t55t Thz CanadianMineralo gist Vol.35, pp. 155l-1570(1997)
THECRYSTAL CHEMISTRY OF HEXAVALENTURANIUM: POLYHEDRONGEOMETRIES, BOND-VALENCE PARAMETERS, AND POLYMERIZATIONOF POLYHEDRA
PETER C. BURNS1 Deparnrent of Geology,University of lllinois at Urbana-Champaigg245 Natural History Building, 1301 W. GreenStreet, Urbana, Illinois 61801, U.SA.
RODNEY C. EWING' Departnmt of Earth and Planetary Sciences,University of New Mertco, Albu4uerque,New Mexico 871i1-1116, U.S.A.
FRANK C. HAWTHORNE Departmentof Geological Sciences,University of Manitoba Wnnipeg, Manitoba RiT 2N2
AssrRAsr
The geomeffy,bond valences,and polymerization ofhexavalent uranium polyhedrafrom 105 well-refined structuresare analyzed.The Utu cation is almost always presentin crystal stnrcturesas part of a nearly linear (UOr)z* uranyl ion that is coordinatedby four, five or six equatorial anions in an approximately planar arangement perpendicular to the uranyl ion, giving square,pentagonal and hexagonal bipyramids, respectively. The Utu-O7" bond length (Oy,: uranyl-ion O atom) is independentof the equatorial anions of the polyhedra;-averagesof all polyhedra tlat contain uranyl ions ffs; I6lIJ6f-Or.= 1.79(3),mg0.-.9 a,= 1.79(4),and t8lu6+-Our= 1.78(3)A. Not a[ r6lu6+polyhedra contain uranyl ions; thereis a continuous seriesof coordiaationpolyhedr4 from squarebipyramidal polyhedrawith uranyl ions to holosymmehic octahedralgeometry. The mUo*and t8lu6+polyhedra invariably contaitl a uranyl ion. The equatorial U6.-0 (0: O,-, OH-) bond-lengthsof uranyl polyhedra dependupon coordhation number; averagesfor all polyhedra are t6lu6+-dq = 2,28(5), rlUot-$* = 2.37(9), afi t8tlJ6+-$q- 2.47(12) A. Cunently availablebond-valence parameters for U& are unsatisfactoryfor determiningbond-valence sums. Coordination-specific bond-valencepaxameters have been derived for U6|, together with parametersapplicable to all coordination geometries.The parametersgive bond-valence sums for Ue of -6 vlr and reasonablebond-valences for Uc,-Ou, bonds.The bond-valenceparaneters facilitate the recognitionof Ua, U5+and U6| catiotrsin refined crystal structures. The crystal-chemicalconsfraints ofpolyhedral polymerization in uranyl phasesare discussed.
Keywords:uranium, uranium mineral, uranyl, polyhedral geomefiy,bond valence.
Somraans
La gdometrie,les valencesde liaison, et le degr6 de polym6risation des polybdrescontenant l'uranium hexavalentdans 105 structuresbien affrn6essont ici analys6s.Dans les structurescristallines, le cation Ua' fait presquetoujours partie d'un ion uranyle, (UOJri quasiment lin6aire et coordonn6 i quatre, cinq ou six anions da:rs un agencementi peu prBs planaire perpendiculairei l'ion uranyle,ce qui mbnei desgroupes carr6s, pentagonaux et en bip)'ramideshexagonales, respectivement. La longueur de liaison Uc'-Oo, (Oy,: atome d'oxygbne faisant partie de I'ion uranyle) est iaddpendantede I'agencementdes anions6quatoriaux des polyBdres; les moyennesde tous les polyBdresqui contiement l'ion uranyle sont: r6u6+-Our=1.79(3), lTlUe-Ou"= 1.79(4), et t8lu6f-Our= 1.78(3)A. Par contre, pas tous les polyddrescontenant t61uil contiennent f ion uraayle. I1 existeune s6riecontinue de polyedresde coordilence, de bipyramide carr6eavec ions d'wanyle b octaddreholosymm6trique. [.es polybdresconGnant r7{JG et rsluo'contiennent sans exception I'ion uranyle. La longueurdes liaisons Utu-$ ({: Oz-,OHJ despolybdres i uranyle d6pendde la coordinence.Les moyennespour tous les polyAdressont t6lu6++q=2,28(5), mUto-S"q =2.37(9),et r8lu6++€q- 2.A1(1r!)A.t*sparametres presentementdisponiblespour les valencesde liaisbn impliquant Utu ne sont pas ad6quatspour en ddterminerla somme.De tels paramBtrespropres e rme coordinenceparticulibre ont 6t6 d6rivdspour
t Presentaddress: Department of Civil Engineeringand GeologicatSciences, University of Notre Dame,Notre Darne,Indiana 46556-4767, U.S.A. E-mail address:[email protected] 2 Present address:Department of Nuclear Engineering and Radiological Sciences,University of Michigan, Ann Arbor, Michigan48109, U.S.A. 1552 TTIE CANADIAN MINBRALOGIST
l'uranium hexavalent,ainsi que desparamdtres applicables i toutesles g6om6triesdes polyddres. Ces parambfres mhnent i des sommesdes valences de liaison pour Ue. d'environ 6 unit6s et desvaleurs raisonnablas pour les liaisonsU6'-Oo, Cesnouveaux parametresfacili0ent I'identification de U+r, Us et Utu dans les compos6sdont la structure a 6t6 affrn6e.Nous 6valuonsles contraintescristallochiniques irnpos6essur la polymdrisationdes polyEdresdes compos6sI uranyle. (Traduit par la Rddaction)
Mots-cl6s: uranium, min6ral uranifdre, uranyle, g6om€ftiedes polyBdres,valence de liaison.
Inrnotuctron work classes.The majority of Ue'phases(106) adopt a structurethat is basedupon infinite sheetsofpolyhedra The Uo' (uranyl) mineralsare major constituentsof that share corners and edges. Burns et al. (1996) the oxidized parts ofuranium deposits,where they are grouped these sheets according to the topological commonly found as tle products of alteration of arrangementof the anionsin the sheet. uraninite (Frondel 1958,Finch et al. L992,Finch & A detailed understandingof the crystal chemistry Ewng l992,Pearcyet aI. 1994).These minerals have andbonding of Uc. will aid in evaluatingthe likelihood recently received renewed interest becauseof tleir ofthe incorporationoffission productsand transuranic significance to the environment. Uranyl minerals are elementsin low quantities into the strucfures of U6+ productsof the oxidation of radioactivemine-tailings; phases@urns et al.1997a).In addition,an appreciation they im.Itactupon the releaseof U and Pb into the envi- of the underlying controls of bond topology is a ronment. In addition, uranyl minerals are prominent necessarystep toward understandingthe relations alteration-inducedphases in laboratoryexperiments on between tle hierarchy of mineral structures and the UO, as well as spentnuclear fuel subjectedto oxidative paragenesisof the minerals. Despite the wealth of dissolution(Wadsten 1,977, Wang & KatayamaL982, crystal-structure data available, the current state Wronkiewicz et al. 1992, Forsyth & Werme 1992, of knowledge of the crystal chemistry and bonding of Johnson& Werme1994. Finn et al. I996.WrorlrjLewicz Ue' lags behind that of most of the lighter elements. et al. 1996).Under oxidizing conditions,such as tlose The coordinationpolyhedra of many cationsimFortant found at tle proposednuclear-waste repository at in minerals have been investigatedin detail using Yucca Mountain, Nevada, the UO2 in spent fuel is theoretical approaches(e.9., silicates: Gibbs 1982, unstable,and the rate of alteration in the presenceof Lasaga & Gibbs 1987, 1988, L990, I99L: borates: water is likd to be appreciable(Murphy & Pabalan Tossell 1986, 1990,Burns 1995;carbonates: Tossell 1995).Spent fuel containsfission products(a.g., Sr, Cs 1986;copper oxysalts: Burns & Hawthorne1995ab), and! andnansuranic elements (e.g., Np, Pu, Am" Cm) but there has been little quantitative theoretical work (Oversby 1994).T\e generally low concentrationsof done on tlte coordinationchemistry of U0*. This is fission productsand transuranicelements in spentfuel becauseUtu complexespresent serious problems when will generally preclude them from forming discrete applying quantum-chemistrymettrods, owing to the phasesduring alteration(Oversby 1994). The formation largenumber of electrons(requiring the useof effective of tle alteration products of UO2,mainly Uto phases, core potentials) and relativistic effects. Despite these will lower the concentration in solution of these dfficulties with fheoretical approaches,some calcula- radionuclides if they are incorporated into the tions havebeen reported for Ue' complexes(Taisumi & structuresofthe alterationproducts. Laboratory studies Hoffmann 1980, Wadt L981, van Wezenbeeket al. have provided evidence for the retention of some 1991, Pyykkd & Zhao 1991,Pyykkd et al. 1.994,Ctaw radionuclidesin the alterationphases of spentnuclear et aI. L995). tuel (Finn et a\.1996). There are about 170 minerals known to contain U BoNpnqcul Uo' Por,vrupna as a necessarystrucfural constifuent. Of these,most contain Uil, the oxidized form of U, althoughUa also The oxidation statesof the 5/actinide elementsare occursin severalmins14ls. The crystal structuresof quile variableowing to the screeningof the 5/electrons 56 Ue. minerals and about 120 synthetic ge+ phases from the nucleus.Uranium can occur as [J+r,lJs+ 91Ucr havebeen reported. As part ofour on-goingexamination in crystal structures,with U& preferredunder oxidizing of ttre strucfural relations in U6" phases,Burts et al. conditions.The Uc' cation usually occursin crystal (1996) have proposed a structural hierarchy for Uo' strucfuresas part of an approximatelylinear (UtuO2)2+ minerals and inorganic phases. The structures are uranyl ion (Evans 1963). Ab initio molecular-orbital organizedon the basisof the polymerization of cation calculations (Craw et al. 1995) have shown tlat polyhedra of higher bond-valence,resulting in sheet, the Utu_O bonding mechanism in the uranyl ion is chain, finite cluster, isolated polyhedron, and frame- primarily by donationof electronsfrom thep orbital of THE CRYSTALCHE!trSTRY OF HEXAVAIJNTI'RANIT]M 1553
(a) (b)
rilx \i1\
UrQq UrQt UrQu
Frc. l. The three types of.Ur$, polyhedra[Ur: (UO")z'uranyl ion, $: Oz-,OHl. u* cations are shown as circles shadedwith parallel lines, and anions are shown as unshadedcircles.
