1091
The Cawdint Mincralogist Vol.33,pp. l09l-1101 (1995)
THESTRUCTURE AND PHYSICOCHEMICALCHARACTERISTICS OF SYNTHETICZIPPEITE
RENAIJD VOCI{TEN" LAURENT VAN HAVERBEKEAUP KAREL VAN SPRINGEL DepanemzntSch.eihmdz - Experimentele Mincralogie, Universiteit Atrh'verpen (RUCA), Middelheim.lannI, 8-2020 Anatterpen" Belgiwn
NORBERT BLATON AND OSWALD M. PEETERS
Laboratoriwn voor analytischechemie en nedicinalefysicochemie, Fatalteit FarmareutischeWetenschappeg KathotiekeIJniversiteit Izaveu Van Evenstrant4, 8'3000 leweu Belgiwn
ABSTRACT
Zppeite was synthezisedby adjustinga UO2SOasolution containingK2SO4to a pH of 3.6 by meansof KOH and keeping it for ?5 hours at 150'C and an approrinat" p"ir*i of 3.5 MPa. The crystalsare yellow and well-formed. Chemicalanalysis rtves K(UO),(SO,(OFD1.H2O; th" *-poiitioo. The strongestlinesof the X-ray pattemconespond to lvalues of 7.06;3.51, i:q, Z.;ae,[ia Z.dS A. in" putt rn is identical to the pattem of natural zippeite. Single-crystalX-ray studiesrevealed the composition K(1O;2SO4(OFU.HrO. The crystals are monoclinic, space group A/c, with a 8.755(3), b 13.987Q\, c t7 J30e) A,p f O+.titlj", andZl 8. ttt" d"nriryDnis 4.8, andD*is 4.7 glcm3.The crystalstructure was solved by Patterson (010). methods.The slucture hasbeen refined to anutrwiigftd residualofb.053. Zppeite possessesa layer structurepaxalld to UO4(OIO3pentagonal bipyramids are the building blocks of a unique pattern of double polyhedrafinked along_theaaxis by edgi*hariirg of OH-groups.The UOo(OlI)3 chainsare joined by chainsof SOapolyhedra into infinite (UO)2(OII)2SOasheets. o yo 3/t, Tllre crystals show a rwo cuojrtomrsoa sheets sandwich planm layers of K+, olf, and Hro il y arrd moderatefr,ioto.6or" U"t*oit 520 and 610 nn" with unresolvedbands at room temlrerafi[e, but with three distinct bandsat j7 K T\e infrared spectrumwas recdrde4 and the most important bands were assigned..Theoptical parameterswere determined.The crystalshave a moderate'solubility,with the solubility product found to be 1032'0.
Kelwords: zippeite,synthesis, srystal structure,physicochemical properties.
Solrtnaanr
Nous avons€ussi i synth6tiserla zipfite en ajustantle pH d'une solution UO2SO4contenant &SO4 A 3.6 en y ajoutant KOH et en la laissantpour75 heuresi 15b;C et envhon3.5 MPa. Les cristauxsonljaunes et bien form6s.Une analysechimique a donn6K(UO)r(SO,(OrD3.H2O. ks raies les plus intensesdu specfiede ditraction sont situ6si une valeur d de 7 .06,3.51, 3.14, 2.86 et Z]ASA.-Ce.p""6 identique i celui de la zip$ite natmelle. Des dtudessur cristal unique par ditfraction X confirment la composition,et montrent"rt qui b appeit" est monoctinique,groupo spanalC2Jc, avec a 8.755Q), b 13.987Q), gc#. r6solula ti.iZOtl) A, p f O+.f:tf l;, Z = 8. La densiGmesur6e D, est 4.8, et la densit6calcul6e D, est 4.7 N-ousavons "stucfure par -eOoO"s de Pattenon jusqud un rdsidu R de 0.053. la zippdite est faite de couchesparallbles i (010). Des pentagonesbipyramidaux UO4(OI03 soil I'unit6 structuralede based'un agencementunique de poly!.dresdoubles align6s le ioog i" t'axe o par partageda$tes iipliquant des groupesOH, ks cha?nesUOa(OH)3 sont li6es par deschatnes de polybdres SOipour forrner dis feriillets de stoechiomdtrie(UOr2(OII)2SO4. Entre deux de ces feuillets se trouvent des couchesde K*' OFi et tt O a une 6l6vationy d'envfuonYEete/q.I** sristauxmontrent une fluorescencemoyenie entre520 et620 r.m"avec des bandesn6n r6soluese bmperature amblante,et trois bandesdistinctes e 77 K. NoUs avons mesur6et interpldtd le spectre infrarouge,et nous avonsaet"rnioe les propri6t6soptiques. Irs cristaux sont mod6r6mentsolubles, et le produit de solubilit6 est 10142'@' (fraduitparlaR6daetion)
Mots-cles: dppeite,synthbse,structure cristalline, propri6t6s physicochimiques. 1092 TTIE CANADIAN MINERALOGIST
