![Tw1 the CRYSTAL STRUCTURE of SCHOEPITE. [(Uo2)8O2(Oh]121](http://data.docslib.org/img/67357a92dc803ce6966c47c7665917c0-1.webp)
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:
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 and intergrown metaschoepite,which has a smaller unit- backgroundeffects. cell volume than schoepite(Christ & Clark 1960).The presenceof metaschoepitecan be inferred from Srnucn;r SonruoN aNp Rnrnvn"mu the smallera cell-edgesfound for the other six crystals (Tablel). The U sites were located in the spacegroup Pbca Precessionphotographs of crystal schob confltmed by direct methodsusing the programSIIEIJilL (4.1); the orthorhombicsymmetry and the spacegroup Plcc, most of the O atomsin the structuralunit were located in agreementwith Christ & Clark (1960). A thin from difference-Fouriermaps. The structure was plate, approximatelytriangular, 0.2 mm on each edge refined to an R index of 6.77o usirg lFl; however, and 0.02 mm thick" was mounted on a SiemensP4 we could not locate all the O atoms in spncegroup Nicolet R3m attomated four-circle diffractometer Pbca, and the O(uranyl) atoms displayed (apparent) equippedwith a graphite monochromatorand MoKc[ positional disorder about the U atoms. Structure X-radiation. Forty diffraction-maxima, 25 of which refinementswere then tried in three subgroups,PbAy were between35 and 60" 20, were centered,and the Pb2p aad F)pa, using the U positions as starting unit-cell dimensions were refined by least squares points. Only in space group 72pa were we able to (Table 2). Following the collection of the intensity locate all remaining O atomsfrom difference-Fourier data, the crystal was re-centerednand the unit-cell maps.The disorderof the O(uranyl) atom$,apparent in parametersredetermined. Differences from previously spacegroup Pbcaowas resolved as discretepositions determinedvalues were within the reported standard in P2pa. As only three (weak) reflections violate deviations, indicating that the crystal had not the D glide in space group Pbca (031, 051, 053; undergonesignificant alterationduring datacollection. all "observed" at -3o), the choice of the non- Data were collectedusing the 0-20 scan-modeand centrosymmetricspace-group, P2pa, is based on a variable scan-rateproportional to the peak intensity achieving a crystal-chemically realistic solution of (minimum and maximum scan-speedswere 1.7 and the structure, rather than on systematic-absence 29.3" 20/min,respectively). A total of 11,147reflec- violations. tions was measuredover the range4o < 20 3 60', with The structure refined to an R index of 3.0Von indexranges 0 < h < 20, 0 < k < 23, -20 ( / < 20. Two Y2pa; however, U(6), U(8) and several O atoms standardreflections were measuredafter every fifty tO(16), OH(2), OH(12)l had unreasonabledisplace- reflections. An empirical absorption-correctionwas ment factors (U"o = 0). In particular, isotropic applied, based on 71 psiscans of each of fifteen displacement-factorswere strongly correlatedfor the sheet-atompairs pseudosymmefically related by a 2-fold rotation axis along [010] p.e., U(l)lu(s), u(z)tu(6), u(3)tu(7), u(4)/u(8), o(r7)to(rg), oH(r)toH(1), oH(z)toH(8), 0H(3)/OH(9), (CSM TABTE2. MISCELTANEOUSINFORMATION FOR SCHOEPITE 91.82) oH( )l oH(ro), oH(s)toH(1.r), oH(6)toH(12)1. This a 61 14.337(3) dystat slze (mm) 0.19 x O.2t x O.O2 is probablythe result of strongvariable correlation due b 16.813(51 mdlation MoKq/ct to the prominent pseudosymmetrycombined with c 14.731141 Tolal no. ot I. 11.147 residualabsorption problems. vtA"t 3551(2) No. of4 8278 The structure was then refined on Fz using the Sp. Gt. nrca Uniqu6refloctions 4635 program Z 4 4(admuthal'% 22.a-2.O SIIELXL-93. Isotropic-displacementfactors 9o 4.87 F(msrge)% 2.8 of O atomsin the planeof the structuralsheets (sheet O 9*' 4.8 w4"lFll * 5.8 atoms),pseudosymmetrically related by t010l2tin spuce p (mm-r) 56,47 A1 lF.l>&Fl o/o 2.7 gronp Pbca, were constrainedto be equal (table 3). & (afldat6) % 6.8 Displacementfactors for all other atomswere refined No. paEmgtOrS 235 Callcont€nta 4{t(UOr}eO:(OHh:l(HrO)rr} independently.This lowered the Rt index slightly to 2.77o.An extinction coefficient was refined but found & =:(l4l-l4l)Elr.l to be negligible. The final wR2index of 5.8Vois based wB, - l2wt*"-rll2tZwl4l\e w - 1Id(F:l + to.o249.l4n4 on all intensity data exceptthe 0 9 0 reflection, which P = l{mulo,Flll+2Ffr13 was omitted becauseof severe overlap (4534 data, ' Blllist& deJong (1936) 235 panmeters). The final minimum and maximum
Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/5/1071/4006270/1071_34_5_cm.pdf by guest on 30 September 2021 L074 TIIE CANADIAN MINERALOGIST
TABLE3. FIML PARAMETERSFOR SCHOEP]TE TABLE 4. BOND DISTANCES6) TOR SCHOEPNE 'u^ u(1)-o(1) 1,77(21 u(51-o(91 1.76(31 ul1, 0.2591(1) 0.5132(1) 0.7683(a 111(4) u(t-ol2l 1.75(21 U(El-O(rg) 1.27(21 ul2l o.0276fi) 0.3775(1) 0.7628| ], 136(4) U(1)-oH(7) z.&(21 U(5)-oH(l)e 2.47(21 u(31 0.2792(1t 0.7461(1) 0.7474(21 123(41 ull!-oHzt 2.38121 u(5)-oH(8) 2.39121 utq', -0.0008(1) o.8127t1t o.7497t2t 122141 u(1)-oH(3) 2.66(21 u(5)-oH(9) 2.78121 utst 0.2797(1t 0.0134(1) 0.74@(2) 1@(4) u(1,-oH(0) 2.39l2l u(6)-oH(12) 2.42|.21 uBt 0.0117fi) o.a772l1l 0.7631(1) 71(3) u(1)-o(17)b 2.2812' u(6)-o(r8) 2,17121 u(n o.2607t1t 0.2460(1) 0.7520|.21 84(3)
Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/5/1071/4006270/1071_34_5_cm.pdf by guest on 30 September 2021 TIIE STRUCTTJREOF SCHOEPITE r075
TABLE 5. BOND-VALENCEARRANGEMENT' IN SCHOEPITE unl ulst ul4l u{st utsl utTt u(8} : ll+H
o{1} 1,92 1.92 o(a 1.83 1.83 o(3) 1.83 1.83 2.O3 o(4) 2.06 2.O8 o(5) 1.96 1.96 2.O9 o(61 1,78 1.78 1,90 o(7! 1.92 1.92 2.03 o(81 1.66 1.66 1.85 o(9) 1.96 1.96 o(10! 1.92 1.92 o(11) 1.78 't.78 1.80 on2I 1,87 't.87 2.O1 o(13) 't.87 1,87 2.@ o(14) 1.74 1.74 1.85 o(15) 2.06 2.06 2.22 o(16) 1.83 'f.83 2.42 o(17) 0.65 o.71 0.75 2,11 o(18) 0.80 0.65 0.63 2.O8 oH(1) o.43 0.46 0.M 1.31 2.',t0 oH(2) 0.54 0.46 0.39 1.39 2.22 oH(31 0.34 o.49 0.49 1.32 2.17 oH(4) 0.59 0.61 1.20 2.O5 oH(51 0.30 0.56 0.60 1.36 2.',t8 oH(6) O.53 0.43 0.46 1.42 2.06 oHFl o.47 o.36 0.56 1.39 2.26 oH(8) 0.53 0.44 0.35 1.32 2.13 oH(9) o.27 o.47 o.52 1.26 2.O4 oH(10) 0.63 o.55 1.18 1.99 oH(11) 0.50 0.39 o.41 1.30 2.O8 oH(12) o.50 0.55 0.50 1.55 2.35 6.28 6.27 6.26 6.23 6.M 6.19 6.23 6.30
' calculatodwith the paramatersof Brown & Wu (1976) ' Bond-valsncesums to O atoms with estlmat€d H-bond contributionsadded, shown only for thoss O atoms involvEd in H-bonding. H-bondvalsncss aro trom O-O distancesfor singls H-bondsand calculatsd H-O distanFs for bifurcated H-bonds;ostimat€d using Figs. 1 & 2 in Brown & Altormatt (1985). Bond-valencssums for OH groups in tho'X+H" column ls calculatedsuch that valencesums to th6 H atoms are unity (cf.TablE81.
