Original paper

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

Olds, Travis A.; Plášil, Jakub; Kampf, Anthony R.; Škoda, Radek; Burns, Peter C.; Čejka, Jiří; Bourgoin, Vincent; Boulliard, Jean-Claude

European Journal of Mineralogy Volume 29 Number 1 (2017), p. 129 - 141

published: Feb 1, 2017

DOI: 10.1127/ejm/2017/0029-2586

BibTeX file

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Abstract

Gauthierite, KPb[(UO2)7 O5 (OH)7]· 8H2 O, 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) Å, b = 14.5368(8) Å, c = 14.0406(7) Å, b = 103.708(6) °, V = 5917.8(6) Å3 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 a = 1.780(5), b = 1.815(5), g = 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: K2 O 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: K0.67 Pb0.78 U7 O34 H23.77. The ideal formula is KPb[(UO2)7 O5 (OH)7 ](H2 O)8, which requires K2 O 1.90, PbO 9.00, UO3 80.74, H2 O 8.35, total 100 wt.%. Raman and infrared spectral data confirm the presence of UO2 2+, OH and molecular H2 O. 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> 3 σ (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).

Keywords

oxidation zonecrystal structureshinkolobweuranyl-oxide hydroxy-hydrategauthieritetopologynew mineral