the O atom into the empty d and/orbitals of the U GgoN{ETRESoF Ue PoLYHEDRA atom. Average Uto-O bond-lengthsin the uranyl ion are -1.8 A, and thus the bond-valencerequirements of EXAFS data the uranyl-ion O atoms(hereafter referred to as Oy,) are largely satisfiedwithout additional bonding. ExtendedX-ray absorptionfine-structure (EXAFS) In crystal structure$,the uranyl ion is coordinated spectroscopyis a useful technique for determining by four, five or six anionsin an approximatelyplanar the speciation of actinide elements in solutions and arrangementessentially perpendicular to the uranyl solids(Nitsche 1995, Reich et al. 1996).EXAFS spectra ion, giving UrSa square, Ur$5 pentagonal and UrS6 readily provide bond-length information for U hexagonalbipyramids (Ur: uranyl ion, $: O2-,OH-), polyhedra althoughfew studiesofminerals havebeen respectively(Fig. 1).The uranylion hasa formal valence reported. Tlpical uranyl-ion Utu-O bond-lengths of2+, andtlus tle fypical bond-valencesassociated ob'tainedfrori EXAFS spectraare 1.78 anct1.79 A with eachUil-S"q (S*: equatorial$) bond are -0.5, (Charpinet al. 1985,Moll et al. 1994, 1995,AJIen et -0.4 and -0.33 valenceunits (va) for Ur$a, Ur$5 and al. L995,1996).X-ray-dffiaction studieshave shown Ur$6, respectively.As the bond-valencerequirements that the two uranyl ion Ua'-O bond-lengthscan differ ofthe equatorialanions are only partly satisfiedby the for a single uranyl ion, but EXAFS techniquesdo not Utu-$"q bond, Ur$a, Ur$5 and Ur$6 polyhedra may resolve ttresetwo distances.EXAFS is usually not polymerize with other Ur$, polyhedra or with other capable of resolving individual equatorial UG-S cation polyhedra to form complex structureswithout bond-lengths,but instead gives an averageequatorial violatingthe valence-sumrule @rown 1981).Because U6r_0 bond-lengfhthat is comparableto the average the equatorial anions are close to being coplanar,and bond-length obtained from X-ray-diffraction studies the Or. bond-valencerequirements are largely satisfied 0M:ollet al. 1994). without further substantialbonding, Ur$" polyhedra may share equatorial edges and corners, commonly X-ray-diffraction data resultingin infinite sheets.In suchcases, the uranyl ion is oriented approximately perpendicularto the sheet, The accuratereflnement of crystal structurestlat and the sheets are most often connected tbrough contain Uto is more dfficult than in tle caseof most weaker bonds to interlayer cations and tlrough H other mineral groups owing to the high absorptionof bonds. X rays by U. Thus,nvmy of the publishedUe structures L554 THE CANADIAN MINERALOGIST
q'@@@ caICnt(8O!@L€,pL r noc|rzl& rrTdcDto'l It ,nd6nn !&rctt(U0rrco*I@pb l9
lrylqtet Ms{Ce0(Oqil@Ph' hlffilh m dtue! C&xTqO,) a pturs&! qCP&@0{G}!@fp){ 2r strldodlre Crnx)t(8iq@I@l 5 rubtlr zl (& lqKrotor(oEl!(rch 5 eLe& MslG!),)@JL@ho B tknF|dElh eftdcx'r(s4D]@eph 7 r.t'mrtikrta tcnt(Tco!)l u rbrlqfflb lbRt orrPSO&L6p)u s sdo!!fu IGDTO{@'d@O! at 6|md!d! ItKbr.q(o&t6ro. 9 e*rUOqg* NaCrrtGrO){Ar)rt(SOdF@p}o 7n tonsilltb Baqrftdt(FJbOAIeOb l0 *rooorr& Ml[COr)(tio!OS)ll@O)! n frns{di?{{O t{d[Crc|bCOr,o(OEl@pL rr sdrtrto Gnt{8lo0@oL a gdll!@ Brl(tjq){s.ot&t€ror 12 :mrtdo cairsKrodtcor)bltrgohz D jdd 6t(nt{so.){o},t@p[ 13 qunu at(nb@tp(otsl(@)' g, 3l tsEdfrc lttcDJGndl@) t4 yerhot'milt€!,b t(&)g@.| Itttdi, q(UorGqEIeO)r lJ $&d& BIP{CnXAorUflPL J2 Elartoltlrlb frdd(ro[)@Oil€O] l5 4l|.ffo Klcn1}l(sor@rl€ro 3l EgEffi B4(nr)Aorl$lq. * u __ _. Rtc .Coryma nr0 *cwa oqd ..M* &tcrorprl 34 tMSCrOJG?Ot)l 5t Cartp. e Batcror)oj 35 &t(I,Otorlorl 59 srrlFa 63 tt(D:X@] 36 &l(tqx@)d 60 Kd&IrO6 84 htcDtoJ n C{tPrplobs 6l l&Iloc 8! ICFI)E(Po.)l@OD. 3t d&IGrO.)oll 6t r{q E5 q('0r)@ot1@P)! 39 ICp!@r)OI 6t rrOlHP al lod(rn@Jlop! & t(Uq)muel s &I& tt t(Dr)D(A!Or)l@O. 4l No.t(IDr)otl 6t &!4ln' s IJ|(n'Xr0.)l@P). 42 flD.fe!fio)t 6 GttP{ n Mst(IrqXBo&l6rO)t 43 erdcne$t${U(q 6t e(t,otot 9t giI Knxsotll&&o} u t@otCrPo.)bGIP)l@rO). 6t re{tlor)ob rdlr{sod(&oL 45 ieKn (so.){rrrol0!pD. 5e cror6m() 93 uKlx):)@o![ 6 tCnt(so0CODt(&O)'r m p4n God 94 MsKntGo0L@O. 17 t(Dt(sod@s)hl@!! 7r a{Pt0rd).)@p}, 9' &tcDrat 18 tcpr(so.)@Le@ot, t2 &crorom&@p)D 96 lEr(Ipibo.l 4t t(rot(slor@OL1@'OL rt B4e)Sroo0@0). ,, lcl},)(@d n c&tcx)ilcq'hlGPl 74 lrdrprloroo.Ic*Ol 9E Mt('or|{vlA)lGp). rl &{cx}r(aqrx6,o} 15 N&GFrepr) 99 ca{(ttot{v0)t 52 oE)dcqxoorhl 75 cror}(c.o.)@tp)" l(n cat(u0r}o{botl 13 rb[(IDtoo!)rl n nhGq,Xr€rl. l0l t&Gq)trord s nd(rptcor)d 1t Mg(t,O2lMoOr)ff10)rr ta, KICD{qo.XoSl@O)rr Jj t(nt0 l)hlcIo), 7e catlrPt ltB cct(Iror@bl x &t(rOtrool s' CquO' t04 0q'LKnt(6or){FP)l@O tt lbIJOc tr UqSq9, tot Rr&rm l:Gh&m(r98qOlrffir(19160!(J):t&F&tr!@(lS6'(4)Bt@(r98rb!6)rryrdgAntEoOy|j) (OTryl6rr4l O98U q)chdff&C.sm(tgtsI (fI ptdlr.r (r9s), (9IpbOgs$ O0):t,i!l&Os66} (u)Etrrar O9s0, O2)Coq6AFntn@OJl9tI03)It Ftb(l9o!),(tOR!6lDqds&Ryu(rgrralGO:ircrer09ea}(fO:slsSror4t O9t3' o7): rr@[email protected] ogea) oc! sh[t.rd, (t99l}o9IDwdnsrd. o90u (atlphst€ral (199!@r): Arndograt (19r) @):@w&CotuO98t,(23Ilrrilhr&Tsytn(rSO,(24):!e'-'ta&CrbO t(2t:Fhlh€r4f096),(26ItU€ld6OS6aI (tA:&en&nffirqds(lt7tl (28r:Dffidlsraf O9!D! (,g}Ms|rierOgB6bI O0):pha&D€dqG983I €r):Ros.dgARrr o97?b} (32II\'ffiO9&D' C33):Vodnmer4r O99t) (J4! c6sr€r4t (198I Gt R!b.dar, (196I (3O: $rryr rrat O972bt€D Aro6t!€fd,O9t6),(J6ll|rs6(r97q,Or:rhl&Cob099r}({0}Efd&@098I(4t):Ftc,h6r4rO9ts}(42} Ehdrrd, o962),(a3I Socefithdr4r (lsrb),(44] Als*€f al ow), (4t:Nidi0dar 0y,9] ({o: ca46dn(nrt(D (4?):Bntra€r4r (190r} (4C)88r@O969L(4D:8hdcrd, (19728I(50):Dt&m€rd, (UqD (Jl):Botoe&O!d[@(1y70\(32ttMdd. 0992),(53): Garydlo9ffb}(54) So0993aI (5t S4atr$Is6rar, (190), (56):r tndu€r4r orrra) (tIt[h6dr4t, o9?r] (5sx carp€drogrta] (19: radnwrral, (l9EE! (6('' ryco97l} (61):l@ddrd (rgct] (62rtF4f@6d. axeof low precision,and high R indices comnonly are 60 reported.Owing to the high X-ray scatteringefficiency of U, as comparedto O, the O atom positions are in 40 many casesimprecisely known, and ttris problem is 20 especially pronouncedin Rieweld refinements using X-ray powder data. o The structuresconsidered here arelisted in Table 1. In eachstructure considered, Uo'bonds to O2-,OH- or 12o IIrO; structureswith F or Cl bondedto Uo'have been excluded from consideration. Structures containing 100 both U+ and Ue (or possibly U*) have been omitted 3' Bo because of the possibility of U&-Uil disorder. 