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The Structure of Richetite, a Rare Lead Uranyl Oxide Hydrate

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The Structure of Richetite, a Rare Lead Uranyl Oxide Hydrate t87 Thz Cannlian M ineralo g tst Vol.36, pp. 187-199(1998) THESTRUCTURE OF RICHETITE,A RARELEAD URANYL OXIDE HYDRATE PETER C. BURNSI Deparfinent of Civil Engineering and Geological Sciences, University of Notre Dame, Notre Dame, Indiana 46556-0767, U.S.A. ABSTRACT The structure of richetite, approximate formula MPb8.57[(UO2)r8Ors(OH)r2J2(H2O)nr,Z = 1, triclinic, a 20.9391(3), b 12.1000(2),c 16.3450(3) A, o 103.87(1), p 115.37(1),y 90.27(1)', V 3605.2 Ar, spacegroup Pl, has been solved by direct methods and refined by fuIl-matrix least-squarestechniques to atr agreementfactor (R) of 8.9Voand, a goodness-of-frt (,t) of 1.79 using 12,383 unique observed reflections (lF.l > 4oo) collected with MoKd, X-radiation and a CCD (charge-coupled device) area detector.The structue contains 36 unique Uo positions, each of which is part of a near-linear (UaOr)z* x16y1 ioo that is further coordinated by five (O, OH-) anions, forming pentagonal bipyramids. The uranyl polyhedra share edges ro form symmetrically distinct but topologically identical o-U3ortype sheets at z = 0.25 and z = 0.75. Although cr-U3o8-type sheetsof uranyl polyhedra occur in several structures,the richetite sheetsare unique in theL arrangementof OH anions. There are 13 partially occupied unique Pb2*sites, two octahedrally coordinated M sites that may contain Fe* or other cations, and 4l unique HrO groups in two distinct interlayers at z = O and z = 0.5. Both the Pb2*and M cations link to uranyl-ion O-atoms from adjacent sheets,and thus provide linkage of the sheetsto the interlayer constituents.An extensive network of H bonds provides additional linkage. Keywords: richetite, uranyl mineral, uranium, structure determination, lead uranyl oxide hydrate. Solrvnrne La structure de la richetite, dont la fo^rmule approximative serait MPbE.sT[(UOr)rEOr8(OH)lzlz(HzO)ou Z = 1, triclinique, a20.9391(3),b 12.1000(2),c 16.3450(3)A, cr 103.87(l), p 115.37(1),y90.27(1)",V 3605.2 43, goupe spatialPl, a 6t6r6solue par mdthode directe et affin6e par moindres carr6s sur matrice endbrejusqu'e un r6sidu R de 8.9Voettn indice de concordance (S) de 1.79 en utilisant 12,383 r6flexions uniques observdes(lF.l > 4ou) avec rayonnement MoKcr, et un d6tecteur I aire CCD (d charge coupl6e). La structure contient 36 positions uniques contenant Utu, chacune d'elles faisant partie d'un ion uranyle (Uc'g;z* presque lindaire, et de plus coordonnd par cinq anions (O, OHJ, pour former des bipyramides pentagonaux. Les polyBdres tr uranyle partagent des arCtespour donner des feuillets de type a-U3Os d z = 0.25 et z = 035 qui sont distilcts en symdtrie mais topologiquement identiques. Quoiqu'on fouve des feuillets de type cl-U,Os de polybdres d'uranyle dans plusiews structures,ceux de la richetite seraientuniques dans leur agencementd'anions OH . tr y a 13 sites uniques de Pbz*d occupation partielle, deux sites M I coordinence ock6drique qui pourraient contenir Fe3+ou autes cations, et 41 groupes uniques de H2O dans deux inter-couchesdistinctes d des niveaux z = 0 et z = 0.5. Les cations Pb2"et M sont li6s aux atomes d'oxygdne des ions d'uranyle de feuilles adjacents,et assurentdonc les liaisons entre les feuillets et les composantslogeant enfte les feuillets. Un r6seau important de liaisons hydrogbne fournit. des liaisons additionnelles. (Traduit par la R6daction) Mols-cl6s: richetite, mindral d'uranyle, uranium, d6termination de la structure, oxyde d'uranyle de plomb hydrat6. IxrtooucnoN mineralsgenerally are poorly characterized,and there is a lack of systematicwork on the structuresand Uranyl minerals are major constituents of the stabilities of these minerals, largely becausethey oxidized portions of urani ,m deposits,where they most commonlyoccur only ascomplex intergrowths of very commonly occur as the products of alteration of smallcrystals. uraninite (Frondel 1958, Finch & Ewing 1992).Despite Richetiteis a rarelead uranyl oxide hydratethat is their importance in environmental issues such as associatedwith uraniniteand various uranyl minerals the weathering of radioactive mine tailings and the in theShinkolobwe uraninm deposits at Shaba,Demo- geological disposal of spent nuclear fuel, uranyl cratic Republic of Congo. Richetite was originally I E-mail address: [email protected] 188 THE CANADIAN MINERALOGIST described by Vaes (1947), and a more detailed TABLE I. CRYSTALI'GRAPEIC DATA FORRICHETTTB description was provided by Piret & Deliens (1984), o (A) 20.9391Q) clrystalsize(m) 0.008x020x0.15 r (A) 12.1000(2) Radietim Mo(a who proposed the formula PbO.4UOr.4HrO on the c(A) 16.3450(3) Total Ref. 23365 basis of analyses done with an electron microprobe and a (') 103.870) UniqueRef, 17810 pf) UniqwlF"l>4or lZSgj