The crystal structure of deliensite, Fe[(UO2)2(SO4)2(OH)2](H2O)7, was solved by direct methods and refined to R1 = 6.24% for 5211 unique observed reflections [Iobs > 3σ(I)], on a crystal that was found to consist of rotational and inversion (merohedral) twins, from Jeroným mine, Abertamy in the Czech Republic. The presence of four twin domains was taken into account in the refinement. The structure is orthorhombic, space group Pnn2, with unit-cell parameters a = 15.8514(9), b = 16.2478(7), c = 6.8943(3) Å, V = 1775.6(1) Å3 and Z = 4. The crystal structure of deliensite contains uranyl-sulfate sheets with a phosphuranylite topology, consisting of dimers of edge-sharing uranyl pentagonal bipyramids linked by corner-sharing with sulfate tetrahedra. The sheets lie in the (100) plane and are decorated by [Fe2+O(H2O)5] octahedra; two weakly bonded H2O molecules are present in the interlayer. The [Fe2+O(H2O)5] octahedron is linked directly to the sheet via the uranyl oxygen atom. Adjacent sheets are linked by hydrogen bonds only. The sheet topology and geometrical isomerism is discussed and a comparison of the composition obtained from electron-probe microanalysis, powder-diffraction data, Raman and infrared spectra of deliensite samples from Mas d'Alary, Lodève, France; L'Ecarpière mine, Gétigné, France; and several localities at Jáchymov, Western Bohemia, Czech Republic is made.

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