5 Structures refined using X-ray data collected from 3 eo single crystals,or neutrondata collected from powdersn E havebeen included wherethe R index is less than1%o. 40 Structures reflned using X-ray data collected from powdershave beenexcluded. General trend.s 60 40 Data have been grouped according to the coordination number of Utu: six, seven or eight, 20 including tle Oy" atoms. Uil-$ bond-length data for all coordination numbers are presentedin Figure 2. 0 oooaoo09 ooo oao There is a completely bimodal distribution of Uto-$ q\qqq.:.!a? 1qq \oqq s';';r$.t.t.I .t cT cI OTOJ$ bond-lengths in both tle mU& and t8lu6rpolyhedra; ooooaaaa oa9 ooa ooNoooFN o{o @N6 the bimodal distribution fe1 tol[Jo+is less pronounced --;.-:-ciNci ci ci ci ci ci ci (Fie.2). U6: o Bond Lengrth(A) Ft(Ja+aTyl ttt(Ja+polyhedra Frc. 2. The distribution ef{J*-$ bond-lengttrsin well-refined structures. The Uc-$ bond-length distributions in t71UGand trlUotpolyhedra (Fig. 2) arecompletely bimodal owing to the presenceof a uranyl ion in everypolyhedron. A1l 2.35 t71u6i and t8lUtu polyhedra are Ur$5 pentagonal bipyramidg and Ur$6 hexagonalbipyramids, respec- 2.30 tively.^Inboth cases,the populationcentered around -1.8 A correspondsto the Ue'-Ou, bonds, and the 2.25 population centeredaround -2.4 A correspondsto "g the Ue'-$* bo-nds.The populationsat -2.4 Aare@er E than at -1.8 A becausethere are more Uo'*$* bonds 9, z.zo than Utu-Oy"bonds in eachpolyhedron. tt l! The averagemUto-Ou" and trUe-S"u bond-lengths 2.15 for all 93 Ur$, polyhedraare 1.79 (o = 0.04) and2.37 6 (o = 0.09)A, respectively.In2S Ur$6polyhedra,the A z.,ro averagel8l(J6t-4u, rn4 t8lu6i+e4bond-lengths are 1.78 (o = 0.03)and2.47 (o = 0.12)A, respectively. t6JU61polyhedra The Utu-$ bond-length distribution for polyhedra 1.70 ',1.751.80 1.85 1.90 1.95 2.00 2.05 2.10 2.15 containing t6lu6!isirregular, and bond len6gthsrange 45 geometriespresented in Figures3 and 4 were slteined 40 using diffraction techniques: as such, ttrey represent long-range avetageconfigurations tlat may in some 35 casesdiffer from local site-geometries.Two possible 30 models explain tle trend shown in Figure 3: (1) the 25 energetics of. a Ur$a square bipyrnmid and a UtoQ6 octahedronmay be similar, and ttre pathway between 20 the two coordination geometriesmay not have a sig- 15 nificant energy-barrier, thus permitting tle range of 10 polyhedron geometries to exist and be observedby (2) A uranyl ion may Ps diffraction techniques. 6u6-9r)'?. o be locally presentin eachpolyhedron, but either static polyhedral Bzog or dynamic disorderresults in tle range of l! '15 geometriesobserved by diftaction techniques. Pyykkit & Zhao (1991) have repofied quasirelativistic ab initio calculations for (UO)G clusterswith Ur$4 and octahedralgeometries, as well asfor geomeftiesintermediate between these two coor- dination polyhedra. The calculations predict a trend similal to that shown in Figure 3, alttrough tle predicted bond-lengths are seriously in error. More significantln the energy of tle cluster was found to vary very little over the range of geometries.Thus, Pyykkit & Znao (1991) concludedthat tle range of aoaoooooooooDoo t6lu6+-Ou,bond-lengthsmay be interpretedas a soft es @Nts@OOOOOFFNNOO - - - ; ; ; J ci .\i .,i c,i ci c\i c,i ci vibration mode of the cubic (UO6)Gcluster. ,A d rA d,A'6,rl d,6,6,r5'!5,$,6,A NNNNNNNT\NF-CINNF-N Where positional disorder is not present, ttre @@NN@OOOOOF-NNO - ; - - - ; ; ; c\i ci c.i .,i ai 6i c.i anisoftopic-displacementellipsoids of anionsare typically nearly spherical,orthey me elongatesubperpendicular (A) U'- o BondLength to the cation-anionbon{ reflecting the relative easeof bond-bending compared to bond-stretching. As ttre FIc. 4. The distributionof Utu-S bond-lengthsfs1 te[Jot typical t6lu6t--Ou,bond-length is -0.5 A shorter than polyhedraia well-refinedstructures. Bottom: square bipyramidalpolyhedra that contain a uranylion, middle: intermediategeometries, top: approximatelyoctahedral geonefties. (a) (b) Note tlat where a uranyl ion is present, the average t6lUil-Ou" bond-lengtl colresponds to the shortest averagetrans 161U6+-+bond-length.The datain Figure 3 plot in three clusters; one corresponds to Ur04 polyhedra tlat contain uranyl ions with a typical Ue4u, bond-lengtl of -1.8 A, one correspondsto a holosymmetric or approximately holosymmetric octahedralcoordination, and the thhd corespondsto a geometry that is intermediate between the two. The (c) gct-$ bond-leng1.hdistributions for eachof thesetbree gloups are shown separatelyin Figure 4. Considering only the sixteen re{Jotpolyhedra that contain typical uranylions (i.e.,Ua,-Or,*1.8 A), theaverageteUe-Ou" and I61U6+$eq bond-lengtls are L.79 (o = 0.03) and 2.28 (o = 0.05)A, respectively. J[g te]IJ6+polyhedral geometries display a trend from the typical Ur$a square bipyramid to a holosymmetricoctahedron @g. 3). The ubiquify of the uranyl ion in tnu6+and t8lu6+polyhedra @g. 2) indi- Frc. 5. Anisotropic displacementellipsoids (50% probability) catesthat a uranyl ion is alwaysenergetically favorable 1e1lor polyhedrain: (a) Nq[(UOrO3], (b) o-Li6UOa" in thosecoordination geometies. 1!s rou$ polyhedral (c) KuLi+UOo,(d) &t@Oro6l. TTIECRYSTAL CHEMISTRY OF HEXAVALENTURANITJM 1557 the typical tqUtu-$"qbond-length, disorder in theuranyl 20 ion positions within the polyhedron should result in anion anisotropic-displicementellipsoids that are 1s tsIU elongate subparallel to the Uto-S bond. Anisotropic- 10 displacemenlparameters are availablefor only a small number of well-refined Ue' structures tlaf contain 5 holosymmehicor near-holosymmetricUo*$6 octahedra, ,e Considerationofthese structuressupports both models for the observed t6lUGpolyhedron-geometry trends. 20 The structuresof Naa[(UO")O:l CW9{,e Hoppe 1986) (Wolf is'- t4u and cr-LiuUOo & Hoppe 1985) borh contain b holosymmetricUG$e octahedra;in botl cases,the F ro anisotropic-displacementellipsoids (Figs. 5a, b) of * o the anionsare consistentwith absenceof significant fi positionaldisorder ofthe anions,and thus arecompatible S with model(1). In contast, the anisotropic-displacement 30 ellipsoids of anions for the holosymmetric UGS. 2s octahedron in tle structure of KrLiaUO6 (Wolf & Hoppe 1987)and the distortedUtu$u octahedron in the 20 structureof QtQO)Oul (Wolf & Hoppe 1986)show 15 lqu considerableelongation parallel to the cation-anion bonds (Figs. 5c, d), and are thereforecompatible with 10 model (2), without contradictingmodel (1). 