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

Abstract

A new arsenite mineral species dymkovite, ideally Ni(UO 2 ) 2 (As 3+ O 3 ) 2 ·7H 2 O (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 mm 2 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), 2 V meas = 20(10)°, 2 V 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 As 3+ O 3 anions are strong, whereas bands of As 5+ O 4 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, As 2 O 3 = 19.57, P 2 O 5 = 0.58, UO 3 = 59.43, H 2 O calc = 13.44, total = 100.00. The empirical formula, calculated on the basis of 17 O apfu, is: (Ni 0.69 Mg 0.26 Fe 0.03 Zn 0.03 ) ∑1.01 U 1.97 (As 3+ 1.88 P 0.08 ) ∑1.96 O 9.94 ·7.06H 2 O. Dymkovite is monoclinic, space group C 2/ m , a = 17.99(3), b = 7.033(7), c = 6.633(9) A, β = 99.62(11)°, V = 827(3) A 3 , Z = 2. The crystal structure was refined from single-crystal X-ray diffraction data ( R 1 = 0.063). The structure is based upon the [(UO 2 )(As 3+ O 3 )] − sheets formed by chains of edge-sharing [UO 7 ] pentagonal bipyramids and (As 3+ O 3 ) triangular pyramids, which are linked through hydrogen bonds involving disordered [Ni(H 2 O) 6 ] 2+ octahedra and additional H 2 O molecules in the interlayer. The strongest lines of the powder X-ray pattern [ d in A ( 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(UO 2 ) 2 [(As 3+ O 3 ) 1.4 (As 5+ O 4 ) 0.6 ]·7H 2 O. 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.