Structural Relations Among Schoepite. Metaschoepite and "Dehydrated Schoepite" 835

831

The Canadian M fueralo gist Vol. 36, pp. 831-845(1998)

STRUCTURALRELATIONS AMONG SCHOEPITE. METASCHOEPITE AND "DEHYDRATEDSCHOEPITE"

ROBERTJ. FINCHI ANDFRANK C. HAWTHORNE

Department of Geological Sciences, Untversity of Manitoba, Wnnipeg, Manitoba R3T 2N2

RODNEYC. EWING

Depanmcnt of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, Michigan 48109, U.S.A.

ABSTRACT

- Schoepite, [(UOr)8O2(OH)rz](HzO),2,transforms slowly in air at ambient temperarureto metaschoepite,UOr,ngrg 1o r't, and crystals commonly contain an intergrowth of both minerals. The transformation may be due to the loss of one-sixth of the interlayer HrO groups in schoepite, and a possible smrctural formula for metaschoepiteis [(UOr)EOr(OfDrz](HzO)ro:The transformation of schoepite (a 14.337,, 16.813, c 14.731 A, P2rca) ro meraschoepite(a 13.99, b 16;72, c 14.73 A, Pbna) is characterized by a 2Vodecrease in the a cell dimension, a slight decreasein the 6 dimension, and little or no change in the c dimension. Unit-cell changesprobably reflect the reorganization of H-bonds. Differences in unit-cell volumes induce strain in crystals in which the transformadon to metaschoepiteis incomplete, and stored strain energy may be-sufficient to rapidly drive the transformation of the remaining schoepite to "dehydrated schoepite" la 6.86, b 4.26, c 10.20 A, Abcm (?)l when partly altered crystals are exposed to an external stress (e.g., heat, sunlight or mechanical pressure). Metaschoepite is apparently stable in air; canary yellow altered crystals commonly consist of a polycrystalline mixture of "dehydrated schoepite" and metaschoepite.The alteration of schoepite to "dehydrated schoepite" occurs in three steps: (l) loss of all interlayer H2O from schoepite, causing collapse of the layers, (2) abmic rezrrangement wirhin the struchral sheetsto a configuration that may be similar !o that of metaschoepite, and (3) funher re-ilrangement to a defect o-UOr(OH)r-type sheet. The complete reaction is [(UOJ8Or(OFt)rzf(lfzO)n '+ 8 [(UOr)O65(OH),.] + l2HrO. We propose that "dehydrated schoepite" forms an omission solid- solution over the compositional range UO3.0.75H2O to UOr.11r9, representedby the general formula (UO2)O0.25-"(OI{)r.5 .2 (0<;r < 0.25).

Keywords: schoepite, metaschoepite, uranyl oxide hydrate, uranium minerals, dehydration, phase transformation.

Sonmaarne

La schoepite, (UOr8Or(OFt)r zf(llzO)n, se transforme lentement en m6taschoepite,UOr.;r11rg (n = 2), dans l'air i temp6- rature ambiante; les cristaux contiennent en g6n6ral un m6lange des deux min6raux. La transformation pourrait bien €tre due l la perte d'un sixidme des mol6cules de HrO pr6sents dans la schoepite. Il est donc possible que la formule stru-cturalede la m6taschoepite soit [(UO)8O2(OH)r2](H2O^)ro.La transformarion de la schoepite (a 14.337, b 16.8L3, c 14.731 A, P2,ca) en mdtaschoepite (a 13.99, b 16.72, c 14.73 A, Pbna) est accompagn6ed'une diminution de 2Vodu parambtre r6ticulaire a, une l6gbre diminution de la dirnension ,, et trds peu (ou pas) de changement dans le parambtre c. Ces changements t6moignent probablement de la r6organisation des liaisons H. Les diff6rences des volumes de la maille qui en r6sultent mdnent tr la formation de contraintes dans les cristaux dans lesquels la transformation en mdtaschoepite est amorc6e mais incompldte. IJ6nergie accumul6e pourrait suffire pour causer une transformation rapide de la schoepite r6siduelle en "schoepite d6shydrat6e" [a 6.86, b 4.26, c 10.20 A, Abcrn (?)l quand les cristaux partiellement alt6r6s regoivent une contrainte externe, due par exemple tr la chaleur, les rayons du soleil, ou bien une pression m6canique. Dans cette situation, la mdtaschoepite semble stable dans I'air. Les cristaux a1t6r6s jaune serin condennent en g6n6ral un m6lange polycristallin de "schoepite d6shydrat6e" et de m6taschoepite.L-altdration de Ia schoepite en "schoepite d6shydrat6e" s'effectuerait donc en trois 6tapes: (1) perte des mol6- cules de HrO de la position interfeuillet de la schoepite, causant un affaissement de ces feuillets, (2) une r6organisation des atomes d I'int6rieur des feuillets pour atl€indre un agencement semblable I celui de la m6taschoepite, et (3) r6organisation plus avalcde, menant tr un feuillet de type ct-UO(OH)2 avec lacunes. La r6action compldte serait donc [(UOr)8Or(OH)rz](HzO)tz '+ 8 [(UOrO0.r5(OH)ri] + 12H2O.Nous considdrons la "schoepite d6shydrat6e" exemple de solution solide par omission dans l'intervalle de composition UO3.0.75HrO b.UOr.flr6, que repr6senteIa formule g6n6rale (UO2)Oo.r5-,(OH)r5*2, (0 (.r S 0.25). (Traduit par la Rddaction)

Mots-cl6s: schoepite, m6taschoepite,oxyde i uranyle hydrat6, min6raur d'uranium, d6shydratation, transformation de phases.