5 Uranyl-ion bond-lmgth OFC.'lOtA@F@oo NNNNNNNNNN@ The averageU6'-Ou" bond-lengths dd:Nc5+6dF06d for Ur$a, Ur$5 @NNNNNNNNNN and Ur$u polyhedr4 as derived from crystal-structure analysis,are L.79(3), I.79(4) and 1.78(3)A, respec- G U*- O BondAngle (') tively. Thus,the uranyl-ionbond-lengths are insensitive to tle numberof anionsthat coordinatethe uranyl ion, Ftc. 6. Bond-angledistributions of the uranyl ion in in the caseswhere the coordinationanions are Oz-and well-refinedstructures. OH-. The averageUr*-.ou" bond lengthsobtained from X-ray-diffraction studies are in agreementwith the valuesobtained using EXAFS spectroscopyfor various the equatorialanions is suchthat the uranyl ion may be structures(above). subperpendicularto the equatorial plane while main- taining roughly equal repulsion between botl Or, Uranyl-ion linearity anionsand the equatorialanions. However, in the case of Ur$5 polyhedra any tilting ofthe uranyl ion relative The distributions of O-Uo.-O bond-anglesin the to the normal to the equatorialplane will result in the uranyl ions of UrS"polyhedra are shown in Figure 6. two Ou, atomsbeing subjectedto different amountsof The uranyl-ion bond-angleis usually linear or close to Coulombic repulsion from the equatorial anions @ig. linear in Ur$a atd Ur$u polyhedra, with most bond 7), thus causingtle uranyl-ion 66ad-angleto depart anglesin therange L79 to 180o.In contast theuranyl-ion from 180". bond-anglesin Ur$5 polyhedra, although being close to linear, show a strong tendency to be somewhat distorted away from 180o,with the maximum of tne distributionin the range L78to L79". urho UrQs urQu A possibleexplanation ofthe bond-angledishibution in UrQ, polyhedrainvolves tle positionsofthe equatorial anionsofthe polyhedra-In general,the equatorialanions and the U6" cation are close to being coplanar,and the uranyl ion is positionedrorrghly orthogonalto a plane drawn throughthe equatorialatoms. However, in many crystal structures, considerations of local bonding apparentlyrequire the uranyl ion to be subperpendicular m00 plane to the containingthe equatorialaniolr. 1o,0" Ftc. 7. The distribution of equatorial anions with respectto caseof Ur$a and Ur$6 polyhedra, the distribution of possibletilting ofthe uranyl ion in U* polyhedra- 1558 TI{E CANADIAN MINERALOGIST 12 cation and anionpositions are regularly usedto analyzn 10 refined structures and give information on cation I HU (Brown& wu 1976) 6 oxidation-states,anion identities and H bonding. The 4 strengthofthis approachis that tle bond length is a 2 T 0 .TT unique function of bond valence@rese & O'Keeffe 8 1991).However, it is not uncommonfor apparently 6 Hu@,"""eo'K*ff",t],. 4 well-refined structures that contain Uet b have 2 a, h bond-valence sums at the Utu position that depart 18 significantlyfrom6vu. '16 The most recently published bond-valence 14 parameteri?rr. given for Utu by Brese & O'Keeffe 12 ^::.il1||[(1991)and Brown & Altermatt(1985) is 2.075A (for 10 8 OonAsto oxygen), and the b.constant is 0.37 A. 6 bond-valenceparameters Ra = 2.059 A a4 However, the 82 and N = 4.3 given by Brown & Wu (1976) are in 30 conrmonusage. Other bond-valence parameters for U6' i614 , are given by Zachariasen(1978). In our study,only ld'rz mu pr*" a o'f""n" teetl 10 reasonablywell-refined structuresare considered,and 8 ttrus cation bond-valence sums for to[JG, tzl{J6taad 6 tt[Jer polyhedrashould cluster around6 va. The distri- 4 2 butions of bond-valence sums at the Uil positions 0 r---lrllhilcalculatedusing the parametersof Brese & O'Keeffe (1991) Brown & Wu (1976)for the structuresof 4 and 2 the phaseslisted in Table 1 are shownin Figure 8. The 0 .rl-I-.ftfl*:wu1e76)bond-valencesums are usually significantly different 6 (Fig.8). In the caseof 4 from the expectedvalue of 6 vu 2 ...I -li:TiiA':' Ir[J6*and 1r1ger,the majority of the bond-valencesuills 0 calculatedvrith both setsof parametersare significantly OOt\OOOFNO\t B@ \0qqqq o6eoso@o@@ ct ct o(o(oFt\ greaterthan 6 va, andthe maximumin eachdistribution vuidllSdd.1drc5 +tA dxo6d,r in tle range 6.4 to dddsidaictdd at ct ctdctct correspondsto bond-valencesums par:rmeters, BondValence (vu) 6.7 vu, depending on the choice of althoughsums greater thanT vu are not uncommon. sumsof bond valencesfor tTU6+and t8lu6r sumscalculated for The average Frc. 8. Thedistribution ofbond-valence parametersof Brese& O'Keeffe position structuresusing the calculatedusing tle tle Ua. in well-refined = parametersof Brese& O'Keeffe(1991) and Brown & Wu (1991)are 6J (c = 0.3)and 6.6 (o 0.3)vu, respec- (r976). tively. The ayeragesums ofbond valencecalculated with the parametersof Brown & Wu (1976) for mUc andt8rutu are 6.4 (o = 0.3) and6.5 (o = 0.2) vr, respec- tively. In most cases,the bond-valenceparameters of A BoNp-VnLsNcEA?PRoAcH Brese& O'Keeffe (1991)result in uranylion U6'-Or" bond-valences in excess of 2.A va, suggesting Hydrous uranyl phasesform during the oxidative (erroneously) that Or" atoms do not participate in dissolutionof UO2and spentfuel in the presenceof any additional bonding. There is a broader lange of water (e.g.,Wadsten L977,Waag &Katayama 1.982, bond-valencesurns for toUr+if oneuses the parameters Wronkiewicz et al. 1992, Forsyth & Werme 1992, of Brese& O'Keeffe(1991) or Brown & Wu (1976), Johnson& WermeL994,Hnn a al 1996,Wronkiewicz with mostfalling between5.6 and7.0 vu.T)heaverage a al. L996)and during the alterationof uraninite@inch bond-valencesum for t6lu6+in structutesis 6.3 (o = & Ewing 1992).Hydrogen bonding plays a crucial role 0.4) w whenusing the parametersof Brese& O'Keeffe in the stability of thesestructures, as in, for example, (1991),and 6.0 (o = 0.3) va whenusing the parameters tle structures of schoepite (Finch er aI. 1996) and of Brown & Wu (1976); however,tlere is no clear ianthinite @urns e/ aL l997b), where adjacentsheets trend in either distribution. of U$, polyhedra are held together only by H bonds. Most Uto environments in crystal structures give Unfortunately, it is usually impossible to determine bond-valencesums that differ significantly from the the positionsofH atomsin uranyl phasesbased upon expected6 vu @ig.8) when using the parametersof X-ray-diffraction experiments. Brese & O'Keeffe (1991). However,a total of nine The bond-valencemethod @rown 1981)has proven t6lu6+cation environmentsgive bond-valencesums in to be a powerfrrltool for the predictionand interpretation tlhe range 5.8 to 6.0 vu. Of these polyhedra,none of bond lengths in solids. Bond-valencesums at both contain a uranyl ion with Uil-Ou" =1.8 A; rather,most 1559 a 5z o c .E o oo o o c g E ? 