is lessened.A cosmetic disadvantageof refining F2"ca. T\ese sites can be divided into two ordered againstF3 is that R indicesbased on Fl are larger than seti, u(1)-u(4) and u(5)-u(8), that are pseudo- for refinementsbased on lF"l using a threshold. In symmetrically related by a 2-fold screw axis along order to compare Fl refinements with refinements 10101in the spacegroup Pbca for the sameunit cell. basedon lf"l with a o(F") tlreshold, tle more conven- Al1 U atoms are coordinated by seven anions in tional R index, basedon ldl valueslarger than 4a(F), pentagonaldipyramidal arrangementsGig. 1). Each is alsoreported as i?1 in Table2. UQz(Q: OH- andO) pentagonaldipyramid consistsof two apical^ 02- anions at distances in the range DnscnrpnoNoF TrrESrRUcruRE 1.74-183A andO-U-O anglesin therange 172-179", and five equatorialanions (O2- and OH-) in the range Cation coordination 2.17-2.78 A (Table 4). A pentagonalarrangement of equatorial anions was predicted as the most stable There are eight symmerically distinct U sites, all configurationaround a (UOr) group by Evans (1963). occupying the general position 4a in space group The apical 02- anions are designatedas uranyl-O
Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/5/1071/4006270/1071_34_5_cm.pdf by guest on 30 September 2021 t076 THE CANADIAN MINERALOGIST I aI
I I
1b.- -.. 1
Frc. 1. Atomic arangement ia the structural sheet of schoepite.O atoms are shown as highlighted circles. OH groups are stippled, and small filled circles are U atoms. View along[001], z * 0.2S.
atoms [O(uranyl)], and bond-valencecalculations Strucnral unit (Table 5) show that the U-O(uranyl) bondshave bond valencesin the range 1.66to 2.06 va (valenceunits). The UQ7 pentagonal dipyramids share edges to The large differencein efficienciesin X-ray scatlering forrn dimersthat further link by sharingedges to form of U and O makes the short U-O(uranyl) distances staggeredribbons along [100] (Ftg. 2); theseribbons among the least accurately determined interatomic fts11slossJink in the [010] directionby sharingedges distancesin the structures of uranyl compounds and cornersof the polyhedra.The result is a strongly determined by X-ray diffraction. The U-O(uranyl) bonded sheet of the form [(uOrsO2(OIDrz] parallel bond lengthsdetermined for schoepiteare comparable to (001) (Fig. 2); this sheet constitutesthe structural to tlose reported for other uranyl-oxy-hydroxide unit of schoepite,and the sheetsstack along [001]. compounds(e.9., Taylor 1971,Aberg 1978,Pagoaga As the sheets are neutralothey are linked together et al. 1987). Bond-valencesums around the U atoms by H-bonding only, tbrough a complex network of in schoepite(Table 5) are in the range6.194.44 vu, H-bonding involving interlayer H2O groups and signifi.cantlyhigher than the ideal value of 6.0 va. This O(uranyl) atoms and OH- groups in the stuctural suggeststhat the U-O bond-valenceparameters are sheet.This explainsthe perfect {001} cleavageparallel somewhat in error, a factor that complicates the to the sheets.Viewed atong [001], the [/ sites are interpretation of hydrogen bonding in schoepite. approximately superimposed,a feature of all uranyl However,if a U-O distanceof 1.74A correspondsto a oxide hydrate minerals (Pagoagaet al. 7987, Ytret bond-valenceof approximately2.0 vu,longer drstances 1985"Piret et al. L983,Piret-Meunier & Ytret 1982, (>L.74 L) and lower bond-valences(4.0 vu) may Mereiter 1979); there is no staggering of U sites indicate O(uranyl) atomsthat *e 6sting as hydrogen- perpendicular to the sheets, as for most high- bond acceptors. temperatureuranates (Loopstra & Rietveld 1969).
Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/5/1071/4006270/1071_34_5_cm.pdf by guest on 30 September 2021 TIIE STRUCTTJREOF SCHOEPITE t077
-r I I a I I I
Frc. 2. Arrangementof UQ7polyhedra in the strucn[al sheet of schoepite.Different shadingsof polyhedrashow the rwo setsof polyhedrarelated by a pseudosymmetry axis,t0r0121. View alongt0011, z = 0.75.Labels refer to the U atom(cf Fig. l).
Interlayer H2O groups are closerto one adjacentstructural sheet, and threeof the H2O groupsarc closerto the other structuralsheet. Twelve O atomswere locatedin the interlayer,none There is no evidencefor disorderor partial occupancy ofwhich aredirectly bondedto any cation.Because the of the interlayer W sites. structural sheets are electrostatically neutral, all the interlayer O atoms must be HrO groups and are Pseudo-syrnmetry designatedas W(l)...W(12).Ten of the twelve H2O groups are located at the apices of two distorted The arrangementof U atoms, O(sheet) atoms pentagons,approximately 2.9 A on edge;the remaining and interlayer H2O groups is strongly pseudo- two H2O groups are located between the pentagonal centrosymmetical.Only the O(uranyl) atoms are not rings (Fig. 3). The H2O groups that make up these pseudo-centrosymmefrical,and their positions about pentagonal rings are not coplanar because of the U atoms are largely responsiblefor the lack of a H-bonding interactionswith the anions of the sheet. center of symmetry. The combined effects of The pentagonal rings mimic the positions of the H-bonding and steric crowding by adjacentO(sheet) meridional anionsin the U(1)$7and U(5)Q,polyhedra atomsand interlayer H2O groupscause the O(uranyl) (Frg.4). The U(5)Q7polyhedron is moredistorted than atoms to deviate significantly from a centro- the U(l)07 polyhedron, and the pentagonal ring symmetricalarrangement in schoepite. associatedwith the U(5)Qt polyhedron is the more distorted of the two (Frg. 3). Each pentagonalring of Chemicalcomposition of schoepite H2O groups circumscribesO(uranyl) atoms from tle U(1) and U(5) polyhedra in the two adjacentsheets; The structuresolution showsthe stuctural formula eachis buckled, suchthat two ofthe five H2O groups of schoepiteto be [(uor)ro2(oH)12](H2o)rz,corre-
Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/5/1071/4006270/1071_34_5_cm.pdf by guest on 30 September 2021 1078
& @---"q.. @-"--@.. ib w'---w.A ..w a: @ 0'.' ;i ..aP.1i9 @wtro tI 'li,@"'q,"' I I "" 'w-- a Y*ib':n6 I FIc. 3. Atomic arrangement I of interlayer H2O groups "@ I in schoepite,with possible intrasheet H-bonds indi- *u cated I as dashed lines I I (3.1 A). Interatomic dis^- I I rances I ,,& greaGrthan ].1 A I '-@' but less tlan 3.4 A are Y shown as dotted lines. & View along[001], z = 0.S. b-l
spondingto the compositionUO3.2.25H2O, in good agreementwith that originally determinedby Billiet & de Jong(1935) from the measureddensity and unit-cell parameters.The composition commonly reported for schoepiteis UO3.2H2O.It was determinedby thermo- gravimetric analysis (TGA) on both natural (Protas 1959) and syntheticmaterial (Bignand 1955,Peters 1967). Our results indicate that the name "synthetic schoepite"for the compoundUO3.2H2O may not be appropriate.
