BLACK POWELLITE from MOLYBDENUM-URANIUM DEPOSIT Galina A

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 mineral have been found in intimate association with uranium minerals of the oxidized zone, including uranyl-arsenate 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 fluorescence 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 intergrowths of metamict powellite 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 examied 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 assemblage); (b) crystalline (uranophane-beta assemblage). Asterisk denotes admixture of clay matter.

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. Kopche 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. pale yellow color of powellite is diagnostic There are no bands of powellite in IR feature of the mineral. Specified structural spectra (Fig. 4a). Characteristic bands of failures can be removed by heat treatment, water molecules and a molybdenum oxide which was performed. indicate substantial transformation of the Black bipyramidal crystals ignited up to mineral. 7000C in air in muffler changed their color Combination of crystal shape of po- to conventional slightly grayish greenish wellite, optical isotropy, X-ray amorphism, 28 New Data on Minerals. М., 2008. Volume 43 a b

c d

and bands of H2O and Mo oxide in IR spec- tures are characteristic of black powellite trum shows the metamict state of powellite from uranate assemblage. This structural crystals associated with uranate. Godovikov failure can be caused by sufficiently strong (1975) reported certain instability of pow- radiation that results in the displacement of ellite, which “under supergene conditions atoms in the crystal structure of the mineral. … is easily transformed into molybdenum Most important lithospheric radioactive acid and hydromolybdates”. elements are U, Th, and 40K whose decay is A term “metamict mineral” introduced accompanied with a-, b-, and g-emisson are in mineralogy by Brögger (1896) implies attributed to such natural sources of radia- notion “mix otherwise” meaning a mineral, tion. -component that can displace atoms in where the arrangement of molecules in the crystal structure is the most effective. For crystal structure differs from that in an ini- example, colored halos around mineral tial crystal. In an extreme case, this results inclusion containing radioactive elements in amorphous state of mineral with pre- in transparent colorless single crystals of served crystal shape, but with optical quartz visualize such effect. isotropy, absence of cleavage, and con- X-ray amorphous (metamict) state is choidal fracture. Thus, the preservation of characterized by higher potential energy as crystal shape with the failure (to complete compared to the same matter but in crys- destruction) of three-dimensional lattice talline state. Heating of metamict mineral periodicity in a crystal structure is the major releases this energy that is recorded on the feature of metamict state. Just these fea- DTA curves as exothermal peak at tempera- Black Powellite from Molybdenum-Uranium Deposit 29 ture (temperature of recalescence) lower of the contact of the mineral with radioac- than that of destruction or melting of the tive elements and variable degree of super- mineral. The similar energy release can gene alteration of powellite (for example, cause exothermal effect on the DTA curve hydration, micro-mosaicism etc.). This fact of uranophane associated powellite, which can be explained on the basis of general fea- lost typical yellowish color (indicating tures and stages of supergene formation of transformation of mineral matter) but pre- uranium minerals (Belova, 2000; Belova and served ability of diffraction. Probably, maxi- Doinikova, 2003; Doinikova et al., 2003). mum transformation leading to complete Uranate are formed at the later oxidized amorphization occurs in powellite associat- stage than uranyl silicates. Uranate charac- ed with uranate. Unfortunately, due to lack terize longer process of uranyl mineraliza- of material, thermal analysis of powellite tion. Hence, powellite associated with associated with uranate was not performed. uranate was longer affected by radioactive Ignition of metamict mineral either medium. Powellite from the uranophane restores initial crystal structure (recrystal- assemblage was shorter affected by urani- lization when minimum fragments of initial um mineralization that resulted only in the structure were preserved in the structure) or failure of the matter continuity (texture) results in crystallization of simple oxides of within crystals but was sufficient for disap- mineral-forming elements. In any case, pearance of anisotropy and ability for dif- amorphous matter is devitrified. The similar fraction of the mineral. G.K. Krivokoneva divitrification of porous, but crystalline mat- (pers. comm.) noted that crystals of pow- ter we observed after ignition of black ellite associated with uranyl silicate are bipyramidal crystals of powellite associated black only on the surface, whereas inside, with uranophane. they are yellowish white. This observation Metamict state is not typical of po - supports our assumption of radiation nature wellite. Therefore, the following remains of black color of the mineral. In other words, undecided: what did lead crystal structure longer contact with radiation leads to both of the examined mineral with heterodesmic failure and metamict radioactive decay that interatomic bonds (degree of Ca-O ionic occurs in case of uranate assemblage. bond is near two times more than Mo-O) to Relatively less irradiated