Abstract:

The crystal structure of furongite, Al4[(UO2)4(PO4)6](OH)2(H2O)19.5, from the Kobokobo pegmatite, Kivu, Democratic Republic of Congo, was solved for the first time. Furongite is triclinic, the space group P1¯, Z = 2, a = 12.1685(8), b = 14.1579(6), c = 17.7884(6) Å, α = 79.822(3), β = 77.637(4), γ = 67.293(2)°, and V = 2746.2(2) Å3. The crystal structure was refined from single-crystal X-ray diffraction data to R1 = 0.0733 for 7716 unique observed reflections, and to wR2 = 0.2081 for all 12,538 unique reflections. The structure of furongite contains infinite uranyl phosphate sheets of composition [(UO2)4(PO4)6]10− which are parallel to (1 0 1). The sheets are constituted by UrO5 pentagonal bipyramids and PO4 tetrahedra which share edges and vertices, and adjacent sheets are linked by a dense network of hydrogen bonds. Running through the sheets and connected mainly to the free apical oxygen atom of PO4 tetrahedra are Al octahedra connected together to form remarkable Al2O5(OH)(H2O)5 and Al4O8(OH)2(H2O)10 clusters. These Al clusters are only bonded to one sheet, and do not connect two adjacent sheets together. The topology of the uranyl phosphate sheets is related to the uranophane anion topology, and can be described as a new geometrical isomer of the uranophane group. Furongite is the first uranyl phosphate reported in nature with a U:P ratio of 2:3.

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