The new mineral geschieberite (IMA2014-006), K2(UO2)(SO4)2(H2O)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 H2O. 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 Na2O 0.23, K2O 14.29, MgO 2.05, CaO 0.06, UO3 49.51, SO3 27.74, H2O 6.36 (structure), total 100.24 wt.%, yielding the empirical formula (K1.72Mg0.29Na0.04Ca0.01)Σ2.06 (U0.98O2)(S0.98O4)2(H2O)2 based on 12 O atoms per formula unit. The Raman spectrum is dominated by the symmetric stretching vibrations of UO22+, SO42 and weaker O–H stretching vibrations. Geschieberite is orthorhombic, Pna21, with a = 13.7778(3), b = 7.2709(4), c = 11.5488(2) Å, V = 1156.92(7) Å3, 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 (401,1İ13) 15 and 3.006 (122) 17. The structure, refined to R = 0.028 for 1882 I > 3σ(I) reflections, contains [(UO2)(SO4)2(H2O)]2− sheets that are based on the protasite anion topology. Sheets are stacked perpendicular to a. Potassium atoms and one H2O molecule are located between these sheets, providing an interlayer linkage. The remaining H2O 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.

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