view file
Transcrição
view file
Canadian Mineralogist (1989) Yol.27, pp. 193-197 REFINEMENT OF THE CRYSTAL STRUCTURE OF LEUCOPHANITE JOEL D. GRICE NationalMuseumof Natural Sciences, Mineral Sciences Division, Ottowa,OntarioKIP 6P4 FRANK C. HAWTHORNE Depaftmentof GeologicalSciences,Universityof Manitoba, Winnipeg,ManitobaR3T 2N2 ABSTRAC"T INTRoDUC"IIoN The crystal structure of leucophanitg [CaNaBeSi2O6F, a 7.401Q), b 7.412Q), c 9.990Q) A, orthorhombic, P21212yZ = 41,was refined using 926 observed(>2.5o1) reflections collected with MoKa X rays and a crystal from Mont Sainl-Hilaire, Quebec;a similar refinement of a crystal from Stokkoy, Norway gavenearly identical results.The structure is similar to that previously determined, but required a shift in origin (+ Yt, *t/n, +3A) and an interchange of tle x and / coordinates. The revised structure refined to R : 3.0 and R* = 2.7V0 based on 926 observedreflections, as compared \irith R- 2070 for tlle samedata and the previous model. Leucophanite is a shell structure basedon the 2dimensional net (515f)that is also tire basis of the melilite-group minerals. The lower symmetry and different cation-orderingpatternsin lzucophanite as compared with the melilite-group minerals are mainly dictatedby the local bond-valencerequirementsof F, which is required by stoichiometry in the leucophanite structure. The crystal strucfure of leucophanitewas reported by Cannille et al. (1967).Theseauthors usedcrystals from Brevig, Norway to measureintensity data using multiple Weissenbergfilms and a microdensitometer.With 586observedreflections,they were ableto refine the structureto a reidual of R = 9.4Vo. We obtained leucophanite crystals from the Desourdy quarry, Mont Saint-Hilaire, Rouville County, Quebec(specimennumber NMNS 53455) and from Stokkoy, Vestfold County, Norway (NMNS 46861).The Saint-Hilaire leucophanitewas analyzed by Chao (1967) and determined to be Cao.esNao.x([email protected], almost pure end-member leucophanite, CaNaBeSi2O6F. The analytical data given by Brdgger (1890) for the Norwegian material also correspond to nearly endmember leucophanite.The structuresdeterminedon crystals from both localities confirm their chemical purity. Keywords: leucophanite, Mont Saint-Hilaire, structure, cation ordering, nets. E><pnnrNtsNIAL Sotrtvnrnn Nous avons affin6 la structure cristalline du leucopha4e [CaNaBeSi2O6F,a 1.4OlQ), b 7,412Q), c 9.990Q) A, orthorhombique, P21212pZ = 41,en utilisant un 6chantillon du mont St-Hilate et 926 r6flexions observ6es (I>2,5o1) avecrayonnementMoKa. Un affinement semblable, r6alis6avecun cristal de Stokkoy (Norvbge),a donn6 desrdsultats presqueidentiques. La structure, qui ressemble i celle qui avait 6td ddterminde, requiert un d6placement de I'origine (+ 1/4,+ rA, + /q) et une permutationdes axesr et /. L'affinernent a atteint un r€sidu R : 3.090 et Rw = 2.7t/o pour les 926 rdflexions observdes,compar6 i un r6sidu d'environ R- 20t/o aver,le m0mesdonn6eset le moddle structural pr6c6dent, Qs min&al poss0deune structrue en coqrrille calqueesur la maille bidimensionnelle (5j5i), qui est aussi celle des mindraux du groupe de la mdlilite. La sym6trie plus faible et les agencementsordonnes diffdrents parmi les cations du leucophane, en comparaison des membresdu groupe de la m6lilite, r6sultent des exigeanceslocales en