Geschieberite, K2(UO2)(SO4)2(H2O)2, a new uranyl sulfate mineral from Jáchymov. (2nd January 2018) - Plášil, J.
Abstract: The new mineral geschieberite (IMA2014-006), K2 (UO2 )(SO4 )2 (H2 O)2, was found in the Svornost mine, Jáchymov, Czech Republic, where it occurs as a secondary alteration phase after uraninite in association with adolfpateraite and gypsum. Geschieberite forms crystalline aggregates of bright green colour (when thick) composed of multiply intergrown prismatic crystals elongated on [001] typically reaching 0.1–0.2 mm across; observable forms are {010} and {001}. Crystals are translucent to transparent with a vitreous lustre. The mineral is brittle, with perfect cleavage on {100} and an uneven fracture. It has a greenish-white streak and a probable Mohs hardness of ∼2. The mineral is slightly soluble in cold H2 O. The calculated density is 3.259 g cm –3 . The mineral exhibits strong yellowish-green fluorescence under both shortwave and longwave UV radiation. Optically, geschieberite is biaxial (–), with β = 1.596(2) and γ = 1.634(4) (measured at 590 nm), with X = a. Electron-microprobe analyses provided Na2 O 0.23, K2O 14.29, MgO 2.05, CaO 0.06, UO3 49.51, SO3 27.74, H2 O 6.36 (structure), total 100.24 wt.%, yielding the empirical formula (K1.72 Mg0.29 Na0.04 Ca0.01 )Σ2.06 (U0.98 O2 )(S0.98 O4 )2 (H2 O)2 based on 12 O atoms per formula unit. The Raman spectrum is dominated by the symmetric stretching vibrations of UO2 2+, SO4 2– and weaker O–H stretching vibrations. Geschieberite is orthorhombic, Pna 21, with a = 13.7778(3), b = 7.2709(4), c = 11.5488(2) Å, V = 1156.92(7) Å 3, Z = 4. The eight strongest powder X-ray diffraction lines are [ dobs in Å (hkl) Irel ]: 6.882 (200) 100, 5.622 (111) 53, 4.589 (211) 12, 4.428 (202) 16, 3.681 (311) 18, 3.403 (013) 12, 3.304 (401, 1̄13) 15 and 3.006 (122) 17. The structure, refined to R = 0.028 for 1882 I > 3σ(I) reflections, contains [(UO2 )(SO4 )2 (H2 O)] 2– sheets that are based on the protasite anion topology. Sheets are stacked perpendicular to a. Potassium atoms and one H2 O molecule are located between these sheets, providing an interlayer linkage. The remaining H2 O molecule is localized within the structural unit, at the free vertex of the uranyl pentagonal bipyramid; this vertex does not link to sulfate tetrahedra. The mineral is named for one of the most important ore veins in Jáchymov – the Geschieber vein.