1069
The CanadianMineralo gist Vol. 36,pp. 1069-1075(1998)
THE STRUCTUREOF BOLTWOODITE AND IMPLICATIONSOF SOLIDSOLUTION TOWARD SODIUM BOLTWOODITE
PETERC. BURNS'
Deparxnent of Civil Engineering and Geological Sciences, (Jniversity of Notre Damz, Notre Dame' Indiann 46 5 5 6-07 67' U. S.A.
ABSTRACT
spacegroup The strucrureof boltwoodite, mo noc,linic,a'7.0772(8),b7.0597(8),c 6.647gQ) L,g LO4.g82(2)",V320.9(1) A3, (S) of p2lrn,hasbeen refined by fuIl-natrix least-squares techniques to an agreement factor (R) of 3.2Vo and a goodness-of-fit (charge-coupled device) l.li using 760 unique obierved reflections (lF"l> 4oF) collected with MoKo X-radiation and a CCD 2' which is supported area detector. The structure determination provided the formula (Ko.sel.{ao.aD[(UoD(Sio:o]I)l(Flzo)r.s, Z= in the absence of a by analyses done with an electron microprobe, and which aUi"t h". the previously accepted formu! tfhe tetrahedra and hydronium ion, the presence of an acid ,ili"ut" g.oop, ald the number of HzO. structure contains silicate parallel to (l00)_at x = t/+ u;nyl pentagonal biplramids that share edges anld corners to form o-uranophane-type sheets that are one N4 one and t'wo H2O groups' aad 3/s.Therc are four distinct partiauy occipied sites in the interlayer that corespond 1o K ofthe interlayer Local arrangements result in eitherNafto o"taieOra ($: unspecified anion) or K$7 polyhedra. Local arrangements group P21l2. The structure ur" noo-""n:fiorymmetric, although the long-range symmet'y of the strucnrre is consistent with space mineral stabil- of boltwooditeiermits substantialvariation of interlayer composition, which is significant from the penpective of in a geological ity. ln addition, boltwoodite may be a key phase in determining the future molitity of certain radionuclides repository containing spent nuclear fi:el.
uranyl mineral' Keywords: boltwoodite, sodium boltwoodite, uranophane, spent nuclear fuel, repository, crystal structure,
Sovnnns y Lastrucore de la boltwoodite, molqalinique, a 7.0772(8), b7.O5g7(8), c 6.il79(T A,9 lw'g82(2f, 320'9(1)A3' goup" de concordancede spaualy21m, a€t€affrnde par moindres carr6s sur matrice entibrejusqu'd un r6sidu R de3.2/a et une mesure d'aire d charge 1.13 en utilisant 760 r6flexions uniques observ6es (lF"E 4op) p'r6lev6es avec rayonnement MoKct et un d6tecteur qui concorde avec les coupl6e (CCD). L'affinement a men6 ) une formule r6vis6eiKo srl.{ao+)looD(siotoE)l(HzO)r.s,Z= 2, jusqu'ici par-l'absence.de I'ion r6sultats'd'analyses par microsonde 6lectronique, mais qui aiffere Oi la formule accept€e des tdtraBdres sificat6s et hydronium, la p'r6senceO'un gloupe silicat6 acide, et le nombre de groupes H2O. La structure contient type uranophane-ct des bipyramidis pentagonalei d uranyle qui partagent des aretes ei des coins pour former des feuillets de contiennent un paraUstesl (100)iux niveaux x =Vq.tTn.ny uqiut.sites distincts partiellement occup6sentre les feuillets; ils un octaddre Na$6 atome de N4 un atome de K, et deux groupes de H2O. l,es agencements locaux ont comme resultat soit quoique-la slm6trie d ($: anion quelconque) ou un polyedre KO?. i". ug"o""."ntr inter-feuillet ne sont pas centros)trndtriques, variations piu. gr*ai 6chelle concorde utic lo group" ,p atal Plm. La sructure de la boltwoodite permet des "rig"*"".-du mindral. De plus, la impo'rtantes en composition dans la position inter-feuilt;q ce qui iniiuence directement la stabilit6 du radioactifs boitwoodite pourrait s'av6rer une phase rds importante dans la quistion de la mobilitE 6ventuelle de certains nucl6ides dsns un d6potoir de d6chetsnucleaires.