valences des liaisons impliquant le fluor, dont la pr€senceest imposeepar la stoechiomdtrie. The crystal structureof leucophanitewas refined for crystals from both Mont Saint-Hilaire, Quebeg and Stokkoy, Norway; only the experiment for the Saint-Hilaire samplewill be describedin detail as thq techniquesand resultsare very similar for both stnrcture determinations. The Saint-Hilaire crystal chosenfor data sollection was ground to an ellipsoid that measures0.23 x 0.23 x 0.13 mm. X-ray-diffraction spots on precessionphotographsare sharp and intense.These photographs indicate orthorhombic symmetry and systematic extinctions consistent with the unique space-gxoupnQtzr (#19). Intensity data were collected on a Nicolet R3m automated four-circle diffractometer using the method of Grice & Ercit (1980. The data relevant to the structure refinement are given in Table 1. Srnucrunr RsrrNEtvrENr (Iraduit par la R6daction) Mots<lds: letcophane, mont St-Hilaire, structure, mise en ordre des cations, mailles. The initial starting parametersfor the atoms were those of Cannillo et al. (1967). Theseparameters 193 194 THE CANADIAN MINERALOGIST refined with isotropic temperature-factors to an R of 20.2s/obut would not refine further., It is interesting to note that evenat this R index, the atomic coordinates had high, but not unreasonable, standaxd deviationsof approximately t 0.001for the cations and t 0.@2 for anions. These standard deviations are in the order of those recorded by Cannillo et al. (1967); of course, theseatomic parametersgaverise to the samereasonablepolyhedra and bond lengths. The magnitudesof the temperature factors were not considered to be fully satisfactory, but most were believable. However, the residual index indicated a sdriouserror in the model. To find the correctstructure,.E-mapswerecalculated from a set of normalized structure-factors. With a set of starting coordinates for the heavier atoms, a seriesof refinementswith difference-Fourier maps was run, and the structural model completed. TABLEl. DATA LEUC0PHNI'II: STRUCIURE-REFINEMENI a ( E ): 7 . 4 0(' r2 ) b([): 7.a]2(2) cU): 9.990(2) The finat R valuesin Table I are for refinement with anisotropic temperature-fastors, and the weighting schemeincorporatedan isotropic, primary-extinction correction. The final positional parameters and equivalentisotropic temperature-faclorsare given in Table2, and the anisotropic temperature-factorsare given in Table 3. Bond lengths and anglesare glven in Table 4, and an empirical bond-valencesrunmation is shown in Table 5. The observedand calculated structure-factors have been submitted to the Depository of UnpublishedData, CISTI, National ResearchCouncil of Canada, Ottawa, CanadaKIA 0s2. Leucophanitefrom Stokkoy, Norway gavealmost identical results, with a slightly improved R of 2.6t/o' The very closesimilarity to the Saint-Hilaire material establishesthe $tructure reported here asthe correct one; the results for this second structure also have been depositedin CISTI. DISCUSSION Comparison of the atomic coordinatesfor the leucophanite structure as determinedby Cawillo et al. (L967) to those reported here may be made by tak958 No. of Fo: Radlation,/lilono: Mo/graphite No. of Fo>2.5d(t): 926 16.0m-' r: ing the coordinates in Table 2, interchanging x and 3.07, Final i: Mln. transmlsslon: 