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The Canadian Mineralogist Vol. 33, pp. I I 19-1124 (1995)
THE CRVSTALSTRUCTURE OF MOTTRAMITE, AND THENATURE OF Gur=Zn SOLIDSOLUTION IN THE MORRAMITE-DESCLOIZITESERIES
MARK A. COOPERAND FRANK C. HAWTHORNE Departrnentof GeolagicalSciences, University of Manitoba"Wirmipeg, Manitoba R3T 2N2
ABSTRACT
The crystalstrucftre of mottramite,Pb(Cua31Zn6./VO4(OID, a 7.667(4),b 6.053(2),c 9.316(4)A, V = qlZ.l3) N' Pnma,Z = 4,has beenrefined to an R index of 3.67ofor 550 observedreflections measured with MoKo X-radiation.Mottramite is isostructuralwith descloizite,Pb-Zn(VO|(OID. The Pb2*cation is coordinatedby sevenanions between 2.45 nd 2.80 A, withafurthertwoanionsat3.lg5A.TheCu2*cationiscoordinatedbysixanionsinaV+2+21-dtstortedoctahedralarrange- 'Ihe ment, with a
Keywords:rnottramite, crystal-structure refinement, descloizite, Jabn-Teller effect.
So&fi\,IAIRE
Nous avonsaffind la structurecristalline de la mottramite,Pb(Cuo.uZfu.dVO4(OID, a7 .667(4),b 6.053(2),c 9.316@)A, V = 432.3(3)A3 , Pnma,Z = 4, jusqu'i un residuR de 3,6Vo en utilisant 550 r6flexionsobserv6es et mesur6esavec rayonnement MoI(o. La motfamite possBdele-mOme motif structural que la descloizite,Pb^Zn(VOd(OH).-lp cation Pb2* est coordonn6 par septanions siur6s d unedistance entre 2.45 et2.80A, et deuxautres h 3.195A. Le cationCu2* est coordonn6 par six anions itanlin agencement12 + 2 + 2l octa6driquedifforme, la distancemoyenne
Clraduit par la Redaction)
Keywords:motfamite, affinenent de la structurecdstalline, descloizite,effet de Jahn-Teller.
INTRoDUciloN no structural discontinuities.As part of our general work on the structuresof Cuz+oxysalt minerals @by & Mottramite,ideally PbCu(VOrOH, is one of a large Hawthorne 1993), we here report a sffucture refine- group of mineralsof the generalformA2(XO)Z,wherc ment of mothamite, and examine the way in which A represents large and intermediate-size divalent the descloizite structure-type accommodatesthe cationssuch as Ca and Pb, Cu and Zn, XrepresentsP, Jahn-Teller distortion associatedwith Cuh in octa- Ass+and V5*, and Z representsOH, F, Cl (Richmond hedral coordination. 1940). It is isostructural with descloizite, PbZn (VO/OH, and severalother minerals of the descloizite Er
TABLE 1. MISCELLANEOUSINFORMATION FOF MOTTRAMITE Table I andare expressed as percentages. The Siemens a (A) 7,687(41 crystal sizo (mm, O.O8x O.OAx 0.05 SHELXTL PLUS (PC version) system of programs , 6.053(2) Radiation MoKa/Graphho was usedthroughout this study. c 9.316(41 Total no. of I 1463 Refinement of the positional and anisotropic dis- v(Ao) 432.319) No. of lFl 688 placement parametersconverged to an R index of Sp. Gr. Pnma No. of lFol > 5a 650 3.6Vo. A tbree-dimensionaldifference-Fourier map // {cmi} 460 8(azimuthal) oh 12,6 + 2,4 was calculatedat this-stageof the refinement,and the D* (g cm{} AJ67 R(obs) % 3.0 single hydrogen atom was found at the 4c position. wR(obsl % 3.7 Subsequentcycles of refinement gave hydrogen Cellcorrtenti 4tPbCuVOrOHl coordinatesthat were not realistic, as indicatedby an a = :(l4l - lF.l)/u14l anomalously short donor-hydrogen bond-length, a - - -exp(-l,A{8ine wR Irwll F"l | F.ll2 EFAb, w - 1I dF.t1 Ul2tl corrrmonfeature of hydrogen positions refined using X-ray data. The "soff' constrainl that the donor- hydrogendistance should be -0.98 A sTasimFosed by adding extra weighted observationalequations to the least-squares matrix. Only the donor-hydrogen diffractometer.Twenty-eight reflections over the range distanceis constraine4 and the hydrogen position is 8' < 2e < 30o were centeredusing graphite-mono- free to seek its optimum position around the donor chromatedMoKa X-radiation. The unit-cell dimen- atom. Refinement of all parametersgave a finat n sions (Iable 1) were derived from the setting angles rndexof.3.6%o and a wR ndex of 3.7Vo.Final position- of the twenty-eight automaticaly aligned reflections al and displacementparameters are given in Table 2, by least-squarestechniques. Data were collectedusing selectedinteratomic distancesand anglesin Table 3, the 0-20 scan method, with a scan range of 3.0'. A variable scan-rateinversely proportionedto the peak intensity wasused, with maximumand minimnm scan- rates of 29.3"29/min and 1.5"20lmin, respectively.A TABLE 3. SELEcTEDTNTERAToMTc DrsrANcEs tA) AND ANGLES(o) lN total of 1453reflections was measuredover tle ftmge MOTTRAM|TE -L0 4' < 20 S 60o,with index ranges < h < 10, -8 < ft v-o(1) 1.750(81 Pb-O(llb 2.501(81 /2 < 0, -13 < / < 0. Two standard reflections were V-O(2)a 1,64 121 Pb-O(l)d 2.7oo$l x2 measuredevery fifty-eight reflections; tlere were no v-o(3) 1.704(10) Pb--O(2)€ 2,7s61121 significant changes in their intensities during data