I Presentaddress: Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439,U.S.A. E-mail address : frnch@ cmt.anl.sov 832

PnevrousWonx IvmopucnoN The InternationalMineralogical Association recog- Minerals containinghexavalent uranium play an nizesthree structurally related hydrated oxyhydroxide important part in the transportand fixation of uranium mineralsof of hexavalenturanium, with compositions in nature; a knowledgeof the phaserelations and that canbe representedby thegeneral formula UO3,(2 paragenesisof theseminerals is an importantpart of + x)HzO(-r < l) (Nickel & Nichols l99l). Schoepite understandingthe geochemistryof uranium(Langmuir was originally describedby Walker (1923), and the 1978,Shock et al. L997,Finch 1997a).The 25 or so crystalstructure was reported by Finch et aI. (1996a). uranyloxide hydrates comprise an importantsubgroup Schoep& Stradiot (1947) describedparaschoepite, of the more than 170 known mineralsof hexavalent 5UO3.9trzH2O,but providedfew data.Christ & Clark uranium@inch 1994,Burns e/ al. 1996).The crystal (1960)reported the presenceof paraschoepitefrom a structuresof mosturanyl oxide hydratesare based on samplemixture that alsocontained schoepite and "de- sheetsof polyhedraof theform [(UO),Q(OH;.1t2'-zro hydratedschoepiteo'. Paraschoepite is consideredto (Christ& Clark 1960,Evans 1963, Miller et al. 1996, form by partial dehydrationof schoepite(Schoep & Bumset al. 1990. Most of theseminerals contain diva- StradiotL947,Chist & Clark 1960),although Christ Ient cationsand H2Ogroups in interlayersites, but (1965)suggested that schoepiteand paraschoepite are severalhave electrostatically neutral sheets and contain polymorphs.Christ & Clark (1960)described a third no interlayer cations.In this latter group, adjacent related mineral, metaschoepite,UO3.2H2O, also sheetsare bonded together through H bonds only formed by partial dehydration of schoepite. (Fnch et al. 1996a,Finch 1997b).Of theuranyl oxide Metaschoepitewas identifiedfrom precessionphoto- hydrateswithout interlayercations, three are closely graphsof apparentlysingle crystalsof schoepite,in related:schoepite, metaschoepite, and paraschoepite. which additionaldiffraction spotsindicated that the fwo Sometimescalled "[JO3 hydrates", these three minerals mineralsare in parallelgrowth. can be representedby the generalformula UOr.(2 + Owing to difficulties in obtainingsuitable single- x)HrO, where.ris lessthan one (Christ & Clark 1960). phasemineral samples, much of what is known about The crystal structureis known only for schoepite, schoepiteis basedon studiesof syntheticanalogues [(UOr)8Or(OH),J(H,O),2(Finch et al. 1996a),and the (Hoekstra& Siegel1973, Smith et al. 1982).Schoepite structuralformula of schoepitecorresponds to x = +0.25 and metaschoepiteare difficult to distinguishon the in thefonnula of Christ& Clark (1960). basisofX-ray powder-diffractiondata alone, and the Schoepiteoccurs at manyoxidized uranium depos- name"schoepite" is commonlyapplied to any minglal its, andit may play a key role in theparagenesis of the or synthetic preparation with a formula close to complex assemblageof uranyl minerals that form UOr'211r6.Synthetic UOtc/11r9 is most commonly whereuraninile has been exposed to oxidizing meteoric preparedby heatingrun productsin water aboveap- water(Frondel 1958, Deliens 1977,Flnch et al. 1992, proximately50"C. When preparedthis way, unit-cell Finch & Ewing 1992).Schoepite and otheruranyl min- parameterscommonly correspond most closelyto those eralsalso have been identified as corrosion products of of metaschoepite(Christ & Clark 1960, Debets & UO, and spentnuclear fuel (Wadsenl977,Wang & Loopstra1963, Finch et al.1997),although mixtures of Katayama1982, Forsyth &Werme 1992,Sunderet al. syntheticschoepite and metaschoepitealso zuecom- 1992,Wronkiewicz et al. 1992,l996,Buck et al. 1997, mon (Petersl967,Fnch et al. L997).Paraschoepite has r998). not beensynthesized and remainsthe leastwell defined During a recentcrystal-structure study (Finch er aL of the three relatedminerals described by Christ & I996a), numerouscrystals of schoepitewere found Clark(1960). to alter spontaneouslyto a mixture of metaschoepite Schoepite,metaschoepite and paraschoepiteare and "dehydratedschoepite," UO,.g.311rO. Alteration struchrally andchemically distinct from the four uranyl took placeover severalhours to daysfollowing ma- hydroxides,a-UO2(OH)2, B-UO2(OH)r, 1-UO2(OH)r, nipulationof singlecrystals for X-ray examination. andU.O3(OH)2 (Hoekstra & Siegel1973,Smith et al. The alteration of schoepitewas first describedby 1982),none of which containsstructurally bound HrO Schoep& Stradiot(1947), and Christ & Clark (1960) groups.When naturalschoepite, metaschoepite, or syn- proposeddehydration as the mechanismby which theticUO3.2HrO dehydrate in air at ambienttemperatures, schoepitetransforms to metaschoepiteand paraschoepite or in waternear 100"C, the phasethat resultsis approxi- (also seeChrist 1965).Fnch et al. (1992)exarnined matelyisostructural with c-UOr(OtI)2 (Taylor & Hurst schoepiteand its dehydrationproducts, but found no 1971),but with a compositionbetween UO3.0.72H2O evidencefor paraschoepiteas describedby Christ & (Peters1967) and UO3.0.9H,O (O'Hue et a/. 1988);the Clark (1960).Because the structuraland paragenetic mostcommonly reported composition is UO3.0.8H2O relationshipsamong schoepiteand related uranium @awsonet aI. 1956,Hoekstra & Siegel1973). Christ & mineralsare still uncertain,we haveexamined the al- Clark(1960) obtained UOr.Q.8HrO by dehydratingnatu- terationof schoepitein moredetail. ral schoepiteover H2SO,, calling the product "dehydrafetl STRUCTIJ'RAL RELATIONS AMONG SCHOEPITE. METASCHOEPITE AN'D "DEHYDRATED SCHOEPITE" 833

