Shinkolobweite, from the Shinkolobwe Mine, Democratic Republic of Congo; a new mineral containing uranium in the rare pentavalent oxidation state

Shinkolobweite, Pb (sub 1.333) [U (super 5+) O(OH)(UO (sub 2) ) (sub 5) O (sub 4.67) (OH) (sub 5.33) ](H (sub 2) O) (sub 5) , is a new lead uranyl oxide-hydroxide hydrate mineral containing hexavalent and pentavalent uranium from the Shinkolobwe mine, Democratic Republic of Congo. Crystals of shinkolobweite are dark reddish-brown prisms up to 0.5 mm in length, occurring on a matrix of massive uraninite associated with fourmarierite, rutherfordine, soddyite, and sklodowskite. Crystals are translucent with subadamantine luster and light bronze-yellow streak, are flattened on {010}, are elongated on [001], and exhibit the forms {100}, {010}, {101}, and {101}. The mineral is non-fluorescent under both longwave and shortwave ultraviolet illumination. It has a Mohs hardness of approximately 2 and exhibits brittle tenacity with perfect cleavage on {010}, imperfect cleavage on {100}, and even fracture. The calculated density is 5.853 g/cm (super 3) based on the empirical formula. Electron probe microanalysis provided the empirical formula Pb (sub 1.290) U (super 6+) (sub 4.876) U (super 5+) (sub 1.166) O (sub 27) H (sub 16.633) based on 27 O apfu and U (super 5+) :U (super 6+) determined by X-ray photoelectron spectroscopy. Shinkolobweite is orthorhombic, superspace group Pnnm(0b0)000, a = 14.4808(4), b = 7.0681(8), c = 11.9423(3) Aa, V = 1222.32(15) Aa3, modulation wave vector [0 1/3 0], and Z = 2. The structure was refined from 8959 reflections to a final R (sub 1) = 0.0736 for all reflections. Uranyl oxide-hydroxide sheets in shinkolobweite adopt the beta -U3O8 topology and possess (3 + 1) commensurately modulated ordering that results from the long-range ordering of U (super 5+) and U (super 6+) in the sheet, as well as the position and occupancy of interlayer Pb (super 2+) cations. Observations of a topological transition between alpha -U (sub 3) O (sub 8) and beta -U (sub 3) O (sub 8) type sheets in shinkolobweite supplements our understanding of U (super 5+) mineral oxidation and stability.