(Traduit Par la R6daction)
cristalline, min6ral Mots-cl1s: bolrwoodite, sodium boltwoodite, uranophane, fuel nucl6aire usag6, ddpotoir nucl6aire, taustulg d uranvle.
I E-mail address: [email protected] 1070 TI{E CANADIAN MINER,ALOGIST
Irrrnotucrrou phasesin determiniagthe future mobilities of someof the radionuclidesunder repository conditions. Boltwoodite, a uranyl silicate of t}le uranophane group, was lust describedfrom pick's Delta mine, E>eennrsN"rAL Emery County, Utah by Frondel & Iro (1956), and has sincebeen identified from severallocalities. The X-ray dffiacrton currently acceptedformula for boltwoodite is (I{3O)KtruOt(SiO4)l@2O) (Srohl& Srnitht98l). Sev- The specimenof boltwooditeselected for study is eral investigatorshave suggested that Na canpartially from near Rdssing,Namibia. The specimencontains replaceK in the structure(Honea 1961. Chernikov er severalclusters of acicular crystalsup to -1 cm in aI.1975, Strunz& Tennyson1983, pu 1990),and length.A crystalfragment with approximatedjmensions Chernikovet al. (1975)described sodium boltwoodite 0.ll X 0.04 X 0.02 mm was selecredfor study.The from anunspecified occurrence ofuranium mineraliza- crystal was mountedon a SiemensPLATFORM three- tion within the Kyzylsai ore field, Chu-Ili Mountains, circle goniomererequipped with a lK SMART ccD southwestBalkhash region, Kazakhstan (pekov l99g). (charge-coupleddevice) detecrorwith a crystal-to- Boltwooditeand sodium boltwoodite are apparently not detectordistance of 5 cm. Burns(1998) discussed the isostructural,although the structuresare protiUty applicationof CCD detectorsto the analysisof mineral closelyrelated. The structureof boltwooditehas mono- strucfures. clinic symmetry(Srohl & Smith l98l), but the original The data were collectedusing monochromatic descriptionof sodium boltwooditeindicates thal its MoKa X-radiationand frame widths of 0.3. in o, with structureis orthorhombic(Chemikov et a/ 1975).How- 40 s usedto acquireeach frame. More thana hemisphere ever,Vochten et aI. (1997)recently synthesized sodium of three-dimensionatdata was collected,and the data bolfwooditeand demonstrated that the struchrreis prob- wereanalyzed to locatepeaks f61 the determinationof ablymonoclinic, with a unit cell thatis similarto thatof the unit-celldimensions. These were refined (Table l) boltwoodite,except that thea dimensionis doubled. with 1662reflections using least-squarestechniques. The structureof boltwoodite was reportedfor a Datawere collected for 3"( 20 ( 56.6"in approximately rwinnedcrystal by Stohl& smith (19g1).The data were 16 hours;comparison of the intensitiesof equivalent of low resolution,with the refinementof the structure reflections collected at different times during the data basedupon 201 independentreflections with a maxi- collection showedno significant decay.The three- mum2e of 35o,and resulted in an agreementindex (R) dimensionaldata were reducedand correctedfor of 1O.9Va.The study showedthat the structureof Lorentz,polarization, and background effects using the boltwooditecontains the a-uranophane-typesheet of Siemensprogram SAINT. An empirical absorption- silicate tetrahedraand uranyl pentagonal6ipyramids. correctionwas done by minimizing the intensity Stohl& Smith(1981) did not loca[ethe H atomsin the variation of symmetricallyequivalent reflections with structureobut assumedthat an (H3O)+hydronium ion is the crystalshape modeled as an ellipseusing the pro- plesent,as suggested by electroneutralityrequirements. gram SADABS (G. Sheldrick,unpublished). A rotal Vochtenet al. (1997)used sums ofbond valencescal- of 1952reflections were collected,of which therewere culatedfor the atomic sitesin the structurereported by 