DETIYDRATED SCHOEPITE r' FAST

!'.-*

,l PRECIPTTATION FAr{TrrrNffEl loxIpATIoNl

TER lI,WA

Ftc. l. Representationof the nanlal phase relationships among schoepite, metaschoepite and dehydrated schoepite, as inferred from natural occrurences ar:d experimental studies (see text). schoepite."'Dehydrated schoepite" also occurs naturally, but it has not achieved official mineral status. It is readily idenffied from powder-diffraction patterns, partly becauseof an intensepeak correspondingto -5.1 A, the 002 diffraction maximum (Fnch et aI. 1992).Fie- ure 1 illustrates the paragenetic relationships among T schoepite, metaschoepite and "dehydrated schoepite", E as inferred from natural occrurences and experimental M P studies(Christ & Clark 1960,Smith et al.l982,Finch E et al. 1992,1996c). R I The dehydrationof schoepiteto UOro0.311rg ir itn"- A iTi versible,with "dehydratul schoepite' remaininguncbangd T U lMs even in humid air, for long periods et al. 1956, ,t @awson R Christ & Clark 1960).UO3.0.8H2O doesnot react in wa- ls E I ter at near-ambient temperatures (Dawson et al. 1956), and reaction in hot water e 1@'C) yields a range of compositions, from approximatelyUO3.0.9H2O near 100oC, Hc. 2. Thermal stability of schoepite (S), metaschoepite (MS), stoichiometric -290oC (Hoekstra to UOr.H2O above & dehydrated schoepite @S) and the UO(OH)2 polymorphs Siegel 1973, O'Hare et al. 1988). The ranges of thermal (a, B, y) in water. Lines are solid where the stability field stability for the UO2(OH), polymorphs, as well as for for each phase is established with a high degree of certainty, schoepiteand metaschoepite,are indicated in Figure 2. and dashed where less certain or metastable. The question Phases with compositions from UQr.Q.8HrO to mark along the tie line between a-UOz(OtD2 and DS indi- UO3.H2O are isostructural with ct-UOr(OH)2, and "de- cates the uncertain nature of this solid solution. The stability studies hydrated schoepite" is the name commonly applied to fields were deduced from experimental and natural occurrences (Cbrist & Clark 1960, Smith et a|.1982), as any ct-UO2(OH)2-typephase within this compositional well as data reported here. range. To explain the irreversible dehydration of schoepiteoSobry (1973) proposed that schoepitemight contain hydronium ions (HrO*); however, Frnch et al. can be distinguished only by precise determination of (1996a) demonstratedthat only neufal HrO groups occur unit-cell parameters(Finchet aL.1997) (Tables 1,2). in schoepite, leaving unansweredthe question ofwhy the In addition, both natural and synthetic samples may con- ransformation of schoepiteto UOr.0.8Hp is irrevenible. tain a mixture of schoepite and metaschoepite, in which casethe two are best identified by using frrll-pattem-fitting DrsuNcutsrmtc ScHoEpnE AND MprascHoepITE methods such as Rieweld refinement Gnch et aI. 1992, 1995,1997). A C-centeredsubcell can be describedfor Where X-ray powder diffraction is the only ana- both schoepite and metaschoepite, with^ approximate di- lytical method available, schoepite and metaschoepite mensions,a'= 7.15, b' = 4.2,c' = 7.35A (Christ& Clark 834 THE CANADIAN MINERALOGIST

TABLE1. UNIT.CELLPAMMETERS REPORTED FOR SCHOEPITE AND SYNTHFTIC UOT'zH'O (A) MINERALS

mineral a b c methodusod rcI. '14.40 '16.89 .14.75 sch@pite rotationphotos t1l sch@pit€ 14.23 16.72 14.62 Debye-Schenermmera t2l sh@pne 14.33 16.79 14.73 pr@€sion 't6.72 I3l metasch@pito 13.99 14.73 pr@ession t31 paraschoepite 14.13 16.83 15.22 Pr&€ssion t3l schoepite 14.U 16.81 14.73 singl+cr)6taldiffnctometer l4l SYNTHETtCUOe.2HrO synlh.temp. a b c methodused '14.872 40.c 13.971 16.696 ditfEctometor tsl "heatods€v€EI hB.' 14.36 16.66 14.74 Debyescherer €me€ 16l tt4.g5 116.71 i1a.ll rrealculaled 16,4 r14.o8 116.71 tu.7q tr@tcutated 16,4 60'C 13.984 16.701 14.673 Debyo-Scherer€meE lgl References[1]Billiet&deJong(1935);t2lProt6(1959);[3]Christ& Clark(1960);[4];Finchefa/.(1996a);tS] Dobets& Lmpstra (1gffi); [6] PeteB (1967rimFhch ot at. (1994; sobry (1973). ' I [s] Noles unilsare In kX. Dataof Petec (1967) but unit-cellparameters are @alculatedby Finch€t a/. (1994 for two phases.

TABLE2. AXIALRATIOS REPORTED FOR NATUFALSCHOEPITE AND RELATEDMINEMLS

a/b clb methodused rel. schoepite 0.852r 0.875 opticalgoniom€ter t1l schoepite 0.8521 0.875 opliml goniomoler 'sch@pite I2l (?) 0.855t 0.900 opliel goniometer t3l schoepile 0.852 o.874 opti€l goniomeler t4l schoepite 0.851r o.874 opti€l goniomeler t5l schoepit€ 0.852 0.873 XRD (rotation& powdor) t6l schoepite 0.851 o.474 XRD (powder) m schoepite 0.85i] o.aT7 XFtD(precession) t8l mot6cho6pite 0.837 0.881 XBD (precession) t8l pa6choepite 0.889 0.904 XRD (prrce$ion) t81 schoepite 0.853 0.876 XRD (4aircledifl Ectometer) tel synthetlc 0.837 0.879 XHD (powder) t101 synthetic 0.862 0.845 XRD (powdeo tl 1l 0.84.:, 0.882 remlculated [11.12] 0.859 0.882 re€lculated 111,121 synthetic 0.837 0.879 XRD (Dowder) I13t Releren@sl1lwalker (1923);[2] Sch@p (1924);I3l Buttgenbach(1924); Ial; Ungemach(1929); [S] patache (1934); [6] Billiet& de Jong (1935);m Prctas(1 959); [8] Chrisl& Ctark(1 960); [9] Finch er at(1996a); [1 O] Deb€rs & L@pstra(1963); Peters(1967); Fincharal. (1997); Sobry(1973). ' [11] [12] Ilsl Nol6 Buttgenbach(1924) reported the oplicilis panllel to (OO1),not (O1O)(Walke|1923; 1960). ' Christ& Clark a/b valuesare twicethose reportedby Walker(t 923),Schoep (1 924), Buttgenbach (1 924) and palrche (1994).