0.619 y and adding Vrto eachand adding % to e. The shift 2.7 lihx. tfansmlsslon: 0.685 Flnal aw: in origin involves one of 64 possibleorigins in space . a :(lFol-lFcl)/tlFol groupP2Qr21,but only.16of thesepossibilitieslead a y . t ' l { ( l F o l - l F c l ) ' z / r w l F o l 2 l t ,w < - 2 ( F o ) to changesin the phases.The interchange of the x and y coordinatesreflects the closeproximity of this structure to tetragonal symmetry. The structure is in fact closely related to the tetragonal structure of TABLE2. LEUCoPHNI1!: PoSITIoNALANDIlERl.lALPARAI4EIERS melilite, Ca2MgSipT. The lowering of symmetry from tetragonal to orthorhombic for the leuu(eq)xIotR2 Slte cophanite structure results from the ordering of Ca ee(2) 0.6462(l) 0.75es(r) ca 0.0788(1) and Na at two distinct sites, as opposed to the sin0.6782(2) 0.2465(2) 2 o o ( 5 ) l,la 0.1021(2) 0 . 3 5 9 2 ( 6 ) 0 . 5 3 0 7 ( 4 ) 28(r0) Be 0.1200(6) gle Ca site in melilite (Smith 1953). The occupancy ( 3 ) 76 sil o.rooo(r) 0 . 3 5 8 0 ( r ) 0 . 0 2 4 r ( r ) for thesetwo sites refined to 1.043(4)for Ca and si2 0.2371(2) 0 . 9 9 r 7 ( 2 ) 0 . 9 9 9 4)( 1 '|7812) 05(7) 0l 0 . 0 9 1 2 ( 4 ) 0.3548(4) 0.8645(2) 0.996(9) for Na, indicating cc rlete order. 02 0 . 1 4 8 3 ( 4 ) 0 r 1 5 8 0 ( 4 ) 0 . 0 8 9 5 ( 3 ) r 0 0 (7 ) 1 0 r( 7 ) 03 0.0986(4) 0 . 9 1 3 6 ( 4 ) o . 4 1 l (l 3 ) The most prominent feature of the leucophanite e8(7) 04 0.0800(4) 0.9129(4) o.9o3o(3) structure (Fig. 1) is the tetrahedral [BeSip6F] sheet 0 . r 7 6 9 ( 4 ) 0 . r 6 2 6 ( 4 ) 0 . 5 9 3 5 ( 3 ) 're 7 ( 7 ) 05 or(6) 06 0.2553(4) 0.5076(4) 0.5892(3) perpendicular to [001]. This strongly bonded struc0..1216(3) 0.3526(4) o.3729(2) 1 5 9 ( 7 ) F ture module is linked into a 3-dimensional structure by more weakly bonding alkali and alkaline earth cations, and thus is a sheet $tructure in the termi nology of Hawthorne (1985, 1986).If we consider (XIOgEI) IE.TPERAIURE-FACIORS TABTf3. LEU@PHAI{ITE : A}TISOTROPTC the sheet as a 2-dimensional net (e.9., Smith 1977' otz 0rr orr Hawthorne & Smith 1986), it is representedby the general symbol (51514); this signifies that there are 2r (3) 25(3) 6(3) 116(3) 8e(3) Ca and one 4-connectedververtices two 3-connected -64(8) 45(8) 52(7) 2 5 0 ( r 0 ) r7r(e) Na 180(9) 0(0) 4(r5) 4e(r8) 0(0) in the net, and that all the shortest circuits from Be 30(18) tex d t a l -2(4) 0(4) 56(4) 73(4) 9 l( 5 ) stl -J LCJ -r r (4) ai,i 69(4) e8(4) stz 68(4) i each vertex are Pentagonal. r1n4) 301) 1 7 ( r)1 0l 12e(12't e 6 0 r) 9 0 ( r)1 It is of interest to consider the substitution of e(lr) r3(10) 6 7 ( r) n0nl) I (12) 02 124(r2') 8 ( 1 2 ) -9(r'r) r09 0(1r) n(12\ n 6 (r r) 06 2) on the basic melilite+ype (515f) net. The cations ' r 1 ( 1 2 ) -8(r2) -290r) n 5 (r ? ) 65(r2) il4(n) 04 31( 10) t 0 ( ' l )r may be written as X2YZ2O7,where minerals 11502) 66n2) 1 r 0 ( r 2 ) -14(r2) melilite 20112) 2'I8 ( 1)1 - 6 ( 1 0 ) r 0 4 ( 1)r r o l( 1 2 ) 99(il ) - 3 ( r 2 ) X: Ca, Na(+Ba, St . . .), I= Mg, N, Zn, Be - 1 8 (1r) (9) 162(12) 1 9 1 0 2 ) 1 2 3 ( r) (+Co, Fe, Ni . . .), Z = Si,Al; Iications occupythe Ideal Fomula: CaNaBeSl:0oF P2r2t2t space Group: zi4 r95 THE CRYSTAL STRUCTI.JRE OF LEUCOPHANITE rArLE4. LEUCoPHAI{ITE: (R)ND ATOLES (o) SELECIEO nftnAloillc oISTNCES Ca squaE antl-DrlsB Ca-01 Ca-04 Ca-06 Ca-old Ct-02e Ca-04d &-05b Ca-F c @!n 2.401(4) 2.440(4\ 2.381(4) 2.349G\ 3.014(4) 2.650(4) 2.396(4) 2.48€G1 z:5I5* 0l -Cn-05b 05b-Ca-04 04 -Ca-F c F c-Ca-01 06 -Ca-04d o4d-C!