Original paper

A new uranyl phosphate sheet in the crystal structure of furongite

Dal Bo, Fabrice; Hatert, Frédéric; Philippo, Simon

European Journal of Mineralogy Volume 29 Number 3 (2017), p. 517 - 527

published: Jun 1, 2017

DOI: 10.1127/ejm/2017/0029-2629

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ArtNo. ESP147052903013, Price: 29.00 €

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Abstract

The crystal structure of furongite, Al4 [(UO2)4 (PO4)6](OH)2 (H2 O)19.5, from the Kobokobo pegmatite, Kivu, Democratic Republic of Congo, was solved for the first time. Furongite is triclinic, the space group P 1,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 singlecrystal X-ray diffraction data to R 1 = 0.0733 for 7716 unique observed reflections, and to wR 2 = 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 Ur O5 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 Al2 O5 (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.

Keywords

furongitetopologykobokobouranyl phosphatecrystal structurealuminium