Gauthierite, KPb[(UO2)7 O5 (OH)7]· 8H2 O, a new uranyl-oxide hydroxy-hydrate mineral from Shinkolobwe with a novel uranyl-anion sheet-topology

Abstract

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, P 21/ c , with a = 29.844(2) A, b = 14.5368(8) A, c = 14.0406(7) A, β = 103.708(6)°, V = 5917.8(6) A3 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 {0 1 0}. 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: K0.67Pb0.78U7O34H23.77. 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 A ( 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).