1' c o @ c o EI 1.80 1.85 1.90 't.95 2.00 2.10 2.15 Ue- O Uranyl Bond Lendh (A) Flc. 9. The Ue'-O bond length ofthe uranyl ion versasthe bond-valencedeficiency ofthe uranyl-ion O-atom in well-refined anhydrousstructures. are holosymmetric or near-holosymmetricoctahedra. O'Keeffe (1991)to provideappropriate bond valences Thus, it must be concludedthat the bond-valence for Utu-Oy, anionsmakes interpretation of H bonding parameter(2.075 L) of-Brese & O'Keeffe (L99L), difficult in hydrous structuresthat contain Ue. when usedwith b = 0.37 Anis inadequafefor U6*sftuc- It is impossible to firlly assess the correct tures that contain the uranyl ion, and this parameter bond-valencerequirements of the Oy,bond in hydrous should not be used to analyze refined structures, to structures,as the Oy, ani6a i1 6srty casesaccepts a H predict Utu-$ bond-lengths,or to interpret H bonding. bond, but the preciseposition of the H atom is usually The useof this bond-valenceparameter where a uranyl not known. However, H bonding is not a factor in ion is presentwill usually result in bond-valencesums anhydrousstructures, and the sum ofbond valencesat that are significantly greater than 6 vu, and the bond- the Oy" position from cations otler tlan Ue may be valence sums at the anion positions will be incorect. calculatedu5ing the parametersof Brese & O'Keeffe The parametersof Brown & Wu (1976) result in more (I99L). The variation of the Ue-Or," bond-lengthwith reasonablebond-valence sums at the Oy" atoms, tle bond valence associatedwith the Utu-Oa" bond althoughthe pararnetersresult in bond-valencesums at (obtainedby subtractingthe sum of bond valencesto tle r7lu6+and t8lu6+positions tlat are usually greater Oy, from cations other than Utu from the formal Ihtn 6.0 v* valence of.2.0 vu) is shown in Figure 9. The data are reasonably well characterized by a straight line Derivation of bond.-valenceparamcters for U6t (P = 82Vo).The equation of the least-squares-fitline indicatesthat a bond valenceof 2.0 va correspondsto a The most serious drawback of tle bond-valence Ue'-O bondleng1hof 1.691A. parametersfor Uo'given by Brese& O'Keetre(1991) The expectedU-O1,, bond-length where tle Oy, is that the bond valencescalculated for Ue.-Oy, bonds atombonds only to Uo'is 1.691A, asthis corresponds are in many cases2.0 vu or greater.Itis commonfor to a bond valenceof 2 vu. Note that this value c4nnot Ou" anionsto be bondedto cations such as Kt or Na-, be derivedusing EXAFS spectroscopyof solutions,as or to accept H bonds (or both), indicating tlat tle the Ou, aniol will accepthydrogen bonds from tle bond-valencerequirements of Oy" anions are in many solvent. This value may be comparedto theoretically instances not met by the Uc'-Oo, bond alone. The predictedbond-lengths. Within the Hartree-Focklimil inability of the bond-valenceparameters of Brese & PyylftO et al. (L994) obtainedU-Oy, bond-lengthsfor 1560 TIIE CANADIAN MINERALOGIST 12 D = 0.506 A. optimal parametersobtained by fitting 10 IrtU to the data for all types of Uc' coordinationpolyhedra 8 areRo = 2.051A, D= 0.519A. 6 The distribution of bond-valencesunr for the Utu 4 positionsin well-refined structures(Iable 1), calculated 2 using the new parameters for each coordination 0 24 number, are shown in Figure 10. The range ofbond- 2. valence sums for t6lUG. t7lu6f and FrU6r iS much 20 narrowerthan for thosecalculated using the parameters 18 of Brese& O'Keeffe(1991) or Brown & Wu (1976) gi6 (Fig. 8). Also, the maximum of eachdistribution is in 914 14u the range5.9 to 6.1.vu. E12 The distribution of bond-valence sums for each ll. 10 polyhedroncalculated with the pammetersRt=2.051 I A, b = 0.519 A, derived for all Uo' coordination 6 geometries,is shownin Figure 11.These parameters 4 tzu6', 2 perform bestfor and pooresffe1t6l(J6r, where the 10 maximum in tle distribution of bond-valencesums is 8 in the range5.7 to 5.8 vz. Optimal resultsare obtained where tle coordination-number-specific parameters 4 tEu areused. 2 The distribution of bond valence for individual Uil-Ou" bonds in well-refined structuresocalculated au?q\ @AqFNO{O@N@Ooo aaaa ct ct at Gt d ct d qt @ ct ct @ N ts using the coordination-number-specificpatameters, is v+,6d Fl ob d d +.t cl + sl d ll 06 d /\ ririd ut rt e qt ci at ci ai d qt qt ci @ BondValence (vu) 12 FIc. 10. The distribution ofbond-valence sunn calculatedfor 10 the Uc. position in well-refiled structures using the 8 parameterstelU#: Ru - 2.074A, b = 0.55! A; mUe: Ru= 2.M5 A, b = 0.510A; ttlgo+'Ru=2.M2 A, , = 0.506A. 0 4 2 0 24 2 the uranyl ion in a vacuumtlat rangedfrom 1.66 to 20 L.72 A, dependingupon t}re parameterizationof the 18 calculation. Van Wezenbeeket al. (L99I) calculateda 9', bond lengtl of 1.67 A, obtained using nonrelativistic 914 Hartree-Fock-Slater calculations^,wittr the predicted 6,, bond-lengtl expanding to 1.70 A where relativistic lt 10 effects are taken into account. Craw et al. (L995) I obtaineda uranyl bond-lengthof 1.663A by using o Harfee-Fock calculations.Accounting for correlation 4 of dynamic electrons gave 2 with approximqtions bond 10 lengthsrangrnC from 1.700to 1.783A. I Bond-valenceparameters for Ue'-$ bondsthat will 6 result in a valence of.2.0 vu for a U&-$ bond-length 4 of 1.691A. and bond-valencesums of 6.Ovu at the 2 Ue position have beenderived. Suchparameters were 0 a I q \ oq cq q.: n a?.q (| q \ oq q q q sought for subsetscontaining t6lu6+,t7lu6+ and t81U6+ aaoaaa@@@@@@@@@otsN v + 6 d rl ob d d +.t Gl + d d r o6 d r polyhedra ttrat occur in tle well-refined structures d D ut ui o ut ct ct o ct ai at @' ct ai ai listed in Table 1. No suitable value of R;; exists if D is BondValence (vu) fixed at 0.37,as doneby Brese& O'Keeffe(1991). However, variation of both R;; and b provides tle of bond-valence sums calculated r6tU6+ Ftc. ll. The distribution desiredresults. For polyhedra, Rii = 2.074 4, for tie Utu position ia well-refined stnrctures usilg b = 0.554 {; for rag* polyhedra, R; = LMs A, the coordination-independentparameters Rr = 2.051A, D = 0.510 A; for relUe*polyhedra, nu = Z.O+Z4,, b =0.s1.9A. TIIE CRYSTAL CHEMISTRY OF HEXAVALENT URANIT'M 1561 gbm o 5 o E30 lt oF--.ciciaqqEqqqeeEEee v4,$,A,.1,6'15'.5'6d,i6cf qq.:c.{c?.qqqNooo or;;-ci BondValance(w) FIc. 12. The distribution of bond-valencesums for U&-Or, bonds calculatedfor well- refined structuresusins the coordination-sD€cificoarameters t6lu6*: R, = 2.074 A. b = 0.554A; mlJ*: Rn= 2.O4SA, r = O.StO A; nrfo-' Ru= 2.M2A, f 1 O.SO6A. TABTE1 U.() BOND.I,.ENGIHSAND BONDVALE{CESTJMS AT TITBIJRANIIJM POSTNO}TS IN SEIaCTEDSTRUCTTJRES Us in Pmgwt st lmidal Coodimio Vdaae$m ueorAt o-Uo() (w) Rfd. Udttoq zcrt z0t8 Ltl3 2.356 2.