New nickel-uranium-arsenic mineral species from the oxidation zone of the Belorechenskoye deposit,Northern Caucasus,Russia:II. Dymkovite,Ni(UO_2)_2(As~(3+)O3)2?7H_2O,a seelite-related arsenite

摘要

A new arsenite mineral species dymkovite,ideally Ni(UO_2)_2(As~(3+)O_3)2?_7H_2O(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(v. Pleochroism is strong:X = very pale yellowish-green,Y ≈ Z = light greenish yellow. In the IR spectrum,bands of As3+03 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_2O_3 = 19.57,P_2O_5 = 0.58,U03 = 59.43,H_2O_(calc)= 13.44,total = 100.00. The empirical formula,calculated on the basis of 17 O apfu,is:(Ni_(0.69)M_(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_2O. Dymkovite is monoclinic,space group Cλlm,a = 17.99(3),b = 7.033(7),c = 6.633(9)A,β = 99.62(11)°,V= 827(3)?~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-sharingU07pentagonal bipyramids and(As~(3+)O_3)triangular pyramids,which are linked through hydrogen bonds involving disorderedNi(H_2O)_6~(2+)octahedra and additional H20 molecules in the interlayer. The strongest lines of the powder X-ray patternd 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_2O. 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.