Twinning and incommensurate modulation in baumoite, Ba-0.5[(UO2)(3)O8Mo2(OH)(3)](H2O)(similar to 3), the first natural Ba uranyl molybdate

摘要

Baumoite, Ba-0.5[(UO2)(3)O8Mo2(OH)(3)](H2O)(similar to 3), is a new mineral found near Radium Hill, South Australia, where it occurs in a granite matrix associated with baryte, metatorbernite, phurcalite and kaolinite. Baumoite forms thin crusts of yellow to orange-yellow tabular to prismatic crystals. The mineral is translucent with a vitreous lustre and pale yellow streak. Crystals are brittle, the fracture is uneven and show one excellent cleavage. The Mohs hardness is similar to 21/2 . The calculated density is 4.61 g/cm(3). Optically, baumoite crystals are biaxial (-), with alpha = 1.716(4), beta= 1.761(4), gamma = 1.767(4) (white light); and 2V(calc) = 42.2 degrees. Electron microprobe analyses gave the empirical formula Ba0.87Ca0.03Al0.04U2.97Mo2.02P0.03O22H11.99, based on 22 O atoms per formula unit. The eight strongest lines in the powder X-ray diffraction pattern are [d(obs )angstrom (I) (hkl)]: 9.175(39)(12 (1) over bar), 7.450(100)(020), 3.554(20)(221), 3.365(31)(004, 202), 1255(31)(123, 30 (2) over bar), 3.209(28)(12 (4) over bar), 3.067(33)(30 (3) over bar, 222, 32 (2) over bar) and 2.977(20)(142). Single-crystal X-ray studies (R-1 = 5.85% for 1892 main reflections) indicate that baumoite is monoclinic, superspace group X2/m(a0g)0s with X = (0,1/2,0,1/2), with unit-cell parameters: a = 9.8337(3), b = 15.0436(5), c = 14.2055(6) angstrom, beta = 108.978(3)degrees, V= 1987.25(13) angstrom(3) and Z = 4. The crystal structure is twinned and incommensurately modulated and is based upon sheets of U6+ and Mo6+ polyhedra of unique topology. Four independent cationic sites partially occupied by Ba atoms are located between the sheets, together with H2O molecules.