TABLE 2. FINAL POSITIONALAND DISPLACEMENT PARAMETEFS FOR MOTTNAMTE glte x uq Urr uu L/s us u8 ut 'll2 Cu 0 0 162(4) 1810) 17781 12718) -11t6) 13(e) 1|Jt Pb 0.13186(7) 1t4 o,17374rc1 19AQl 188(2) 27ABt 't42t2t 0 2t2t 0 v 0.s778(3) 3t4 0.1939(21 r38(6) 160(9) 145(11) 119(9) 0 9(9) 0 o(11 o.8847(g) 0.607(1) o,2s64l7t r96(17) 199127, 172tS3t 216127) 11t241 -IS|.2A! -64t27t o(2) 0.063(1) 3t4 0.0g6tr) 232tul 170tEOt 299Q4t 226t40t 0 53(461 0 't68127t o(3) 0,703(1) 3t4 o.0gli1) 1S1(3Sl 184t671 169(411 0 -r 1(36) 0 oH o.a312't 1t4 0.06111) 1361241211142t 62t44) 1471 ) 0 -r8(3S' 0 H O,71t2t ',lt4 -0.02(11 200
'Ut- Utxlf TIIE CRYSTAL STRUCTTJREOF MOTTRAMITE LL21
TABLE4, BONDVALENCEITABLE FOR MOTTMMITE ground,polished and analyzedby elecfronmicroprobe using a CAMECA SX-50 accordingto the procedure of Hawthome et al. (L993). The results (mean of ofi) o.324t 0.35't 1.164{ o.2*{ 10 points) aregiven in Table 5. The unit formula based on 5(O,Otf with OH = I apfu (atomper formula unit), 0,16 1.39 0.38 2.O3 is Pb1.sa(Cus.rtZns))(YOi(oH), ignoring minor o.o5tt- components.The crystal was found to be fulI of very o(3) O.1921- O.21 1.31 r.90 smallinclusions (usually < 1 p-).The compositionof OH 0.5oel- 0.4o 2.O2 the largest inclusion (-5 ttm) is given in Table 5. 1.97 When reducedon the basis of 13 anions,the formula correspondsfaidy closely to that of vanadinite,ideally ' calculatgd from ths cuwBs ol Brown & Alt8rmatt ( l 985) Pb5ryOr3cl, althoughthe presenceof someCu2* and Zn suggestsanalytical contaminationfrom the host mottramite.This accountsfor the slightly low (Cu+Zn) and a bond-valenceanalysis in Table 4. Observedand sum of 0.93 apfu relattve to Pb and V derivedfrom the calculated structure-factors are available from the compositionof mottramite(Table 5). Depository of Unpublished Data, CfSTI, National ResearchCouncil, Ottawa,Ontario KlA 0S2. Canox CoonoNenor.l
Cunncel CoNposrrroN The Cu site is coordinatedby four oxygen anions and two hydroxyl groups (Table 3) in a distorted The crystal used in the collection of the X-ray octahedral arrangement(Fig. 1a). Nearly all CuQ6 intensity data was subsequentlymounted in epoxy, (0: O'-, OH, H2O) octahedrain mineralsand synthetic inorganic solids are stongly distorted away from TABLE5. CHEMICALCOMPOS]TIONT holosymmetric symmetry owing to Jahn-Teller AND UNITFORMULAE OF MOTTFAMITE relaxationassociated with the electronicdegeneracy of AND AN INCLUSIONIN MOTTRAMITE a de elecfton configuration in a holosymmefricocta- "TMOTTFAMITE INCLUSION hedralfield. Usually, this distortionresults in an axially (Eby Hawthorne 1993), Vzoo 2',t.70 17.81 elongated octahedron & As.Ou o.29 0.30 designatedas a [4 + 2] distortion. However, such a CUO 15.49 1.29 pattern of distortion is not always the case.There are z;no 2.31 0.31 a significant numberof instanceswhere the CuQ6octa- FsO 0.15 0.07 hedron has two short bonds, two intermediate-lengttr CaO 0.18 bonds,and two long bonds@urns 1994).Mottramite is Pbo 56.24 76,14 one such structure; the intermediate-length bonds HrO 2,14 (2.102A) are very closeto the meanof the long and cl ;;. shortbonds (2.117 A), a 12+ 2 + 2l-coordination. QeCl -o,4'l The V5+ is coordinatedby four oxygen anionsin a Sum 98.54 g?.34 distorted tetrahe&al arrangement.The
O(1) o(2)
Ftc. 1. Oblique views of selectedcoordination polyhedra in mottramite: (a) the CuQ6polyhedron; (b) the PbQepolyhedron. Oxygenatoms are largehighlighted circles, H atomsare small unshadedcircles, Cu2. is ttre diagonal-lineshaded circle, and Pb is the cross-hatchedcircle.
