| Popis: |
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 is found in the zone of hydrothermal alteration of U–V-bearing carbon-rich silicified schists, in association with quartz, calcite, alumohydrocalcite, allophane, crandallite, kyrgyzstanite, ankinovichite and an unknown Al–OH-mineral. Nickelalumite formed by hydrothermal alteration of U–V bearing carbon-rich silicified schists. 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 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. Electron-microprobe analysis gave Al2O3 39.94, SiO2 0.17, SO2 12.16, V2O3 0.29, FeO 0.15, NiO 8.00, ZnO 6.21, (H2O)calc. 31.87, sum 98.79 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 and Fe for Ni and V for S. 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. |