INTRoDUCttoN further characterizedby its ffiared and luminescence spectr4 its solubility product,and optical paramelers. Zppeite is a secondaryuranium-bearing minslal belonging to the group of basic uranyl sulfates.This SnsmEsts minsral is ofrecent fonnation and occursas a vellow earthy efflorescenceand glassy crust on the walls of In order to synthesize a sufficient quantity of mins v/6pffigs containing uraninite. The mineral has zippeite,several experiments were necessary. A 50 mL a hardnessof about 2 and is easily soluble in acids. solutionof 0.075M UO2SO4was mixed with 1.5g of Its formation results from the oxidation reaction of K2SO.and adjusted to a pH of 3.6 by meansof KOH. uraninite in a H2SO^medium, which in turn is forrned This solution was introduced into a 170 mL teflon- by the oxidationof metal sulfidesin a relativelv humid lined bomb, resulting n a 30Vo degree of filling. atmosphere. The bomb was heatedfor 75 hours at 150'C. which T,rppateis most commonlyassociated with gypsum resultedin an approximatepressure of 3.5 MPa. The and uranopilite [(UOr)6(SOt(OH)r0.12H2O],and crystals obtained were washed several times with less, commonly with johannite [Cu(UOr)r(SOr), distilled water and subsequentlyair-dried. They were (-O.pz.SttzOJand uranophane [Ca(H3O)2\!UO); large enoughand sufficiently well-formed for a single- (SiOr2l.3H2Ol. Numerouslocalities are describedin crystal study. A scanningelectron micrograph (Ftg. 1) the literature (Frondel 1958).There is someconfusion showsthat the crystalsare monoclinic, with a plate-like concerning its composition, structure, and related habit and truncatededges. The height of the crystals properties.Nov6cek (1935) describedthe mineral as rangesfrom 0.1 to 0.25 mm, andthe width rangesfrom 2UO3.SOr.zH2O,v/ith n being 5 or 6, whereasHess 0.05 to 0.1 mm. The SEM photographin Figure I (1924)proposed the formula 2UOr.SOr.3HrO.Traill shows that the morphology of the crystals can be (1952) describeda syntheticzippeite phaseas a basic consideredas a combinationof the forms {101} and uranyl sulfate (UO)2SO4(OI!2.4H2O, and Frondel {010} andtwo domes. (1958) proposedthe formula 2UOr.SOr.5HrO.More recenfly, Frondel et al. (1976) found, on the basis of Cns&trcAr,ColposrnoN accuratechemical analytical datA that the structure of zippeite contains potassium, and proposed that The air-dried syntheticcompound was dissolvedin zippeite is a K-bearing basic uranyl sulfate with the 6 M HCl. The KrO content of the resulting solution formula&(UOr6(SOr3(OII)10.4H2O. was determinedby atomic absorption spectrophoto- As natural zippeite is not suitablefor single-crystal metry using a Philips Pye Unicam model PU 9200 studies, the aim of our researchwas to synthesize atomicabsorption spectrophotometer. The UO, content zippeiteunder moderate pressure and temperature,and was also determinedspectrophotometically by means to use the resulting well-formed crystals for a of a Pye Unicam SP8-100 ultraviolet spectrophoto- determinationof its structme.This syntheticzippeite is meter using arsenazoItr as the reagent, the optical
Ftc. 1. Scanning elecffon micrograph of the syn- thetic crystals. The size bar is divided into 10 pm units. TTIE STRUCTUREOF ZPPEITE 1093
TABLE I. CHEMICAL COMPOSIION OF SYNTTIETICZPPEITE From the TG curves,a total loss of.6.L7VoH2O was deduced.In the range 30o - 150oC, 1.97 molecules Oxide wa$tt% Atomic qwtiti* (x l0') Alomic rdo of H2O were losl whereasbetween 150oand 450'C, 0.41 moleculesof water were lost. The total numberof K,O 7.56 802.5 l.r3 uo, 75.90 2653 1.88 water moleculeslost over the whole rangeis therefore so, 10.63 1327 O.94 2.38. The loss of SO3 takes place in the region HrO 6.t7 6500- 900'c. Total 100.51
Kr rr(UOJr ss(SO4)os(OI03.0.88HrO X-nav Powpm.-Dnrn-q.cnoNDere
