Nickelalumite, ideally NiAl4(SO4)(OH)12(H2O)3, a new-old mineral from the Kara-Tangi uranium deposit, Kyrgyzstan

Abstract

Nickelalumite, ideally NiAl4(SO4)(OH)12(H2O)3, is a newly approved mineral from the Batken region, Kyrgyzstan, where it occurs in the Kara-Tangi and Kara-Chagyr uranium deposits. It formed in a zone of hydrothermal alteration of U–V-bearing carbonaceous siliceous schists, in association with quartz, calcite, alumohydrocalcite, allophane, crandallite, kyrgyzstanite, ankinovichite and an unknown Al–OH-mineral. It occurs as aggregates of colourless to pistachio-green radiating bladed crystals from 0.05 to 0.50 mm long. It is vitreous to transparent in thin flakes, has a white streak, and shows no fluorescence under long-wave or short-wave ultraviolet light. Cleavage is perfect parallel to {001} and no parting was observed. Mohs hardness is 2, it is brittle and has a splintery fracture. The calculated mass density is 2.231 g cm–3. In transmitted plane-polarized white light, nickelalumite is non-pleochroic, biaxial, α = 1.542(2), γ = 1.533(2), β could not be measured due to the almost negligible thickness of the flakes. EPMA chemical analysis gave Al2O3 39.94, SiO2 0.17, SO3 15.20, V2O3 0.29, FeO 0.15, NiO 8.00, ZnO 6.21, (H2O)calc. 31.87, total 101.83 wt%, H2O was determined by crystal-structure analysis, and the empirical formula is as follows: (Ni0.55Zn0.39V0.02Fe0.01)Σ0.97(Al3.99Si0.01)Σ4.00 (SO4)(OH)12(H2O)3 based on 4 (Al + Si) cations. There is considerable variation in substitution of Zn, Cu, Fe and V3+ for Ni and V5+ for S6+. Nickelalumite is monoclinic, P21/n, a = 10.2567(5), b = 8.8815(4), c = 17.0989(8) Å, β = 95.548(1)°, V = 1550.3(2) Å3, Z = 4. The crystal structure of nickelalumite was refined to an R1 index of 5.66% and consists of interrupted [NiAl4(OH)12] sheets intercalated with layers of {(SO4)2(H2O)3}; nickelalumite is a member of the chalcoalumite group.