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525 The CanadianMineralo gist Vol. 32,pp. 525-532(1994)

THECRYSTALSTRUCTURE OFWICKENBURGITE, PbrGaAllAlSiloO2Tl(H2O)s, A SHEETSTRUCTURE ANITA E. LAM erll LEE A. GROAT Departmcnt of Geological Sciences, IJniversity of British Columbia, Vancower, British Colutnbia V6T IZ4

MARK A. COOPERam FRANK C. HAWTHORNE Depantnentof GeologicalSciences,Universityof Manitoba. Winnipeg,ManitobaR3T 2N2

ABSTRACT The crystalsrrucrureof wickenburgite,Pb3caAl2sir0o zt1fzo)2,a 8.560(3),c 20.190(6)L, v lp;aLzg) A3, spacegroup P37c,Z = 2, hasbeensolvedby direct methodsand refined to an R index of 3.17obasedon 1042uniquereflectionsmeasured with MoKcx,X-radiation on an automatedfour-circle diffractometer.There is one unique Pb2+position coordinatedby eight indicatethatall of these considerations anions,four in therange2.3U2.53A andfour in therange2.97-3.36A; bond-valence interactionsmustbe consideredto be bonds.The four closeanionsall lie on one sideof the Pb2+cation,indicating stereoactive lone-pair behavior.There are six distinct Si positions,all tetrahedrallycoordinatedby oxygen atoms,one Ca position octahedrally coordinatedby three oxygen atoms and three (H2O) groups, and two distinct Al positions, one octahedrally and [AlSi5$15]link by sharing coordinatedand the other tetrahe&ally coordinated.Two sheetsof stoichiometry[CaSi5Q17] tetrahedronverticesto form a thick slab orthogonalto [001], and further intrasheetlinkage is provided by hydrogenbonding. lntercalatedbetweentheseslabsare intemrpted sheetsof corner- ald edge-sharing(PbQJ and (AlQ) polyhedra.The strong bonding within the thick slabsand the weakerinterslablinkage providedby the t6lAl and t8lPbaccountfor the {001} cleavage of wickenburgite. Keywords:wickenburgite,crystalstructure,1ead,stereoactivelone-pair.

SoMMeRs a 3.560(3),c 20.190G)A, Nous avonsrdsolu la structurecristalline de la wickenburgite,PbrCaAl2Sir0O27(H2O)3, directesjusqu'i un rdsiduR de 3.lVoen utilisantlM2 r6flexions V 1281.2(9)A3, groupespatialP3lc, Z = 2, pu mEthodes uniques,mesur&s avecrayonnementMoKcl et un diffractombtreautomatis6.Il y a une position uniqueoccup6epar le Pb2+, qoutt" autresentre 2.97 et336 A.l*s i coordinencehuit, quatreanions6tant situ6sh une distanceentre 2.30 eLz.$ L, "t Otreconsid6r6esde vraies liaisons. considdrationsdes valencesde liaisonsmontrentque toutesces interactionsdoivent Les quatreliaisonsles plus courtessontregroup6essur un m6mec6t6 du cation Pb2+,indication de la st6r6oactivit6d'une paire d'6lectronsisol6s.Il y a six positionsdistinctesde Si; danstous les cas,il s'agit d'une coordinencetdtra6driqueavecdes atomesd'oxygbne.Le calciummontreune coordinenceoctaddriqueavectrois atomesd'oxygbneet trois groupesIlrO, et I'aluminium occupeun site h coordinenceocta6driqueet un autre l coordinencet6tradrique.Deux feuillets de stoechiom6trie et [Alsi5Qr5l sont 1i6spar partagede sommetsde t6trabdrespour former une plaquetteperpendiculaireA t001]. [CaSi5Q17] Des liaisons hydrogbnecompldtentla connexion entre les feuillets. En intercalation entre ces plaquettessont des feuillets interrompusde polyddres@bQ) et (Al0r) A coins et e ax0tespartagds.Les liaisons fortes i f int6rieur des plaquetteset les liaisonsplus faibles entrecelles-ci,fourniespar I6lAl et t81Pb, rcndentcomptedu clivage 1001) de la wickenburgite. (Traduit par la R6daction) Mots-cMs:wickenburgite,structurecristalline,plomb, paire d'6lectronsisol6sstdr6oactive.

526

TIIE CANADIAN MINERALOGIST

INTRoDUC,noN Wickenburgitewas first describedby Williams (1968) from severalprospectsnear Wickenburg, MaricopaCounty,Arizona. It is an oxide-zonemineral associatedwith phoenicochroite, mimetite,cerussite, crocoite,hemihedrite,ajoite, vauquelinite,descloizite, laumontite,and shattuckite.The formula was given as Pb,CaAlrSi,9O24(OH)6, but the presenceof (OH) as distinct from (HrO) was only assumed.Penna& Giese(1969)proposeda structurefor wickenburgite basedon the spacegrotp P62c,but a detaileddescription was not published.As part of a generalstudy (by LAG) of the crystal chemistryof the lead oxysalt minerals,the crystal structureof wickenburgitewas examined.Fortuitously,the structurewas solvedindependentlyat the University of British Columbiaand the Universityof Manitoba;a combineddescriptionis presentedhere. E>l

Ftc. 4. The (PbQ8)-(A106) intemrpted sheetof polyhedra in wickenburgite; the (PbQs) polyhedraareorthogonal-dashshaded,andthe (AlQ6)octahedraareline-shaded.

TTIE CRYSTAL STRUCTLEEOF WICKENBI]RGITE

Ar(1)

531

c

Ftc. 5. The wickenburgitestructurcviewed orthogonalto [001]; legendas in Figs. 2 and 4. Note the altemationof slabsof tetrahedraand sheetsof (PbQ) and (AlQ6)polyhedra.