X-ray powder-diffraction data were recorded at 40 kV alLd 20 mA using CuKcrl radiation (1, = 1.5406A) and a Guinier-Hdgg camerawith a diameter density-being measuredat 662.5 nm (Singer & of 100 mm. Silicon powder (NBS-640) was used as a Matucha 1962). The SO1 conlent was determined calibration standard.The relative intensities of the gravimenically as BaSOr-. The water content was diffraction lines were measuredwith a Carl Zeiss Jena determinedby thermogravimetricanalysis, which will MD-100 microdensitometer.The results are given in be discussedseparately. Table I summarizesthe results Table2. The X-ray patternof the syntheticmaterial is of the chemical analysis and the computed atomic identical with the data given by Traill (1952) (JCPDS ratios. 8-138) and Frondel(1958) (JCPDS 29-1062), so the From the oxide composition,the chemical formula compoundcan be idenffied definitively as synthetic was calculatedby the classicalresidual oxygen method zippeite. based on 11 atoms of oxygen, the result being: K1.13(UOr1.88(SOrg.e4(OHL'0.88H2Oor, ideally, Srnucnins DE"rERvm.{ATroN KCJO)2SO4(OI{)3.H2O.The crystal-structureanalysis showedthatz=8. Many crystals are twinned. Crystals suitable for The thermal stability of the synthetic crystals was diffraction work were soughtby microscopicexamina- investigatedby thermogravimetry(IG) combinedwith tion underpolarizedlight. Only crystalsthat extinguish differential seanning calorimetry (DSC). A Dupont were selected.Crystal quality was further DSC 910 and TGA 951 apparatuswas used with assessedby noting the quality of X-ray-diffraction an applied heating rate of 5'CYmin and a N2 flow of spots on WeissenbergfiIms. Appropriate peak-shape 30 ml-/min. The TG curve and its derivative are and width on diffractogramsrevealed that the selected representedin Figures2A and 28, respectively. crystals are untwinned and have good crystallinity. Weissenberg photographs showed a monoclinic C-centeredlattice (systematicabsences for h + k = 2n + | for all reflections, and I = 2n + L for hOl reflections). An orthorhombic pseudocell with the same dimensionsas the unit cell was describedby Frondel et al. (1976),but the very weak supersfructure diffraction-maxima (requiring doubling of the ortho- rhombic b-axis) could not be detectedeither with an overexposed(200 h) Weissenbergphotograph or with a four-circle ditractometer. An orthorhombic space- group that fit$ the observedintensities could not be found. A transparent, yellow, plate-like crystal was mounted on a single-crystaldiffractometer. Unit-cell dimensionsand an orientationmatrix for datamensure- ment were obtained ftom a least-squaresrefinement using the settingangles of24 centeredreflections in the range 19 < 20 < 25o. Accurate cell parameterswere obtainedby least-squaresrefinement of measuredand calculated (sin O)2-values of. 24 reflections (Stoe 1988a). Two reflections were selected and used as intensity standardsto monitor the measurementsat o 2oo m @ t@ TmD{d@fc) l-hour intervals. The intensity of the standardpeaks <3Vo (13 FIc. 2. Thermo (A) and differential thermo vuied during data collection days). The gravimetric @) curvesof the syntheticcrystals. intensity data were correctedfor Lorentz-polarization 1094 TIIE CANADIAN MINERALOGIST
TABLE 2. X-{AY POWDER-DIFFRACTION DATA: SYNTHETIC ZPPEITE Table 2. (rcntinued)
ht . Fudol et ar. (19O (Stoe1988b), and absorption effects (Stoe 1988c), the restraintsfor the sulfate and uranyl groups.The final latter using V scans.The experimentalconditions for unweightedR factor was 0.0531,and the resultsof the data measurementotogether with selecteddata about refinementare included in Table 3. Voids calculatedin the crystal, are listed in Table 3. The structure was the residual structure with the progfiun PLATON solved by Patterson methods using SI{ELXS (Spek1990) show a cavity of 19 A3 at the positionof (Sheldrick 1986). Fourier map and structure-factor the highestpositive difference-Fourierpeak,-a value calculationrecycling methodswere then usedto locate thal is smaller than the expectedvohrme (40 