Nickelalumite, ideally NiAl 4 (SO 4 )(OH) 12 (H 2 O) 3 , a new-old mineral from the Kara-Tangi uranium deposit, Kyrgyzstan

Nickelalumite, ideally NiAl 4 (SO 4 )(OH) 12 (H 2 O) 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 Al 2 O 3 39.94, SiO 2 0.17, SO 3 15.20, V 2 O 3 0.29, FeO 0.15, NiO 8.00, ZnO 6.21, (H 2 O) calc. 31.87, total 101.83 wt%, H 2 O was determined by crystal-structure analysis, and the empirical formula is as follows: (Ni 0.55 Zn 0.39 V 0.02 Fe 0.01 ) Σ0.97 (Al 3.99 Si 0.01 ) Σ4.00 (SO 4 )(OH) 12 (H 2 O) 3 based on 4 (Al + Si) cations. There is considerable variation in substitution of Zn, Cu, Fe and V 3+ for Ni and V 5+ for S 6+ . Nickelalumite is monoclinic, P 2 1 / n , a = 10.2567(5), b = 8.8815(4), c = 17.0989(8) Å, β = 95.548(1)°, V = 1550.3(2) Å 3 , Z = 4. The crystal structure of nickelalumite was refined to an R 1 index of 5.66% and consists of interrupted [NiAl 4 (OH) 12 ] sheets intercalated with layers of {(SO 4 ) 2 (H 2 O) 3 }; nickelalumite is a member of the chalcoalumite group.