Abstract
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.