Abstract
Lakebogaite, ideally CaNaFe23+H(UO2)2(PO4)4(OH)2(H2O)8, is a new Ca-Na-Fe uranyl phosphate mineral from a quarry in Upper Devonian granite near Lake Boga, northern Victoria, Australia. It is associated with Na-analogue of meurigite (IMA 2007-024), torbernite, and saléeite on a matrix of microcline, albite, smoky quartz, and muscovite. Lakebogaite occurs as bright lemon-yellow transparent prismatic crystals up to 0.4 mm across. The crystals have a vitreous luster and a pale yellow streak. Mohs hardness is about 3. The fracture is even to conchoidal. In transmitted light, the mineral is pale yellow with very weak pleochroism: X = yellow, Y = grayish yellow, Z = grayish yellow: dispersion r > v, strong. Lakebogaite crystals are biaxial (+), with slightly variable refractive indices within the ranges: nα = 1.650(2)-1.652(2), nβ = 1.660(4)-1.664(3), nγ = 1.681(3)-1.686(2), measured using white light, and with 2Vmeas = 80-85° and 2Vcalc = 70-74°. Orientation: Y = b; crystals are elongated along [010], resulting in straight extinction. The empirical chemical formula (mean of nine electron microprobe analyses) calculated on the basis of 30 anions is (Ca1.00Na0.80Sr0.10)Σ1.90(Fe3+1.85Al0.30)Σ2.15(UO2)1.80 (PO4)4.07(OH,H2O)10.12. Lakebogaite is monoclinic, space group Cc, a = 19.6441(5), b = 7.0958(2), c = 18.7029(5) Å, β = 115.692(1)°, V = 2349.3(7) Å3, Z = 4. The seven strongest reflections in the powder X-ray diffraction pattern are [dobs in Å (I) (hkl)]: 6.60 (100) (110), 3.16 (40) (514̅, 604̅), 4.07 (20) (404̅), 3.80 (20) (314̅), 3.56 (20) (020, 312), 3.31 (20) (114, 220), 2.797 (20) (006). The crystal structure was solved from single-crystal X-ray diffraction data and refined to R1 = 0.038 on the basis of 5222 unique reflections with F > 4σF. It comprises pairs of edge-shared UO7 pentagonal bipyramids that are inter-linked via corner-sharing with PO4 tetrahedra, to form chains parallel to the c-axis. Each UO7 polyhedron also shares one of its edges with another PO4 tetrahedron. The (UO2)2(PO4)4 chains are cross-linked via corner-sharing between the PO4 tetrahedra and Fe3+O4(OH)2 octahedra. The octahedra join together by corner-sharing via OH anions to form chains parallel to b. The Na+ and Ca2+ cations, and 4 water molecules occupy eight-sided channels along [010]. The remaining water molecules occupy large ten-sided channels directed along [001] and intersecting the [010] channels. The mineral is named for the nearest township.
© 2015 by Walter de Gruyter Berlin/Boston
