Redetermination of durangite, NaAl(AsO₄)F

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 (Na_0.95Li_0.05)(Al_0.91Fe³⁺_0.07Mn³⁺_0.02)(AsO₄)(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 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 O^2- 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].