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Lakebogaite, CaNaFe₂³⁺H(UO₂)₂(PO₄)₄(OH)₂(H₂O)₈, a new uranyl phosphate with a unique crystal structure from Victoria, Australia

¹ School of Earth Sciences, University of Melbourne, Parkville, Victoria, 3010, Australia
² Geosciences, Museum Victoria, GPO Box 666, Melbourne, Victoria, 3001, Australia
³ CSIRO Minerals, Box 312, Clayton South, Victoria, 3169, Australia
⁴ Mineralogisch-Petrographische Abt., Naturhistorisches Museum Wien, Burgring 7, A-1010 Wien, Austria

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: n = 1.650(2)–1.652(2), n_β = 1.660(4)–1.664(3), n = 1.681(3)–1.686(2), measured using white light, and with 2V_meas = 80–85° and 2V_calc = 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 (Ca_1.00Na_0.80Sr_0.10)_1.90(Fe_1.85³⁺Al_0.30)_2.15(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) ų, Z = 4. The seven strongest reflections in the powder X-ray diffraction pattern are [d_obs in Å (I) (hkl)]: 6.60 (100) (110), 3.16 (40) (51, 60), 4.07 (20) (40), 3.80 (20) (31), 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 > 4F. 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.

Key Words: Lakebogaite • new mineral • uranyl phosphate • Lake Boga quarry • north-western Victoria • Australia • crystal structure • (UO₂)₂(PO₄)₄ chains