Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Jakub Plášil

A novel sheet topology in the structure of kamitugaite, PbAl[(UO₂)₅(PO₄)_2.38(AsO₄)_0.62O₂(OH)₂](H₂O)_11.5

Journal of Geosciences, volume 62 (2017), issue 4, 253 - 260

DOI: http://doi.org/10.3190/jgeosci.246

Kamitugaite is a rare supergene uranyl phosphate of aluminum and lead occurring at the Kobokobo pegmatite in the Sud-Kivu Province, Democratic Republic of Congo; its structure has remained unknown until now. Based on single-crystal X-ray diffraction data carried out on the type specimen of kamitugaite (no. 13986, Royal Museum for Central Africa, Tervuren), it is triclinic, space group P-1, with a = 9.0296(8), b = 10.9557(8), c = 15.8249(15) Å, α = 89.585(7)°, β = 85.349(8)°, γ = 84.251(7)°, V = 1552.5(2) ų and Z = 2. The structure was refined from diffraction data to R = 0.1074 for 2697 unique observed reflections. The structure of kamitugaite is based upon infinite sheets of uranyl and phosphate polyhedra, stacked perpendicular to c; these sheets result from edge-sharing of UO₇ and UO₈ bipyramids, forming chains approximately parallel to b, which are linked by (P,As)O₄ tetrahedra. Such a sheet has not been observed in minerals or synthetic compounds and is related to the phosphuranylite topology; the ring symbol is 6¹5⁴4³3⁴. There are two distinct interlayer complexes in kamitugaite: one involving Pb²⁺ and H₂O groups and another involving octahedrally coordinated Al³⁺ and isolated H₂O groups. Adjacent sheets are linked a) through the Pb²⁺-O and H-bonds, and b) via H-bonds only in case of the interlayer with Al, the bonding differences being largely attributable to the very different stereochemistry of Pb²⁺ compared to Al³⁺. The unique combination of these two elements is probably a key reason for the scarcity of kamitugaite.