-Otd 0ld-Ca-02e 02€-ca-06 Be tetrahedrcn ii,iiii fr# gi,xili *-H"on 1.642(6) 1.598(6) r.577(5) 1.670(5) I.&Z- 7 5 .(rr ) 5 5 . 6 ( l) s'l l EtrahedDn ila square anil_prl9 Ia-03 Na-F tla-010 Na-02d !ln-03e Ia-05e l{a-05d Na-F a @an 2.398(4) 2.729(41 2.569(4) 2.479(4) 2,921141 2.530(4) 2.3/.ll4) 2.461(4) 2]553- 05 -Be-06 F -6e-05 F -Be-06 04d-Be-05 04d-Be-06 04d-Be-F @an F -tta-ozd 02d-Ia-03 03 -l,la-Ole ole-tia-F 06d-Na-F 03e-tra-02d F a-Na-03 05a-Na-0te i I { i*n sfl-02 Sll-ola Sll-03€ sil-06d 1.659(3) r.596(3) r .6se(4) 'r .60r(3) T3Z6- st2-04 st2-02b Sl2-03s st2-05d man 1.618(3) 1.662(3) 1.658(3) t.612(3) TldB- t ) l 5a vertex, and Z cations occupy the 53 vertex; this type of vertex occupancy seemsto be the norm for incorporation of divalent cations into polymerized tetrahedral modules. Notably, gugiaite contains Be at the 5a vertex. This contrasts strongly with leucophanite, in which Be occupies a 53vertex and Si occupiesa 5avertex. Thus we have the questionof why leucophanite is orthorhombic (and not tetragonal), and also why the differeut ordering schemein the tetrahedral sheet is different. One notable aspectof leucophanite is that it has both Na and Ca in equal amoutrts as intramodule cations, and that theseare ordered in the structure. By itself, this cannot be the determinative factor for the orthorhombic symmetry, as synthetic .,sodamelilite", NaCaAlSi2O, (Louisnathan 1970), is tetragonalwith the melilite strucfure, and hasNa and Ca disordered at a single site. The occurrenceof Be in the sheetcannot be the primary reason gugiaite, Ca2BeSi2OT(Kimata & Ohashi l9g2), has the tetragonal melilite structure. Zachariasen(1930)suggestedthat the presenceof F in leucophanite causes the orthorhombic symmetry. This is probably the case,but doesnot addresstle reaons(s)for the different ordering schemesof the leucophanite and melilite sheets.Furthermore, it is uot clear why F could not disorder within a tetragonally symmetricalsheer. In a gugiaite-compositionmelilite-Iike sheet,F cannot oscupy any of the bridging anion positions in the sheetbecauseoflocal bond-valenceconstraints. Only a single nonbridging aniou position is left to accommodatethe F. This nonbridging anion (O2 in the nomenclatureof Smith 1953)is bonded to one Si and three Ca atoms; hence it also cannot be replaced by F, again becauseof local bond-valence restrictions. However, if the B€ occupies a 53 vertex, tlen the local environment of the O, anion (one 0la-sll-02 o3e-sll-02 03e-Sll-01. o6d-sir-02 06d-Sl l-0la 06d-sl1-03e man 112.9(21 105.0(2) 110.9(2) r04.6(2) .l16.2(2) 106.2(2, IM:t- 512 tetnhedmn 02b-s't2-04 107.8(2) 03s-sl2-04 lll.3(2) 0 3 q - s l 2 - 0 2 b1 0 5 . 3 ( 2 ) osa-srz-04 il2.0(2) osd-sl2-ozb 111.5(2) 05d-Sl2-03q 108.7(21 rean nlI?