1960, Finch 1994). This subcell correspondsto the ap- & Clark 1960). Systematic absencesare diagnostic be- proximate positions of U aloms at (0,0,0) and (0,0,ta), and cause schoepite displays diffraction aspect Pbca, the strongest diffraction-maxima for both minerals are whereas metaschoepite. displays diffraction aspect those corresponding to this subcell. These sfrong dififrac- Pbna. The two minerals are therefore readily distin- tion-maxima give rise to a pseudohexagonalnet in hftO guished in frOlprecession photographs, in which h = 2n precessionphotographs, a characteristicof most uranyl for schoepite arLdh + I = 2n for metaschoepite.In addi- oxide hydrates (Christ & Clark 196O,Finch 1994). tion, hkl precession photographs of schoepite and The subcell in schoepitehas nearly the same dimen- metaschoepi[e are distinctly different. Both minera]s are sions in the plane of the sheets(a' -b' plane) as does characterized by the same apparent systematic absences cr-UO2(OH), and, by analogy, "dehydrated schoepite" in frkOphotographs, and both display the same strong (Taylor & Hurst 1971, Smith et al. 1982). subcell reflections: those for which h =2n and k= 4n, Although their X-ray powder patterns are closely The next most intense group of diffraction spots in hftO similar, schoepiteand metaschoepiteare readily distin- photographs of both schoepite and metaschoepite are guished by single-crystal diflraction techniques(Christ those for which ft = 2n and k = 4n X 1. In schoepite, STRUCTURAL RELATIONS AMONG SCHOEPITE. METASCHOEPITE AND "DEHYDRATED SCHOEPITE" 835

Flc. 3. Comparisonof fttOprecession photographs of (a) schoepiteand (b) metaschoepite. theseappear as satellitesbeside the strongsubcell re- is known to oxidize to schoepiteor metaschoepite flectionsin ZK) photographs(Fig. 3a),whereas they do (Schoep& Stradiot1947, Guillemin & Protas1959, not appearas satellites, but betweenthe subcellreflec- Burnset al. 1,997)(Fig. 2), andthe X-ray powderpat- tions along4* in photographsofmetaschoepite (Fig. tern for paraschoepitereported by Christ & Clark 3b). When mountingcrystals of good optical quality for (1960) can be interpretedas a compositepaftern of precessionphotography, crystallo$aphic orientations metaschoepite,"dehydrated schoepite", and ianthinite are readily determinedoptically becausethe optic (Finch et aI. 1997). Whatever the true nature of planesofboth schoepiteand metaschoepite are parallel paraschoepite,we wereunable to identify it in any sam- to (010)(Walker 1923, Christ & Clark 1960).Mounting plesthat we studied. crystalsfor precessionanalysis with the rotationaxis perpendicularto the planeof the optic axis (parallelto EXPERIMENTAL a) allows oneto obtainboth hle0and hll photographs and thus to unambiguouslydistinguish schoepite and Singlecrystals of schoepitewere removedfrom two metaschoepiteor to identiff a compositepattern (c/. museumsamples (91.62 and B3616) originally from Fig. I of Christ& Clark 1960). the Shinkolobwemine, Shaba,southern Congo. Both Christ& Clark (1960)stated that they obtained preces- samplescontain uraninite, uranyl carbonates,uranyl sion photographsof paraschoepitethat weredistinctive; oxidehydrates and uranyl silicates. Schoepite crystals however,they did not reproducea photograph,reporting were taken directly from the surfacesf sample83616, only an X-ray powderpattem for paraschoepitewith ad- whereascrystals were extractedfrom the interior of mixed "dehydratedschoepite". We were unable to 5ample91,.62by breaking it open.Unit-cell parameters obtainany precession photographs that could be anrib- of singlecrystals were measured either on a four-circle uted to paraschoepite.Some crystals of schoepitewith diffractometeror by precessionphotography (Table 3). darkinclusions commonly produced precession photo- Precessionphotographs were taken for two of three graphs indicating inclusions in parallel growth with principalorientations (ftft0 photographs and either 0ft1 schoepite(or nearlyso). The most common inclusions we or ft01photographs). foundare becquerelite, founnarieriteo vandendriesscheite, During a structuralstudy of schoepitn(Fnch et al. andianttrinite. Tanthinils, [U*r(UcOr)oO6(OH)4](HrO)e, 1996a),cleavage fragments from severalcrystals of 836 THE CANADIAN MINERAIOGIST

TABLE3. MEASUREDuNtr-cELL PARAMFTERS(4) FcR scHoEptrE AND MFfAScHoEptrE cRysrALS

@mmeni6 a3616 's-c(2) '14.o 16.72 14.7 metschoeprte.broad mulma sc-f 14.074(n $.717(4 14.7O('l\ metashoephe,stable(@ted) sc-o 14.17(1) 18.74(1) 14.68(2) slable(coated) s scr(1) 14.26 16.72 14.69 'sc-d(2) beloE alleEtionto metNh@pito: sd2) 14.296(3) 16.n5gl 14.713(4) stable(coated) 14.301(3) 16.788(4) 14.712(4\ '6c-d(1) alteredto DS and MS 14.308(3) 16.793(2) 14.706(3) stable(coated) s-b 14.31 16.76 14.71 allor€dto DS and MS 91.62