&3 2.nt 2.176 l?t.l 4.r2 I z.sn z.vn 2319 2.131 2.w 2.16 25n 164.1 5.r2 usq 1.933 2.V24 2,505 2.48 2.303 2.135 2.3s3 173.0 52? 2 [JVO, 2.050 z.Vn 2325 2206 2206, 23M 23M 179.' 52tt 3 u0rro 2.65 2.W5 2246 2246 2.300 2.3N 2.53t 17t.9 4.95 4 U! in Oclshrdnlcod.frdim rl3{r,lr Naf.q 2.t53 2.t53 2.t5t 2.151 2.145 2.145 4.96 5 BauFelJOc 2.t65 2.t65 2.165 2.t65 2.165 2.165 4.92 6 KUO, zl,lt 2.lA 2.l,lt 2.14 2.r4 2.14t 4.9t 7 U+, UeddUo Pd$dBirMscdlom sftcdg uo(A) FU3O| 2.60 2.0t0 2.021 22X' zxt 239E 239t 525 1.t38 t.ttt 2.tt2 236t 236t 2295 229s 5.96 z.At 2.0n 2.0t$ 2.0tt 22t5 2275 5.03 c-lllor 2.C15 2.075 2.1 2.156 22Ji' 2257 2544 5fI 9 2.V14 2.0/14 2.11E 2.723 Lt4E 2.1t0 2206' 5.40 IJNbP, 2.301 2fi1 2.312 2.303 z.gt 2.39t 2.303 4.30 l0 tNhPE 23gg 23@ 22n 2m 2.371 2371 z.EU 4.6 ll l{uot(Po& z2r9 2.& z.ln 2.nr LAL 2.y3 23tt 431 t2 L7A VO 22e' 2.419 2.t62 2J61 257t 5.91 UMqilI Z05E z.qtE 2.394 2A;2 2324 2292 2324 5.t7 l3 n@as: (l) Secdcnaar r9a, @):ItidmA$frld(1992I (31:Dtrtuad- (1992\(4):Cordfnlestal (tc8t),(O:Adlph.hbGtar (l9t9} (6) Gmrtaal (r9?l} O):Dido&Pmdl(l9l! (t): Iaopcra09To), (9IhqdB097t! O0Ihscn&Gde(1994I(ll): Bus&sr4r(1994! (U): Bddaidstal(190{), (13): c!6c$ otaf (19t3). r562 THE CANADIAN MINERALOGIST shown in Figure 12. The maximum in the dishibution Specific examplesof valencedeterminarton of U occurs at 1.60 to L.65 vu, indicating that the Oy" in structures atoms usually receive bond-valencecontributions in addition to the U6*-Oy, bonds. Only a small number The structure of p-U3O3 is consideredto contain of structures have Oy" anions whose bond-valence Us' and ge*, with tle formula U5+2U6+O8(Loopstra requirementsare met by the Ue.-Or, bond alone.More 1970). There are three U atoms located in general commonly, an Or" atom acceptsH bonds or bonds to positionsin the spacegroup Cmcm (Loopstra 1970); large monovalent or divalent cations. Note that those two are coordinated by seven anions in pentagonal- Us-Ou" bondswith valencesof -1.0 va correspond bipyramidal arrangements,and one is in octahedral to holosymmetlicor near-holosymmefic octaledra, coordination(Table 2). Thereare two reasonablepossi- rather than to uranyl ions. bilities here:eitherboth pentagonalbipyramids contain Ue and the octahedroncontains Ua", or onepentagonal ITqFRRNGTrD VeLm.IcEor URANTUM bipyramid contains Uo' and the other two polyhedra N CRYS,IAL SIRUCTIJRES contain Us*. The bond-valencesums at each site are U(1) = 5.25, UQ) = 5.96and U(3) = 5.03 vu (Iable 2), Uranium is only known to occur as U& and Uo' consistentwith the presenceof two U5*and one U& in in mins14ls,although Us* has been reported in about the structureof p-U3O8. 30 inorganic structures.Furthermoreo the recentdeter- The structure of cu-U3Osis also considered to mination of the crystal structure of ianthinite @urns contain both U* and Ue', although no uranyl ion is et al. L997b) suggeststhat it may contain Us*. The present in the refined structure. There are two U bond-valence method is often used to determine positions in the structure, which crystallizes in the oxidation stafesof cationson the basisof bond lengths spacegroup C2rnm 1977).Both sites are from refined structures.Although there areparameters Q,oopstra coordinatedby sevenanions in a pentagonal-bipyramidal available for the calculation of bond valencesfor Ua arrangement,and both polyhedra contain near-linear andUs* 1981),it is necessaryto fust identiff @rown O-U-O clusterswith bond lengtls of -2.O7 A. The the valenceofthe U cationin order to selectthe correct absenceof a uranyl ion with a Ue-O bond length of set of bond-valenceparameters. Here we demonstrate -1.8 A suggeststhat neither site contains exclusively that the new bond-valenceparameters Ru - 2.051A and U6'. The bond-valencesu'ns for the sitesare U(1) = b = O.519A, tlat were derived specifically for Uo., 5.27 and U(2) = 5.40 vu, indicating tlat each site distinguish between {J'rt, lJ5+and Ue' in well-refined probably contains both Us* and UG, and as ttrere are sftuctures. twice as many U(2) sites as U(1) sitesin the structure, this gives a total U valence of 16.07 va, which is Coordinationgeometry and bond.-valencesurns consistent witl the formula U5+2U6'O8.However, for Us* note that thesebond-valence sunu are also consistent with Ua - Uto disorder and the formula UaU6.2Os,as Geometriesofpolyhedra arepresented in Table2 U4'- U6. disorderwould result in bond-valencesums for seven structurestlat are reported to contain Us+. of 5.3 va per sitein this case. The U$ cationoccurs either in octahedralcoordination, or in a pentagonal bipyramidal coordination that The structuresof UMrOr @usch & Gruehn 1994) contains 4 asar'-linsar(Us+Or)tt ion witl a Us*-O and UM6O16@usch et aI. 1994) are eachreported to bond-length of *2.05 A. fne latter coordination contain Ua, and the U atoms are coordinated by geometry is similar to the Ur$, polyhedron that is sevenand eight O atoms,respectively. The polyhedral common for Uto, except that the uranyl ion Uo'-O bond-lengthsare summarizedin Table 2, and tle bond-lengtlis -1.8 A. bond-valence sums calculated for the uranium sites are4.30 and4.06 va for UNb2Orand UM6O16, respec- = The bond-valencepar4meters Ru 2.051 A and tively. These results show tlat the bond-valence = -6 b 0.519A give bond-valencesums of va for U& parametersalso successfullydistinguish U+'. in various coordination geometries@ig. 11). It is informative to calculate the bond-valence sums The structureof U(UOJGOa)r@6nardetal. 1994) with tlese paxametersfor the U positions in phases is reportedto contain both Ua and Uet, making it one soltaining U5+;these values axegrven in Table 2. The of the relatively few structures known to contain U bond-valencesums for the Us* sites are all close to 5 witl both of these valence states. There are two U va, with a rangefrom 4.82 to 5.26 vu.T\is is consistent positions in the structure, which crystallizes in the with the fact that thesestructures contain Us*, and that spacegroup PI; both are positions coordinatedby ttre bond-valenceparilreters are effective in distin- sevenanions in pentagonal-bipyramidalarrangements. guishing U5r and U6+.We will now considerslamples Only the U(2) site contains a uranyl ion, which is to demonsfratefurther how the bond-valenceparameters consistent with tle presetrceo1 g0* (Table 2). T\e may be usedto distinguishvalences of uranium. bond-valencesums calculated for the U sites are U(L) 1563 = 4.31 and U(2) = 5.91 va.These values are consistent TABLE3. STJMMARYOF POLYHDRAL