525
The CanadianMine ralo gist Vol. 32,pp. 525-532(1994)
THECRYSTAL STRUCTURE OF WICKENBURGITE, 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, University of Manitoba. Winnipeg,Manitoba R3T 2N2
ABSTRACT
The crystalsrrucrure of wickenburgite,Pb3caAl2sir0o zt1fzo)2, a 8.560(3),c 20.190(6)L, v lp;aLzg) A3, spacegroup P37c,Z = 2, hasbeen solved by direct methodsand refined to an R index of 3.17obased on 1042unique reflections measured with MoKcx,X-radiation on an automatedfour-circle diffractometer.There is one unique Pb2+position coordinatedby eight anions,four in therange 2.3U2.53 A andfour in therange 2.97-3.36 A; bond-valenceconsiderations indicate that all of these interactionsmust be consideredto be bonds.The four close anionsall lie on one side of the Pb2+cation, indicating stereoactive lone-pair behavior. There are six distinct Si positions, all tetrahedrallycoordinated by oxygen atoms,one Ca position octa- hedrally coordinatedby three oxygen atoms and three (H2O) groups, and two distinct Al positions, one octahedrally coordinatedand the other tetrahe&ally coordinated.Two sheetsof stoichiometry [CaSi5Q17]and [AlSi5$15]link by sharing tetrahedronvertices to form a thick slab orthogonalto [001], and further intrasheetlinkage is provided by hydrogenbonding. lntercalatedbetween these slabs are intemrpted sheetsof corner- ald edge-sharing(PbQJ and (AlQ) polyhedra.The strong bonding within the thick slabsand the weakerinterslab linkage providedby the t6lAl and t8lPbaccount for the {001} cleavage of wickenburgite.
Keywords:wickenburgite, crystal structure,1ead, stereoactive lone-pair.
SoMMeRs
Nous avonsrdsolu la structurecristalline de la wickenburgite,PbrCaAl2Sir0O27(H2O)3, a 3.560(3), c 20.190G)A, V 1281.2(9)A3, groupespatial P3lc, Z = 2, pu mEthodesdirectes jusqu'i un rdsiduR de 3.lVoen utilisant lM2 r6flexions uniques,mesur&s avec rayonnementMoKcl et un diffractombtreautomatis6. Il y a une position unique occup6epar le Pb2+, i coordinencehuit, quatre anions6tant situ6sh une distanceentre 2.30 eLz.$ L, qoutt" autresentre 2.97 et336 A.l*s considdrationsdes valencesde liaisonsmontrent que toutesces interactionsdoivent "t Otreconsid6r6es de vraies liaisons. Les quatreliaisons les plus courtessont regroup6es sur un m6mec6t6 du cation Pb2+,indication de la st6r6oactivit6d'une paire d'6lectronsisol6s. Il y a six positionsdistinctes de Si; danstous les cas,il s'agit d'une coordinencetdtra6drique avec des atomesd'oxygbne. Le calciummontre une coordinenceoctaddrique avec trois atomesd'oxygbne et trois groupesIlrO, et I'aluminium occupeun site h coordinenceocta6drique et un autre l coordinencet6tradrique. Deux feuillets de stoechiom6trie [CaSi5Q17]et [Alsi5Qr5l sont 1i6spar partagede sommetsde t6trabdrespour former une plaquetteperpendiculaire A t001]. Des liaisons hydrogbnecompldtent la 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, fournies par I6lAl et t81Pb,rcndent compte du clivage 1001) de la wickenburgite. (Traduit par la R6daction)
Mots-cMs:wickenburgite, structure cristalline, plomb, paire d'6lectronsisol6s stdr6oactive. 526 TIIE CANADIAN MINERALOGIST