a
Ftc. 4. The arrangementof interlayer H2O groups (harched circles) relative to the UQr polyhedra of tle adjacent structuralunit. Two of the more regular pentagonalrings are shown (center) witl six H2O groups thit are not membersof the rings tW(5) and W( 11 )l : note that half of the H2O groups overlay anion positions in tle adjacent sheet.The other half of the H2O groups not in registry with the anions of the sheet match up with the anions of the other adjacent sheet (not shown in this view). --4 The shadingof the polyhedrais the sameas for Figure 2. l- b/2 View along[001],3 = 4375.
Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/5/1071/4006270/1071_34_5_cm.pdf by guest on 30 September 2021 TIIE STRUCTUREOF SCHOEPITE LW9
Hypnocnq Boxpnvc nt ScHosprE between the H2O sheet and the structural unit. This can only be true if half the structural-unit - HzO It is usually not possibleto locate H atomsdirectly interactionsare of the D-H..,A* type andthe otherhalf from X-ray diffraction data by structure refinement areof theA*...H-D type. In otherwords, the total bond- and difference-Fouriermaps for suchhighly absorbing valencecontribution to the HrO sheetmust be balanced materialas the uraniumoxy-hydroxy-hydrate minerals. by an equal bond-valenceconfributtonfrom the H2O For schoepitein particular, this is unfortunatebecause sheetto the structuralunit. The positioningof the H2O the chemicalcomposition and perfect {001} cleavage groupsrelative to the OH groups @igs. 5, 6) suggests indicate that H bonding must be the mechanism that there must be an H-bonding interaction between whereby the sheetsof the structural unit are linked. each OH group and its opposingH2O group. Each of Furtherrnore,the similar physical and crystallographic the twelve OH groups in the structural unit acts as properties of schoepiteand metaschoepite(Christ & a donor to a nearby H2O group, and these twelve Clark 1960, Debets & Loopsfra 1963) suggestthat H-bondsfrom the structuralunit to the H2O sheetmust these structures are distinguished primarily by be balancedby twelve H-bonds (D -+ A*) from the differences in their arrangements of interlayer H2O sheetto the structuralunit. This conclusionis in H-bonding. Despite theseproblems, we can get some accord with observed U-O(uranyl) distances, and idea of the interlayer H-bonding in schoepitefrom the suggeststhat O(uranyl) atoms act as acceptoranions locations of the O atomsof the HrO groupsand from for H bonds from the H2O groups of the interlayer fhe stereochemicalcharacleristics of HrO groups and sheet. their associatednetworks of H bonds (Hawthorne As tle coordinationof eachO atomin an H2Ogroup 1992,1994). is expectedto be approximately tetrahedral,several O atomsin the structuralunit can be eliminatedfrom Ge ome t ric al char ac t e ri stic s consideration as H-bond acceptors. First, the two O(sheet)atoms O(17) and O(18) cannot act as Figure 4 shows the arangement of H2O groups acceptorsbecause these atoms are not displacedtoward relative to the adjacentpolyhedra of the structuralunit. the H2O sheetand are well shieldedfrom H-bonding Some of the H2O groups map out the peripheral interactionsby surroundingO(uranyD atoms. Second, vertices of the underlying U$r polyhedra whereas four O(uranyl) atoms are doubffirl acceptors,as they other H2O groups do not show this correspondence; occupypositions inside the five-memberedHrO rings, the latter groupsmap out the peripheralvertices of the and are thereforenot stereochemicallysuited to accept overlying polyhedra.The H2O groups form a slightly H-bondsfrom the H2O groupsof theserings; theseare puckeredsheet parallel to {001}. Thereis a coopera- the four O(uranyl) atoms bonded to U(1) and U(5), tive puckeringbetween the sheetof the structuralunit respectivelyO(1), O(2), O(9) and O(10) (Fie. 1). and the inlerlayer sheet of H2O grorps, such that all Eliminating these six O atoms from consideration H2Ogroups lie between2.5 and,2.9A from OH groups leavestwelve O(uranyl) atomsin the sffucturalunit as of the structural unit. These short distances must potential H-bond acceptors. representH bonds linking the structural sheetsto the interlayerHrO groups.Within the HrO sheet adjaceqt IsoLatedH2O groups H2O groupsare separatedby distancesof 2.8 to 3.4 A (Frg. 3). The shorter distances represent H-bonds Two H2Ogroups, W(5) andW(l1), arenot members within the HrO sheet.Some constaints on the bonding of the pentagonalrings (Fig. 3), and cannot be interactionswithin the HrO sheet can be inferred by tetrahedrallycoordinated. These two H2O groups act examining the stereochemicalcharacteristics of the as acceptorsonly for the adjacentOH groups in the HrO groupsthemselves. structuralunit, OH(5) and OH(11),respectively, and are donorsto nearby O(uranyl) atoms.W(5) acts as a H-bond interactionsbetvveen the H2O layer donorto O(3) and0(16). W(l1) actsas a donorto two and the structural unit of the three O(uranyl)atoms, O(5), O(7) and O(12). Which two of the three act as acceptorscannot be There aretwelve symmetricallydistinct H2Ogroups ascertained,as all threeseem equally likely. The W(11) and twelve symmetrically distinct OH groups per H-bonds may be disorderedor bifurcated (or both), formula unit. Thus there are thirty-six D-H with three arrangementspossible in each case. The (donor - hydrogen) and thirty-six (equivalent)H...A* H-bonding interactions between W(11) and its (hydrogen...acceptor)bonds (A* is used here to associatedO(uranyl) atom! are weal<,distances being emphasizethat, in the caseof bifurcated bonds, each on the order of 3.0 to 3.2 A (Table 6). Thus W(5) and H..r{ interactioncontributes one-half to A*). As eachH W(11) account for four of the fwelve H-bonds that atom is involved in an equal number of D-H and emanatefrom the HrO sheet to the structural unit, H../* bonds,there must be an equal number of D-H leaving eight H-bondsto be assigned. and H../* bonds both within the HrO sheet and Figure 5 illustratesthe structuralrole that W(5) and
Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/5/1071/4006270/1071_34_5_cm.pdf by guest on 30 September 2021 1080 TIIE CANADIAN MINERALOGIST (a)
)f
{ T I c 2
I I V w(5) E& w !, I t oH(1.t)4 Ftc. 5. H-bond interactions between the structural unit and the two H2O groups that are not mem- bers of the pentagonal rings, W(5) and W(11). W(11) is shown witl all three possible D --> A l- O-l interactions;see text. H2O groups are shown as hatched circles: other shadingsas for Figure l. (a) View along [010], Y - 0.75:'(b) View along 11001,r = 0.2s. o(5) is behind the plane of the illustration. and the W(11) -+ O(5) bond is t t shownas ar arow. t I I I -L t