X)LYMAIZATION IN with U(1) coutaining U& and U(2) containing Uc', as reportedby B6nard et al. (1994). mIJa The structure lle of UMo2Os is reported to contain qJ+ lle lq54o lle cfoo b U& and Motu (Cremers et al. 1983). The structure rAUe - lle l,le Te*Oo l€ contains one U position tlat is coordinatedby seven - lc,3e le Pe04 9c 3crls anionsin a pentagonal-bipyramidatanangement (Table eh 2e 7o lilear 2s 2). There is an approximately O-U-O cluster -lc with U-O bond-lengthsof 2.O6A in the polyhedron. AseG 6o ls The polyhedral geometryis similar to that observedin -lc various polyhedra that contain Us* (Table 2), and the c - rb? o@fds, o - bond-valencesum calculatedfor the site is 5.I7 vu.T\e ofpollmizdio" structureof U2MoO3,which is reportedto contain Us* (Serezhkinet al. L973),has fwo similar coordination polyhedra about the U atoms,and bond-valencesurns areU(1) = 4.92 andU(2) = 5.12 vu (labte2). Thus,the structureof UMo2Ospossibly containsU$, rather than structures containing infinite sheets of polyhedra. Ua asreported (Cremers et al. L983),and the chemical Thesestructures account fot 78Voof U mineralsfor compositionmay be morecomplicated than 1tr61iodicated. which stuctures axeknown, many of which may form The examples provided above demonstrate the owing to the oxidative dissolutionof spentnuclear fuel. efftcacyof our revisedbond-valence parameters for Ue' The nature of the polymerization of polyhedrain each for deterrniningthe valencestates of U atomsin refined of the 106 stuctures basedupon sheetsis summarized structures. As such, they should be useful in the in Table 3, where the mode of polymerization is analysisof new crystal structures. indicatedby a letter, and either occursby the sharingof edges(e) or corners(c) ofpolyhedra and the numbers CoNsrnanqrsoN PoLyMERZATIoNoF PoLyrGDRAtrr indicatethe numberof structuresthat containeach type Uo'-BeARINcSrnucrunss of polymerization. The sharing of edgesbetween two Ur$a polyhedra Important factors sel6elling the polymerization of in a sheetonly occursin the structureof Cs4[OOr)rOt]; polyhedra in any oxide or oxysalt structure include tle scarcity of Ur$a polyhedra that share edges is cation valence, cation coordination number, and the presumably due to the Coulombic repulsion of tle lenglhs of edgesof polyhedra.The rules of Pauling U6' cations, which are separatedby 3.56 A only in (1960) indicate that cation polyhedrawith low coordi- Csa[(UOJsOr].Where Ur$a polyhedraarc presentin a nation numberand high valencewill tend not to share sheet,it is much more commonfor the Ur$a polyhedra elements of the polyhedra; in caseswhere tley do, to share corners only, as in some sheets with the the sharing of cornerswill be more favorable than the autuniteanion-topology (Fig. 4b ofBurns et al. 1996), sharing of edges or faces of the polyhedra. In most or the Ur$a polyhedra shareedges with Ur$5 polyhe- structures that contain Uo', the U6t cation forms a dra. In contrast, Ur$5 polyhedra usually polymerize (Ue'O)z* uranyl ion. It is commonfor uranyl polyhedra with other UrQ, polyhedrain sheetsby sharingedges; to share edges witl cation polyhedra that contain this includesall of the Ur$a, Ur$t, and UrS6polyhedr4 cations with valencesof up to 5+, and also with otler although the sharing of edges with other Ur$5 uranyl polyhedra indicatingthat the uranyl ion behaves polyhedrais most common.Where Ur$6 polyhedraare more like a divalent cation than a hexavalsntcation. presentin a sheetpolymerization occurs by sharingedges Where tle sharing of edges of polyhedra does with eitherUrS5 or Ur$6polyhedrabut neverby sharing occur,the degreeof mismatchbetween the ideal length edgeswith Ur$a polyhedra,presumably owing to the of edges of the polyhedra is an important factor in large mismatchof ideal edge-lengthsof the polyhedra. determining the stability of the structure. Where a The details of how other polyhedra of higher uranyl polyhedron shares an edge with another bond-valencecations polymerize with Ur$, polyhedra polyhedronofhigher bond-valenceothe sharededge is arealso summarizedin Table3. The modeof polymeri- always an equatorial edgebecause the bond-valence zation is dependent on both cation charge and requirementsof the Oy, anionsare largely satisfiedby polyhedronsize, which, in tum, is dependenton cation the UG-Ou"bond (-L.7 vu). Assumingthat the equatorial radius and tle number of coordinating anions. In the anions and tle U0* cation are coplanar, and that the caseofthree-coordinaled cations in sheets,ttre triangles bond angles are ideal, estimates of ideal equatorial usually sharean edgewitl a Ur$opolyhedron.Both edge-lengtls of polyhedra for Ur$a, Ur$5 al;d Ur$6 C&$3 and 5s+$, share edgeswith Ur$6 polyhedra, polyhedra are3.22, 2.79 and2.47 A, respectively. whereasB3*$3 triangles share edgeswith both Ur$t The structuralhierarchy ofUa phasespresented by and Ur$6polyhedra, or they shareonly corners witl Burns e/ aI. (1996) includes 106 phasesthat have Ur$5 polyhedra. L564 TIIE CANADIAN MINERALOGIST Severalfour-coordinated cations occw in the sheets a Moo'$, hexagonal bipyramid that shares edges of Uc' phases(Iable 3), andthe modesof polymerization with Ur$6 polyhedra in the structure of umohoite, are dependentupon cation valence.Where the cation [(UO)(Mood](HzO)al (Makarov & Anikina 1963) valence is 6+, ttre tetrahedrashare corners only witl @g. lOa of Burns et al.1996). Ur{, polyhedra; edge sharing witl Ur$" polyhedra Considering each of the Utu structures that are does not occur, presumably owing to Coulombic basedupon infinite sheets,the following observations repulsionbetween the leftahedrally coordinatedcation may be made: andUa'. The only known exceptionin a Uo'phaseis (1) Ur$a polyhedracommonly sharecorners only with found in the structure of &IOOJ(SOa)31, which otler uranyl polyhedra, and where Ur$a polyhedra contains finite clustersin which SSatetrahedra share shareedges witl otler uranyl polyhedra, it is almost polyhedra(Mikhailov edgeswith Ur$5 et al. 1977). invariably with Ur$5 polyhedra. Wherethe cationcharge is 5+, the polyhedrashare botl (2) No structure contains either a Ur$5 or a Ur$6 corners and edgeswith Ur$, polyhedra. The As5'0a polyhedron tlat shares a corner only with anotler tetrahedroncommonly occursin sheetsthat are based uranyl polyhedron; where polymerization occurs, it (Fig. upon tle autunite anion-topology 4a of Burns et involves the sharing of edgesbetween polyhedra. poly- al. 1996),where it sharesonly cornerswith Ur$a (3) Uror polyhedra most commonly shareedges with hedra.In the shuctureof Mg[(UO)(AsO)]2(H2O)a,the other Ur$5 polyhedra althoughedge-sharing with both Ast*40 tetrahedra share edges with Ur$5 polyhedra Ur$aor Ur$6polyhedra also is common. @achetet al. L99l), and the sheethas the uranophane (4) Ur$6 polyhedracommonly shareedges with either anion-topology@urns e/ a/. 