Extending the mineralogy of U 6+ (VIII): Leishmanite, Ba(H 2 O) 4 [(UO 2 ) 3 O 2 (OH) 3 ] 2 ·3H 2 O, a new mineral from Valais, Switzerland

Abstract The new mineral leishmanite (IMA2026-002), Ba(H 2 O) 4 [(UO 2 ) 3 O 2 (OH) 3 ] 2 ·3H 2 O, was found in the small uranium occurrence near Nendaz, Valais, Switzerland, where it occurs as elongated tabular crystals (up to 0.1 mm) in a peculiar assemblage with parauranophane and ‘biotite’ in a thin quartz veinlet, concordant with the schistosity of a very finely bedded paragneiss; the origin of the mineral is most probably from the retrograde metamorphic fluids. The streak is pale yellow. Crystals are translucent with vitreous lustre. The tenacity is brittle, the Mohs hardness is ∼2½, and the fracture is curved. The mineral has D calc. of 5.217 g·cm –3 for the ideal formula. The mineral is optically biaxial (–) with α = 1.755(3), β = 1.796(3) and γ = 1.799(3); 2V is 28(2)°; orientation is X = b, Y and Z lie in {010}; pleochroism: X colourless, Y yellow, Z yellow; X < < Y < Z . Electron microprobe analyses provided (Ba 0.91 Pb 0.07 K 0.06 ) Σ1.04 (H 2 O) 4 [(UO 2 ) 6 O 4.02 (OH) 5.98 ]·3H 2 O. Leishmanite is orthorhombic, Pmn 2 1 , a = 12.079(2), b = 15.096(3), c = 7.155(2) Å, V = 1304.7(5) Å 3 and Z = 2. The crystal structure ( R 1 = 0.0392 for 6324 I > 3σ I reflections) contains uranyl-oxo-hydroxo sheets of both the α-U 3 O 8 (protasite) topology with the PD stacking sequence and β-U 3 O 8 topology with the DRU stacking sequence. Between the sheets, Ba 2+ is coordinated by three H 2 O molecules, and an additional four H 2 O molecules are not bonded to any metal cation. The structure and chemical composition of leishmanite are discussed with respect to the paragenetic relationships within alteration associations of uraninite, based on the bond-valence approach.