t t ,l T f1 7 c 2
; w(l1) _l- "a &w.u) t,t. *ta ,' ,9 tr^
l+b
Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/5/1071/4006270/1071_34_5_cm.pdf by guest on 30 September 2021 TIIE STRUCTUREOF SCHOEPITE 1081
t Frc. 6. H-bond interactions between the structural c unit and the pentagonal rings of the H2O sheet. Intrasheet H-bonds 2 between H2O groups me shown as heavy solid lines. D -> A interactions from OH goups in the l_ structural unit to H2O groups are shown as heavy dashed lines. D -->A tnteracttonsfrom HrO groups to O(uranyl) atoms in the structural unit are shown as double dashed lines. Bifurcated H-bonds ate shown ema- nating from inferred H positions for W(3), W(4), w(9) and w(10). w(5) and W(11) are omitted. Shadingsand bondsas for Figure 5. (a) View along (b) I010l,Y=0.5,showing the more regular of the two pentagonalrings and the associatedH-bonds: the H-bond arrangement for the more distorted pentagonalring is similar. (b) View along [100], l-I x * 0.25, showing the arrangementof the two pentagonalrings and the c H-bonding interactions betweenthem due to the 2 bifurcated H-bonds (see text). I
'+ b/2 +
Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/5/1071/4006270/1071_34_5_cm.pdf by guest on 30 September 2021 1082
TABLE6. POSSTBLEH-BONDS {A) ron-tUrenleVen nro O(uranyl)acceptors. These donor-acceptor trios are: tNscHoEPlTE (<3.3 A) w(3)-H...o(3)and o13; W(4)-H...o(13) and 0(16); !?11)-oH(l1 2.68(31 W(71-OH(7) 2.S4t3l w(9)-H...o(6)and o(12); w(10)-H...o(6) and o(7). w1t-u12) 2.8O(3t w7',-w8l 2.89(3) Thus we have accountedfor eleven of the twelve wtlt-wt8t 2.s7(31 wlt-w2t 2.90(3) requiredH bondsemanating from the H2O sheetto the M1)-O(l4lur 2.98(31 W(7}-o(5lu 3.05(31 structuralunit, oneeach from W(l), W(2),W(3), W(4), Wz',-oHl2l 2.81(31 !v(8l.OH(8) 2.74t31 W(7), W(9) andW(10), andtwo eachfrom W(5) and w2t-wl6t 2.83(21 w8l-w121 3.14{3} w(11). W2r-vt/(71 z.W|zl wB'Fw1l 2.87(3) The twelfth H bond is somewhatproblematic. At w2)-wt10t 2.94(31 w8t-w4l 3.06(3, Wl2)-O(8)u 2.77(31 vv(8)-o(151 2.80(31 least one of the three H2O groups, W(6), W(8) and W(12), must act as an H-bond donor to the structural w(3)-oH(3) 2.89(3) tY(g)-oH(g) 2.66(3) unit. Each of these three H2O groups occupies a wl3)-M10) 2.77(31 wgt-wt4l 2.9e(3) w(3)-rvt6) 2.89(21 wst-w12t 2.81(31 position adjacentto a neighboringH2O ring (Ftg. 3). M3)-O(3)u 3.31(3) uv(9)-0(12)u3.13(3) O(15) completes the tetrahedral environment about M3)-O(13)u 2.97(3) W(91-O(8)u 2.96(3) W(8) but, despitethe W(8)-O(15) distance(2.86 A), tY14)-oH(4) 2.90(31 wl1o)-OH(1012.78(3) the shortO(15)-U(8) bond length (1.74 A) castsdoubt y/4.4r-wgt 2.98(3) w(lo)-w(3) 2.77(31 on a strong H-bond interaction between W(8) and w4t-wat 3.06{3} w10t-w2t 2.94t31 O(15).Another likely H-bondacceptor is O(11),with t7l4)-O(13)u 3.08{3} W(1O)-O(6)u3.03(3) a bond distanceof 1.80A to U(6); however,O(11) is t'Yl4)-O(l6)u3.11(3) wllO)-O(7lu 3.18(31 3.0 A from W(12) anclis not in an optimal position M5)-OH(5) 2.78(31 1/(11)-OH(l1)2.67(3) with regardto the expectedtetrahedral environment of M5l-O(16)u 2.95(31 W(l1)-o(7lu 3,12(3) W(12). ThusW(12)-O(11) represents a weakH-bond M5)-o(3)u 2.88(31 Ml1)-o{12)u 3.02(3) interaction.This ambiguity in the position of this last W(l1)-O(5)u 3.0€(3) H-bond cannotbe resolved,and H-bondsfrom W(8) M0)-oH(6) 2.EO/2I w12t-oHt12t 2.74t21 and W(12) may both be bifurcated.Figure 6 showsthe ttt Sl-Wl2l 2.83e1. w(121-w(8) 3.14{3} proposed H-bonding interactions between the two M0l-w(31 2.a9al w12t-w9t 2.81(31 interlayer HrO rings and the structuralunit. w(6)-w(7) 2.89(3) w12l-wt1t 2.80t2t M12)-O(11) 3.05(3) To restatethe role of the O(uranyl) atomsas H-bond acceptors,the O(uranyl) atoms, O(8), O(11) and O(14), ' u = uranylorygsn atom act as acceptorsfor one H2O group each. O(uranyl) Atrowstndlcate Inlened D-A ratatlonshlos. atomsO(3), o(5), 0(6), OQ), O(12),O(13) and 0(16) act as acceptorsfor two H2O groups each. The six o(uranyl) atoms,o(l), o(2),o(4), o(9), o(10) do not W(11) play in the schoepitestructure. The uranyl ions act as H-bond acceptors;the role of O(15) is uncertain. to which these two HrO groups are H-bonded are Ten of the twelve H2O groupsact as H-bond donorsto displacedquite noticeablytoward W(5) and W(11). the structural unit, with W(6) acting as a donor only The combined effects of the short OH-H,O bonds to other H2Ogroups within the H2O sheet. and the tilting of the uranyl ions result in iignificant puckering of the structural sheet, especially along H-bond interacrtonswithin the H2O layer 10101Gie. sb). No unique solution to the H-bonding arangement Pentagonalrings betweenadjacent HrO groupscan be derived,although donor-acceptor relationships within the H2O sheet The ten remaining HrO groups are all membersof can be constrainedby the H2O-O(uranyl) atom the pentagonalrings (Fig. 3). Three of these,W(1), arrangementsdiscussed above. Within eachpentagonal W(2) and W(7), must act as H-bond donors to riog, ull five H-bonds must have the samedirectional O(uranyl)atoms O(14), O(8) andO(5), respectively, as (or rotational) sense.For each ring, two rotational theseO(uranyl) atomsoccupy positions that complete sensescan be defined;viewed down [001], theseare the tetrahedralcoordination around the O atoms of "clockwise" (C) and "anticlockwise" (A). Thus four thesethree H2O groups.Also, O(14), O(8) and O(5) combinationsare possible for the two symmetrically haverelatively long bonds(>1.75 A) to their associated distinct pentagonalrings: C{, A-A, C-A and A{, U atoms(table 4). all of which ,ue compatible with the H-bonding Four H2O groups,W(3), W(4), W(9) and W(10), interactionsbetween the H2O sheetand the structural cannot donate H bonds to single O(uranyl) atoms, unit as described above. The H-bond interactions as there are no O(uranyl) atoms in the appropriate betweenthe pentagonalrings are more limited. W(6), positions.The fourH-bonds emanatingfrom thesefour which does not act as a donor to an O(uranyl) atom, HrO groupsto the stuctural unit are bifurcated,with must donateto W(2) in the next pentagonalring, and the H atomslying approximatelymidway betweentwo W(8) may donateto W(12) in the neighboring ring.
Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/5/1071/4006270/1071_34_5_cm.pdf by guest on 30 September 2021 THE STRUCTT]REOF SCHOEPITE 1083
TABLE7. ANGLESAROUND INTERLAYER H,O GROUPSIN SCHOEPITE
oH(1)-W(1)-W(12)102.9(8)" oH{7}-W(7)-W(6) [email protected](8)' oH(4)-W(4)-W(9) 108.8(S)' oH(l0)-w(10)-w(3) 109.8(10)' oH(1)-W(1)-W(8) s4.8(8)" oH(7)-W(7)-W(21 98,7t7t: oH(4)-W(4)-W(8) 112.2(91' oH(1O)-W(rOFW(21108.6(91" '129.4(S)' oH(1)-W(1)-O(14)141.5(9). oH(71-w(71-o(6) 136.0(9)' oH(4)-W(4).O(13) 124.8(8)" oH(10)-w(10)-0(6) w(12)-W(1)-W(8) 97.2(8)" w(6)-w(7)-w(2) [email protected](7)' oH(4)-W(4).O(16) 124.3(9)" oH(10)-w(10)-o(7) 125.6(10)o w(121-w(1)-o(14)r09.4(9)' w(6)-w(7)-o(6) 102.0(8). w(s)-w(4)-w(8) 107.4t9)' w(3)-wo0)-w(a 102.6(8)' w(8)-w(11-o(14) 101.3(8). w(2)-w(7)-o(5) 114.8(8)' w(9)-w(4)-o(13) 64.1(81" w(31-w(r0t-o(o) 71.3(7)' w(91-w(4)-o(10) 120.6(91" w(31-w(10)-o(7) 123.1(10)' oH(2)-W(2)-W(6t 83.4(8). oH(81-w(8t-w(12) 78.8(8). w(8)-w(4)-o(13) 122.211Of w(2)-w(1ot-o(61 12o.7(9ro oH(2)-W(2)-W(71 93.0(8)" oH(81-w(8)-wrl 99.8(9)" w(8)-w(414(16) 77.3t7','" w(2)-w(10Fo0 73.217f oH(a-w(2)-w(10) [email protected](8)" oH(sl-w(8}-w(4) 94.7(8). o(13)-W(4)-O(1e) 64.5(7). o(6)-w(10)-o(7) 64.6171" oH(2)-w(2)-O(8) 142.3l.8f oH(81-w(8)-o(16) 141.7(10)' w(6)-w(2)-w(7) 162.917't" w(12)-Wt8l-W(rl 102.7(8)o oH(5)-w(6)€{10} 140.4(8}" oH(l1)-W(fi)-O(7) 132.9(10)' w(8)-w(2)-w(10) 96.8(7)" w(12)-W(8)-w(4) 91.2171. oH(6)-W(5)-O{3) t48.S(9}" oH6t)-W(itl-062) i,r5.3fl0). w(8)-w(2!-o(8) 70.3(n' w(l2)-w(81-o(151 65.4(71' o(16)-W(5)-0(3) 70.1|'7l. o(121-w(11)-O(7) 86.1(81. w(7)-w(21-w(10) I I 0.2(81" w(11-w(8)-w(4) 106.1(8)" o(61-w(11)-o(12) 76.9(81' w(7)-w(2)-o(8) 98.4(7)' wfil-w(81-o(15) r 10.4(10)" o(6t-w(11}€(7t 68.4(81' w(10)-w(2)-o€) 108.8(8)o w(4)-w(a)-o(15) 99.2(9). oH(l11-W(11)-0(6) 135.2(9)'
oH{3}-W(3)-W(r0} r04.3(8)" 0H(9)-W(9)-W(4) 101.4(10)" 0H(B)-W{6FW(2) 93.4(8)" oH(12)-w(12)-W(8) 90.0(81" oH(3).W(3)-W(6) 104.9(8)' oH(9)-W(9)-Wfi2) 108.1(r0)' oH(6)-W(6)-W(3)106,7(8)" oH(l2)-W(12)-Wt9l 112.0(9)' oH(3)-W(31€(31 137.9(7)' oH(9)-W(9)€(12) 147.8(10)' OH(0)-w(01-w(7) 106.4{91" oH(l2)-W(r2)-W(1) 107.6(8r oH(3)-W(31-O(131145.0(8). oH(9)-w(9)-O(0) 160.1(10)' W(2).W(6)-W(3) 1m.4(7)" w(81-w(12FW(9) 91.6(7)' w(10t-w(3)-w(6t 104.3(8)' w(41.w(9)-w02) 89.8(8)' w(2t-w(6)-W(7) 139.4{8}' w(1)-w(12)-W(8) 132.7$1" w(10)-w(3)-o(3) 117.9(9)o w(4)-w(9)-o(12) 109.2(10)' W(3)-W(6)-W(7) 10€.8(8)" w(l)-w(12FW(9) 119.4(91' w(10)-w(3)-o(13) 45.7(7t" w(41-w(9)-o(61 66.0(7)" w(l)-w(12)-O(11) 64.3(0)" w{6)-w(3){(3) 06.0(6)" w(12)-W(9)4(12) 05.1(7)' oH(l2)-W(l2)-0(11) 134.2(81' w{6}-w(3}-o(13) I 10.0(91. w(12)-w(9)€(6) 101,1(10)' w(9)-w(12)-O(11) 110,2(91' o(3).w(3)-o(13) 62.1?to o(0|-w(9,€r2t 57.417to w(8)-w(ra-0(111 72.Ol7f
The two H2Ogroups, W(2) andW(12), eachact as an acceptorfor three adjacentHrO groups,rather than the CoweprsoN wmr RSIATEDSrnucnrnrs two expectedto complete a tetrahedralenvironment. The pseudo-centrosymmetricalrelationship among the The twenty-one known uranyl oxide hydrates O atomsinvolved in H-bonding is evident in Figure 7 display close structural similarities to schoepite [an approximatecenter of symmetry is located at the (Burns er al. 1996, Baran 1992, Cejka & Urbanec center of the figure, between 0(6) and O(13)l; 1990, Smith 1984, Deliens L977b, Sobry 1973, however, the hferred H-bonding interactions violate Peters 1967, Protas 1959). The uranyl oxide Pbca symmetry.This suggeststhat it is the pattern of hydrates are all sheet structures with one perfect H-bondingin schoepitethat is primarily responsiblefor cleavage. Crystals are optically negative, with the reduction in the symmetry ftom Pbca to Pz(a, similar X-ray powder-diffraction patterns, and they as it displacesthe O(uranyl) atoms from their ideal commonly have pseudohexagonalhabits. Their unit positionsin Pbca. cells can be described in terms of a primitive The H-bond contributions to the O(uranyl) atoms pseudohexagonal cell (Deliens 1977b, Chript are addedto the bond-valencesums in Table 5, and & Clark L960):,ao"* = 4.1, c6"* = 7.0 to 7.5 A, the estimatedbond-valence for interlayer H2O groups y = I20o. A reduced C-centered orthorhombic are given in Table 8. TheseH-bond contibutions are subcell (a-, b*, c*) can also be defined from the estimates,but they indicate that the H-bonding primitive pseudohexagonalcell (compare with arrangementproposed for schoepiteis reasonable. Pagoaga1983):
Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/5/1071/4006270/1071_34_5_cm.pdf by guest on 30 September 2021 1084 TIIE CANADIAN MINERALOGIST w(2) w(12) W(5) r lffi"l I T il i-.),U-\^'i' .l' Y\'|:'iotte1W Y t i i ,Jffi*t't I *ttr@-;;.d.*."^ a I -g(14) , i o(gfui""y6"%;,--DL'(o' z--ill61,'','w *,n,f';t:l'A*,,o, -' ,/- " .,W-Ij'&;t 1rW: I Qoet orr/' \ o,''rilli-^ottzi-".* -i-'o,r, t \ .jr,|-'o'-'' *[, w(l1) t w(8) I I ,-r..O(5) \,r l- b/2-l FIc. 7. View down [001] showingthe H-bond interactionsamong the HrO groupsand betweenH2O groupsand the O(uranyl) atomsthat act asH-bond acceptors.The H2Ogroups are hatched circles at z = 0.5. O(uranyl) atomsabove the H2Osheet are shownas highligbted circles; O(uranyl) atomsbelow tle H2O sheetare shown as dashedcircles. Arows indicate inferred D -->A interactions;hollow arrowsindicate ambiguous intrasheet H-bonds. The H-bondinginteractions forW(8) andW(12) are uncertain. One of three possible biircated H-bond arrangementsis shown for W(11). The rotational senseof the intrasheetH-bonds shown is "A{", one of four possibleanangements (see Table 6 and text for discussion).