1996).The F $n teftahedron Ur$5 or Ur$6polyhedra but never\rith UrQapolyhedra- also occurs in sheetsthat are basedupon ttre autunite (5) The most important factor in determiningthe mode anion-topology, where it shares corners with Ur$a of polymerization between Ur$, polyhedra and other polyhedra. In addition, P:*$n tetrahedra commonly cation polyhedrais cation charge. occur in sheets based upon tle phosphuranylite (6) Thosecation polyhedra (excluding Utu polyhedra) (Fig. anion-topology 8a ofBurns et al. 1996),where with high-charge cations (6+) and low coordination polyhedra. the teftahedrashare edges with Ur$6 number (<6) do not commonly shareedges with UrQ, The Ass*-O and F*-O bond lengths,inferred from polyhedra. (Shannon sumsof effective ionic radii 1976), are I.69 (7) Polyhedracontaining pentavalent cations regularly and 1.53 A, respectively.Assuming ideal bond-angles shareedges with Ur$5 and Ur$6 polyhedra, but they of tlese tetrahedra,ideal edge-lengthsofAss*$o and alsocommonly share only cornerswith UrS" polyhedra. Ps'$o tetrahedra arc 2.76 and 2.50 A, respectively. (8) Cation polyhedra witl lower-charge cations (3) Thus, the ideal edge-lengthsof As5*0aand F*04 tetra- almost invariably share edges with Ur$5 or Ur$6 hedra are best matchedto Ur$5 and Ur$6 polyhedra, polyhedra but never with Ur$4 polyhedra. respectively.The edge-lengthof the As*04 tetrahedron (9) An importantgeometical factor for the polymeriza- polyhedron, is incompatiblewith a Ur$6 which explains tion of polyhedrais the edgeJengthmismatch between why no structurescontain As5*0atefrahedra in sheets the ideat polyhedra, which is mainly due to cation phosphuranylite with tle anion-topology. Where the size. Small degreesof mismatch favor edge-sharing, tetrahedron contains a cation with a valence of 4+, whereaslarger degreesof mismatchfavor the sharing fte tetrahedron always shares an edge with a Ur$t of corners. polyhedron.Most examplesinvolve the Si4+04tetrahe- dron, which has an ideal edge-lengthof 2.64 A, a fair Acrotowr-mcmtm,us matchfor the ideal Ur,fr polyhedron.All uranyl silicate sheetstrucnres contain sheetsthat are basedupon the This work was supported in part by the Natural (Fig. uranophaneanion-topology 6a of Burns et aL Sciencesand EngineeringResearch Council of Canada 1996),where each SiQa tetrahedron shares an edgewith in the form of a Post-DoctoralFellowship to PCB, and polyhedron one UrQ5 and a corner with another. by tle Office of BasicEnergy Sciences (Grant No. DE- The only five-coordinated polyhedra tlat occur in FG03-95ER14540,M.L. Mller andR.C.Ewing). The phases the sheetsof U6+ are NblS5 and Vs*$5 square manuscriptwas improved following thoroughrevi,ews pyramids. pyramids In all cases,the square shareedges by Dr. RobertFinch and an anonymousreferee, and the polyhedra, with Ur$5 and all but one occur in sheets editorial work of Drs. John Hughes, W.H. Maclean basedupon the francevillite anion-topology@ig. 7b of and Robert Martin. Burnse/aL L996a). Excluding Ua, polyhedra with cations of higher Rnrnnwcrs bond-valenceand coordination number greater than five are rare. The only instancesare two examplesof ALcocK,N.W., RosBRrs,M.M. & Browu, D. (1982): Mf$u octahedra"each of which share edges with Actinidestructural studies. 3. Thecrystal and molecular Ur$5 polyhedra in sheets based upon the iriginite structuresof UO2SOa.H2SO4.5HrOand (NOzSOJz. anion-topology (Fig. 7e of Burns et al. 1996), and HrSO4.4HzOJ. Chem. Soc., Da.lton Trms. 1982, 869-87 3. 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Chem.52E, 129-137. & -(1986): Neuesliber Oxouranate(VI): NaaUOsund IIUO5. Rev. Chim. Mindrale 23,828-848. & - (1987):Eia neuesOxouranat (VI): KrLi4fUO6l.MiteinerBemerkungiiberRby'-iafUO6lund ReceivedMarch 18, 1997, revised manuscript accepted' CsJ-i4VJO6l.Z Anorg.Allg. Chem.554,34-42, May 18,1997. t57L TheCanadian Mburalo gist Vol. 35,pp. 157l-1606 (1997) RECOMMENDEDNOMENCLATURE FOR ZEOLITE MINERALS: REPOFT OFTHE SUBCOMMITTEEON ZEOLITESOF THE INTERNATIONAL MINERALOGICAL ASSOCIATION,COMMISSION ON NEWMINERALS AND MINERALNAMES DOUGLASS. COOMBS1(Chairnan) GeologyDepartmznt, university of Otago,P,O, Box 56, Duned.in, New Zealand ALBEMOALBERTI Istiato d.iMineralogia" (JniversftA di Fenara, CorsoErcole I" d.'Este,j2, 144100Ferrara, Italy TIIOMAS ARMBRUSTER Laboratoimftr chcmischeund mburalogische Kristatlographie, Universitdt Bern" Freiestrasse 3, CH-i012Bem, Swiaerl'anl GILBERTOARTIOLI Dipartimentodi Scienzed.ell.a kna. [Jniversitd.di Milana,vin. Botticelli, 23, I-20133 Milarc, Italy CARMINECOLELLA Dipartimentodi Ingegneriadei Materiali e dellaProd.azionc, Universitd Fedcrico II di Napoli, PiazzpleV Tecchio,80, I-80125 NapolL ltaly ERMANNOGALLI Dipartimcntod.i Scienze dellaTbrra, Universitd di Modena,via S.Eufetnia, 19, 141100 Modena, Italy JOELD. GRICE MineralSciences Division, Canadian Musewn of Naturc,Ottawa" Ontarin KiP 6P4,Canada FRIEDRICHLIEBAU Mineralogisch-PetrographischesInstint, IJniversitdtKiel, Olshaasenstasse40-60, D-2300 Kiel" Germany JOSEPHA. MANDARINO (retiredfrom Subcommittee,December I 994 Depanmcntof Mineralagy,Royal Ontario Mweum, Toronto, Onnrio MSS2C6 Canada HIDEOMINATO 5-37-17 Kugayam+ Suginami-ku,Tolqo 168,Japan ERNESTH. MCKEL Division of Exploration and Mining, CSIRO,Private Bag, Wembley6014, WestemAu*tralia" Aastralin ELIO PASSAGLIA Dipanimento di ScienzedellaTerra, Universitd.di Modena"via S. Eufemia 19,I-4II0a Modenu haly DONALD R. PEACOR Departnent of Geological Sciences,University of Michigan, Ann Arbor Michisan 48109, U.S.A. SIMONAQUARTIERI Dipartim.entodi ScienzeitellaTena, Universitd ili Modeno, via S. Eufemiu 19,141100 Modena"Italy ROMANORINALDI Dipartinznto di Scienzedella Terra, Universitd.di Perugia I-06100 Perugia" Italy MAITCOLMROSS U.S. Geological Suney, MS 954, Reston, Wrginia20192, U.SA. RICHARDA. SHEPPARD U.S. GeologicatSurvey, MS 939, Box 25M6, Federal Center Denve4 Colorado 80225, U.SA. EKKEHARTTILLMANNS Institutflr Mineralogie md Kristallographie, UniversitdtWiett Althanstrasse14, A-1090 Vmna, Austria GIOVANNAVEZZALIM Dipartimento di Scienzed.ella Tena, Ilniversitd. di Modena, via S. Er{emia 19, 141100 Moden'alta$ I E-mnil address: [email protected]