$hod 14.06 16.74 14.58 cr)€talheated in waten65(f8)"C for 19 d. '14,72 sch@ 14.30 18.24 subsequentlyheat€dlnalrat120'Cfor1hr, shoa 14.308(2) 16.808(3) 14.705(4) slable(not mted) shob 14.337(3) 16.813(5) 14.731(4\ stable(not ruted) t Standarddeviations are t1o in the last decimalpla@; siandard deviations lor prtresion daia | are nol reported. 6ct(1) and sc{(2) are th€ sme smpl6. unit-cellpaEmeteB of sc€(2) werenot deteminedas p@isely as for s€{t, due to the poorqualityot precaslon photographs ' aftertEnslomation to meta$h@pite. Ec4(1) and&4(2) are the sam€cr)€tal, but datams rmileted on sc-df2)atler six months. schoepitehad been extracted from sampleB3616. Sev- X-ray intensity dala ftom one crystal of metaschoepite eral monthslater, many of the schoepitecrystals that that clearly displayed diftaction asryt Pbna (sc-f, sample remainedin the samplehad altered. The coresof most 83616) were collected on a single-crystal dffiactometer alteredcrystals were composedof a fine-grainedyel- (Iable 3). We attempted to determine the structure; how- low powder,whereas crystal rims remainedmore or ever, the qualiry of the data was too poor to allow a lesstranslucent yellow. Thesealtered crystals were re- reasonable solution ofthe structure. Only the U posi- movedfrom sampleB36I6,andX-ray powderdiftaction tions could be determined (Table 4), and these were datawere collectedon the powderedmaterial from the used in our structural model of metaschoepite for coresof the alteredschoepite crystals. To avoid possible Rietveld refinement of unit-cell parametersfrom pow- frirtherreaction,the altered material was not ground Unit- dered samples containing metaschoepite. cell parametersof the phasescomprising the altered materialwere determined by RieWeldrefinement (Finch Rssu-rs et al. 1995)using known structuresas starting models (faylor & Hurst 1971,Finch et al. 1996a),and uranilm Spontaneousalteration of schoepite in air (25"C) positionsonly for metaschoepiteCfable 4); atomicparam- eterswere not refined. We found that the relative stabilities of schoepite ln orderto examinethe stability and possible trans- from the two samples (91.62 and B3616) were differ- formationof schoepitein waterat elevatedtemperatureo ent. Nearly all crystals of schoepitefrom the surface of severalsingle crystals of schoepitefrom sample91 .62 sample B3616 altered following manipulation and re- wereheated at 65 + 8oCin deionizedwater for sevento moval of cleavage fragments. Altered crystals became nineleendays. Precession photographs of thesecrystals clouded or opaque, including crystals extracted for X- weretaken after heating.In addition,a singlecrystal ray analysis and many crystals remaining on the (schoc,dimensions: 0.20 x 0.30x 0.05mm) of inclu- sample. Alteration was inhibited in some crystals extracted from B3616 by coating them with hair spray, TABLE 4. and these did not alter further for more than two years (Table 3). In contrastoschoepite crystals from sample d 14.074(3) b 16.717(3\ 14.697(13) c 91.62 were not coated, and these did not alter during Sito v z this study (after more than two years of storage in air; u(1) '0.01e6(5) .0.1450(4 0.24711Z) all crystals ltsm sample 91.62,however, have now altered). Unit-cell parameters and axial ratios of schoepite u(21 0.0059(s) -0.3749(n 0.2457m crystals from sample 91.62 we consistentwith those re- u(3) 0.2680(5) .0.2403(7) 02231 (7) ported for schoepite (Tables 1, 2), whereas unit-cell u(4) 0.2670(5) .0.0091(7) 0.2651(7) parameters of schoepite from sample B3616 are more ' Crystal sc-/(gample &3816); atomlc posfions d6tomlned by dicct methods. variable, with values ranging from those of schoepiteto Final B index of 19.5%; isotroplc displacoment pametere only. Z = 8. those characteristic ofmetaschoepite (Table 1, Fig.4). As noted above, crystals from sample B3616 were ob- sion-freefresh schoepitefrom sample91,62 was tained from the surface of the sample, whereas crystals mountedon a glassfiber andheated in air for onehour from sample 97.62were extractedfrom its interior. at 720"C.Precession photographs were takenof this Although measured unit-cell parameters vary con- samplebefore and after heating. tinuously betweenthose of schoepiteand metaschoepite STRUCTURAL RELATIONS AMONG SCHOEPITE. METASCHOEPITEAND'DETIYDRATED SCHOEPITE" 837