812unique reflections (Rnn = 3.lVo),with 760 classed Stohl& Smith(1981) ro arguethat SiO3OH groups are as observed(lF.l ) 4or). presentin the structure,rather than a hvdroniumanion. Infrared spectraobtained for syntheticboltwoodite are Chemicalarwlysis consistentwith thepresence of SiO3OHin thestructure (Vochtenet al. 1,997). Chemicalanalysis was donein wavelength-disper- Wronkiewiczet aL (1996)identtfied both bolt',voodite sion (WD) modeon a JEOL 733 electronmicroprobe and sodiumboltwoodite 6n samplesof UO2heated in usingTracorNorthern 5500 and 5600 automation. Data unsaturateddrip testsunder oxidizing conditionsthat weredesigned to modelthe behaviorof spentnuclear TABLE 1. MISCELI-ANEOUS INFORMATION fuel in a geologicalrepository. In addition,boltwoodire CONCERNING BOLTWOODITE anduranophane form whenspent nuclear fuel is altered a (A) 7 .0772(8) Crystal size (m) 0.1 I x 0.04 underoxidizing conditionsin contactwith waler derived b( ) 7.os97(E, x0.02 from the proposednuclear wasle yucca c (A) 6.@79(n Tobl ref. t952 repobitoryat P (') 1U.982(2) Uniqre ref 8t2 Mounfoin,Nevada (Finn er al. 1995,1996). Ongoing v( ) 320.9(l) je. 3.to/o experimentsindicate that a variety of radionuclides, Spregrovp P2tlm Uniqw lF"l>46p 760 includingthe fission products eoSr and l37Cs, F(000) 370 Finaln 32o/o arebeine p (nm'') 24.5 ,9 t.l3 retainedto a largeextent with theproducts of alteratioi Do(dm) 4.144 (Finn er al. 1996), suggestingthat uranophane-group Unit ell cmtentu: 2{(K*Nao)tOOlSiqODlGIrO)rr} =>(F"l-lFJEtF"l mineralsmay be incorporatingradionuclides x into their = crystal structures.Thus, theseminerals mav be kev S IfyffJ-Fjl(z-z)1ts, for z obeavatim md z prmetqs THE STRUCTURE OF BOLTWOODITE lo71
o/o) reduction was done with a conventional PAP routine. TABLE 2. ELECTRONMICROPROBEDATA (wL ON BOLTW@DITE The operating voltage was 15 kV, and the beam current 12 Stsuct" timit l2 was 0.20 pA. To prevent sample burn-up and to NaO 2.59 2.74 Na*a 0.37 0.39 0.42(3) Na volatilization and migration, a beam 30 pm in diam- K2O 5.69 5.1,4 K 0.53 0.48 0.s6(2) eter was employed, the broadestthe sample size would CaO 0.06 0.18 C€ 0.01 0.0r Si 1.05 1.06 I There was no apparentchemical Tening using the sio2 14.48 14.46 allow. uor 6E.E5 6822 U 1.05 r.05 1 back-scatter elechon detector. Data for all elements in HrO* 825 8.17 Hp 2.0(1) the sample were collected for 25 s or O.5OVa precision, Tobl 99.92 98.91 whichever was attainedfirst. A 100-senergy-dispersion * etculated broed on stoichiometry scan indicated no elements with Z greater than 8, other ** fmula calculated ming total etim = 3 than those reported here. The following standardswere used for the electron-microprobe analysis: albite (NaKc,), sanidine (I(Ka), diopside (CaKct), sanbomile P2rlm,whereas the K atoms and H2O groups located in (SiKa) and synthetic UOz (UMa). Data for standards the interlayer require a reduction of the symmetry to were collecled for 50 s or 0.25 Voprecision, whichever P2r.The structure was solvedtn P21lm using Patterson was attained frst. K counts were corrected for an over- methods. The initial structure-model contained all of the lap with U. The results of two analyses,performed on atomic sites that correspond to the sheets of silicate and two crystals of boltwoodite in parallel growth, are given uranyl polyhedra, but no interlayer sites.Refinement of in Table 2. HzO was calculated by stoichiometry from the atomic positional and isonopic-displacementparam- the results of the crystal-structureanalysis. eters resulted in an R of 10.3Vofot the observed reflec- tions. Refinement after conversion of the displacement Srnucnrne SoltmoN ANDREFINElvfil.tr parametersto an anisotropic form gave an