Nollmotzite, Mg[UV(UVIO2)2O4F3]·4H2O, the first natural uranium oxide containing fluorine

Abstract

Nollmotzite (IMA2017-100), Mg[U^V(U^VIO₂)₂F₃O₄](H₂O)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_1.06Cu_0.02)_Σ1.08[U^V(U^VIO₂)₂O_3.85F_3.15][(H₂O)_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) ų 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^VIO₂)₂F₃O₄]^2- sheets of β-U₃O₈ topology and contains an interlayer with MgF₂(H₂O)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.

Keywords: new mineral; nollmotzite; pentavalent uranium; uranium oxide fluoride.