Redetermination of durangite, NaAl(AsO(4))F

The crystal structure of durangite, ideally NaAl(AsO(4))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 (Na(0.95)Li(0.05))(Al(0.91)Fe(3+) (0.07)Mn(3+) (0.02))(AsO(4))(F(0.73)(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 AlO(4)F(2) octa­hedra parallel to the c axis. These chains are cross-linked by isolated AsO(4) tetra­hedra, forming a three-dimensional framework. The Na(+) cation (site symmetry 2) occupies the inter­stitial sites and is coordinated by one F(−) and six O(2−) anions. The AlO(4)F(2) 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(AsO(4))F], to tilasite [CaMg(AsO(4))F].