R of 6.OVo. Within the symmetry consEaints of the space group Scattering curyes for neutral atoms, together with P2/m, there are four symmetrically distinct sites in anomalous dispersion corrections, were taken from In- the interlayer, as shown by the difference-Fourier map turnnrtonal Tablesfor X-Ray Crystallography, Vol. N (Fig. 1). Local site-geometries and electron densities (Ibers & Hamilton 1974).The SiemensSIIELXTL Ver- indicate that thesesites correspondto K and Na cations sion 5 syslem of programs was used for the determina- and two H2O groups, as labeled in Figure 1. The tion and refinement of the crystal structure. interlayer sites were added to the structure model. Systematic absencesof reflections indicated that the Refinement of the occupancy factors for the interlayer space group is either P21/m or P21. Stohl & Smith sites indicated that each is partialy occupied, and the (1981) concluded that the sheetsof silicate tetrahedra constraint that the occupancy of the Na and H2O(7) sites and uranyl pentagonal bipyramids are consistent with is identical was imposed on the model (see below).
HrO(7)
Fra. l. Difference-Fourier map at t = 0 projected along [100]. The calculation was done without the interlaver constituents included in the model. The contoul interval is 0.5 e/A3, with the lowesi contour line corresponding to 2 el A3 . to12 TI{E CANADI,AN MINERALOGIST
TABLE 3. FINAI ATOMIC PARAMETERSFOR BOLTWOODITE u2 u* UD ur" Uts u 0.52463(s) % 0.63810(5)t4tQ) ?A7(3) 1rQ) 110(2) 0 s8(2) si 0.4320(s) % 0.t48t(, 178(0 296(16) r22(t3) 122(13) 0 6r'02) K o.M6{r) 0.037(2) 0.gt(2) 4rQn Na' 0 0% 626Q9) c(l) 0.786{D % 0.668Q) 27e(20) U4143) 327(sr) 27s(48) 0 E(36) 0 o@ 0263(t, % 0.5n0) 27s(t9) 19s(40) 377(s2) zss@q 0 62Q4) 0 o(3) 0.566(1) % 0.9EEO) 214(rq 3u@3) 155(30 183(3e) 0 102(33) 0 o(4) 0.524q9) 0.0740(8) 0.688E(9) 28r(14) 630(43) @Q4) 1s3(2t 1E2t loE(2O -6(2r) oH(5) 02M(2) % 0.030Q) 407Q5) 366(5t s36(6T 3A!s8) 0 t7'/(46) 0 Hp(o' 0.r1(3) 0.92s(3) 0201(3) 639(74) Hp(aq 0.938(7) % 0.24s(9) l 110(19a "Ue-U*ffx10' ""uj = uaA2 x t6 "wupocy&cton: K = 0.2s(1),Na= O2l(2), Hp(O = 0.s3(3)"S,o(Z) -021(2)
Refinement of the entire model, including anisotropic- P21/m. Arefrnement conductedin spacegroup P2l gave displacement parameters for ths a1srnsgonrained within an R factor of 3.3Vo. However, the refinement was the sheets of silicate and uranyl polyhedra, isotropic- unsatisfactory because of high correlation among displacement parameters for the interlayer sites, the parameters,and severalof the anisotropic-displacement occupancy factors for the interlayer constituents, and a parameters became non-positive-definite. Therefore, weighting scheme for the structure factors, resulted in a results for the space group P2 1/m are reported, although final R of 3.2Vofor the 760 observedunique reflections this spacegoup is consistentonly with the long-range (lF,l > 4on) and a goodness-of-fit (.t) oi t.13. to ttr" structure. The final atomic-positional parameters and final cycle of refinement, the mean parameter shifl/esd anisotropic-displacementparameters are given in Table was 0.000, and the maximum peaks in the^final differ- 3, selected interatomic-distancesand angles are given ence-Fouriermaps were 2.09 wtd -2.M e/A3. The posi- in Table 4, and a bond-valence analysis is provided in tions of the interlayer constituents represent a long- Table 5. Observed and calculated structure-factorsare range averageconfiguration with severalpossible Iocal available from the Depository of Unpublished Data, arrangements(see below). Some of the local arrange- CISTI, National Research Council, Ottawa, Ontario ments violate the center of symmetry in space group KIA OS2.
