New nickel-uranium-arsenic mineral species from the oxidation zone of the Belorechenskoye deposit, Northern Caucasus, Russia. Pt. II. Dymkovite, Ni(UO₂)₂(As³⁺O₃)₂.7H₂O, a seelite-related arsenite

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 x 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. D_calc is 3.806 g cm^-3. Dymkovite is optically biaxial (-), α = 1.625(2), β = 1.735(5), γ = 1.745(3), 2V_meas = 20(10) , 2V_calc. = 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, H₂O_calc = 13.44, total = 100.00. The empirical formula, calculated on the basis of 17 O apfu, is: (Ni_0.69Mg_0.26Fe_0.03Zn_0.03)_Σ1.01 U_1.97(As³⁺_1.88P_0.08)_Σ1.96O_9.94.7.06H₂O. Dymkovite is monoclinic, space group C2/m, a = 17.99(3), b = 7.033(7), c = 6.633(9) Aa, β = 99.62(11) , V = 827(3) Aa³, 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 Aa(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.