aro o (2.a,o"r).sin(120)= 7.0 A subcell; however, these structures can also be b* =Qh.=4'lA representedin this way by redefining their unit cells cm = ch"*=7 .0 to 7.5 A (layerspacing) such that the structural sheetsare parallel to (001),o (Miller et al.1996). The unit cells of most ufanyl oxide hydrates are integral multiples of this reduced orthorhombic Fourmarierite subcell.Most fall into oneof two groups:(1) thosefor which b = n2brci(2) thosefor which b = n3b* (z is an The structure of schoepiteis strikingly similar to integer, usually I or 2). Schoepiteand the two uranyl that of fourrnarierite,Pb[(ttor4o3(oll)4].4H2o @iret oxide hydrate minerals containing Pb, fourmarierite 1985); cell parametersare similaroand the structural and curite, fall into the first group; for schoepite, sheetin schoepiteis topologica[y identical to that in a"=2,a^, b"= 4b-, c"=2c-. The uranyloxide hydrates fourmarierite(Figs. 2, 8). Schoepiteand fourmarierite containing alkaline earths, such as becquerelite, are the only two minerals known with this type of billietite, compreignacite,protasite and wiilsendorfite, sheet arrangement(Bums er al. L996). If two of the fall into the (larger) secondgroup. Other uranyl oxide OH groups in schoepitetOH(6) and OH(12)l are hydratesare known with unit-cell parametersthat are replacedby O2-, the compositionof the sheetchanges 161 simFle multiples of the reduced orthorhombic to that of fourmarierite. The negative charge on the
Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/5/1071/4006270/1071_34_5_cm.pdf by guest on 30 September 2021 TTIE STRUCTUREOF SCHOEPITE 1085
TABLE8. ESNMATEDBON},VAI.ENCE SUMS TO INTEFLAYERH,O GROUPA' Becquerelite,billietite and protasite t t{'A (w'l D-W lwl Z lwl W1| wt12l.O.A2tol14lO.89OH(l):0.21;Wl8l:0.'10 2.OA The three protasite-groupminerals, becquerette, Wl2t W(10):0.87rO(8):0.81OHl2ltO.17tlfi6tt0.fl;mTt0.16 2.17 Ca[(UO)6O.(OH)6].8H2O,billietite, Ba[(UO)6Oa t//|.3l !y{6):0.84;'ft}: 0.80 OH(31:0.15; wt1olr 0.19 (OH)61.4-8H2O,and protasite, Ba[(UO)3O3(OH)2] wl4l !v(8):0.92;(bl:0.80 OH(4):0.'t5;wtg):0.11 .3H2O,are structurally related (Pagoagaet al. 1987). wlsl O(3):0.S4;0(16):0.87 oH(51O.1S 1.89 Although they display many similarities to the wl0) Wzt: O,83iltllTt O,e4 OH(010.36; M3):0.16 2,19 structuresof schoepiteand fourmarierite,the structural W7l !t12):0.85;0(61:0.93 OH(7):0.13;lVtO):0.16 2,O7 sheets in the minerals of the protasite group are Wq lt(l):0.84; O(15):0.84 OH(g):0.19; lvt4):0.08; !44121:O.O3t 1.98 significantly different from those (Frg. 8). Schoepite w(9) M4): 0.89; (bl: 0.80 Or{lgt O.22t W12lt O.18 2.Og and fourmarieritehave corner-sharingU$7 polyhedr4 w(10) tvl3):o.81; (b): O.8O oH(lol:0.'19;Wtzl:0.13 whereasthe protasite-groupminerals have only edge- (b)r Wll1) O(12):0.91; 0.80 OH(!11:0.22 1.93 sharing UQ7polyhedra @agoagaet al. L987). As a M12l lv(g):O.82; (b)t o.80' OH(121:0.20; Wl'|lr0.18 2.OO result, the triangular o'holes"in the structuresheets of ' E€ffnd€dushg Rg, 2 in Bevn & Attomat (1980).Btur€ted t+bonds€nrlhto O.B the protasite-groupminerals are isolated rather than rub tF do@ &O gruo, s[ o0E ,,14Hsdltudons €leddod suat!that vdmr w to the H ddns m udty (€t Tsbh O. Int+ls:pr Fbondsfiom ,aetal.!8btl@ foblr 3) forming the "bow tie" dimers in schoepite and qd lho H-bon&tgBhM h Flg.7. t O) Ind€t6 blturialedbonds (ct Tabls8, Flg.7). fourmarierite(Frg. 8). This differencein the two types t Bond{qlsn@€tlnds tur bnegtod H-bonde|ton W(12)ro O(11)& W(8)and ftom w(1r) io o(q aocr. of structural sheet is the main structural distinction betweenthe mineralsof the protasiteand fourmarierite groups(Miller et aL.1996). These two types of structural sheetcan be readily distinguishedon the basis ofunit-cell parameters.The structural sheet in fourmarierite is compensatedby a-b plane is^parallel to the structural sheets,and a is interlayer Pb2+cations. Fourmarierite contains eight n x (-7 A) fo-r both types of sheet. Howevet, interlayer H2O groups as compared to twelve in b is n x (-6.15 A) for the protasitegroup, but is z x schoepite.Four H2O groups are bonded to two Pb (-8.2 A) in schoepiteand fourmarierite (compare the atoms, forming [PblH2O)4]4r' dimeric groups. Each reduced cells of Pagoaga 1983). For the known [Pb2(H2O)4]4r'dimer in fourmarierite replaces eight structuresof both groups,n = 2 alongD. The reasonfor H2O groupsin the schoepiteinterlayer. The remaining the different b dimensionscan be understoodby noting four HrO groups in fourmarierite occupy interlayer that the ratio of uranyl ions to O(sheet)atoms differs sitessimilar to the W sitesin schoepite. for the two types of sfructuralsheet. For the protasite-
