Gauthierite, KPb[(UO₂)₇O₅(OH)₇]·8H₂O, a new uranyl-oxide hydroxy-hydrate mineral from Shinkolobwe with a novel uranyl-anion sheet-topology

Gauthierite, KPb[(UO₂)₇O₅(OH)₇]·8H₂O, is a new uranyl-oxide hydroxy-hydrate mineral from the Shinkolobwe Mine, Democratic Republic of Congo, Africa. It occurs on a matrix of uraninite-bearing quartz gangue associated with soddyite and a minor metazeunerite–metatorbernite series mineral. It is a product of oxidation–hydration weathering of uraninite. Gauthierite is monoclinic, P2₁/c, with a = 29.844(2) Å, b = 14.5368(8) Å, c = 14.0406(7) Å, β = 103.708(6)°, V = 5917.8(6) ų and Z = 8. Prismatic crystals have pronounced lengthwise striations and reach about 1 mm in length. Gauthierite is yellowish orange with a light orange streak and vitreous lustre. The Mohs hardness is ~3 to 4. It is brittle with an uneven fracture and perfect cleavage on {010}. The calculated density based on the empirical formula is 5.437 g/cm³. Optically, gauthierite is biaxial (−), with α = 1.780(5), β = 1.815(5), γ = 1.825(5) (white light), 2V_meas. = 58(1)°; dispersion is extreme (r ≫ v). The optical orientation is X = b, Y ≈ a*, Z ≈ c (or X = b, Y ^ a = 14° in obtuse β); it is pleochroic with X very pale yellow, Y and Z orange-yellow; X ≪ Y ≈ Z. Electron microprobe analyses (average of 9) provided: K₂O 1.29, PbO 7.17, UO₃ 82.10, H₂O 8.78 (structure), total 99.34 wt.%. The empirical formula (based on 34 O a.p.f.u.) is: K_0.67Pb_0.78U₇O₃₄H_23.77. The ideal formula is KPb[(UO₂)₇O₅(OH)₇](H₂O)₈, which requires K₂O 1.90, PbO 9.00, UO₃ 80.74, H₂O 8.35, total 100 wt.%. Raman and infrared spectral data confirm the presence of UO₂²⁺, OH⁻ and molecular H₂O. The eight strongest powder X-ray diffraction lines are [d_obs in Å (hkl) I_rel]: 7.28 (020,400) 49, 3.566 (040, −802, −204) 67, 3.192 (622, −224) 100, 2.541 (−842, −244) 18, 2.043 (406) 14, 2.001 (662, −264, 14 · 2 · 0) 23, 1.962 (426, −146) 14, and 1.783 (12 · 0 · 4, −10 · 4 · 6) 17. The crystal structure of gauthierite (R = 0.0567 for 6997 reflections with [I > 3σ(I)]) contains uranyl-(hydroxo)-oxide sheets with a novel topology that is similar to that of vandendriesscheite, but with a unique chain sequence UDPDPDUPUP, P₄(UD)₆. Adjacent sheets are linked through K⁺ and Pb²⁺ cations (the lone 6s² pair on Pb atoms is stereoactive).