TABTE 4. SELFCTED TNTERATOMTC DTSTANCFS (A) AND TABLE 5. BONDVAIENCE (w) ANAIYSIS FqRBoLTwOODm ANGLES E) FOR BOLTWOODITE u si Kr* Na" u-o(2) 1.802(e) K-o(2x 2.60(r) o(D LS79 0.177(0.M94)) 0.078,+ (0.016r+) 1.759 u-o(1) 1.812(e) Ka(l)e 2.77(1' o.o9o(0.025 '+) u-o(3) 2270(8) K-o(2)h 2.78(r) oa) 1.610 0181(0.0782+) o.14r2l.Q.EOa+) 1.922 U{(4)a,b 2.312(s)ra K-IIp(OI 2.92(2) 0.173(0.096'+) q3) 0.93 t.vt9 1.7D, u-o(4ts,d 2.449(6)rJ K-HAOg 2.e4Q) aI
(b)
T b I
k_c sinpl
Fto.2. (a) Sheet of silicate Erahedra and uranyl polyhedra that occurs in the structure of boltwoodite at x = lq and 3/ projected along [100]. The uranyl polyhedra are shaded witl crosses, and the silicate tetrahedra are shaded with parallel lines. (b) The sheet anion-topology corresponding to the sheet shown in (a) derived using the method of Buns et al. (7996).
identical to the sheet in the bolnvoodite strucfltre; these Thus,the silicale tetrahedron is an SiqOH group,as is correspond to a-uranophane (Ginderow 1988), cupro- found in the structuresof o.-uranophane(Ginderow sklodowskite (Rosenzweig& Ryan 1975),sklodowskite 1988),B-uranophane (Viswanathan & Harneit 1986), (Ryan & Rosenzweig 1977), and kasolite S.osenzweig cuprosklodowskite(Rosenzweig & Ryan 1975),and & Ryan 1977), and to Mg[(UO2)(AsOa)]2(H2O)a in sklodowskite(Ryan & Rosenzweig1977), n contrast which arsenateratherthan silicate tehahedra are present to the SiO+ tetrahedronreported by Stohl & Smith (Bachet et al. l99l). (1981).The presenceof a SiOgOHgroup in the struc- The polyhedron geometries within the sheets of ture of boltwooditeis supportedby infrared spectra polyhedra in boltwoodite are consistent with those in (Vochtener al.1997). other well-refined uranyl structures.The Uft cation is strongly bonded to two O atoms, forming a nearly Interlnyer linear [177.8(4)"] (U-6*O2)2+uranyl ion (Ur) with
cating that not all H2O(6) bond to two K, whereaseach K is probably bonded to two H2O(6).