A3) for a the position of the other atomsin the asymmetricunit. water molecule. The K position is divided into two half-occupied The crystal stucture was also refined in the non- positions Kl and K2, which representfwo closely centosymmetric space-groupCc, resulting in a higber spacedalternative positions for the sameatom. They R value, 0.066. This confirmed the choice of A/c, refined in a satisfactory manner, although their which was madeon the basisof the intensity statistics. displacementfactors arc relatively large. The elecfion Generalcalculations were performedusing PARST densityof the hydrogenatoms was within the statistical Q.{ardelli 1983). The lists of structure factors and backgroundlevel, and ttrus no attempt was made to anisotropic displacement parameters have been locate these atoms, The structure-factorcalculations depositedwith the Depository of UnpublishedData, were carried out witl the program SHELXL93 CISTI, National ResearchCouncil of Canad4 Ottaw4 (Sheldrick 1993) using neufral-atomscattering faclors Ontario KlA 0S2. Table 4 contains the refined and anomalous-dispersioncorrections. The structure positionaland equivalent isotropic temperature-factors. was refined with rigid bond and approximate The structure and numbering scheme of atoms are isotropiciry constrainrs(DELU, ISOR in SI{ELX93, illustratedin the PLUTON (Spek 1992)plots shownin respectively) for all atoms and appropriatedistance- Figures3 and4. TIIE STRUCTLIREOF ZPPEITE 1095 TABLE 3. CRYSTAI,DATA AND RESIJLIS OF STRUCTUREREFINEMENT The structuralformula for zippeitededuced from the OF SYNTHEIICZPPMTE refinement is K(UO)2SOa(OII)3'H2O.The density, measured in toluene using a pycnometer at room (RYSTAL DATA temperature,is 4.8 g cm-3 (mean of five determina- Crystalsizs (m) 0.12x0.0/6x0.038 tions), in good agreementwith the calculatedvalue of a (A) 8.755(3) , (A) B.9E7A 4.696 g cm-3,which is basedon the structuresolution. c (A) n:7&(n B(f 104.13(3) v (A3) 216Q) Srnuc"n;neDFScRIPTIoN Fmnula weiglt 744 1000) 2ffi Spa€ gmup ah Zipperte possessesa layer structure.The U and S z E coordinationpolyhedra are locatedin layersparallel to Fmrula ouH5K1S1U2 D"(d@) 4,696 (010) and are interconnectedparallel to b through K+, D.(dP') 4.8 OH-, and a water molecule(Fig. 4). pm-' 3138 The coordinationpolyhedron around uranium is a INTBNSITYMEASUREMENT pentagonalbipyramid similar to that found in other Difbactoaela Sbe STADI4 uranyl sulfate compounds(Niinisttt et al. L978, 1979, Motrmars g40hiF and references cited therein). Each U atom is Radiatim Mora (l = 0.71069A) smb?e t'-m surounded by two apical uranyl oxygen atomsand by ze-rdgeo 2.0- 50.1 tbree equatorial hydroxyl grorrps and two equatorial ll,t mses -10 <, s 10 -16st316 oxygen atoms of sulfate. The UOa(OIf3 polyhedra -21s ts2l form zigzag chains along the a axis by sharing Nunba of FflstiG m$r€d t4E16 Nunbs of mique Bfl*tim 1t66 equatorialOH-pairs (Ftg. 4). In this way, a polymeric Ru 6.t2 chain is.built up from dimersof uraniumatoms bonded NumbsrofE0ectim *,lth I > 3d(4 ?eardfd(tNisi6) 0.047ad 0.134 by double OH-bridges.These linked doublepolyhedra are connectedto other UO4(OI{)3chains on both sides REFINEMENTOF THE STRUCTTJRT by chainsof S polyhedrato form the layersparallel to RefiDEo@r@ f2, p = l/tl0d'oz) + 0.665 Plt P = (F,2 +2 FJB (010). Zppeite is the only compoundreported to date Fimri 0.0t31 that contains this pattem of linked double uranyl 5 (tumss of Fit) 13 (156 p6amb$) polyhedra.A comparable,also layered,but by far less (A/o)Im <0.1 (tttm-0.@) ap^ <4-ll 23 densearrangement of doublepolyhedra has been found Aptu (?A-') -{1 in johannite (Mereiter 1982), another basic uranyl exti6ti@ r 0.@030(1) @ffici€d containstwo F d = lFctl + 0.