Plavnoite, a new K-Mn member of the zippeite group from Jachymov, Czech Republic

The new mineral plavnoite (IMA2015-059), ideally K_0.8Mn_0.6[(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 Jachymov 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 marecottite, magnesiozippeite, blatonite and gypsum. The mineral occurs as reddish to reddish-orange thin blades, elongated on [0 0 1] and flattened on {0 1 0}, which are intergrown in globular aggregates up to 0.5mm 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 {0 1 0} 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 2 U atoms per formula unit, apfu) K_0.77(Mn_0.51Zn_0.04Ni_0.03Mg_0.02)_Σ0.60[(UO₂)₂O_1.08(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) Aa³, β = 104.052(18) , V = 2122.0(8) Aa³ 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.