Structural formula
The structure refinement indicates the formula I (Ik.soNao.aD[(UO2XSiO3OH)](HzO)r.s for the crysral csrp studied. This formula is different from the currently acceptedformula in the absenceof an hydronium ion, the presenceof hydroxyl in the silicate tetrahedron,and lo the amount of HzO. The results of the electron-micro- k_b_ l probe analysisfor crystals of boltwoodite from the same o) specimen(Table 2) confirm the K:Na ratio that resulted from the structure model.
DrscussloN
This study demonstratesthat either K or Na can be accommodated in the interlayer between the sheets of silicate and uranyl polyhedra in the structure of boltwoodite, although this is achieved by the cations assuming different positions that result in different coordination polyhedra that are consistent with cation size. It is significant that both the K cation and the con- siderably smaller Na cation occur in the interlayer in the samecrystal, owing to the presenceof appropriately sized coordination polyhedra for both cations that only require modifications of the interlayer H2O positions, rather than the alignment of the sheets of silicate and uranyl polyhedra. The complex paragenetic relations of uranyl miner- als are further complicated by the presence of solid so- lution within single crystals. Mineral structuresthat are compatible with a range of compositions will tend to have larger stability fields than those that are compo- sitionally inflexible. The determination or prediction of Ftc. 3. Possiblelocal arrangementsofinterlayer constituentsin such fields of stabiliry will require the study of crystals the structure of boltwoodite. The interlayer at.r = 0 is shown with a range of compositions, rather than thermody- projected along [00] onto the sheerofuranyl polyhedra at x = r/q. (a) Projection showing the arrangement of all namic measurementsdone for end-member composi- interlayer sites, each of which is panially occupied. (b) Ar- tions (e.9., Nguyen et al. 1992). rangement that occurs when Na sites are occupied. Boltwoodite and sodium boltwoodite are phasesthat (c) Possible arangement that occurs when K sites are oc- are likely to form in substantial quantities when spent cupied. (d) Alternate arrangement tlat occurs when K sites nuclear fuel is corroded under unsaturated oxidizing are occupied. (e) Arrangement compatible with a transition conditions in contact with silica-bearing watero such from a Na-rich to a K-rich region. as at the proposed repository at Yucca Mountain (Wronkiewicz et al. 1996,Finn er al. 1995, 1996).T\e flexibility of the boltwoodite structure that permits Occupancyof the K and H2O(6)sites is locally cation substitution in the interlayer has been demon- correlated,such that one ofthe arrangementsshown in strated by the present study. Note that it may be possi- Figure 3 occurs.The K cationis bondedto four Ori, ble for substantial amounts of Cs to substitute into the atoms,the OH(5) group,and two H2O(6)groups, grv- interlayer of boltwoodite or sodium boltwoodite, ing a KS7 polyhedronwith
AcrNowI-ElceluIEr,{Ts Horwa, RM. ( I 96 1): New data on bolwoodite, an alkali uranyl siltcate. Am. Mineral. 46, 12-25. This research was funded by the Environmental (1974): Management Sciences Program of the United States IBERS,J.A. & Hervro-roN, W.C., eds. International Tabks X-ray Crystallography,IV. The Kynoch Press, Departrnent of Energy Robert for @E-FG07-97ER14820). Birmingham, U.K. A. Gault of the Canadian Museum of Nature carried out the electron-microprobeanalysis of the crystals of bolt- NcuyEN, S.N., Snve, R.J., Wern, H.C. & ANpnsws, J.E., Jn. woodite. Reviews by Drs. R. Vochten, N. Blaton, (1992): Standard Gibbs free energies of formation at the F. Demartin, and C. Gramaccioli, and editorial work by temperature 303.15 K of four uranyl sitcates: soddyite, Dr. R.F. Martin, helped improve the quality and clarity uranophane, sodium bol[woodite, and sodium weeksite. of this manuscript. l. Chem. Th.ermodyn. A, 359 -37 6.
