New Data on Minerals. М., 2008. Volume 43 23 BLACK POWELLITE FROM MOLYBDENUM-URANIUM DEPOSIT Galina A. Sidorenko, Natalia I. Chistyakova All-Russia Institute of Mineral Resources, Moscow, [email protected] Olga A. Doinikova Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, RAS, Moscow, [email protected] Nikita V. Chukanov Institute of Problems of Chemical Physics, RAS, Chernogolovka, Moscow oblast, [email protected] Irina S. Naumova, Vladimir I. Kuzmin All-Russia Institute of Mineral Resources, Moscow, [email protected] New data on black powellite from a Mo-U deposit, South Kazakhstan are given. Bipyramidal crystals of the min- eral have been found in intimate association with uranium minerals of the oxidized zone, including uranyl-arsen- ate mica (uramarsite) and uranyl silicate uranophane-beta. X-ray diffraction, infrared spectroscopy, differential thermal analysis (DTA), analytical scanning electron microscopy (ASEM), electron microprobe, X-ray fluores- cence analysis (XRFA), and laser spectrography have been performed to examine the mineral. Two varieties of powellite have been identified: crystalline in uranophane and amorphous in uranate. The causes of black color of powellite are discussed. This coloration of powellite can be prospecting guide for deposits of radioactive elements. 9 figures, 10 references. Keywords: powellite, uramarsite, uranophane, uranate, Mo-U deposit Bota-Burum, South Kazakhstan. Tetragonal bipyramidal black crystals established in various samples. The compo- of 0.3–1 mm in size, whose chips are sitions of the mineral from uranyl-silicate translucent in red attracted attention of and uranate assemblages are as follows, mineralogists, when they studied oxidized wt. %: 26.91 CaO and 73.12 MoO3; and 26.63 zone of the Bota-Burum Mo-U deposit, CaO and 73.69 MoO3, respectively. These South Kazakhstan. Isolated crystals are correspond to the composition of powellite sealed in crystalline uranate (Fig. 1) and the formula CaMoO4. Iron less than 1% (Sidorenko et al., 2007). Intergrowths of was detected in the first analysis that is these variably oriented crystals cover a caused by oxide films; insignificant content surface (Fig. 1c) or heal cracks in host car- of other elements (Cu, U, Pb, Y, As) is due to bonatized felsite porphyry. Regular black the admixture of uranyl and clay minerals. bipyramids were identified as inclusions in The luster of crystals ranges from radiant aggregates (Fig. 2) of amber- adamantine to resinous, more frequent sub- coloured uranophane-beta (matrix mineral metallic. Microscopically, bipyramidal was determined by X-ray diffraction). crystals with adamantine luster are charac- The composition of black mineral from terized by imperfect and rough faces. various assemblages (various mineral matri- However, there are some differences in ces) was examined using electron micro- optical properties of powellite from various probe, X-ray fluorescence analysis (XRFA) assemblages. The crystals included in ura- and laser spectrography. The compositions nophane-beta (call them “silicate”) are of the black crystals from various assem- anisotropic, whereas those disseminated in blages (uranate and uranyl-silicate) are the uranate are isotropic. Infrared spectra of same; Ca and Mo are mineral-forming ele- powellite from these assemblages are sub- ments. X-ray element-distribution maps for stantially different (Fig. 4). Such distinction a sample of uranate assemblage are shown in IR spectra reflects the structural state of in Fig. 3. The chemical composition of these the mineral and can be resulted from both black bipyramids was investigated with a variable conditions of formation and post- JXA-8100 (JEOL, Japan) electron micro- crystallization history of the mineral. This probe. Similar contents of Ca and Mo were fact caused separated description of each 24 New Data on Minerals. М., 2008. Volume 43 abc Fig. 1. Black powellite from uranate assemblage. (a, b) Isolated bipyramidal crystals included in uramarsite; (c) intergrowths of crystals cov- ering surface of the rock. Fig. 2. (a) Inclusions of bipyramidal crystals black powellite in segregations of yellow b-u r a n o ph a n e; a b (b) bl a c k po w el l it e asso ciat ed w it h t al c from uranophane-beta assembl age. Fig. 3. (a) BSE image of inter- growths of metamict po- wellite crystals (grey) with grains of uranate (white) in silicate matrix (black); (b–f) X-ray element-distribution maps of U, As, Ca, Mo, and a b c d e f variety of powellite. Anisotropic Ca-molyb- isomorphic impurity; (3) structural damages date with IR spectrum of powellite was stud- caused by radioactive emanation (for exam- ied in more detail. Despite initial object for ple, smoky coloration of quartz), which can research, isotropic variety was examied in be removed by ignition to restore primary less detail due to extremely small amount. color of mineral. The aim of this study is to explain rare black color of powellite. There are few rea- Black powellite sons for change of typical color of a mineral: associated with uranyl silicate (1) disseminated mineral