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 long-prismatic, 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 cm−3. 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, YZ = 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)∑1.01 U1.97(As3+1.88P0.08)∑1.96O9.94·7.06H2O. 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 (R1 = 0.063). The structure is based upon the [(UO2)(As3+O3)] sheets formed by chains of edge-sharing [UO7] pentagonal bipyramids and (As3+O3) triangular pyramids, which are linked through hydrogen bonds involving disordered [Ni(H2O)6]2+ octahedra and additional H2O molecules in the interlayer. The strongest lines of the powder X-ray pattern [d in Å (I)(hkl)] are: 8.93(100)(200), 4.463(34)(111, 400), 3.523(23)(020), 3.276(21)(220), 3.008(26)(11-2), 2.846(27)(112, 221, 31-2). Dymkovite is a Ni-dominant, almost arsenate-free analogue of seelite, Mg(UO2)2[(As3+O3)1.4(As5+O4)0.6]·7H2O. The mineral is named in honor of the Russian mineralogist Yuriy Maksimovich Dymkov (b. 1926), a specialist in U mineralogy, the geology of U deposits, and problems of mineral formation, who was one of the first researchers of the U ores of the Belorechenskoye deposit.

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