Original paper

New nickel-uranium-arsenic mineral species from the oxidation zone of the Belorechenskoye deposit, Northern Caucasus, Russia: II. Dymkovite, Ni(UO2)2(As3+O3)2·7H2O, a seelite-related arsenite

Pekov, Igor V.; Levitskiy, Viktor V.; Krivovichev, Sergey V.; Zolotarev, Andrey A.; Chukanov, Nikita V.; Bryzgalov, Igor A.; Zadov, Aleksandr E.


A new arsenite mineral species dymkovite, ideally Ni(UO2)2(As3+O3)2·7H2O (IMA no. 2010-087), was found at the Belorechenskoye deposit, Adygea Republic, Northern Caucasus, Russia. It is a supergene mineral associated with rauchite, annabergite, and goethite in cavities of a dolomite vein with primary uraninite (pitchblende), nickeline, and gersdorffite. Dymkovite forms longprismatic, lath-shaped to acicular crystals (≤0.5 mm long, ≤0.05 mm thick), which are elongated along [010]. They are combined in sprays or open-work, chaotic groups up to 1.5 mm across; crusts up to 2 × 2 mm2 and up to 0.05-mm-thick also occur. Dymkovite crystals are transparent and bright yellow, whereas crusts are translucent and light yellow to light greenish-yellow. The luster is vitreous. The mineral is brittle, the Mohs' hardness is ca. 3. Cleavage was not observed. Dcalc is 3.806 g cm3. Dymkovite is optically biaxial (-), α = 1.625(2), β = 1.735(5), γ = 1.745(3), 2Vmeas = 20(10)°, 2Vcalc. = 32°. Dispersion is strong, r > v. Pleochroism is strong: X = very pale yellowish-green, Y ≈ Z = light greenish yellow. In the IR spectrum, bands of As3+O3 anions are strong, whereas bands of As5+O4 anions are very weak. The average chemical composition (electron microprobe) is (in wt%): MgO = 1.11, FeO = 0.24, NiO = 5.40, ZnO = 0.23, As2O3 = 19.57, P2O5 = 0.58, UO3 = 59.43, H2Ocalc = 13.44, total = 100.00. The empirical formula, calculated on the basis of 17 O apfu, is: (Ni0.69Mg0.26Fe0.03Zn0.03)Σ1.01U1.97(As3+1.88P0.08)Σ1.96O9.94·7.06H2O. Dymkovite is monoclinic, space group C2/m, a = 17.99(3), b = 7.033(7), c = 6.633(9) Å, β = 99.62(11), V = 827(3) Å3, Z = 2. The crystal structure was refined from single-crystal X-ray diffraction data (R


dymkovitenew mineral speciesarseniteseelitecrystal structureoxidation zonebelorechenskoye depositadygea