(Table3), manyofthese areaverage values ofcell pa- polycrystallinemalerial were surroundedby a deepyel- fturetersfor the two mineralsintergrown in one crystal. low cloudedrim. Theserims had developedcleaya5e Individual diffraction-maximafor inlergrownschoepite fracturesparallel to (010) (Fig. 5). Precessionphoto- andmetaschoepite are not readilyresolvable with film graphsof the rims of alteredcrystals demonstrate tlat data or a four-circle diffractometer.However, our they commonly contain intergrown schoepiteand dif[ractometerdata indicate asymmef ical peaksowing metaschoepite,and that their unit-cell parametersare to thepresence of two phasesin severalcrystals. Crys- closestto thoseof metaschoepite.In a previousstttdy, tals consistingof both mineralsare usually optically schoepitecrystals 1to6 sample9I.62 that had been homogeneousand cannotbe identifiedby optical ex- groundand Ieft in a sealedglass vial for threemonths also decomposedto a mixture of metaschoepite,"dehydratedschoepite" and minor schoepite(Finch et al. 1992). The spontaneousalteration of powdered 15.5 schoepitefrom sample91.62 at roomtemperature con- trastswith the relativestability of singlecrystals from the samesample, a differenceprobably caused by grind- 15.0 ing. Nevertheless,alteration of schoepitecrystals from !ISs sample83616 yieldedthe sameproducts as the altera- C m-i+54 tion of powderedschoepite from sample91.62 (Fnch et a,P al. 1992).There has been no furthervisible changein u.s s 5ampleB3616 after more than two years."Dehydrated (A) - . .ag-t schoepite"does not replacethe remaining metaschoepite. a This observationis consistentwith previousfindings ^otr 14.0 oO ir 1 Pf; that metaschoepiteand dehydratedschoepite are stable MS with respectto further change(Christ & Clark 1960, Fnch et al. 1992). One crystal of schoepitefrom sample83616 13.5 [crystalsc-c(7), Table3] alteredspontaneously over- 16.65 16.85 night at 2O"Cto metaschoepitefsc-c(Z)l during a b 6) ten-hourX-ray exposure,As a result ofthe transformation,the crystalchanged from translucentsulfur yellow to deepgolden yellow with numerousinclu- sions.We proposethat beforetle transformation,this Ftc. 4. Unit+ell parametersof schoepite and metaschoepiteftom 'Iable 3 (filled symhls) and values reported elsewhere (hol- crystalcontained minor metaschoepite.Though addi- low symbols; Table 1). Damonds and squares represent tional spots attributable to metaschoepiteare not minerals; open squres are for natural schoepite (S), evidentn tbehkl precessionphotograph taken of this metascho€pite (MS) and paraschoepite (PS) (Christ & Cla* crystal before the transformation,the existenceof 1960) (Iable l). Circles reprresentsynthesis products @ebes metaschoepiteis indicatedby the shorteneda cell di- & Loopsnra 1963, Petem 1967, Sobry 193). Estimated uncer- mension before the transformation(Table 3). The ainties for diftactomet€r data indicarc tlo (closed-enderror transformationof this crystalwas characterizedby a ban) (Iable 3); unceraindes for p'recessiondata are estimafe* 2Vodecrease in the a cell dimension,from 14.26A to {.157o of the measuredunit-cell pararneters(open-ended er- 14.0A; thereis no measurablechange in theb or c cell ror ban). Estimated uncertainties for the a and c cell edgesare less than the extent of the datapoints at the scale shown Un- dimensions(Table 3). The appearanceof weakdiffrac- certainties for data from other snrdiesare not indic€Ed Values tion-spotsinthe hOlphotograph with /zand I both odd for paraschoepite@S) are shown for completeness;however, indicatea space-groupchange from P2tca (pseudo- the srrfirs ofpaftNchoepite is unc€rtain- (Nore: Horizontal axis Pbca) for schoepiteto Pbna for metaschoepite.The is greatly expanded compared to the vefiical axis.) strongestdiffracted intensities in hkOphotographs of both schoepiteand metaschoepite are for h = 2n andk = 4n.The nextmost intense diffraction-spots in theE&0 aminatisn; if a sufficient component of each mineral is photographof schoepitediffer from thosein the hle0 presentin a crystal however,precession photographs may photographof metaschoepite.Although there is no displaya disinctivecomposite patern (Chis & CIrk 1960). changein systematicabsences, the hl6 photographsof X-ray powder-diffractiondata from powderedmate- schoepiteand metaschoepiteare distinctive (Fig. 3). rial obtainedfrom crystalsin sampleB3616 that had Our resultsindicate that most (if not all) crystalsfrom altered in the laboratory indicate that the resulting the surfaceof sampleB3616 that had altered spontane- materialis a mixture of metaschoepiteand "dehydrated ouslyin thelaboratory consist of intergrownschoepite schoepite",in somecases with minor schoepite.Crystals andmetaschoepiteo whereas crystals taken from the inte- from sampleB3616 did notdecompose homogeneously. rior of sample91 .62 containedlittle or no metaschoepite Instead, small regions that had decomposedto (Table3). 838

"F -{}.srrnm &

Flc. 5. Schematic representation of an altered crystal of metaschoepite plus "dehydrated schoepite". Light regions represent "dehydrated schoepite" (DS), and dark regions are metaschoepite (MS). Dashed lines indicate the (010) cleavage developed in metaschoepile following rransformation from schoepite. The size is indicated for a typical altered crystal from sample 83616.

Schoepitealteration in water (-65oC) alteredcrystals from sample83616. Spotbroadening, sfieakingand film fogging preventedus ftom determin- Schoepitecrystals heated in deionizedwater being unit-cell dimensionsof schoepiteand metaschoepite camebrittle anddeveloped abundant inclusions. Most separately,where both arepresent, such that we report of the crystalsaltered in this way haveopaque cores, averagevalues only. althoughtheir rims remain fianslucentand optically The averagea cell dimensionof crystalsheated in continuous.Optical examinationsindicate that many water is 14.0 A, and none was found with 4 greater inclusionsare fluid, but fine-grainedopaque inclusions than 14.1A. Th" au"rugeb cell dimensionis fO.ZA, were observedin severalsamples (comprising less than regardlessof durationof heating.The c cell dimension -5 voI.Vo).The optic axialangles (2Vo) of alteredcrys- wasdfficult to measureprecisely, owilg to sfreaking, tals range between 65o and 80o (estimateswithin but valuesrange from 14.6A to 14.8A. Evidencefor approximately* 5o)oconsistent with the optical proper- the -I0.2 A c cell dimensioncharacteristic of "dehy- ties reportedfor schoepite(Christ & Clark 1960). dratedschoepite" is not apparent(Christ & Clark 1960, Precessionphotographs of crystalsheated in water Finch et al. 1992).However, if present,inclusions of closelyresemble the compositepattern in precession "dehydratedschoepiteo' would not be apparentin pre- photographreported by Christ& Clark(1960) and pho- cessionphotographs if not crystallographicallyoriented tographs66tained from the rims of alteredcrystals on with metaschoepite.The poorquality of mostft01 pho- sample83616. Photographs taken with theX-ray beam tographs made systematic absencesdifficult to parallel to the structuralsheets (ft01 or Oklphotographs) determine.Diffraction spotscorresponding to subcell display streakingparallel to c*, suggestingdisorder. reflections in ftftOphotographs (ft = 2n andk = 4n) and Minor streakingperpendicular to c* alsois evident,but ftOlphotographs (ft = 4n andI = 2n) remainthe most streakingin LkOphotographs is relativelyminor. Pre- intense,as for unalteredschoepite (and metaschoepite). cessionphotographs exhibit more fogging rhan photogfaphs Onecrystal, after being heated-for nineteen days, had a of unalteredcrystals. Diftaction spotsare broad compared very shortc dimen-sion(14.6 A) andan intermediatea to thoseof rrnalteredcrystals, but aresimilar to thosein prre- dimension(14.06 A). T\ehl

c ,?