INTRoDUC,noN collected accordingto the procedureof Hawthorne& Groat (1985).In the following text, the data for the Wickenburgitewas first describedby Williams UBC procedureare given, followed in parenthesesby (1968) from severalprospects near Wickenburg, the data for the UMAN procedure.A total of MaricopaCounty, Arizona. It is an oxide-zonemineral 2973(2957)reflections were measuredover two(two) associatedwith phoenicochroite,mimetite, cerussite, asymmetricunits out to a maximum 20 of 600.Eleven crocoite, hemihedrite,ajoite, vauquelinite,descloizite, strongreflections uniforrnly distributedwith regardto laumontite,and shattuckite.The formula was given as 20 were measuredat l0o intervals of y (the azimuthal Pb,CaAlrSi,9O24(OH)6,but the presenceof (OH) angle correspondingto rotation of the crystal about its as distinct from (HrO) was only assumed.Penna & diffraction vector) from 0 to 350o.These data were Giese(1969) proposed a structurefor wickenburgite usedto calculatea thin-plate empirical absorption- basedon the spacegrotp P62c,but a detaileddescrip- correctionwith a 10' glancing angle,the R(azimuthal) tion was not published.As part of a generalstudy indexbeing reduced from 7.2(11.8)7oto 2.6(2.3)Vo; (by LAG) of the crystal chemistryof the lead oxysalt this correctionwas then applied to the whole data-set. minerals,the crystal structureof wickenburgitewas As noted above,several authors have repofied wicken- examined.Fortuitously, the structurewas solvedinde- burgite as hexagonal.We testedthe possibleLaue- pendentlyat the University of British Columbiaand group symmetries(compatible with the existenceof a the Universityof Manitoba;a combineddescription is vertical c-glide, as indicated by the symmetrically presentedhere. absentreflections) by comparingthe merging R(sym- metric) values.For the Laue group 6lmmm,four asym- E>
TABLE 1. MISCELLANEOUS INFORMATION: systemSHELXTL PC Plus. The R indices are of the form given in Table 1 and are expressedas percent- ages.Inspection of the diffractedintensities showed that the Laue group is 3m, rather that 6lmmm as a{A) 8.560(3) 8.546(1) requiredby the spacegroups P63lmmc and P62c c 20.190(6) 20.168(4) assignedby Williams (1968) and Penna& Giese 14Ar) 12e1.219) 1275.9a41 (1969),respectively. The distributionof E values Sp.Gr n1c indicatedthe absenceof a centerof symmetry,and Cell content 2lPqCaAl(AlSiroO"r;1H,O).1 systematicextinctions showedthe presenceof a vertical c-glide. The structurewas solved by direct Radlmono MoKalcraohite methodsin the spacegroup P31c. The solutionwith Total I 2573 2957 the maximum figure of merit revealedmost of the F(azimuthall 7.2-2.6oh 1.t.8-2.3.A cation positions;the remainingcation and the anion F(morgo) TRIG 2,1o/o 2,4% positions were located on difference-Fouriermaps, HEX 3.9% 7,664 and the structurewas refined to -8Vo for an isotropic Total lF.l 861 1109 displacement-factormodel. Conversionto anisotropic lF"l > 4oln 833 1042 displacementfactors for the Pb position resultedin convergence R Fiml F(obs) 5.0o6 3.1% at an index of 5.0(3.1)Vo.Attempts to refine anisotropicdisplacement factors for the other Final wR(obs) 5.6% 3.4% atomsshowed no improvement.This resultsfrom F =:(lF"l-l4l)/tlF.l problemsassociated with the very high absorptionfor -lzwllF.l-lF,l't'lzwftlh,w= wR 1 this mineral, togetherwith the fact that wickenburgite TTIE CRYSTAL SIRUCTURE OF WICKENBI,'RGITE 527
TABIE 2. ATOMIC PARAMETERSI FOR wlCKENzuRGrTE TrcE 3. SELECTS IMMTOMC DISTNCS tAI ND NGES (6' IN WCGSURGM urc|",W
2.63121 2.53111 s(2FO(3) 1.e8t6t 1.€2(21 o.291Slt) 0.26861,|) 12111 bo(3,a bo{E) 3.13(3) 3.08t1) sl(a-(Xal,6,i x3 1.6Bt3t 1.62(2) 0.2920(1) 0.2686{1) 1t(1) Pb-oo)b 2.4212:t 2.3€(1)
