Les phosphates d'uranyle et d'aluminium de Kobokobo. IX. L'althupite AlTh(UO2)[(UO2)3O(OH)(PO4)2]2(OH)3.15H2O, nouveau minéral ; propriétés et structure cristalline

Abstract

Althupite occurs in pegmatitic rocks with beryl and columbite at Kobokobo, Kivu, Zaire, in association with numerous uranyl phosphates. It appears as thin tablets, with maximum length 0.1 mm along [001], flattened on (100), yellow and transparent. Triclinic, space group P-1, a = 10.953(3), b = 18.567(4), c = 13.504(3)Å, α = 72.64(2), β = 68.20(2), γ = 84.21(2)°, V = 2434(1)Å3, Z = 2. Calculated density : 3.98 gr.cm-3. Strongest lines of X-ray powder pattern : 10.2 Å(10)100, 5.09(7)200, 5.80(5)-121, 4.41(5)2-21 and 123, 2.896(5)044 and 324. Biaxial negative 2V = 34°, α = 1.620, β = 1.661, γ = 1.665. Not fluorescent under UV lights. Strong dispersion r < ν. Chemical composition : AI2O3 1.75, ThO2 9.05, UO3 68.65, P2O5 9.73, H2O 10.81 %. Structure was determined by X-ray diffraction (R = 8.2 % for 4 220 reflections). It consists of [(UO2)3O(OH)(PO4)2]n3n- layers which characterize the phosphuranylite structural group. Layers are connected by two joined octahedra Al2(OH)6(H2O)4, one tri-capped trigonal prism ThO3(OH)3(H2O)3 and one pentagonal bi-pyramid UO2(H2O)3O2. Structural formula : AlTh(UO2)[(UO2)3O(OH)(PO4)2]2(OH)3. 15H2O.