@lrFo?t1sh(20)l-l/4 sulfate. The asymmetricunit of zippeite U atoms,Ul andU2. The U2 anm" which is positioned at neady the samey and z position as Ul but is shifted by ca. al2 n x, is responsible for the observed pseudosymmetry. The U-U distances within the dimersare Ul-Ul =3.709(2),U2-U2 = 3.69(2), and U1-U2 = 3.699(2) A. notl U atoms form distorted uranyl groups(O1 + O1') and(O2 + 02' ), respectively, in which the averageU-O bond length is 1.77(3) A and the ayerage angle O-U-O is 168(6)'. The TABLE 4. FINAL COORDINATES AND EQI'IVALENT TSOIROPIC distancesbetween U and the^equatorialO atomsrange from 2.25(1) to 2.48(l) A. Least-squaresplanes Labol O@p@y through the equatorial atoms demonstratethat these UI 1.0 0.1668(1) 0.01573(t 0.08tr9(4) 0.0134(2) atoms deviate significantly from a coplan4'arrange- u2 1.0 0.6665(1) -0.0$?nt 0.08618(t 0.015(2) sl OJ 0J0@0) -o.trn(4) 025@0) 0.0141(9) ment. The distancesof the oxygen atoms from these s2 OJ o.m(o) 0.0016(4) 015000(0) 0.0130) planesrange from -0.37(1)to 0.38(l) A for theoxygen ol 1.0 o.tmQ, 0.1418(9) o.uoa o,ot52{7) ot' 1.0 0.198(2) -0.1@2(8) 0.1163(r) 0.014(l) atomsaround Ul and from -O.l l(l) to 0.08(l) A for 1.0 0.651(2) 0.r0i2(8) 0.r11q0 0.0r6q8) ol l! 0.64r(2) -{.1353(8) 0.055(8) 0.014(1) thosearound U2. The distancesfrom the least-squares 03 1.0 0393(2) -{.028{D 0.0459(7) 0.0157(7) plane 4.052(7) and -{.132(1) A, a4 t.0 -{.099(2) -{.69(9) 0a4qD 0.0120) to Ul and IJZ are 1.0 0J5S7(9) -O.(r/0r(O 0.1955(3) 0.0145(9) respectively.The U-O3 and U-O4 distances[average, 06 t.0 0,62q8) 0.0636(0 0r048cj) 0.0l5q8) 07 1.0 0.3889(8) 0S49(O 0.1975(4) 0,0164C/) 2.32(4) Al are comparableto the valuesfound in-other G 1.0 -o.12(r) -0.05699) orofi(o 0.01?(2) (Aberg 05 0.44$19) 02408(6) 0,ry7(4, 0.01s{7) compoundswith a double hydroxyl bridge K2 OJ 0.428(l) 025s4<6) 0.0r08(t 0.o20q8) 1969,Viossat et al. L983,Mereiter 1982,PerinL976, 09 1.0 0,85Q) 0ri9l(8) 02625(8) 0.0191(9) o.. 1.0 0.68(2) 0255(3) 0.&241' 0,VB(6) kgros & Jeannin1975a b). The distancesfrom U to^the sulfateoxygen atomsare longer I [average:2.47(L)N. av*= 04 oxygen atomsare bondedto three 12r2,u uoiolat.al The 03 and *. The oxyg@ of 0D walq ool@le. $ ei6 B.S.D. i! p@[br€6. uranium atoms. The sum of the angles is 347.9(6)" r096 TIIE CANADIAN MINERAI,OGIST FIc. 3. Schematicrepresentation of the crystal structureof slnthetic zippeite. aroundO(3) and 359.9(6)'axound 04, not far from a Two (UO)2(OH)2SO4 sheets sandwich planar rigonal planar configuration.In accordwith the elec- layersof K+, OH-, and H2Opolyhedra at b = VqarLd 3A. foneutrality principle, thesetwo oxygen atomsshould fts linkage between the sheets is dominated by be part of hydroxyl groups.Analysis of the bond dis- hydrogenbonds and coulombic interactionsresulting tancesshows that eachhydroxyl oxygen (O3 and 04) in the micaceouscleavage of the mineral. is surroundedonly on one side of the layer by three The K+ cation is disorderedover two positions,KL oxygen atoms at distances plagsible for hydrogen andl(2, eachwith a fractional (507o)occupancy. The bonds[2.57(2),1:65(2),2.93(2) A to 03, and2.76(2), K+-K+ distanceis 1.15 A. Bond-valences lms are 2.90(2),2.93(2)A to O4l. Bond-valencesums for 03 L.I4 va for Kl and 1.02 vu for K2. Each potassium (1.86vu) and04 (2.2I vu),basedonthe coefftcients of ion is coordinatedby an irregular polyhedronof eight Brown & Altermatt (1985) and taking into accountthe oxygen atoms as shown in Figure 4, in which K2 is appropriatevalence units for H-bonds derived from omitted for clarify. One of these oxygen atoms, 09, the plot versusO...H distance,are in good agreemetrt does not belong to a uranyl or sulfate group, but is vdth this statement.Bond-valence snms for Ul and U2 situatedin an interlayer position and bondsto the K+ (6.9 and 6.7 va) are rather high. CoDfigurationsmost with a bond length .f 2.62iij A It oc*pil;rtt"-;tti similar to theseticoordinated oxygen atoms occur in no disorder,but somecaution is necessaryhere because uranyl germanates(.egros & JeanninL975Ub). the final difference-Fourier