to lV rsl s\

b* A rg I \l

httt o t+ I I rO I f 0l " 4'f a (MS) (DS) ts $ t a 0t

1) 1$ at I

Flc. 6. Precessionphotographs of crystalschoc after heating in air at 120'C for onehour (a) ftlO photograph;(b) ft01photograph; (c) Representationofthe ft*Op'recession photograph in (3a) with weak ditraction spos enhanced.The patternindicates the intergrowthof two phases,one with an c-UOr(Otf2-type sheet:"dehydr*al schoepite'(phase 1, filled spots),and onewith a structual sheetthat may resernblethat ofmetaschoepite@hase 2, hollow spots).Reciprocal axes are indicatedat right and labeled'DS" for phaseI and 'WS" for phase2.

lowing the transformationof schoepiteto metaschoepite Figure 7 showsthe structuralenvironment of the clearlyreflect stnrcural rearrangementparallel to the sheeB two weaklybonded H2O groups in schoepite,W(5) and (Fig. 3), probablythe resultof modifiedH-bond anange- W(l1). EveryH2O group in schoepiteis H-bondedto mens. If tlp transformationto metaschoepiteis dueto the an OH group within the structuralsheet. W(5) and lms of one-sixttroftlrc inerlayerHp groupsfrom schoepite, W(11)act as H-bond acceptors tq OH(5) andOH(11), then a pssible structuralformula for metaschoepiteis respectively;they also act as donorsto two (or three) IOO:)aOz(OfDtl(HrO)ro,equivalent to UOr./11r9. uranyl O atomsin the adjacentsheeto each at a distance STRUCTURAL REI.AI'TONS AMONG SCHOEPITE. METASCHOEPITE AND "DETTYDR{ED SCHOEPITE" 841

t 1 I l- l I , I c 2

l Y &w(l1) &w(5) *t. ," !.*'. t'