powellite from the metamict, i.e., amorphous state with the uranyl silicate assemblage preserves three- preservation of crystal shape. As aforemen- dimensional order of crystal structure with tioned, transition from crystalline to metam- high mosaicity. ict state as a rule is caused by the irradiation The distinction in mineral assemblages of the own matter of the crystal. In our case, containing bipyramids of powellite is the mineral matrix composed of secondary caused by irregular formation conditions of uranium minerals is a source of radioactive secondary uranium minerals that were emanation because black crystals of po - reported by E.V. Kopchenova and wellite are dipped in it. K.V. Skvortsova (1958), who studied miner- Close sizes of powellite grains from dif- alogy of this deposit for a long period: ferent above-described mineral matrices “Nasturane-sulfide mineralization at the and quite identical shape of bipyramidal Bota-Burum deposit hosted in altered felsit crystals indicate simultaneous formation of volcanic rocks oxidize along highly jointed powellite from different assemblages at the zones that causes extremely irregular sec- deposit. According to the shape of powellite ondary mineralization”. Therein and in crystals, the mineral predates uranyl miner- description of stages of supergene uranium alization. Different structural state of po - mineralization (Belova and Doinikova, wellite (from uranate and uranyl silicate 2003), there was reported that uranium assemblages) can testify to varied duration hydroxides and silicates precipitate in neu- 30 New Data on Minerals. М., 2008. Volume 43 tral and weak-acid media, whereas uranium incorporate by isomorphic substitution into micas, in acid. the structure of the other minerals. Summarizing the results obtained, we Probably, small size (few mm) of powellite conclude that black color of powellite is crystals in comparison with volume of host related to radiation effect upon the mineral uranium mineralization played important resulted in the transformation of its struc- role. Such significant structural failure was ture from micro-blocky to the complete loss favored by polyvalency of Mo. of three-dimensional order. In other words, The documented amorphization of po- blocks of coherent scattering (BCS) wellite should attract attention of mineralo- decrease and powellite becomes metamict. gists because metamictization of calcium Radiation medium leads to the failure of molybdate indicates the possibility of simi- definite molybdenum coordination in po- lar transformation for other minerals. wellite. In the mineral structure, character of chemical bond changes and coordination References polyhedron of molybdenum is distorted with invariability of hexavalent Mo in Belova L.N. Formation conditions of oxidation zones of uranium deposits and uranium min- supergene zone. We suggest that this phe- eral accumulations in the supergene zone // nomenon is similar to darkening of phos- Geol. Ore. Dep. 2000. V. 42 (2). P. 103–110. phate (or arsenate) uranate as a result of (In Russian). pyrophosphate (pyroarsenate) complexes Belova L.N. and Doinikova O.A. Conditions of in the crystal structure of this mineral formation of uranium minerals in the oxida- (Belova et al., 1992). The change of black tion zone of uranium deposit // Geol. Ore. color of powellite to characteristic greenish Dep. 2003. V. 45 (2). P. 130–132. (In Russian). Belova L.N., Gorshkov A.I., Doinikova O.A., yellow clearly testifies to radiation origin of Dikov Yu.P., Lyubomilova G.V., and Sivtsov structural failure of this mineral and its A.V. New data on U4+-bearing mica // Dokl. unusual color. Akad. Nauk. 1992. V. 325 (1). P. 139–141. (In Thus, black color of powellite can be Russian). considered as an indicator of radioactivity Brögger W. Die Minerallen der Sienit-pegmatit- of host rocks. Such specific color of pow- gänge der südnorwegischen Angitung Ne- ellite can serve as a prospecting guide for phelin syenite // Zeitschr. Kristallogr., Mineral. 1896. Bd. 16. deposits of radioactive elements. Doinikova O.A., Belova L.N., Gorshkov A.I., and Intimate association of powellite with Sivtsov A.V. The uranium blacks: genetic secondary uranium minerals, whose condi- problem and mineral composition // Geol. tions of formation are known, testifies to the Ore. Dep. 2003. V. 45 (6). P. 452–465. (In stability of powellite in near-neutral and Russian). acidic media. This allows recalling numer- Feklichev V.V. Diagnostic spectra of minerals. ous publications concerning reasons of Moscow: Nedra. 1977. 288 p. (In Russian). Godovikov A.A. Mineralogy. Moscow: Nedra. metamictisation of minerals, where role of 1975. 520 p. (In Russian). hydration was noted. In our case, revealed Kopchenova E.V. and Skvortsova K.V. Uranium- micromosaicity favored this process and bearing powellite // Proc. Second Internat. supergene solution along with other ele- Conf. Application of Atomic Energy for ments introduced uranyl, which was Peace. 1958. V. 3. P. 192–193. (In Russian). radioactive source in an interblock space of Lazarenko E.K. Course of mineralogy. Moscow: crystallites. Vysshaya Shkola. 1963. 560 p. (In Russian). The conclusion on the existence of Sidorenko G.A., Chukanov N.V., Chistyakova N.I., Bebeshko G.I., Zadov A.E., and Naumo - metamict powellite is an interesting result vaI.S.Uramarsite (NH ,H O) (UO ) (AsO ,PO ) of this study. Up to date, metamictisation 4 3 2 2 2 4 4 2 •6H2O: A new mineral of the metaautunite was associated with radioactive elements group // Dokl. Earth. Sci. 2007. V. 415A (6). (U,Th), which form proper minerals or P. 970–974. (In Russian).