Extending the mineralogy of U ⁶⁺ (IV): Uranyl phosphate sheet of novel topology in the crystal structure of ranunculite

Abstract Ranunculite is a rare supergene hydrated aluminium uranyl phosphate reliably reported only from the type locality – the Kobokobo pegmatite in the Sud-Kivu province, Democratic Republic of Congo; its structure has remained unknown until now. Based on 3D electron diffraction data, ranunculite is monoclinic, with a C -centred unit cell: a = 11.1812(7) Å, b = 17.9281(5) Å, c = 17.91548(16) Å, β = 98.350(4)°, and V = 3553.2(2) Å 3 ( Z = 4). The structure ( C 2/ c ) was refined kinematically to R 1 = 0.4114 for 1697 unique observed reflections. The structure of ranunculite is based upon infinite uranyl-phosphate sheets of novel topology. The two-dimensional representation of the structural unit consists of hexagons (occupied by U 6+ ), pentagons (occupied by U 6+ ), squares (occupied by Al 3+ ) and triangles (occupied by P 5+ ). Those sheets are stacked perpendicular to c ; the interplanar distance is ~9.5 Å. They result from the clusters of edge-sharing uranyl hexagonal and pentagonal bipyramids linked by Al-octahedra and PO 4 tetrahedra. The decoration of the sheets is unique but somewhat resembles the arrangement (of U-clusters, squares and triangles) observed in bijvoetite and lepersonnite topologies; the ring symbol is 6 1 5 1 4 2 3 2 . In the interlayer, there are two Al 3+ -hosting sites (one [6]- and [5]-coordinated; the pyramidal one is only partially occupied), as well as isolated H 2 O groups. There is an extensive network of hydrogen bonds; adjacent sheets are either held by hydrogen bonds only or by tetramers of Al-polyhedra when occupied (through shared O19). This arrangement most probably causes a poor crystallinity of ranunculite (which gives rise to stacking faults observed in the powder diffraction data).