map show-stwo small The double polyhedra chains are joined by SOa positive peals (at approxiraately 2 eA4) at short bridging groupsinto an hfinite (UO,2(OII)2SO4layer. distances(1 A) from 09. The 09 position could have The sulfate lefrahedra are distorted from a regular been refined rvith a very high temperature-factor tetrahedron. The angles range from L02.7(4) to encompassingthe surrounding small peals, or the 118.9(4)' [average:^109(5)'], and the averageS-O latier could have beenrefined as separateatoms, each distanceis 1.47(1)A. Bond-valencesums are 6.02 vu vdth fractional occupancy.It is importantto emphasize for S1 and6.10 vz for 52. thatthereis someexcess electon densi8 inthe vicinitv TIIE STRUCTTJREOF ZPPETIE 1097 \- ^l o ,2?, 9-w"?txos ,{ \ 4o'@ ) \ xtzfs Ko07 \\ 'ft,-l-//F: s7\'(o' o' *N( w?'/(- -tF. NtrY.^.i- t#e2Ro \,9\ /;\\ 'Q")- o \lF Flc. 4. Schematicrepresentation of the crystal structure of synthetic zippeite: a cross- sectionthrough the (UOTIOII)2SO4layer in the a-c plane. of the O9 position, but refining these positions as only obtained from an inspection of interatomic separateatoms with low fractional occupancy-factors distances.Two of them,o-ol' [2.8](4) A] andG02' was judged to be meaninglessin a map dominatedby t2.82(3) Al, me good candidatesfor hydrogen-bond heavy atoms like uranium. The distances O9-K distances.The other four distances[o-o1, 3.04@); p.Arclq and 2.!89(17) Al are close to the K-OH O-O2, 3.13(4); O-O3, 3.16@); O-O5, 3.09(4) Al distance (2.689 A) in potassium hydroxide (Jacobs can only representweak bonding interactions at fte et al. 1985). Combined with the fact that the bond- mosL valence sum to 09 is 1.37 vz, this fact sfrongly The compositionderived from the crystallographic suggeststhat O9 belongsto an OH group. In this way data is in good agleement with the one found by the local charge-balancealso is ensured. chemical analysis, with the exception of potassium. The oxygen (O) of the only water molecule in.the The higher . content of potassium in the chemical sftuctureis locatedat a flrlly occupiedsite witlh a high analysiscan be explainedby the adsorptionof small displacement-factors.As the hydrogenatoms could not quantitiesof K+ ions on the crystal surfaces,which is a be locate4 information about hydrogen bonds can well-known phenomenon. 1098 The powder X-ray diffraction spectrumand index- a fact that canbe explainedby the decreasingquantum ing schemewere calculatedfrom the refined atomic efficiency of fluorescencewith increasingtemperature. parametersby the programDISPOW of the NRCVAX This effect reflects the improved probability for de- package(Gabe et al. 1989).The results are given in activation by extemal conyersion due to increased Table2. frequencyof collisions at elevatedtemperature. More- over, a differentiationinto three separatewell-resolved LrJN{trIFscg.{cE SpscrRA bandsat 535"557 and 583 nm is observedat77 K. Based on the relation E = hcflu. in which hc = Observationson the fluorescenceof naturalzippeite 1.98631023 Jm, E canbe expressedin eV asE = 1.24 axenot uniform. Some naturally occurring specimens x l}a I l, (1 eV = 1.602I x 101eD. With this expres- fluoresce golden yellow, whereas other specimens sion, the band gap energyEn betweenthe conduction fluorescebright yellow to yellow-greenor olive green, and valencebands was calculatedat the most intensive and some fluoresceonly weakly. Frondel (1958) and peak (535 nm) at77 K as 2.3 eV. At this temperature, Robbins(1983) reported that somesamples of zippeite zippeite must be consideredas a mineral with well- do not fluoresceat all and that the degreeofhydration pronouncedinsulator properties. is responsiblefor the variation in the fluorescence. The slnthetic zippeite described here shows a Nrnenno SpscrRUM moderateyellow-green fluorescence both under short- and long-wave ultaviolet radiation. The fluorescence (qVa et al. (1985) studied the infrared spectraof spectra were recorded by means of a Perkin Elner zippeite extensively.According to these authors,the MPS 448 specfrofluorimeterat298 and77 K, using an infrared spectraof different samplesof zippeiteare not excitation wavelength of 380 nm. Both spectra are uniform owing to the greatvariability of waterbonding representedwith the sameamplification in Figure 5. in the stucture. The same auttrors also observed The fluorescenceaI 298 K is characterized,by a important differences in the band wavenumbers broad band between 520 and 620 nrr., having a depending on the mahix in which the sample was maximum at 551 nm. Unlike in tle fluorescence recorded(KBr disc or nujol). spectra of other uranyl compounds(Vochten er cL For this reason,we recordedthe infrared specfium 1993), no separatefluorescence bands could be of synthetic zippeite r$ing an ATI Mattson Genesis detectedin the 298 K specftum.The intensity of the FTIR spectrometerequipped with a Specac diffrrse spectrumat 77 K is muchhigher than the one at 298 K reflection unit. The spectrumis displayedin Figure 6. 420 820 Emission(nm) Ftc. 5. Fluorescencespectra of syntheticzippeite at room temperature(o) and at 77 K (.). TTIE STRUCTI,]REOF ZPPEITE 1099 100 % I r 80. S m i60 t t a n c40 4000 3500 3000 2500 2000 1500 1000 500 -1) Wavenumbers (cm FIc. 6. Fourier-transform infrared spectrum of synthetic zippeite. Table 5 lists the observedbands, together with those different OH-deformationmodes. A relatively shong obtainedby dejka et aI. (1985) and the assignments and broad band is found at 3172 cm-Land is assigned accordingtb thoseauthors. With the exceptionof some to a H2o-stetching vibration. According to Farmer & additional bands from the synthetic compound, the Russell (1971), a broad band below 3200 cm-r is specta agreevery well. There are also some shifls in observedwhen small, strongly polarizing cations are wavenumberthat may be explainedby the difference presentin the structure.This effect is due to the fact in the recordingmatrices used. that coordination of these small cations to water In the OH-stetching region, we observetwo bands, moleculesin the structurecauses the latter moleculesto at 3540 and 3625 cm-l, whereaseejka et a/. (1985) form stronger hydrogen bonds with other water observedonly one band at 35M cm-I. According to molecules.As a consequenceothe wavenumberof the Wilkins & Itn (L967), two OH-sfretching vibrations H2O-strerchingvibration is lowered. The remaining may occur in layered silicates at3625 and 3540 cm-r. bands can be explained as overtonesor combination The bandsat 1442 and 829 cm-l -av be attributedto bands. Opncar Dera Becausethe crystals are too small for direct mea- sruementof 2V, the value of 2V. is calculatedas 59". The crystals are biaxial negative with pleochroic colors: X colorless, I pale yellow, Z yellow (Z is of refraction, A$igEa@t O!6@ed &$a a a! Ohs6ved &iu a aL, parallel to the elongation).The indices determinedby means of liquids with known index o? 306 434 6\ 438 @-r 03 SO. 10&, @-1 l06t @-1 t 0.002, D, SO. 584 5El 1165 1155 of refraction,are a 1.625t 0.001,B 1.710 624 6n 60H 1442 andy 1".740+ 0.002.The compatibilityindex CI = 1 60H 6n 69 6 lt2o r60B t6u - (KlKd, calculated using the Gladstone-Dale 60H E29 D rlro ,172 (1981), 03 UO2 m4 9rrw) UOH 3v0 3504 constantsof Mandarino the mean index of 01 SO. 1m5 1005 M refraction, and the density, is 0.0016,which indicates that the data for synthetic zippeite belong to the . Cejta a aJ. 0985). category"superiot''. 1100 TIIE CANADIAN MINERALOGIST Sonrnu-rrylND SoruBLrty Pnooucr p uo;* + p Hro = fUotplol{or* * q H* When zippeite is dissolvedin aqueoussolution, it With these reactionswe can associatethe formation dissociatesaccording to the following scheme: constants0* defineOas l(Jo)o(oH')2{fl.ft1+1a K(UO)2SO4(OII)IHzO = K+ + zUO? 1on= + SO?