On eithersideofthe slaboftetrahedraarelayersof (PbQ) polyhedraand AlQ6octahedra(Fig. 4j. The (AlQ6)octahedrashareedgeswith the (PbQJ polyhedra,and the @bQ) polyhedrashareboth edgesand cornersto form an interrupted sheet of cation andanionpolyhedra. The constitutionof the entire structureis shownin Figure 5. Thick slabs of tetrahedra[+ (CaQu)octahedral alternatewith sheetsof (PbQs)and (AlQ6) polyhedrato producea layered strucfure.The strong bonding within the slabsof tetrahedracontrastswith the relatively weak bonding betweenthe slabs, and this accountsfor the {001} cleavageobservedin wickenburgite.

Hesse& Liebau(1993)reportedthe solutionof the crystal structureof wickenburgitein the spacegroup F-62c;thesefindings were publishedin the form of an abstract,and hencenot many details are given. However,there is sufficient information to determine that the structuresand the materialsare significantly different. First, the averagingstatisticsfor different Laue groupsindicate that our data are far more compatible with trigonal rather than hexagonalLaue symmetry. Secondly,our structurecannot have P62c symmetry.The linkage of the tetrahedraseemsto be the samein both structures.However,in our structure, Ca lies in the plane of the sheetof tetrahedra,and is octahedrallvcoordinated.In the structureof Hesse&

532

TTIE CANADIAN

MINERALOGIST

Liebau (1993),Ca is describedas an interlayercation hydrogenbond from O(10) acrossa six-membered with trigonal prismaticcoordination.If Ca in our ring of polyhedrato the O(4) anion, and another structureis displacedthrough one face of its coor- hydrogenbondfrom O(10)to O(8) in the adjacentslab dinating octahedroninto an interlayerposition, it then of tetrahedra.The hydrogenbonding thus provides has trigonally prismatic coordination,and requiresre- additional cohesionwithin the slab of tetrahedrain arangementof the Pb atoms.Possibly,there is some wickenburgite. kind of (probablyfusrorder) phasetransitionbefween the two structures.Certainly, the formula reportedby AcrNowt-sucElt{ENTs Hesse& Liebau (1993),H2Pb3CaAl2SileO2s(H2O)3, differs significantly from that reported here: We thank T. Scott Ercit for discussionsin the PbrCaAlrSiroOzT(HrO)t, and Hesse& Liebau (1993) courseof this work" and our two reviewersfor their reportedthe presenceof "Si-OH" groups.Thus we constructivecomments.This work was supportedby conclude that the crystals and their structuresare the Natural Sciencesand EngineeringResearch somewhatdifferent. Council of Canadavia OperattngGrantsto LAG and FCH, and Major Equipmentand InfrastructureGrants Cmurcer Fonvru-a . to FCH. Inspectionof the bond-valencetable (Table 4) shows that the sum of the incident bond-valence aroundthe O(10)anionis 0.33v.u.,whereasthe correspondingsumsaroundall the other anionsare close ( > 1 . 8 6v . u . ) t o 2 . 0 v . u . , t h e i d e a l v a l u ef o r t h e 02- anion.This indicatesthat the O(10)anionis actually (HrO), and thus the formula of wickenburgite mustbe rewriftenasPbrCaAl2Sil0O27(H2O)3. HyonocsNBoluryc

REFERINCES BnowN, I.D. (1981): The bond-valence method: an empirical approach to chemical structure and bonding. /r? Structffe and Bonding in Crystals II (M. O'Keeffe & A. Nawotsky, eds.). Academic Press,New York (1-30). Cnovmn, D.T. & Lnpnu,qN, D. (1970): Relativistic calculation of anomalous scattering factors for X rays. J. Chem. Phys.53,1891-1898.

The (HrO) group [the O(10) anion] is strongly bonded(bondvalence= 0.33 v.u.) to Ca, and hence & MaNN, J.B. (1968): X-ray scatteringfactors computedfrom numericalHarnee-Fock wave functions. must participatein a hydrogen-bondingschemeif the Acta Crystallogr. AA, 321-324. bond-valencerequirementsof the H atoms are to be satisfied.Inspectionof the possibledonor-acceptor distancesaroundthe O(10) site showstfie O(4) and Hawruonxn,F.C. & GRoAr,L.A. (1985):The crystalstrucO(8) anionsas hydrogen-bond acceptors.The resultture of wroewolfeite, a mineral with [Cur(OH)u (SOr(H2O) sheets. Am.Mineral.70, 1050-1055. ing schemeis illustrated in Figure 6. There is one HEssE,K.-F. & Lrcreu, F. (1993):Crystalstructureand chemicalcompositionof wickenburgite,H2cat6plPryll - rhe first silicatewith {18,22-}14(A12Sir0)O2sl.3H2O loop-branchedvierer double-layers.Z. Kristallogr., Suppl.Issue7, 18 (abstr.). PpNNa,G. & GESE,R.F.,JR.(1969):The crystalsructureof wickenburgite,Pb3CaAl2Sil 0O24(OH) 6. Geol. Soc.Am., Sp ec.Pap. I2l, 232-233(abst.). Snm:, J.V. (1977):Enumerationof 4-connected3-dimensional nets and classificationof frameworksilicates.I. Perpendicularlinkage from simple hexagonalnet. Am. Mineral.62,703-709. WrLLrAMs, S.A. (1968):Wickenburgite,a new mineralfrom Aizona. Am. Mineral. 53, 1433-1438. a---------* Ftc. 6. The proposed scheme of hydrogen bonding in wickenbursite.

ReceivedJuly 15, 1993, revised rnanuscriptaccepted. Januan 23, 1994.