The new mineral plavnoite (IMA2015-059), ideally K0.8Mn0.6[(UO2)2O2(SO4)]·3.5H2O, is a member of the zippeite group. It was found in the Plavno mine, in the eastern part of the Jáchymov ore district, Western Bohemia, Czech Republic, where it occurs as a supergene alteration phase formed by hydration–oxidation weathering of uraninite in hydrothermal U-veins. It was found to be associated with marécottite, magnesiozippeite, blatonite and gypsum. The mineral occurs as reddish to reddish-orange thin blades, elongated on [001] and flattened on {010}, which are intergrown in globular aggregates up to 0.5 mm across. Crystals are transparent with a vitreous to silky lustre. The streak is pale orange. The mineral is non-fluorescent under both long- and short-wave ultraviolet (UV) radiation. The Mohs hardness is about 2. Crystals are brittle with perfect {010} cleavage and uneven fracture. The density calculated from the empirical formula is 4.926 g cm−3. Optically, plavnoite is biaxial (+), with α = 1.740(5), β = 1.770(5), γ = 1.850 (5) (measured in white light). The measured 2V is 64.6(4)°; the calculated 2V is 65.3°. Dispersion could not be observed; no pleochroism was observed. Electron-microprobe analyses yielded the empirical formula (based on 2U atoms per formula unit, apfu) K0.77(Mn0.51Zn0.04Ni0.03Mg0.02) Σ0.60[(UO2)2O1.08(OH)0.92(SO4)0.96(SiO4)0.24](H2O)3.50. Plavnoite is monoclinic, C2/c, a = 8.6254 (16), b = 14.258(3), c = 17.703(4) Å, β = 104.052(18)°, V = 2122.0(8) Å3 and Z = 8. The structure (R1 = 4.99 % for 989 reflections with I > 3σ[I]) contains UO7 pentagonal bipyramids and SO4 tetrahedra forming sheets of the well-known zippeite topology. The interlayer region contains infinite zig–zag chains of corner-sharing Mn2+Φ6 octahedra (Φ = O, H2O) with K-centred polyhedra. The K atom sits at the partially occupied, mixed K/O site, the non-shared corner of the Mn2 octahedron. The mineral is named after the type locality – the Plavno mine.

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