Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Fabrice Dal Bo, Frédéric Hatert, Simon Philippo

New crystallographic data and formula revision of phuralumite, Al₂[(UO₂)₃(PO₄)₂O(OH)](OH)₃(H₂O)₉

Journal of Geosciences, volume 62 (2017), issue 2, 87 - 95

DOI: http://doi.org/10.3190/jgeosci.233

The crystal structure of phuralumite, Al₂[(UO₂)₃(PO₄)₂O(OH)](OH)₃(H₂O)_9, from the Kobokobo pegmatite (Kivu, Democratic Republic of Congo), was refined from single crystal X-ray diffraction data. Four samples have been investigated in this study, and the crystal structure was to R₁ = 0.0566, 0.0544, 0.0661 and 0.0353, for 5739, 5953, 6007 and 5793 unique observed reflections, for the samples VC2048, VC2048, VC2054 and VC2055, respectively. Phuralumite is monoclinic, space group P2₁/n, a = 9.4407(3), b = 20.8596(8), c = 13.4326(4) Å, β = 107.905(4)°, V = 2517.40(17) ų, and Z = 4 (sample VC2048). The structure consists of [(UO₂)₃(PO₄)₂O(OH)]^3- layers, parallel to (010), which are connected by Al³⁺ ions and H₂O molecules. The uranyl phosphate sheets show the phosphuranylite anion topology, while the Al³⁺ ions occur in 5- and 6-fold coordination and are connected together to form Al₄O₄(OH)₆(H₂O)₄ clusters. The crystallographic data obtained from the four structural models converge and confirm the previously determined structure for phuralumite. However, these new data also show some discrepancies in the bond-valence analysis, especially in the assignment of the OH^- groups and water molecules. As a consequence of this study, the structural formula of phuralumite, previously reported as Al₂[(UO₂)₃(PO₄)₂(OH)₂](OH)₄(H₂O)₁₀, must be modified to Al₂[(UO₂)₃(PO₄)₂O(OH)](OH)₃(H₂O)₉.