(1998): ReFEREwcES PEKov, I.V. Mincrals First Described on the Territory of the Former Soviet Union. Ocean Pictures, Moscow, Russia. BAcHEr,B., BRAssy,C. & CoussoN,A. (1991):Structure de MetruOz)(AsOr)lz.4HzO.Acta Crystallogr.C47, Pu CoNcrnN (1990): Boltwoodite discovered for the fint time 2013-20t5. rnCfuna. Acta Mineralogica Stnica L0, 151-160. BRFSE,N.E. & O'KEEFFE,M. (1991):Bond-valence parameters RosENzwErc, A. & RYAN, R.R. (1975): Refinement of the for solids.Acra Crystallogr.B,47,192-197 . crystal structure of cuprosklodowskite, Cu[(UOz)z (SiO3OH)21'6H2O.Am Mineral. ffi, M8453. BmNs,P.C. (1998): CCD areadetectors of X-raysapplied to the analysisof mineralstructures. Can. Mineral.36, 847-853. & - (1977):Kasolite, Pb(JO2XSiO4).H2O. Cryst. Struct. Commun. 6, 617-621. Ewnrc, R.C. & Hawrnonrp, F.C. (1997):The crystalchemistry of hexavalenturanium: polyhedron RvAN, R.R. & RosENzwEIG, A. (1977): Sklodowskite, geometries,bond-valence par'emeters, and polymerization MgO'2UO3el$iOr'711r0. Cryst. Struct. Commun. 6, of polyhedra.Can. Mtureral. 35, l55l-1570. 61t-615. Mu-rn, M.L. & Ewnrc,R.C. (1996):Un minerals Srom-, F.V. & Srwrs, D.K. (1981): The crystal chemistry of ald inorganicphases: a comparisonand hierarchy of crys- the uranyl silicate minerals. Am. Minzral. 66,61U625. tal structnres.Can. Mineral.34, M5-880. SrRUNz, H. & TsNi.rysoN, C. (1983): Boltwoodit aus S.W. CmRr\'rKov,4.A., SHASKT{,D.P. & Gnvnnovn, I.N. (1975): Affika. D er Aufs chlw s 34, 497 -50 I. Sodiumboltwoodrte. DokL Acad Sci. USSR,Eanh Sci. Sect.22l,14+146. VrswnuenuN, K. & HARNETT,O. (1986): Refined crystal structure of p-uranophane, Ca(UOz)z(SiO:OH)z.5HzO. Evets, H.T.,Jn. (1963): Uranyl ion coordination.Sciencel4l, Am- M tucral. 7 l, | 489- | 493. 154-1s8. VocHTEN,R., Br-nrou, N., PBrrens, O., Vel SpnntcsL K. & Fnw,P.A., Bucx, E.C., Hon, J.C. & Bnrrs,J.K. (1995): Spent VAN HATERBEKB,L.(1997): A new method of synthesisof fuel's behaviorunder dynamic drip tests.Global '95, boltwoodite and of formation of sodium boltwoodite, Versailles,France. uranophane,sklodowskite and kasolite from boltwoodite. Can. Minzral. 35, 7 35-741. HoH,J.C., Wom, S.F.,SLATER, S.A. & Berrs, J.K. (1996):The release of uranium,plutonium, cesium, stron- Wnorrcwrc4 D.J., B,lrns, J.K., WoLF, S.F. & Bucn E.C. tium, technetiumand iodine from spentfuel underunsatu- (1996): Ten-year results from unsaturateddrip tests with ratedconditions . Radiochim. Acta 74, 65-71. UOz at 9fC: implications for the corrosion of spent nuclear fael. J. NucI. Mater.238,78-95. FRoNDEL,C. & Iro, J. (1956):Boltwoodite, a new uranium silicats.S c ience lA, 931.
Gntornow, D. (1988):Structure de I'uranophanealpha, Ca(UOdz(SiO:OH)2.5H2O.Acta Crystallogr. C44, Received March 24, 1998, revised manuscript accepted 421424. July 18, 1998.