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: (Ni_0.55Zn_0.39V_0.02Fe_0.01)_Σ0.97(Al_3.99Si_0.01)_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) ų, 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.