Crystal structure of vanuralite, Al[(UO₂)₂(VO₄)₂](OH) . 8.5H₂O

Vanuralite, Al[(UO₂)₂(VO₄)₂](OH) . 8.5H₂O, is a rare supergene uranyl vanadate that forms during hydration-oxidation weathering of uraninite in oxide zones of U deposits. On the basis of single-crystal X-ray diffraction data it is monoclinic, space group P2₁/n, with a = 10.4637(10), b = 8.4700(5), c = 20.527(2) Aa, β = 102.821(9) , V=1773.9(3) Aa³ and Z = 4, D_calc. = 3.561 g cm^-3. The structure of vanuralite (R = 0.058 for 2638 unique observed reflections) contains uranyl vanadate sheets of francevillite topology of the composition [(UO₂)₂(VO₄)₂]^2-. Sheets are stacked perpendicular to c, and an interstitial complex ^[6]Al(OH)(H₂O)₄(H₂O)_4.5; adjacent structural sheets are linked through an extensive network of hydrogen bonds. Vanuralite is the most complex mineral among uranyl vanadates, with 961 bits/cell. The scarcity of occurrences is probably caused by the less common combination of elements present in the structure, as well as the relatively high complexity of the structure (compared to related minerals), arising namely from the complicated network of H-bonds.