++ 3OH- + HrO vq.Y With this dissociation,a solubility product K,o can be Djoglc et al. (1986) discussedthe most significant defined as uranyl-hydroxo complexes and list their formation constants, poo. Owing to the presence of these Iqe= tK+l.tuor2+l{sonz-1.1or4r complexes,thti concentrationof the free uranyl ions will be lowered considerably. The total uranyl The solubility was measuredby adding distilled concentation, being the concenftationof all uranyl- water to a suffisient amountof zippeite,after which the bearingspecies together, may be expressedas: solutionwas stirredfor at least L week to give the sotd - solution sy$tem the opportunity to equilibrate. Cvo2= IUO{I + >ptQO)oQH)zn Acrovowr.nocmamlrs TABLE 6. SOLL.'BII.ITY DATA ZPPBIIB We thank the National Fund for Scientific Research Mcdi@ pH pKlp kdio riFE& Sohbllfty, (NFWO) for the financial support awardedto RV and H:O 4Jt 0.d144 653.4 4t37 LVH. We also thank Dr. F. Mees of the Laboratory 0.003 M K2so4 4.45 0.01n 546.0 39.81 0.m7 M Ktol 451 0.f6.9 430J 39.r9 of Mineralogy, Penography and Micropedology of 0.(n0 M K2so. 4.71 0.609 L?/.4 3&6? the University of Ghent for determining the optical 0.(N0 M K2SO{ 4j[ 0.1u8 114.0 3333 parameters.We are also indebted to an anonymous Extlpoldi@ 4Lfi referee,to R.F. Maftin, and to the AssociateEditor, Dr. R.C. Rouse,for their valuable commentson the t U!ib: 10-6 mVL. manuscripl TIIE STRUCTIJREOF MPEITE 1101 Rmnnsvcrs NuNrsro, L., Torvorrn, J. & VALKoNEN,J. (1978): Uranyl(U) compounds. I. The crystal structure of ammonium uranyl sulfate dihydrate, (NH4)2UO2 AsERq M. (1969): The crystal sfucture of IQO)2(OII)2 Clr(IlrO)l. Acta Chem.Scand- 23,79l-8L0. (SO)2.2H2O.Acta Chem Scand.L32, @7-651, & - (1979): Uranyl(V! BnowN, LD. & ALTERMATT,D. (1985): Bond-valence compounds.tr. The crystal structureofpotassium uranyl parametersobtained from a systematicanalysis of the sulfatedihydrate, K2UO2(SO/2'2H2O. Aaa Chzm"Scand. inorganic crystal structure database.Acta Crystallogr, B/.t,24+247. 433.621,-624. NovAcBK R. (1935): Study on some secondaryuranium dBrra, f., Mnazr, Z. (L985): & Ungarrc, Z. Sekund6mi minerals.CesM Spol.eunst Nauk VestnikUrdl 7, Cl2. minerdly urano ve SblrMch N6rodniko Muzeo v Praze. Cas.Nar Muz, Radaprirodaved. 154, 92-1,05. PeRRIN,A. (1976): Structure cristalline du nitrate de dihydroxo diuranyle t6trahydrat6.Acta Crystallogr. B,32, DJocIc, R., Spos, L. & Bnamce, M. (1986):Characterisation 1658-1661. of uranium(Vl) in seawater. Liwnl. Oceanogr. 31, lL22-rt3L. RoBBD.IS,M. (1983): The Collecnr's Book of Fluarescent Minerals. Van NostrandReinhold Co., New York, N.Y, FARMm.V.C. & RussBrr J.D. (1971):Interlayer complexes in layer sitcates. The structure of water in lamellm ionic Susorucr, G.M. (1986):SHELX86. Programfor tIrc Solutian solutions.Trans. Faraday Soc,67, 2J37 -/7 49, of Crystal Stntctures.University of Gdttingen,Gdttingen, Germany. FhoNDH,,C. (1958): Systematicmineralogy of uranium and thorium. U.S. Geol, Surv.,BaIl,lMA, -(1993): SHEDh9j. Programfor thc Refinenentof Crystal Stnrcnre* University of Gdttingen, Giittingen, Iro, J., HoNeA,R.M. & Wmrs, A.M. (1976): Germany. Mineralogy of the zippeite grolup, Can. Mincral. 14, 429436. Smcm, E. & Merucsa, M. (1962): Erfabrungenmit der Bestimmung von Uran in Erzen und Gesteinen mit HFss, F.L. (1924): New and known minerals from the Arsenamfr . Z. Anal. Chem.l9l, 248-253, Utah{olorado carnotite region. U.,S.Geol. Surv., Bull. 750-D.63-78. SpBrqA.L. (1990): PLATON - an integrated tool for tle analysisof the resultsof a singlecrystal structuredetermi- Gagq E.J.,LB PecB,Y., Cturu.am, J.-P.,LE" F.L. & WtnrE, naron. Acta Crystallogr,A46, C34 (abst.). P.S. (1989): NRCVAX - an interactive program system - for sffuctureanalysis. J. Appl. Crystalbgr.22,384-387. - (L992):PLWON A VersatileMolecular Graphics Tool (vercionlan 1993).Bijvoet Centerfor Biomolecular JAcoBs,H., Krct