l-b

FIc. 7. Atomic arrangement and H-bond interactions between the structural sheet in schoepite and the two HrO groups not members of the pentagonal H2O rings, W(5) and W(11). View along [100],.r = 0.25. O(5) is behind the plane of the illusration. of -2.9 A (Fig. 7). Removalof W(5)and W(1 1) there- approximatelyequidistant from the OH groupsin the fore leavesthe correspondingOH groups(and uranyl O adjacentsheet. In order to establishH-bonds from atoms)underbonded. If thebond-valence contribution OH(ll) andOH(5) to uranyl O atomsil the adjacent of OH(5) andOH(ll) to their respectiveH atomsre- sheet,the sh-eetsmust puckerby -2.4 A, or approxi- mainsapproximately 0.8 valenceunits (vz) eachothe mately 1.2 A cooperativepuckering of both sheets. removal of W(5) and W(11) leaves those H atoms Accompaniedby slightshifting alon-g [00], this puck- underbondedby -0.2 va. ExpectedO-0 distancesfor ering will leavea distanceof -3.0 A betweenOH(II) H-bondinteractions are less than 3.1 A (Buut lg72).H- andOH(5) and the threeuranyl O atomsin the adjacent bondsassociated with O-O distancesof 3.1 to 2.95 A sheet,and O-H-O anglesof approximately135'. Such andO-H-O anglesin therange 110 - 135'correspond a configurationis consistentwith bond-valencecontri- to an H-O bond strengthof 0.05 to 0.07 vz (Brown butionsof 0.05 to 0.07 vu from the uranyl O atomsto 1976u b). If the H atomsfrom OtI(5) and OH(II) eacb ttreH atomsand is comparableto multipleH-bonds in form a trifurcatedH-bond of -0.07 uz to thethree uranyl otherphases (Brown 1976b). O atomsin the adjacentsheet, their bond-valencere- The remainingten HrO groupsoriginally in schoepite quirementscan be satisfied, remainwithin the interlayer after the transformationto The nearestO atomsthat may act as H-bond accep- metaschoepite.There is a complexnetwork of H-bond- tors for OH(5) and OH(II) are uranyl O atoms to ing amongthese ten interlayerH2O groups, as well as which W(5)and W(ll) areH-bond donors in schoepite. betweenthem and the structuralsheets (Finch er al. Each of two groups of three uranyl O atoms,O(5), 1996a).The loss of one or more of theseinterlayer H2O O(7),O(12), and O(3) O(14),0(16), definea planethat groupswill profoundlydisrupt H-bonrling interactions is approximately5.0 A from OH(ll) andOH(5), re- (Finch 1997b), and spontaneousdehydration of spectively.Each group ofthree uranyl O atomsforms a metaschoepiteis not expectedat ambienttemperatures. nearly equilateraltriangle, with eachuranyl O atom This explainsthe apparentstability of metaschoepitein 842 THE CANADIAN MINERALOGIST air (Christ& Clark 1960,Finch et aI. 1992,1996b,c) metaschoepite(Fig. 6c).The "metaschoepite-like"do- and also explainswhy thereis little or no changein mains (phase2) are laterally continuouswith the (002)Iayer spacing when W(5) and W(ll) arelost from cr-UOr(OH)2-typesheets of "dehydratedschoepite" schoepite(Table 3, Fig. 4). The unit-cell volume of (phase l). The structural relationship between the metaschoepiteis smaller-thanthat of schoepite,owing sheetsin schoepiteand those in ct-UO2(OH)2is shown to an approximately0.3 A decreasernthe a cell dimen- schematicallyin Figure 8. The structural sheet of sion.The shortera dimensionin metaschoepitemay schoepiteis essentiallyan ordered c-UOr(OtI)2 defect- reflect shifting andpuckering of the layersas well as structurewith 1/8 of the anionpositions in ct-UO2(O$, changesto the configurationof interlayerHrO groups. vacantand 1/8of the anionsshifted toward the vacan- Only slightdistortions of thepentagonal HrO ringsand cies; differencesin the U positionsbetween the two small sffis in the positionsof uranyl O atomsare re- sheettypes are minor (Fig. 8). quired to makethe schoepitestructure conform with Our resultsindicate that the completestructural rear- thePbna symmefi'y of metaschoepite. rangementrequired to convert schoepiteinto the Differencesin tle unit-cell volumesof schoepite a-UO2(OH)r-typestructure of "dehydratedschoepite" and metaschoepitemust inducestrain in crystalsthat proceedsin threesteps. The first two stepsare rapid: (1) containboth phases.This strainmay promotefurtler lossof all interlayerHp; (2) concomiantrearrangement transformationof crystalsthat havepartly convertedto of the structuralsheet to a metastableintermediate struc- metaschoepitewhen manipulated or otherwisestressed ture with a metaschoepite-likesheet (phase 2). The first (e.S.,byexposure to heator sunlight),which would two stepsare represented by a single reaction: explain why some schoepitecrystals spontaneously lruOr)BO2(OII)r2](H2O)r2+[(Uo2)8olOH)n]+ lzHro (l) decomposeto "dehydratedschoepite" at room tempera- The third steprequires relaxation of the structural manipulated (Christ ture when in the laboratory & Clark sheetsfrom the configurationof the metastableinter- 1960,Finch et al. I996a).Domains already transformed mediateto the configurationof the sheetin "dehydrated to metaschoepitedo not decomposein this way,hence schoepite"(phase 1): the incompleteconversion of crystalsto "dehydrated [(UOr)8Or(OH)rr]+8[(UOr)Oo.zs(OH)'.s1. Q) schoepite"when they alter spontaneously (Fig. 5). Another effect of the transformationof schoepiteto The right-handside ofreaction (2) representsa struc- metaschoepiteis embrittlementof crystalsand the ap- hrre-derivativeof c-UO2(OIf2 in which anionvacancies pearanceof a (010)cleavage. This secondarycleavage are disordered.The composition of "dehydrated is manifestedas subparallelfractures in metaschoepite schoepite"on the right is UO3.0.75H,O,close to that (Fig. 5), which may explainstriations parallel to a ob- first reportedby Dawsonet al. (1956),UOro0.311r9, and served on the (001) face of paraschoepitecrystals essentiallythe sameas reporled by Peters(1967) for a (Schoep& Stradiot 1947).T\e (010) cleavagemay phaseproduced by thermaldecomposition of synthetic help reduce strain by acting as shearplanes along UOt 211rgbetween 250" and400'C:UOt'9.7211r9. which adjacentdomains within a crystalmay contract The replacementof someOH groupsby O atoms, differentially along [100]. Cleavageplanes are also combinedwitl vacanciesin the structuralsheets, re- potentialpathways for H2Oreleased from schoepite.As quiresthat hydrogen-bondingin "dehydraledschoepite" only H-bondslink structuralsheets and H2O groups in is substantiallyreduced compared to that in stoichiomet- schoepiteand metaschoepite,embrittlement may also ric cr-UO2(OIfr.The proportionof anionvacancies in be a manifestationof changesin H-bondingand tle es- "dehydratedschoepite" is 1:8,Le., one anionvacancy tablishmentof sheet-to-sheetH-bonds in metaschoepite, per unit cell in "dehydratedschoepite". When placedin asobserved in curite(Mereiter 1979). contactwith water,some of thesevacancies may be filled by OH groups,as UO3.0.9H2Ois the composi- Tran"sformationof schoepiteto "dehydratedschoepite" tion attainedin waterbelow 150"C(Protas 1959, Peters L967,O'Hareet aI. 1988)ithe correspondingstructural The short c dimensionof crystal schoc after one formulais [(UO)Oor (OH) 13]. The proportionof anion hour at 120'C indicatesthat heatingin air causedcom- vacanciesis reducedfrom 1:8in UO3.0.75H,Oto 1:20 pletecollapse oflayers in schoepite.This snucnral collapse in UO3.0.9H2O.The reductionin vacanciesenhances must reflect the loss of all inteflayer H2Ogroups from H-bondingbetween adjacent sheets and explains why schoepite,for which tn c = 7.36 A, andrecrystallization of "dehydratedschoepite" reacts in water to give stoichio- an cr-UOz(OF[)r-typelayer structure, with rn c = 5.1 A. metrica-UO2(OH)2 at highertemperatures @awson ef Thecollapse of layelsin schoepiteoccun rapidly and com- al. 1956,Smirh et al. 1982),but doesnot hydrateat any pletelyas all interlayerHp is lost.Domains within the temperature(Hoekstra & Siegel 1973).The observed strucuralsheets apparently undergo redrangement from compositionalseries between "dehydrated schoepite" and the schoepiteconfiguration before alteration to a sheet ct-UO2(OH)2can be explainedby an omissionsolid-solu- structurethat may be similar to that of metaschoepite, tion with thegeneral formula [(UO)O65-,(O[I)15,1, with asindicated by weak diffraction-spotsthat resemblethe .r varying from zeroin'dehydrated schoepite"lnO.25 n diffraction pattern in precessionphotographs of c-UO2(OH)2. STRUCTLIRALRELATTONS AMONG SCHOEPITE. METASCHOEPITEAND ''DEHYDRATED SCHOEPITE" 843

Ftc. 8. Schematic representation of the structual sheet in schoepite (Finch et al. 1996a) illusrating its relationship to the structural sheet in stoichiometric o-UOdOH)z Gaylor & Hurst 1971) (right). Filled circles are uranyl ions (uranyl O atoms not show:r); open circles are O atoms of the sheets(Oz- and OHJ: solid circles representoccupied positions, dotted circles are vacancies in o-UOr(OH), that are replaced by a single O atorn in the schoepite-type sheet. The structural sheet in "dehydrated schoepite" has lhe same s6mFosition as the schoepitesheet (UO;0.75H2O), and therefore must consist predomi- nantly of five-coordinated uranyl ions; however, the high degree of order that gives rise to tle larger unit-cell in schoepite and metaschoepite is absent in "dehydrated schoepite', in which anion vacanciesare disordered, giving rise to the a-UO(OH)r-type unit cell.

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