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

Abstract

Jezekite (IMA2014-079), Na-8[(UO2)(CO3)(3)](SO4)(2)center dot 3H(2)O, is a new uranyl carbonate-sulfate mineral from Jachymov, 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, cejkaite, schrockingerite and ubiquitous gypsum. It is a supergene, low-temperature mineral formed by hydration-oxidation weathering of uraninite associated with post-mining processes. Jezekite is hexagonal, space group P-62m, with unit-cell parameters a = 9.0664(11), c = 6.9110(6)angstrom and V = 491.97(12)angstrom(3), 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]. Jezekite 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 similar to 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/cm(3). The mineral is optically uniaxial (+), with omega = 1.484(2) and epsilon = 1.547(2) (589 nm). It is non-pleochroic. The chemical composition of jezekite (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)(S1.01O4)(2)center dot 3H(2)O (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 jezekite are [d(obs)angstrom(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 jezekite (R = 0.043 for 444 reflections with I-obs > 3 sigma[I]) contains finite uranyl tricarbonate clusters linked through the Na-O bonds to form sheets of the composition {Na-2[(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 Jezek (1877-1950), a prominent Czech mineralogist and crystallographer