photoelectron ll5370) X-ray spectroscopy Q{PS). The structure r (') N27(r) Fiml x 8.9 of richetite has never been determined, owing to the v(n 3@52 FiEls 1.79 Dd 6,l3gd small size of the crystals, the high absorption of X rays SpaF group PI by the crystals, and the large unit-cell. As part of F@ 5540 ongoing work on uranyl minerals (Burns et al.1996, p(m'') 50.6 1997a,b, c, Burns 1997), a structure model has been Unit+u cooimb:Mfhrn[(Uor)'O'(o0r]dHro)n' obtained for richetite using X-ray-diffraction methods x='(FJ-FJ/>[FJ and a CCD area detector. S- tIadFJ-F.tr(z-z)l*, fq z obrwacim 6d r pa@€tq8 ExpenrvreNTAl A specimen containing richetite from the plate-glancing angle was selected after trying angles Shinkolobwe mine, Shaba, Democratic Republic of in the range 1 to 5' because it provided the best Congo was provided by Mr. William Pinch. The refinement results. Additional information pertinent to crystals occur as platy aggregates that are seldom larger the data is given in Table l. than0.2 mm in diameter, each with a pseudohexagonal outline, and with interpenetrating twins common. Chemical analysis X-ray dffiaction A cleavage flake of richetite, selected from the same specimen as the crystal used for the X-ray studS was A small crystal of richedte, flattened on {001}, was glued to a plexiglass disk and coated with carbon. The mounted on a Siemens PLATFORM goniometer crystal was mdyzed using a Cameca SX-50 electron equipped with a SMART CCD (charge-coupled device) microprobe. Energy-dispersion spectra indicated that detector, with a crystal-to-detector distance of 5 cm. Pb and U are the only major cations present. Five The SMART CCD detector is a two-dimensional area points were analyzedusing the wavelength-dispersion detector with an active-imaging area 9 cm in diameter. mode, a beam 20 pm in diameter, an excitation voltage The detector is equipped with a phosphor screen, of 15 kY a beam current of 20 nA, and UO2 and PbTe located immediately behind a beryllium window, to as standards (Table 2). The structure determination convert the X-ray photons to optical photons that are indicated that additional cations may be present in carried through a fiber-optic taper to the CCD chip. The small quantities (see below), so the crystal was detector provides improved resolution, sensitivity to analyzed for Al, Ba, Bi, Ca, Co, Cs, Cu, Fe, Mg Mn, weak reflections, and shorter data-collection times Na, Ni, R S, Sc, Sr, Th, Ti, V and Zn,but all were than a scintillation counter mounted on a serial found to be below their corresponding detection-limits. diffractometer (Burns 1998). The data were collected using monochromatic MoKa X-radiation, a frame width (in to) of 0.2o, and a TABLE 2. CHEMICAL C1CMPOSITIONOF 30 s exposure per frame. More than a hemisphere of RICIIETITE(wI 7o) three-dimensional data was collected, and the data were PointI Point2 Point3 Point4 Point5 analyzed to locate peaks for least-squaresrefinement of Uor 77.30 77.7t 77.83 77.02 77.31 the unit-cell dimensions (Table 1). The data were Pbo t7.22 t6.76 t7.04 16.89 t7.37 collected for 3o ( 20 < 56.5", and contained minimum and maximum indices -24 < h < 27, -16 < k < 16, -21 < I < 11. The data were collected in approximately fwenty hours, and the intensities of standard reflections Srnucruns SoLUTIoNAND REFINEMENT showed no significant change during data collection. The data were reduced and filtered for statistical Scatteringcurves for neutralatoms, together with outliers using the Siemens program SAINT. The anomalous-dispersioncorrections, were taken from data were corrected for Lorentz, polarization, and International Thblesfor X-Ray Crystallography,VoL N background effects.An empirical absorption-correction (Ibers & Hamilton 1974).The SiemensSHELXTL was done based upon 438 intense reflections. The Version 5 system of programs was used for the crystal was modeled as a (001) plate; reflections with determinationand refinement of the sfructure. a plate-glancing angle of less than 3o were discarded _ Attemptsto solvethe structurein the spacegroup from the data set, which lowered the R..u* of Pl were unsuccessful.The structurewas solved by the 438 intense reflections from 10.0 to 2.7Vo.The direct methodsin the spacegroup Pl. The location THE STRUCTIJ'RE OF RICHETITE 189 of 36 U atoms was extracted from the solution, and AII 36 uranyl ions are coordinated by five [Oz-, refinement of their positional par4meters gave an (OH)l anions, giving UrQ5 pentagonal bipyramids -25Vo. agreement index (R) of A difference-Fourier (Q: unspecified anion). The <U-{"q> (Q-*:equatorial Q) map calculated at this stagerevealed the location of Pb bond-lengths range from 230 to 2.48 A, values within cations and some anions. Additional anions and two the range observed for Ufit polyhedra in well-refined additional cation sites (designated trrl) were gradually structures @vns et al.
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