Nollmotzite, Mg[U(V)(U(VI)O(2))(2)O(4)F(3)]·4H(2)O, the first natural uranium oxide containing fluorine

Nollmotzite (IMA2017-100), Mg[U(V)(U(VI)O(2))(2)F(3)O(4)](H(2)O)(4), is a new uranium oxide fluoride mineral found in the Clara mine, Black Forest Mountains, Germany. Electron microprobe analysis provided the empirical formula (Mg(1.06)Cu(0.02))(Σ1.08)[U(V)(U(VI)O(2))(2)O(3.85)F(3.15)][(H(2)O)(3.69)(OH)(0.31)](Σ4.00) based on three U and 15 O + F atoms per formula unit. Nollmotzite is monoclinic, space group Cm, with a = 7.1015 (12) Å, b = 11.7489 (17) Å, c = 8.1954 (14) Å, β = 98.087 (14)°, V = 676.98 (19) Å(3) and Z = 2. The crystal structure [twinned by reticular merohedry; refined to R = 0.0369 with GoF = 1.09 for 1527 unique observed reflections, I > 3σ(I)] is based upon [U(V)(U(VI)O(2))(2)F(3)O(4)](2–) sheets of β-U(3)O(8) topology and contains an interlayer with MgF(2)(H(2)O)(4) octahedra. Adjacent sheets are linked through F–Mg–F bonds, as well as via hydrogen bonds. The presence of fluorine and pentavalent uranium in the structure of nollmotzite has potentially important implications for the safe disposal of nuclear waste.