o.146(3' 0.601(3) o.172111 26(6t 0.160(2) 0.493(2t 0.169(1 | 2OI3} DBscpupnoN oF THESTRUCTURE: o.4as(3) 0.166(31 o.107(1) 24t6) CanoN Coononqanox 0.4s7{1) o.169(11 o.106(1) 16121 0,612{3) o.'t37(31 o.316(11 0.519{1) 0.136(1| 0.314.(1t lXiii sipositions o.134121 o.671(3) 0.o43tl) 20141 0.12€(11 o.689(1) o.o41(1) 10(2) There are six distinct Si positions,all tetrahedrally coordinatedby oxygen anions.Four of thesesites are o.197(4) 0.036(31 o.346(1) 36(0) o.191t2) o.o31(2) 0.347(1) 1Al2l at special positions (Table 2), and the remaining two occupythe generalposition. Someof the tetrahedra o(9t 0,087{3) o.241(31 o.076(1) 24t61 0,083nl 0.243(1) o.076(1' 7l2l are quite distorted,and the Si-O distanceslange from bond lengthsis o(101. 0.229t41 0.197(4t o.221t21 45{7) 1.58 to 1.64 A. The precisionof the 4,229121 0.107(2) o.218(1) 2713) quitelow (0.01-0.03A), primarily becauseof absorp- o(111 0.463(2) o,214121 0.436(1) 15(3t tion and twinning problems;hence, although some o.441111 o.211(11 0.438(1) '1212) Si-O valueslie outsidethe rangenormally observedin ol12t 0.605(2) o.a44t2l 0.400(1) 11141 silicates(e.s., Si(3)-O(2) = 1.51(6)A, Table3), the 0.60s{11 o.397(1) differenceis not statistically significant becauseof ' Upp6r wl@: UBC; low6t valusr UMAN; 9 Uq - U{ x ld the low precision.Moreover, the short and long Si-O .. Al(11; UBc: 0.97 Al + O.03 Fs: UMAN| 0.81 Al + o.tg Fd bonds do seemto correlate with the bond-valence 'O(10) = HzO requirementsimposed by the basic structuralconnec- tivity (Table4).
Al positions is not hexagonalor trigonal at room temperafure,as shownby the complextwinning. Positionalcoor- There are two distinct Al sites,both at specialposi- dinatesand equivalentisotropic displacement factors tions (Table2) on a three-foldaxis. Al(1) is coor- are given in Table 2, selectedinteratomic distances dinatedby six anionsin a fairly regular octahedral and anglesin Table 3, and a bond-valenceanalysis is arrangement.Site-scattering refinement indicates the given in Table 4. Structurefactors may be obtained presenceof a small amount of a strongerscatterer of from the Depositoryof UnpublishedData, CISTI, X rays (assumedto be Fe3+);the resulting site- NationalResearch Council. Ottawa. Ontario K1A 0S2. populationis 0.81Al + 0.19Fe3+. Al(2) is coordinated 528 T}TE CANADIAN MINERALOGIST
TABLE 4. BOND-VALENCE. CALCULATION(v.u.} FOR WICKENBURGM ca Alll) Al(2) si(l) sl(2) si(3) si(4) si(51 S(6)
o(1) 1.06 1.88
o(2) t.to 1,12 2.27 o(31 0.314* 1.O3 1.96 o(4) o.97at 1.OO 1.97 0(6) 0.11 1.094t o.97 2,17 o(6) f .o3Bl l.os 2.12 o(7) 0.45 o.51.t 1.06 2.O2 o(81 1,03t1 o.97 2.OO o(9) 0.62 0.3541 1.OO 2.OO 0.13 0(101 0.338J 0.33 o(11) O.39 0.6041 0.97 1.86 o(12! O.O8 o.72ar 1.O9 1.95 0.06 2,06 2.97 4.41 4.11 4.12 4.O3
' calculaied with the curves of Brown (1981 ) for the UMAN distances
by four anionsin a fairly regular tetrahedralarrange- bonds?With regardto the bond-valencesum around ment. Pb (Table4), the situationis not definitive.However, theseweak interactionsare neededif the anion sums Caposition are to be satisfactory,as omission of the two longe^st Pb-O(12)bonds listed in Table3 (3.15and 3.35A) There is one unique Ca position, the special resultsin a bond-valencesum of 1.81 v.u. around Wyckoff position2a on a three-foldaxis. This posi- O(12).Thus all eight Pb-Qdistances listed in Table 3 tion is surroundedby six anions; three O(9) are Q2-, shouldbe consideredas bonds. and three O(10) are HrO anions, at approximately equaldistances from the centralcation (-2.35 A), in agreementwith that expectedfor octahedrallycoor- dinatedCa.