FOURMARIERITE BECQUERELITE FIc. 8. Outlined UQ7 polyhedra in the structural units of fourmarierite, Pb[(UO)4O3(OID[.4H2O, and becquerelite, Ca[(UO2)6Oa(OII)6].8H2O.The two sheettypes are distinguishedby the different arrangementsof triangular holes, which are paired in fourmarierite("bow ties"), and isolatedin becquerelite.Fourmarierite has tle sameanangement ofpolyhedra as schoepite.U atomsare indicatedby filled circles; O(uranyl) atomsa.re omitted,
Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/5/1071/4006270/1071_34_5_cm.pdf by guest on 30 September 2021 1086 TIIE CANADIAN MINERALOGIST
group minerals, this ratio is 3:5; for schoepiteand RnrsRENcEs fourmarierite, it is 4:7. Thus one additional O(shee0 atom is required for every traelve uranyl ions in the ,A.Bsnc,M. (1978): The crystal structue of hexaaqua- fourrnarierite-typesheet as comparedto the protasite- tri-phydroxo-trlr-oxo-triuranyl(Vl) nitrate tetrahydrate, rype sheet.The additional-2.05 A every -8.15 A IQO)3O(OH)3(H2O)6INO3'4H2O.Acta Chem. Scand. along b is therefore required to accommodatethe A32.101-107. additional O(sheet)atoms in schoepite [2]-coordinated ARNBERG,L., HovMOLLER,S. & WssrNaAN,S. (1976): On and fourmarierite. the signfficance of 'hon-significant" reflections. Acra There are no known uranyl oxide hydrate sffuctures Cry stallo g r. A.35,497 499. basedon a hybrid ofthe two typesofsheet. Howeveq this could occurby meansof stackingdisorder along D. BARAN,V. (1992): Uranium-Orygen Chetnistry. Nuclear This may explain some of the difficulties associated ResearchInstitute, ReZ,Czech Republic. with resolving the structuresand obtaining consistent and accuratecell-dimensions for uranyl oxide hydrate BENARD,P., LouoR,D., DAclDtx, N., BRANDEL,V. & GoNE.r, minerals such as masuyite and vandendriesscheite M. (1994):U(UO)(PO)2, a new mixed-valenceuranium @eliens1977b, Christ & Clark 1960,Frondel 1958). orthophosphate:ab initio structure determination from powder diffraction data and optical and x-ray photo- Ianthinite electronspecta. Chzm. Materials 6, 1049-1058. BTcNAND,C. (1955):Sur les propri6tdset les synthbsesde Schoepitecan form by oxidation of ianthinite quelquesmin6raux uranifbres. Bull. Mindral. 78, 1,-26. (Deliens 1977a, Gfillemin & Protas 1959). The structureof ianthiniteis unknownbut may be similar to Brllrnr, v. & DE JoNc, W.F. (1935): Schoepiet en that of billietite @inch& Ewing1994). The conversion Becquereliet. NatuurwetenschappelijkTijdschrift Ned- of ianthinite to schoepite occurs with little or no Indie 17" 157-162. apparentstrain. Oxidation proceedsas thin fllaments of schoepite appear within ianthinite and grow BnowN, I.D. & Arrsnuerr, D. (1985): Bond-valence preferentially ilong b (Schoep& Stradiot 1947). As parametersobtained from a systematic analysis of the the degree of oxidation of ianthinite increases,the Inorganic Structure Data Base. Acta Crystallogr. B4l, filaments of schoepitecoalesce until the entire crystal )44-247. of ianthinite has been replacedby schoepite.This is accompaniedby a changefrom dark purple ianthinite & Wu, Kerc Ktry (1976): Empirical parameters to yellow schoepiteand by a continuousincrease in for calculating cation-oxygen bond valences.Acta C stallogr. 832, 1957-1959. 2Vo from approximately60' in ianthinite to approxi- ry mately75o in schoepite(Schoep & Stradiot1947).T\e J., Cesas,I., Cpne,E., EwrNc,R.C., Fwcq R.J. & U4 ions polyhedra BnuNo, in ianthinite may occupy UQ, in Wenur, L.O. (1995):The assessmentof tle long+erm the structural sheet,as reportedrecently for synthetic evolution of the spent nuclear fuel matrix by kinetic, U(UO)(PO)2 (B6nardet al.1994).This is compatible thermodynamic and spectroscopicstudies of uranium with a protasite-typesheet and a Ua:Ue ratio of 1:5. minerals. /n Scientific Basis for Nuclear Waste ManagementX\ruI (T. Murakami & R.C. Ewing, eds.7. ACIC.TOWIIDGEMENTS Materials ResearchSoc. Proc. 353. 633-639.
The authorsare gratefirl to G. Mast of the Geology BrnNs, P.C.,Mnmn, M.L. & Ewnc, R.C. (1996):Uranyl Museum at the Colorado School of Mines and to minerals and inorganic phases:a comparisonand hierar- Dr. M. Deliens of the Institut Royal des Sciences chy of crystalstructures. Can. Mineral.34, 845-880. Naturelles de Belgique and the Musde Royal de l'Afrique Centrale, Tennrren, for the loan of the dura, J. & UnseNEc,Z. (1990): SecondaryUranium schoepite samples. This work is supported by Minerals. Academia,Czechoslovak Academy of Sciences, hague, CzechRepublic. the Natural Sciences and Engineering Research Council of Canada.Early parts of this work were CHRIsr, C.L. (1965): Phasetransformations and crystal supportedby the Swedish Nuclear Fuel and Waste chemistryof schoepite.Am . Mineral. 50, 235-239. ManagementCo. (SKB). RIF gratefully acknowledges an NSERC International Fellowship, and FCH was & CI-ARK,J.R. (1960): Crystal chemicalstudies of supportedby NSERC Operating, Major Equipment sorneuranyl oxide hydrates.An Minzral. 45, 1026-1061, and Infrastructure grants. Financial support for RCE was provided by DOE-BES (grant no. DesE'rs,P.G. & Loopsrne,B.O. (1963):On the uranatesof DE-FG03-95ER1450). The manuscript benefitted ammonium.tr. X-ray investigationof the compoundsin from careful reviews bv S. Menchetti and C. the system NH3-UO3-H2O, J. Inorg. NucI. Chem. 25, Gramaccioli. 94s-953.
Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/5/1071/4006270/1071_34_5_cm.pdf by guest on 30 September 2021 TIIE STRUCTTJREOF SCHOEPITE 1087
Duim.[s, M. (1977a):Associations de min6raux secondaires Mrnsn, M.L., FNcn, R.J,, Buws, P.C. & EwD{c, R.C. d'uranium i Shinkolobwe(r6gion du ShabUZa'ire).Bull. (1996): Description and classification of uranium oxide Min€ra\.100,32-38. hydrate sheet topologies. J. Materials Research (in press). (1977b): Review of the hydrated oxides of U and Pb, with new X-ray powder data.Mineral. Mag. 41, PAcoAGA,M.K. (1983): The Crystal Chemistryof the Uranyl J l-) /. Oxifu HydrateMirurals. Ph.D. thesis,Univ. of Maryland, CollegePark, Maryland. EvANs, H.T., Jn. (1963): Uranyl ion coordination. ,Science 141,154-158. Arpr-euar, D.E. & STEWART,J.M. (1987): Crystal structures and crystal chemistry of the uranyl oxide EwD{G,R.C. (1993): The long-term perforrnanceof nuclear hydrates becquerelite, billietite, and protasite. Ar". waste forms: nahral materials - three case studies. .In M inzral. 72. 1230-1238. Scientific Basis for Nuclear Waste ManagementXVI (C.G. Interante& R.T. Pabalan,eds.), Maerinls Research Pe1tsns,J.-M. (1967): Synthbseset 6tude radiocristallo- Soc.Proc. 294. 559-568. graphiqued'uranates synthdtiques du type oxyde double d'vranyl. Mdm. Soc.Roy. Sci.Li|ge, sdr. 5, l4(3), 5-57. Ftucu, R.J. & Ewnqc,R.C. (1992):The corrosionof uraninite under oxidizing conditions. J. Nucl. Matertals 190, PREr, P. (1985): Structure cristalline de la fourmari6rite, 133-156. Pb(UO)4O3(OI{)4'4H2O.BulL MiniraL l08, 659-665. & - (1994): Formation, oxidation and ------^^: DR n'Ns,.M', PRE'I-MEr Im, J' & GSRMAn'I'G' alteration of iarthinite. 1'l scientific Basis for Nucleai wasteManasement XVI (A. Barkan& R.A. vJ Pb2t0o,5o6(olD'1'a\9:nouveau !11?: ?:ryllt: et structurecristalline' Bull' Mindral' Konynenburg, eds.),Maerials Research soc.proc.3d" ltrygipi:P"dt6s 625_630. 106,299-304. (1982): Mm.nn, M.L. & Ewnqc,R.C. (1992): Weathering PRE'I-MBUNTB,J. & PIRET,P. Nouvelle d6termina- of natural uranyl oxide hydrates:schoepite polytypes and tion de la structure cristalline de la becquerelite.BzlL dehydrationeffects. Radiachim Acta 58159,433443. Minlral.105, 606-610.
FoRsym, R.S. & WERME,L.O, (1992): Spentfuel conosion Pnores, J. (1959): Contribution tr l'6tude des oxydes and dissolution.J. Nucl. Mateials 190. 3-19. d'uranium hydrat6s.Bull. Minlral. E2, 239-272.
Fh.otrout,C. (1958): Systematicmineralogy of uranium and ScHoEp, A. (1932): The specific gravity and chemical tlrorium. U.S. Geol. Surv..Bull. 106/,. composition of becquereliteand schoepite.Mus. Congo Belge,Ann. (Mintralogie) I, Fasc. fr,, 5-7. Gun:,:epmq,C. & PRorAs,J. (1959): Ianthinite et wyartite. BuIl. Mindral. E2. 80-86. & Srneotot, S. (1947): Paraschoepiteand epiianthinite, two new uranium minerals from Hawttonrr, F.C. (1992): The role of OH and H2O in oxide Shinkolobwe@elgian Congo). Am Mineral. 32, 34+350. and oxysalt mineral*,Z Kristallogr, 20l, 183-206. Surru, D.K., Jn. (1984): Uranium mineralogy. /n Uranium (1994): Structural aspectsof oxides and oxysalt Geochemistry, Mineralogy, Geology, Exploration and crystals.Acta Crystallogr.850, 481-510. Resources@. DeVivo, F. Ippolito, G. Capaldi & P.R. Simpson, eds.). Institute of Mining and Metallurgy, IInsHrsLD, F.L. & RasNovrcH, D. (1973): Treating weak london, U.K. (43-88). reflexions in least-squarescalculatious. Acta Crystallogr. 429.510-513. Sonnv, R. (1973): Etude des uranateshydrat€s. I. Propri6t6s radiocristallographiquesdes uranateshydrat6s de cations Hoarsrna, H.R. & Sncer., S. (1973):The uraniumtrioxide - bivalents.J. Ircrg. Nucl. Chem.3S,1515-1524. watersyst€m, J, bwrg, Nucl, Chzm.35,76l:779. Srnoes-GescoyNE,S., JorrysoN,L.J., BEELEY,P.A. & Jom.rsoN,L.H. & WmuB, L.O. (1994): Materials charac- Ss:.ncm, D.M. (1985): Dissolutionof used CANDU teristic aad dissolution behavior of spent nuclear fuel. fuel at various temperatues and redox conditions. 1z Material* Res.Soc. Ball. W(12), 2+27 . Scientific Basis for Nuclear WasteManagement D( (L.O Werme,ed.). Materinls Research Soc. Proc,50,317-326. LoopsrRA,B.O. & Rmvs-o, H.M. (1969): The structuresof some alkaline-earth metal uranates. Acta Crystallogr. Tavron, J.C. (1971): The structureof the o form of uranyl D.25.787-79t. hydroxitle.Acra Crystallogr.827, 1088-1091.
MsRErrEn, K. (1979): The crystal structure of curite, WADSTEN,T, (1,977): The oxidation of polycrystalline lPb656(H2O,OII)41 t@Or8O8(OID6l r. Tsche rmaks M tuwral. uranium dioxide in air at room temperature.I. Nucl. Petrol. Mitt. 26, n 9-292. Materials&.315.
Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/5/1071/4006270/1071_34_5_cm.pdf by guest on 30 September 2021 1088 TTIE CANADIAN MINERALOGIST
WAIxER,T.L. (1923):Schoepite, a newuranium mineral from WnoNrmwrcz,D.J., Barns, J.K., Gemnc, T.J.,VELEcIas, E. Kasolo,BelgianCongo.Are.Mineral.8,67-69. & Tem, B.S. (1992): Uranium releaseand secondary phase formation during unsaturatedtesting of UO2 at WaNc,R. & Kerayaue, J.B. (1982):Dissolution mechanism 90'C. J. Nurl. Maerials L90,107-127. for UO2 and spent fuel. Nucl. Chem.Waste Management 3. 83-90.
WILsoN, A.J.C. (1976): Statistical bias in least-squares ReceivedJanuary 3, 1996, revised manuscriptaccepted refinement.Acta Crystallogr. 432,994-996. June 3, 1996.
Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/5/1071/4006270/1071_34_5_cm.pdf by guest on 30 September 2021