New nickel-uranium-arsenic mineral species from the oxidation zone of the Belorechenskoye deposit, Northern Caucasus, Russia: I. Rauchite, Ni(UO2)2(AsO4)2·10H2O, a member of the autunite group

Abstract

Rauchite, ideally Ni(UO 2 ) 2 (AsO 4 ) 2 ·10H 2 O (IMA no. 2010-037), a new arsenate mineral species of the autunite group, was found at the Belorechenskoye deposit, Adygea Republic, Northern Caucasus, Russia. It is a supergene mineral associated with dymkovite, annabergite and goethite in cavities of a dolomite vein with primary uraninite (pitchblende), nickeline and gersdorffite. Rauchite forms pseudo-tetragonal lamellar crystals (the main form is {001}) up to 0.5 mm across, typically split, like a fan or open book, and their clusters or crusts as large as 2 mm. Rauchite is transparent to translucent and light yellowish-green. The lustre is vitreous. The mineral is brittle, the Mohs’ hardness is ca. 2. The cleavage is {001} perfect. D calc is 3.427 g cm −3 . Rauchite is optically biaxial (−), α = 1.550(3), β = 1.578(1), γ = 1.581(1), 2 V meas = 40(5)°, 2 V calc = 36°. The average chemical composition (mean of eight electron-microprobe analyses) is (in wt%): MgO = 0.71, CoO = 0.07, NiO = 5.38, ZnO = 0.08, P 2 O 5 = 1.08, As 2 O 5 = 20.26, UO 3 = 54.22, H 2 O calc = 17.10, and the total = 98.90. The empirical formula calculated on the basis of 22 O apfu is: (Ni 0.76 Mg 0.19 Co 0.01 Zn 0.01 ) ∑ 0.97 U 2.00 O 4 (As 1.86 P 0.16 ) ∑2.02 O 8 ·10H 2 O. Rauchite is triclinic, space group I -1, a = 7.100(3), b = 7.125(3), c = 19.955(8) A, α = 92.406(14), β = 94.924(14), γ = 90.420(6)°, V = 1004.7(7) A 3 , Z = 2. [parameters of the reduced P cell are: a = 7.100(3), b = 7.125(3), c = 10.751(4) A, α = 106.855(7), β = 104.366(7), γ = 90.420(6)°, V = 502.4(4) A 3 , Z = 1]. The crystal structure was refined from single-crystal X-ray diffraction data obtained at 153 K ( R 1 = 0.089). The structure is based upon autunite-type [(UO 2 )[AsO 4 ]] − layers with Ni 2+ coordinated by six H 2 O molecules and located in the interlayer space. The strongest lines in the powder X-ray diffraction pattern are [ d in A( I )( hkl )]: 9.97(100)(002), 6.641(22)(003), 4.936(62)(004, 01-3, −111), 4.533(41)(−112), 3.539(93)(020, 200, 20-1, 01-5, 02-1), 3.388(43)(20-2, 015, 02-2, 105), 2.488(27)(220, 2-21, −125, 1-25, 22-2, −222), and 2.233(27)(1-31, 3-10, 13-1, 31-1, 31-2, 2-24, 13-2, 21-7). The structure of rauchite corresponds to the 1 A -type stacking arrangement of uranyl arsenate layers in the autunite group of minerals and synthetic compounds. The mineral is named in accordance with the naming rules accepted for the autunite group as the hydrated analogue of metarauchite, Ni(UO 2 ) 2 (AsO 4 ) 2 ·8H 2 O.