O(2) with and without this long distanceas a bond are regard,it ls importantto note the different behaviorof 1.65 and I.55 vu, respectively,not consideringthe the O(2) anion in descloiziteClable 6): the H-bond is contribution from the H-bond. Again, this situation is weaker in the latter mineral, a consequenceof the rather inconclusive, as if the bond-valencefrom the different elecfonic structuresof Zn and Cu2+. H-bond is included in the sum aroundO(2), the values are close to being ideal, both with (2.03 va) and with- Srnucnrns Topolocv out (1.93 ua) this interaction.The samesituation holds for the analogousdistance in descloizite(Table 6). The structural unit of mottramite is shown in The H atom is bondedto the OH oxygenatom, with Figure 2. Chains of edge-sharingoctahedra of the the distanceconstrained bv the refinementto be close general form [MQa] (M: octahedrally coordinated to 0.98 A. Ther" is a H-bondwith the O(2) anion,as cation)extend along the Yaxis, the doublerepeat ofthe suggestedby the arrangementof non-H bond-valences tran r edge-sharingoctahedra defining the & dimension in the structure (Iable 4). The H...O(2) distance, as -6.1 A (Hawthorne1990). These chains are cross- 1.90 A, suggestsa strong H-bond, in accord with the linked by VOa tetrahedrathat also link along the length low bond-valencesu'T around the O(2) anion when of each chain, forming a fairly open framework. The the H-bond is omitted from consideration.In this interstitial Pb2+cations occur in the intersticesof this frameworkGrg. 3) andplay a key role in satisfyingthe local bond-valence requirements of all anions in the structure. Hawtlorne (1990) gave details of TABLE 6. BOND.VALENCEITABLE FOR DESCLOIZITE isostructuralminerals and other MIQ' minerals with relatedstructures based on [MQa]chains.
o(1) o,28-t o.4Fl 1.19't 2,O4 0.192t Soltr) SolrmoN AroNc rrm MorrnaMrrs-DEsclomrE JorN 0.16 1.53 0.06*l* The question of solid solution befween Cu2* and o(3) 0,2741+ O,21 1.12 1.87 other divalent cations in inorganic crystals is very OH O.48e!- O.41 o.7'l ?.oa interesting.There are three distinct situations: (l) no 1.00 solid-solution,(2) completesolid-solution, or (3) com- plete solid-solution with a phase transition at some ' calculated from the curyes ol Brown & Altormatt ('l 986) intermediatecomposition. The structuralconstraints on TIIE CRYSTAL STRUCTUREOF MOTTRAMITE tt23 leb-{ T c I
Frc. 2. The structural unit in mottramite projected down [100]; CuQ6polyhedra are random-dot-shadd VOa polyhedraare cross-shaded.