Pb oosition o(11)k
There is one unique Pb position in wickenburgite, a generalposition with the z coordinatefixed at zero to fix the origin of the unit cell. The Pb atom is sur- roundedby eight anionsbetween 2.26 and 3.35 A. ThesePb-Q (Q: undefinedanion) distancesfall into O(3)a two groups:four shortbonds with 2.26 DsscnprroN or ttts SrRucrtRE: consideredthe distinct types of 63 nets with regardto CoLrvscrrvrY oF PoLyHEDRA the direction in which ttre constituenttetrahedra point (a: up, or d: down); this is an importantfactor in the The eleganceof Nature'spatterns never ceases to linkage of suchtwo-dimensional nets in the third amaze,and wickenburgiteis no exceptionin this dimension.The net of Figure 1ais different from those regard.The S(1), S(4) and Si(6) tetrahedracombine consideredby Smith(1977) in tfiat oneof the nodesin with Ca octahedra(Fig.2a) to form a corner-sharing eachsix-membered ring is occupiedby an octahedron sheetof six-memberedrings of polyhedra,formally rather than a tetrahedron.However, the up-down denotedby the net symbol63. Smirh (1977)has questionis still relevant; the only difference is that a F__ a _# Frc. 2. Sheetelements of the structureof wickenburgite,projected along [001]: (a) the [CaSirQrr]sheet; O) the [AlSi5Q15]sheet. The (SiQ, tetrahe&aare random-dot shaded,the (CaQ6)octahedra are 4a-netshaded, and the (AlQ) tetrahedraare regular- dot shaded. s30 l<- a --+l Flc. 3. The slaboftetrahedra in wickenburgite,projected along [001]; legend as in Fig. 2. tetrahedronhas one free vertex to point up or down, Figure 2 link together in the c direction by sharing whereasthe octahedronhas three free verticesthat can opposing tetrahedral vertices to form a thick slab point in variouscombinations of up and down. The net (Fig. 3). As is apparent in this view, the nvo 63 sheets in Figure 2a has the configurationdudud(3u), and the of the slab essentially overlie each other, and the link- stoichiometry[CaSi5Q 1r]. age between the two sheets occurs through tetrahedra The Si(2), Si(3) and Si(5) tetrahedracombine with that lie on three-fold axes; hence the 7-O-fbridging the Al(2) tetrahedronto form a secondsheet (Fig. 2b7. angles are constrained to be 180o, unlike the T-A-T Again,it is basedon a 63net, andis an [AlSirQrr]net bridging angles within each sheet, most of which with the configuration ududuu. The two sheetsof cluster around 138' (Table 3). Ar(1) k_ a __>l Ftc. 4. The (PbQ8)-(A106)intemrpted sheetof polyhedra in wickenburgite; the (PbQs) polyhedraare orthogonal-dashshaded, and the (AlQ6)octahedra are line-shaded. TTIE CRYSTAL STRUCTLEEOF WICKENBI]RGITE 531 Ar(1) c Ftc. 5. The wickenburgitestructurc viewed orthogonalto [001]; legend as in Figs. 2 and 4. Note the altemation of slabs of tetrahedraand sheetsof (PbQ) and (AlQ6)polyhedra. On eitherside ofthe slaboftetrahedra are layers of Hesse& Liebau(1993) reported the solutionof the (PbQ) polyhedraand AlQ6octahedra (Fig. 4j. The crystal structureof wickenburgitein the spacegroup (AlQ6)octahedra share edges with the (PbQJ poly- F-62c;these findings were publishedin the form of an hedra,and the @bQ) polyhedrashare both edgesand abstract,and hencenot many detailsare given. cornersto form an interruptedsheet of cation However, there is sufficient information to determine and anion polyhedra. that the structuresand the materials