N. Yang, R. M. Thompson, M. D. Wenz and M. B. Andrade

Abstract: The crystal structure of durangite, ideally NaAl(AsO4)F (chemical name sodium aluminium arsenate fluoride), has been determined previously [Kokkoros (1938). Z. Kristallogr. 99, 38-49] using Weissenberg film data without reporting displacement parameters of atoms or a reliability factor. This study reports the redetermination of the structure of durangite using single-crystal X-ray diffraction data from a natural sample with composition (Na0.95Li0.05)(Al0.91Fe3+0.07Mn3+0.02)(AsO4)(F0.73(OH)0.27) from the type locality, the Barranca mine, Coneto de Comonfort, Durango, Mexico. Durangite is isostructural with minerals of the titanite group in the space group C2/c. Its structure is characterized by kinked chains of corner-sharing AlO4F2 octahedra parallel to the c axis. These chains are cross-linked by isolated AsO4 tetrahedra, forming a three-dimensional framework. The Na+ cation (site symmetry 2) occupies the interstitial sites and is coordinated by one F- and six O2- anions. The AlO4F2 octahedron has symmetry -1; it is flattened, with the Al-F bond length [1.8457 (4) Å] shorter than the Al-O bond lengths [1.8913 (8) and 1.9002 (9) Å]. Examination of the Raman spectra for arsenate minerals in the titanite group reveals that the position of the band originating from the As-O symmetric stretching vibrations shifts to lower wavenumbers from durangite, maxwellite [ideally NaFe(AsO4)F], to tilasite [CaMg(AsO4)F].

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