F-c-{ T ta
Frc. 3. The structue of motramito projecteddown [010]; legendas in Figs. 1 and 2. ttu TIIE CANADIAN MINERALOGIST this type of solid solution have not beenexamined, in ACKNOWITDGEMENTS general. The fact that there is complete Cu2+;* Zn solid-solution across the mottramite-descloizitejoin We thalk Horvard T. Evans, Jr., and an anonymous (Van der Westhuizenet al. 1986), together with the reviewer for their comments on this manuscript. This availability of structural data on near-end-member work was supported by the Natural Sciences and compositions (Hawthorne & Faggiani 1979, this Engineering Research Council of Canada in the form study), gives us the opportunify to examinehow the of Operating, Infrastructure and Major Equipment descloizite structure-typeaccommodates the geo- gants to FCH. metrical requirementsof relaxationassociated with the degenerateor near-degenerateeg state in holo- symmetricallyor regularly coordinaied[6Jgu2+. RSFFRm{cEs In descloizite (Table 6), the ZnO6 octahedronhas four long bonds [bv (bond valence)= 0.27 vu) and two BRowN, I.D. & Arrunuerr, D. (1985): Bond-valence short bonds (bv = 0.48 vz), with the latter in a trans parametersobtained from a systematic analysis of the orientation.The four long bonds are to the O(l) and inorganic crystal structffe database.Aaa Crystallagr. O(3) anions.O(l) is [4]-coordinated,with an incident B41.24+247. bond-valencesum of 2.M vu, and O(3) is [4]-coor- dinated,with an incident bond-valencesum of 1.87 vu Bums, P.C. (1.99$: fhe Stereochemistryof Oi+ Orysalt Clable 6); note that in mottramite (Table 4), the same Minerals: an Ab Initio Molecular-Orbital Approaclu bond-valence sums occur, although the individual Ph.D. thesis,Univ. of Manitoba Winnipeg, Manitoba. incident bond-valencesare different. The two shon bonds are to OH, the oxygen anion of which is & HAwTHoRNE, F.C. (1995): Coordination- geomeffy structual pathways in [4]-coordinated (Cu2+,Pb,H).The O(2) anion is Cu2+oxysalt minerals. Can.Mineral. 33. 889-905. [4]-coordinated,but is not linked to the Zn cation: exsludingthe H bond, its incident bond-valencesum is Cnolcn, (1970): 1.79vu, and thus it needsa H-bond of average D.T. & Lmmuell, D. Relativistic calcula- strengtl tion (-0.21 vu) of anomalousscattering factors for X rays.J. Chent to completeits bond-valencerequirements. Phys.53, 1891-1898. If Zn is replaced by Cu2*, as in motramite, the strucfure has to change around the Cu2+ site to & MANN, J.B. (1968): accommodatethe geometrical X-ray scattering factors requirementsassociated computedfrom numerical l{arree-Fock wave functions. with the (pseudo-)Jahn-Teller instability of Cu2+in Acta Cry stalb gr, A2A, 32L-324. regular octahedral coordination. Octahedrally coor- dinated Cu2+ is usually coordinared by a 14 + Zl- EBy, R.A. & HAwuroRNB,F.C. (1993): Sfuctural relationsin axrangementof anions,whereas Zn in descloizitehas a copper oxysalt minerals. I. Structural hterarchy. Acta [2 + 4]-anangementof coordinatinganions, a configu- Crystallogr.M,9,28-56. ration that is energetically less favorable for Cu2+ & Hawthorne 1995). @urns The anion arrangement Hawnror.rw, F.C. (1990): Structunl in MI6lTt4l0n around Cu2+adjusts by shortening one pair of long minerals.Z. Kristalbgr. 192, 1-52. bonds [to the O(1) anion] and lengtheningthe other pair oflong bonds the O(3) anion], thus producing [to - & FAccrAM, (1979): a + 2 + 2]-coordination, R. Refinementofthe srructure [2 a reasonablycommc]n typs of descloizite.A cta Crystallogr. E.35,7l7 -720. of coordination of Cu2+ in oxysalt minerals. Comparisonof the bond-valencetables of descloizite UNcARETfl, and mottramite(Iables 4, 6) showshow this changeis L., Onrnrr, R., Borraza, P. & CZAMANSTB,G.K. (1993): accommodated.The Cu2+-O(3) Li: an important componentin bond in mottramite i gneousalkali amphiboles.Am Mineral, 7E,7 33 -7 45. is lengthenedrelative to that in descloizite,reducing the bond-valenceincident at O(3); this is compensated Rrcmroxo, W.E. (1940): phosphates, by a correspondingshortening of the V-O(3) bond in Crystal chemistryof the arsenates,and mottramite relative vanadatesof the type A,/Oa(Z). Am, to descloizite. In tum" the bond- Mineral.25.441479. valence sum around the V5+ cation in mothamite is maintainedclose to its ideal value by lengtheningthe VaN nrr. Wssruuran, V-O(2) bond, hence reducing the incident bond- W.A., DB Bm:rylr, H., Tonorrre, E.A.W. & Borse, B.J.V. (1986): The valenceat the O(2) anion; descloizite- this is compensatedby an mottramite seriesof vanadatesfrom the Otavi Mountain increase in the strengtl of H-bondingto tle O(2) anion Land, SouthWest Africa: an X-ray study.Mineral. Mag. in motramite Clable 4). Thus a [2 + 2 + 2]-distortion 50,t37-t40. occursmound the Cuz+cation in mottramite.and this is accommodatedby further cooperative bond-valence ReceivedFebruary 27, 1995, revised matwscrtpt accepted adjustmentsin the structure. Janc 17. 1995.