Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Jakub Plášil, Jan Hloušek, Anatoly V. Kasatkin, Dmitry I. Belakovskiy, Jiří Čejka, Dmitry Chernyshov

Ježekite, Na₈[(UO₂)(CO₃)₃](SO₄)₂·3H₂O, a new uranyl mineral from Jáchymov, Czech Republic

Journal of Geosciences, volume 60 (2015), issue 4, 259 - 267

DOI: http://doi.org/10.3190/jgeosci.203

Ježekite (IMA2014-079), Na₈[(UO₂)(CO₃)₃](SO₄)₂·3H₂O, is a new uranyl carbonate-sulfate mineral from Jáchymov, Western Bohemia, Czech Republic. The new mineral was found on samples from the Geschieber vein in the Svornost mine. It occurs as a crystalline crust composed of thin, bladed prismatic crystals of yellow to sulfuric yellow color on a gangue along with andersonite, čejkaite, schröckingerite and ubiquitous gypsum. It is a supergene, low-temperature mineral formed by hydration-oxidation weathering of uraninite associated with post-mining processes. Ježekite is hexagonal, space group P-62m, with unit-cell parameters a = 9.0664(11), c = 6.9110(6) Å and V = 491.97(12) ų, Z = 1. Crystals are thin blades elongated along [001]. Crystals exhibit the forms {001}, {1-11}, {100} and {010}, commonly forming twins/intergrowths with a twin plane parallel to [001]. Ježekite is light yellow to sulfuric yellow and has a very pale yellow streak. It exhibits a bright greenish white fluorescence under both long-wave and short-wave UV. It is transparent with a vitreous to pearly luster. The mineral has a Mohs hardness ~2; it is brittle, with uneven fracture and a perfect cleavage on {001} and along [010]. The calculated density based on the empirical formula is 2.966 g/cm³. The mineral is optically uniaxial (+), with ω = 1.484(2) and ε = 1.547(2) (589 nm). It is non-pleochroic. The chemical composition of ježekite (wt. %, electron-microprobe) is: Na₂O 27.92, SO₃ 18.49, UO₃ 32.85, CO₂ (calc.) 15.08, H₂O (calc.) 6.17, total 100.51, which yields the empirical formula Na_7.88(UO₂)(CO₃)₃(S_1.01O₄)₂·3H₂O (based on 22 O apfu). Prominent features in the Raman spectrum include the O-H stretching vibrations, symmetric stretching vibrations of (UO₂)²⁺ ions, and stretching and bending vibrations of symmetrically non-equivalent CO₃ groups and highly disordered SO₄ tetrahedra. The eight strongest powder X-ray diffraction lines for ježekite are [d_obs Å (I_rel.) (hkl)]: 7.861(59)(100), 6.925(20)(001), 5.193(100)(101), 4.534(44)(110), 3.415(23)(201), 2.751(17)(112), 2.728(20)(211), 2.618(25)(300). The crystal structure of ježekite (R = 0.043 for 444 reflections with I_obs > 3σ[I]) contains finite uranyl tricarbonate clusters linked through the Na-O bonds to form sheets of the composition {Na₂[(UO₂)(CO₃)₃]}^2- parallel to (001). The adjacent sheets of polyhedra are also linked through Na-O bonds to the six Na2 atoms and highly disordered sheets of composition {[(SO₄)₂(H₂O)₃]}^4- into a sandwich-like structure. The new mineral is named after Professor Bohuslav Ježek (1877-1950) a prominent Czech mineralogist and crystallographer.