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Tw1 the CRYSTAL STRUCTURE of SCHOEPITE. [(Uo2)8O2(Oh]121

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Tw1 the CRYSTAL STRUCTURE of SCHOEPITE. [(Uo2)8O2(Oh]121 tw1 The CanadianMfuzralogist Vol.34,pp. 1071-1088(1996) THECRYSTAL STRUCTURE OF SCHOEPITE. [(uo2)8o2(oH]121(H2o)12 ROBERT J. FINCHI, MARK A. COOPERATTO FRANK C. HAWTHORNE Deparxnentof GeologicalSciences, University of ManitobaWinnipeg, Manitoba R3T 2N2 RODNEY C. EW]NG Departmentof Earthand. Platrctary Sciences, University of NewMexico, Albu4uzrque, New Mexico, 87131-1116, U.S,A. ABSTRACT Schoepite,[(UOrsO2(OII)r2](H2O)p,isorthorhombic,a1,4.337(3),b1,6.813(5),c14.731@)4,V3551(2)A3,spacegroup ?2pa, Z = 4. The sfucture has been solved by direct methodsand refined on 4 to a weightedR index of 5.8Vobased on 4534 uniquereflections measured with MoKcl X-radiation on a single-crystalditfractometer (equivalent to an R index of 2.77a for d > 4od). The refinementindicates that the formula containseight more H2Ogroups per unit cell than previouslyassumed. The structue consistsof neutral [(UOrsO2(OH)121sheets of edge-and corner-sharingUS7 pentagonal dipyramids (0: O, OID, hydrogen-bondedto each other through interstitial H2O groups. These sheetsare topologically identical to those found in fourmarierite.The [(UO)6O2(Off 12]sheets are interleaved with almostplanar sheets of interlayerH2O groups. There are twelve symmetrically distinct H2O groups in the interlayer sheet;these are arrangedin two pentagonalrings with 1v76linking H2O groups.H-atom positionswere not resolved but an H-bonding schemeis suggestedon the basis of stereochemicaland bond- valencearguments. The strucfrre displays strollLgPbca pseudosymmetry,especially among the U atoms.The lower symmefry is primarily due to H-bond interactionsbetween interlayer H2O gtoupsand O(uranyl) atornsof the structuralsheet. Keyvvords:schoepite, crystal structure,uranium, hydrogen bonding, uranyl oxide hydrate. Sotrltlaans La schoepite,IQO'sO2(OII)121(H2O)12, estortlorhombirye,al4.33T(3),b 16.813(5),c 1,4.731(a") A,V 3551(2)A3, groupe spaaalP2,ca, Z = 4. Nous en avonsaffine la structurepar m6thodesdirectes en utilisnt I((4$4 reflexionsuniques mesur6es avec rayonnementMoKcr par ditfractom6triesur cristal unique),jusqu'd un r6sidu R de 5,8Va(l'6quivalent d'un indice R de 2,7Vopour F. > 4oFJ. L'affinement montre que la formule contient huit groupesH2O de plus par maille 6l6mentaireque la formule acceptdene f indique. la structure contient des feuillets [(UO)8O2(OFD12Jneutres de dipyramidespentagonales UQr i ardteset i coins parag6s (0: O, OH), interlids entreeux par liaisonshydrogbne assurdes par les groupesHrO intentitiels. Ces feuillets sont topologiquementidentiques d ceux de la fourmarierite.lrs feuilles [(UOr8O2(OfDl2] sont intercal6savec des feuillets presqueen plan de groupesH2O. Il y a en tout douzegroupes H2O distincts dansce feuillet interlitF, agenc6sen deux anneauxpentagonaux 1i6s par deux groupesHrO. Nous n'avons pas affin6 la position desatomes H, mais nousproposons quandmOme un schdmade liaisonshydrogbne fond6 sur argumentsst6r6ochimiques et sur les valencesde liaison. La structure montre une forte pseudo-sym6triePlca, surtoutparmi les atomesU. La sym6trieinfdrierre est surtoutdue aux interactionsdes liaisonsH entreles groupesHrO desfeuillets interlitds et les atomesd'oxygdne des groupes uranyle du feuillet structural. (Traduit par la R6daction) Mots-cl6s: schoepite,structue cristalline,uranium, liaison hydrogdne,oxyde d'uranyle hydrat6. Inrtopucnorv Schoep & Stradiot 1947) and UO3'2H2O(Christ & Clark 1960). The related mineral paraschoepite, Schoepitewas originally describedby Walker 5UO3.9YzH2O,was describedby Schoep & Stradiot (L923); its formula has been reported as 3UOr.7HrO (L947). The relationship between paraschoepiteand (SchoepL932),4UO3.9H2O @illiet & de Jong 1935, schoepiteis uncertain (Christ & Clark 1960, Christ 1965).A third relatedminssnl, metaschoepite, may be a lower hydratethan schoepite(Cbdst & Clark 1960). X-ray diffraction studies of synthetic UO3 hydrates I E-m.ail address: [email protected]. indicate only one phaserelated to schoepite;however, Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/5/1071/4006270/1071_34_5_cm.pdf by guest on 30 September 2021 l.W2 TI{E CANADIAN MINERALOGIST ffiared spectroscopyand thermogravimetricanalysis crystal of schoepite and checked optically before commonly suggest a second synthetic modification mounting on a glass fiber. After three days on the (Hoekstra& Siegel1973). The chemicalcomposition diffractometer, this crystal (sc-a) decomposedat its and structure of schoepitehave been the subjectsof core to a polycrystallinepowder, leaving only a donut- muchdiscussion (Baran 1992, Finch et al. tOgi,Cejka shapedfragment. Two more cleavageftagments were & Urbanec1990). Schoepite occurs at many oxidized removedfrom the