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Extending the mineralogy of U6+ (IV): Uranyl phosphate sheet of novel topology in the crystal structure of ranunculite

Published online by Cambridge University Press:  02 September 2025

Jakub Plášil Open the ORCID record for Jakub Plášil [Opens in a new window] ,
Nathan Steciuk ,
Gwladys Steciuk Open the ORCID record for Gwladys Steciuk [Opens in a new window] ,
Jiří Sejkora Open the ORCID record for Jiří Sejkora [Opens in a new window] ,
Markéta Jarošová Open the ORCID record for Markéta Jarošová [Opens in a new window] ,
Jan Rohlíček ,
Simon Philippo ,
Ivan Němec  and
Alexander Matthies
Jakub Plášil*
Affiliation:
Institute of Physics of the CAS, Prague, Czech Republic
Nathan Steciuk
Affiliation:
Independent Researcher, Nancy, France
Gwladys Steciuk
Affiliation:
Université de Lorraine, CNRS, Nancy, France
Jiří Sejkora
Affiliation:
Department of Mineralogy and Petrology, National Museum, Prague, Czech Republic
Markéta Jarošová
Affiliation:
Institute of Physics of the CAS, Prague, Czech Republic
Jan Rohlíček
Affiliation:
Institute of Physics of the CAS, Prague, Czech Republic
Simon Philippo
Affiliation:
Section Minéralogie, Musée d’Histoire Naturelle, Luxembourg
Ivan Němec
Affiliation:
Department of Inorganic Chemistry, Faculty of Science, Charles University, Prague, Czech Republic
Alexander Matthies
Affiliation:
Independent Researcher, Nancy, France Independent Researcher, Frieberg, Germany
*
Corresponding author: Jakub Plášil; Email: plasil@fzu.cz
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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) Å3 (Z = 4). The structure (C2/c) was refined kinematically to R1 = 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 U6+), pentagons (occupied by U6+), squares (occupied by Al3+) and triangles (occupied by P5+). 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 PO4 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 61514232. In the interlayer, there are two Al3+-hosting sites (one [6]- and [5]-coordinated; the pyramidal one is only partially occupied), as well as isolated H2O 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).

Keywords

ranunculitealuminium uranyl phosphateelectron diffractioncrystal structuresheet topology

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Article
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Mineralogical Magazine , Volume 90 , Issue 1 , February 2026 , pp. 164 - 175
Copyright
© The Author(s), 2025. Published by Cambridge University Press on behalf of The Mineralogical Society of the United Kingdom and Ireland.

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