are significantly The constitution of the entire structureis shown in different. First, the averagingstatistics for different Figure 5. Thick slabs of tetrahedra[+ (CaQu)octa- Laue groups indicate that our data are far more com- hedral alternatewith sheetsof (PbQs)and (AlQ6) patible with trigonal rather than hexagonalLaue sym- polyhedra to produce a layered strucfure.The strong metry. Secondly,our structurecannot have P62c bonding within the slabs of tetrahedracontrasts with symmetry.The linkage of the tetrahedraseems to be the relatively weak bondingbetween the slabs, the samein both structures.However, in our structure, and this accountsfor the {001 } cleavageobserved in Ca lies in the plane of the sheetof tetrahedra,and is wickenburgite. 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- hydrogenbond from O(10)to O(8) in the adjacentslab dinating octahedroninto an interlayer position, it then of tetrahedra.The hydrogenbonding thus provides has trigonally prismatic coordination,and requiresre- additional cohesionwithin the slab of tetrahedrain arangement of the Pb atoms.Possibly, there is some wickenburgite. kind of (probablyfusrorder) phasetransition befween 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 OperattngGrants to LAG and FCH, and Major Equipmentand InfrastructureGrants Cmurcer Fonvru-a . to FCH. Inspectionof the bond-valencetable (Table 4) showsthat the sum of the incident bond-valence REFERINCES aroundthe O(10)anion is 0.33v.u., whereas the corre- spondingsums around all the other anionsare close BnowN, I.D. (1981): The bond-valence method: an empirical (> 1.86v.u.) to 2.0 v.u.,the idealvalue for the approach to chemical structure and bonding. /r? Structffe 02- anion.This indicatesthat the O(10)anion is actu- and Bonding in Crystals II (M. O'Keeffe & A. ally (HrO), and thus the formula of wickenburgite Nawotsky, eds.). Academic Press,New York (1-30). mustbe rewriftenas PbrCaAl2Sil0O27(H2O)3. Cnovmn, D.T. & Lnpnu,qN, D. (1970): Relativistic calcula- HyonocsNBoluryc tion of anomalous scattering factors for X rays. J. Chem. Phys.53,1891-1898. The (HrO) group [the O(10) anion] is strongly bonded(bond valence= 0.33 v.u.) to Ca, and hence & MaNN, J.B. (1968): X-ray scatteringfactors must participatein a hydrogen-bondingscheme if the computedfrom numerical Harnee-Fock wave functions. bond-valencerequirements of the H atoms are to be Acta Cry stallo g r. AA, 321-324. satisfied.Inspection of the possibledonor-acceptor distancesaround the O(10) site showstfie O(4) and Hawruonxn,F.C. & GRoAr,L.A. (1985):The crystalstruc- O(8) anionsas hydrogen-bondacceptors. The result- ture of wroewolfeite,a mineral with [Cur(OH)u ing schemeis illustrated in Figure 6. There is one (SOr(H2O) sheets.Am. Mineral. 70, 1050-1055. HEssE,K.-F. & Lrcreu, F. (1993):Crystal structure and chemicalcomposition of wickenburgite,H2cat6plPryll {18,22-}14(A12Sir0)O2sl.3H2O- rhe first silicatewith loop-branchedvierer double-layers.Z. Kristallogr., Suppl.Issue 7, 18 (abstr.). PpNNa,G. & GESE,R.F., JR. (1969): The crystalsructure of wickenburgite,Pb3CaAl2Sil 0O24(OH) 6. Geol. Soc.Am., Sp ec.P ap. I2l, 232-233(abst.). Snm:, J.V. (1977):Enumeration of 4-connected3-dimen- sional 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 ReceivedJuly 15, 1993, revised rnanuscriptaccepted. wickenbursite. Januan 23, 1994.