sampleand examinedby precession uranium deposits,and it may play a key role in the photography.One of these (sc-D) decomposedon the paragenesisof the complex assemblageof uranyl precessioncamera in a fashion similar to crystal sc-a. mineralsthat form whereuraninite has been exposed to The second crystal (sc-c) changedfrom translucent oxidizing meteoric water (Finch et al. 1992, Deliens yellow to opaqueyellow during a ten-hour exposure, 1977a). but remained intact. A precessionphotograph taken Recently, there has been renewed interest in the after this change showed significantly broadened paragenesisand structure of uranyl oxide hydrates, diffraction-spots,changes in the diffracted intensities, particularly schoepite,ianthinite and becquerelite,as atd a 2Vodecrease in the a cell edee ftom 1,4.29A they not only occur as products of the secondary to -14.0 A. fnis is consistentwlttitle alterationof alteration of uraninite under oxidizing conditions schoepiteto metaschoepite(Christ & Clark 1960). (Finch & Ewing 1992, Frondel 1958), but are also Subsequentcrystals taken from sampleMRB B3616 prominent phases in laboratory experiments on were coatedwith hair sprayafter extractionin order to alteration of the UO2 of nuclear fuel (Johnson & preventalteration. This was partly successfirl,and the Werrne 1994, Forsyth & Werme 1992, Wronkiewicz coated crystals remained translucent;however, data et al. L992, Stroes-Gascoyneet al. 1985, Wang & collectedfrom four coatedcrystals were inadequateto Katayama 1982, Wadsten 1977). Details of the solve the structuresatisfactorily. The most reasonable occurence of uranyl oxide hydrate minerals are an solution and refinementwere obtainedusing datafrom important test of the extrapolation of results of crystal sc4(2), but bond lengths and displacement short-termexperiments to periodsrelevant to nuclear- factors were not reasonable.At tttis point, a second wastedisposal @wing 1993).Moreover, they provide schoepite-bearingsample (CSM 91.62) wasexamined. important constraintson models used to predict the This sample consisted of a coarsely crystalline long-term behavior of spentnuclear fuel (Bruno er a/. matrix of intergtown schoepite,becquerelite, vanden- 1995). driesscheiteand ianthinite, in contact with altered uraninite and veined by soddyite and uranophane. E:<pr,ruvmNrar Cleavage fragoents were taken from inclusion-free crystals of schoepitethat had grown within a cavity. We examinedschoepite crystals from two museum Two of these were mounted on glass fibers and samples,and datasets were collectedon sevenof these examined both optically and by precessionphoto- (Table l). Five of thesecrystals were extractedfrom graphy. Thesetwo crystalswere not coated,and they sampleMRB 83616, in which a matrix of fine-grained did not alter during the datacollections; horvever, both (-1 ttm) rutherfordine surrounds large (1-2 mm) schoa and schob eventually became polycrystalline blocky crystalsof yellow schoepiteand amber-colored approximatelyone year after extraction from sample becquerelite.A cleavagefragment was taken from one csM 91.62. TABLE1. UNIT.CELLPARAMETERS FOB SCHOEPITE CRYSTALS EXAMINED DURING THIS STUDY a b c Sp. cr. Vol. (A3) remarkf MFB 83616 sc-a 14.301(3) 16.788(4) 14.712(41 Ph-a 3532(3) decompoaad t sc-d 14.308{3) 16.793(2) 14,706(3} Pb-a 3533(2) (C) UpositlonBonly r scd(2) 14.296(3) 16,775141 14.71314't P,ca 3528131 (Cl &=7%,9oor U, sc-€ 14.17(11 16.74('11 14.68(21 Pb-a 3482(9, (C) U pogitlonsonly sc-f 14.074(71 16,717a7't 14.7011l. Pbna 3458(6) (C) U positionsonly csM 91.62 schoa 14.308{2) 16.808(3} 14.705141 Pbca 3536(2) & =8%, poor Ua schob 14.337(3) 16.813(5) 14.731141 P2,ca 3551121 &=2,7% finalsolution * Crystalsmarked with (C) were coqtedwith hair spray aft6r mounting. t crystalssc{ and 6ar docomposedduring precassion examination ' sod and sc4(2) ate the sqm6 crystal, but data were recollocted on v4(2) alter 6 months Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/5/1071/4006270/1071_34_5_cm.pdf by guest on 30 September 2021 TIIB STRUCTUREOF SCHOEPITE LW3 Data for crys+,alschoa proved inadequatefor diffraction-maximaat least every 50 20 from 7 to 60", structure solution, with problems similar to those and chosensuch that the diflraction vectors spanned observed for crystal sc4(2). Data for the second one quadrant of the Ewald sphere.The crystal was cleavagefragment (schab) were then collected, and modeledas a {001} plate,and reflections with a plate- a more precise absorption-correctionwas obtained glancing angle less than 7o were discarded. The (seebelow). Crystalschob also had the largestunit-cell absorption correction reduced R(azimuthal) from volume amongthe crystalsexamined (Table l). We 22.8Voto 2.0Vo.The remaining 8278 reflections suspect that this reflects the lack of significant were corrected for drift, I-nrentz, polarization
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