micro-admixture (for example, pink coloration of quartz is The crystals are uniaxial, with refractive caused by the microphase of goethite); (2) index more than 1.780. The density of po - Black Powellite from Molybdenum-Uranium Deposit 25 wellite measured with microvolumetric IR spectrum recorded with a SPECORD method by Vasilevsky ranges from 4.023 to 75 IR spectrometer is typical of powellite 4.195 g/cm3; the average value (six mea- (Fig. 4) characterized by insignificant surements) is 4.128 g/cm3 that is much less admixtutre of silicate (talc). than in handbooks [4.54–4.23 (Feklichev, Differential thermal analysis (DTA) of 1977); 4.25–4.52 (Lazarenko, 1963)]. The powdered black crystals indicated that a density becomes typical of powellite broad exothermal peak with total weight 4.249 g/cm3 after one hour ignition at 600oC. loss of 1.46% (Fig. 5) is observed at DTA X-ray powder diffraction pattern of the curve at heating up to 9000C and the further crystals from silicate assemblage corre- ignition results in textural and structural sponds to well-crystallized powellite with ordering. This exothermal peak can be insignificant admixture of -uranophane probably caused by that the thermal effect (X'Pert PRO Panalytical diffractometer; removes stress and elevated potential ener- CuK radiation). The tetragonal unit-cell gy accumulated by crystal. dimensions of black powellite are: а = Analytical scanning electron micro- 5.227 Å, с = 11.428 Å,с/a = 2.186. Sharp scopic (ASEM) study using a JSM 5300 X-ray diffraction pattern was remained after JEOL scanning electron microscope ignition of powder up to 700oC; no changes equipped with INCA energy dispersion of the unit-cell dimensions were document- system was performed to explain black ed: а = 5.229 Å , с = 11.430 Å, с/a = 2.186. coloration of powellite. The assumption of Si, respectively. 26 New Data on Minerals. М., 2008. Volume 43 Fig. 4. IR absorption spectra of powellite: (a) metamict (uranate assem- blage); (b) crys- talline (urano- phane-beta assem- blage). Asterisk denotes admixture of clay matter. x disseminated highly dispersed mineral interesting. The secondary emission image phase in powellite resulted in color trans- shows sheath-shaped character of bipyrami- formation was checked. No extraneous dal microcrystal having wide solid layer at mineral phases were identified in pow- the surface, disoriented mosaicism in the ellite. intermediate part, and powder-like porous Imperfect surface of faces of tetragonal structure in the core (Fig. 7). The dispersivi- bipyramid is clearly seen under electron ty of crystallite increases toward the core microscope. These faces are covered by that is like loose submicroblock (polycrys- heads of variable-sized microcrystals talline) segregation. The composition of cal- (Fig. 6). The inner structure on the fractured cium molybdate is unchangeable in the surface of bipyramidal crystallites is very whole volume. Black Powellite from Molybdenum-Uranium Deposit 27 ab In addition to Ca and Mo, traces of U or yellow (Fig. 8). Solid surface on chip of Pb were locally detected. E.V. Kop che no va ignited crystal (Fig. 9) indicates compaction and K.V. Skvortsova (1958) identified these of micro-blocky structure of the matter trace elements in powellite from Bota- observed in black crystals before ignition Burum. We suggest that this fact is caused (Fig. 7). Point analyses did not show by failure of textural homogeneity of crys- changes in composition of the surface of tals during post-crystallized alteration. ignited crystal. Small admixture of Al is Such continuous suprergene alteration led observed in the grain core; in wide marginal to micro-mosaicism and porosity of mineral zones Al peak slightly increases and matter within crystal. The solutions pene- insignificant Si peak appears; these ele- trating bipyramidal powellite crystals along ments are probably related to the primary microfractures destroyed the central part admixture of talc. composed of numerous nuclei. Black powellite associated with uranate Disorientation of microcrystals generated during growth and further natural selection Isolated powellite crystals (Fig. 1) were of larger blocks terminating growing single identified in uramarsite (NH4,H3O)2(UO2)2 crystal favored disintegration. Supergene (AsO4,PO4)2•6H2O, uranium mica discov- destruction of the powellite crystal was ered at Bota-Burum (Sidorenko et al., 2007). gradual with introduction of trace elements Black bipyramids are included in lamellar appeared in solutions during supergene uramarsite. transformation (U, Pb, As etc.). We can con- Microscopically, in immersion liquids, clude this on the basis of the texture black crystals of powellite recovered from revealed by ASEM study of crystal chip. lamellar uranate are isotropic. Despite Is it a combination of intrastructural fea- tetragonal bipyramidal shape, powellite tures that is an original cause of unusual crystals do not demonstrate X-ray diffrac- black coloration of powellite? According to tion pattern, thus they are X-ray amor- the handbook (Lazarenko, 1963), the typical phous.