Original paper

Plavnoite, a new K–Mn member of the zippeite group from Jáchymov, Czech Republic

Plášil, Jakub; Škácha, Pavel; Sejkora, Jiří; Kampf, Anthony R.; Škoda, Radek; Čejka, Jiří; Hloušek, Jan; Kasatkin, Anatoly V.; Pavlíček, Radim; Babka, Karel

European Journal of Mineralogy Volume 29 Number 1 (2017), p. 117 - 128

published: Feb 1, 2017

DOI: 10.1127/ejm/2017/0029-2583

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Abstract

The new mineral plavnoite (IMA2015-059), ideally K0.8Mn 0.6[(UO2)2O2(SO 4)]·3.5H2 O, 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 hydrationbn–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 [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 a = 1.740(5), b = 1.770(5), g = 1.850 (5) (measured in white light). The measured 2 V is 64.6(4)°; the calculated 2 V 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) K0.77 (Mn0.51 Zn0.04 Ni0.03 Mg0.02)S 0.60 [(UO2)2 O1.08 (OH)0.92 (SO4 )0.96 (SiO4)0.24 ](H2 O)3.50. Plavnoite is monoclinic, C 2/ c, a = 8.6254 (16), b = 14.258(3), c = 17.703(4) Å, b = 104.052(18)°, V = 2122.0(8) Å3 and Z = 8. The structure ( R 1 = 4.99 % for 989 reflections with I> 3 s [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 (F = O, H2 O) 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.

Keywords

new mineralplavnoitechemical compositionuranyl sulfate hydratecrystal structurezippeite group