Abstract
The new mineral plavnoite (IMA2015-059), ideally K0.8Mn0.6[(UO2)(2)O-2(SO4)]center dot 3.5H(2)O, 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 0.5 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 {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 alpha = 1.740(5), beta = 1.770(5), gamma = 1.850(5) (measured in white light). The measured 2V is 64.6(4)degrees; the calculated 2V is 65.3 degrees. 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(Mn0.51Zn0.04Ni0.03Mg0.02)(Sigma 0.60)[(UO2)(2)O-1.08(OH)(0.92)(SO4)(0.96)(SiO4)(0.24)](H2O)(3.5 0). Plavnoite is monoclinic, C2/c, a = 8.6254 (16), b = 14.258(3), c = 17.703(4) angstrom, beta = 104.052(18)degrees, V = 2122.0(8) angstrom(3) and Z = 8. The structure (R-1 = 4.99% for 989 reflections with 1 > 3 sigma[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+phi(6) octahedra (phi=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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