Cejkaite, the triclinic polymorph of Na4(UO2)(CO3)3 -: a new mineral from Jachymov, Czech Republic

Cejkaite, a new mineral from Jachymov, NW Bohemia, Czech Republic, forms a thin earthy efflorescence over a calcite vein associated with disintegrated uraninite. The color is pale yellow to beige, the streak is light yellow, and the luster is vitreous. The broad secondary mineral association includes andersonite and schrockingerite. Chemical analysis (by ICP-MS and TG) gave (in wt%): Na2O = 21.39, MgO = 0.15, FeO = 0.53, UO3 = 53.93, and CO2 = 24.00 (calculated by difference). The simplified chemical formula is Na4UO2(CO3)(3). The mineral is triclinic, space group P1 or P (1) over bar, a = 9.291(2), b = 9.292(2), c = 12.895(2) Angstrom, alpha = 90.73(2), beta = 90.82(2), gamma = 120.00(1)degrees, V = 963.7(4) Angstrom(3), Z = 4, D-meas = 3.67(1) g/cm(3), and D-calc = 3.766(5) g/cm(3). The strongest seven lines in the X-ray powder-diffraction pattern [d in Angstrom(I)(hkl)] are: 8.022(92)(1 (1) over bar0, 010, 100), 5.080(57)((1) over bar 02, 0 (1) over bar2), 5.024(60)((1) over bar 12, 1 (1) over bar2), 4.967(68)(012, 102), 4.639(100)(1 (2) over bar0, 2 (2) over bar0, 110), 3.221(63)(004), 2.681(60) (330, 114, 030, 300). Optical data could not be measured due to the extremely small grain size, but the calculated mean refractive index is 1.5825. Crystal size varies from 0.2 to 0.6 mum and shows an indistinct hexagonal outline. Thermal decomposition of synthetic cejkaite proceeds in three main steps. DTA endotherm at 430 degreesC corresponds to the decomposition of the uranyl tricarbonate groups. IR spectrum of cejkaite confirms the presence of crystallographically nonequivalent (CO3)(2-) groups and the absence of water. The average U-O bond length in (UO2)(2+), calculated from nu(3) = 848 cm(-1), is RU-O similar to 1.81 Angstrom. A model based on the crystal structure of trigonal Na-4(UO2)(CO3)(3) was adopted and applied to solve the cejkaite crystal structure by the Rietveld method (7238 unique reflections, R-p = 0.076, R-wp = 0. 104). Uranium is eight-coordinated, and forms a [UO2O6] skeleton with almost linear O-U-O that is roughly perpendicular to an irregular cycle formed by six 0 atoms that, in turn, belong to three more-or-less regular and planar CO3 groups. Atoms Na1, Na1a, and Na2 are octahedrally coordinated, whereas Na3 is pentagonally coordinated. The mineral name honors Jiri Cejka for his notable contributions to the crystal chemistry of U minerals.