- Be and three Ca atoms) is ideal for the incorporation of F. Thus we seetJratthe determinative factor for the difference in the ordering scheme among tetrahedra in leucophaniteis the needto incorporate F into the sheetand to satisfy its local bond-valence requirements. The next problem is more difficult: why is leucophaniteorderedand orthorhombic rather tlan disordered and tetragonal? The 53 vertices are occupiedby th Be * /z Si, suggestingthe possibility of ordering and a lowering of symmetry. GehIenite, CarAl(AlSi)O7, has lzN + /zSi occupying the 53 vertices and yet is tetragonal (Louisnathan l97l). However, Louisnathan(1971)also remarked on the occurrenceof optically biaxial gehlenite, and the presenceof very fine-scaletwinning in somegehlenite, suggestingthat incipient ordering occurs on a very fine scale.In beryllosilicates,Be rarely if ever disorderswith Si. This being the case,the occurrence of Be at the 53 vertex in the melilite-like sheetwill require these two vertices to be symmetrically distinct, lowering the symmetry of the structure. Given the opportunity, the Ca and Na also order to produce TASLE5. BONIFYATEilCE SIJIiIiIATIOII' FORI"EUCOPMXITE [a 0t M 03 04 05 06 0.2n 0.326 0.085 3:i93 0.162 0.187 0.080 0.293 0.303 0.149 0.207 I '1.t39 B€ r Brom (1981). st(2) 1.072 0.908 0.910 0.90t 0.910 0.45s 0.490 0.54{r 1.057 1.950 3,947 o.re8 3:i3i o.4ar Otl St(l) I.0lt 1 ,027 3.849 1.827 2.056 2.087 1.905 1.959 2.115 0,867 r96 THE CANADIAN MINERALOGIST @Na OGa ["{1e" nsit ffisiz Frc. 1. A Z-axis projection of tle leucophanitestructure showing only the upper sheet of tetrahedra within the unit cell. The lower tetrahedra, consistingof the same topologrcal 2-dimensional net (SiSr4)is offset slightly in x and y coordinates. the final structure. However, the determinative factor for the existenceof leucophaniteis the local bondvalence requirements around the F that must be incorporated into the structure for overall electroneutrality. Examination of the bond-valencevalues for leucophanite (Table 5) showsthat the deviations from regularity in the various coordination polyhedra can be ascribed to local bond-valence requirements. In particular, the Si-O bond lengths to the Si-O-Si bridging anions are lengthenedto allow for the additional bonding to the intramodule Ca and Na. Note also how the relative ordering of Ca and Na has occurred such that Ca can form more and stronger bonds to those anions that are most underbonded in the tetrahedral sheet [the nonbridging O(l) and the Si-O-Be bridging O(4), O(5) and O(Q anionsl, whereas Na forms more than one bond to those anions that are most strongly bonded in the tetrahedral sheet[the Si-O-Si bridging O(3) anion]. The control of this local connectivity and geometryis the local bond-valence requirements, around both Ca and Na. Thus leucophanite can be thought of as a cation-ordered derivative of the melilite-type structure, the determining factor for the existence of which is dependent on the stoichiometric need to incorporate F into the structure. ACICNIOWLEDGEMENTS The authors thank Robert I. Gait of the Royal Ontario Museum, Toronto, and H. Gary Ansell of the Geological Survey of Canada, Ottawa, for providing crystalsof leucophanite.Financial support and *as p.ovided to F.C.H. by the Natural Sciences Engineering ResearchCouncil of Canada. RSFERENcES Bnoccrn, W.C. (1390):Die Mineraliender Syenitpegmatitgange der subnorwegischenAugit-,,und Nephelinsyenite. Leukophan. Z. Kristallogr, Mineral. 16, ?'46'n8. Browll, I.D. (l9Sl): The bond-valencemethod: an empiricalapproachto chemicalstructureand bond\ng. In Structure and Bonding in Crystals II (M. OtKeeffe& A. Nawotsky, eds.).AcademicPress, New York. Cam.u,r,o,E., Grusnrrerrt,G. & Ttzzott, V. (1967): The crystalstructureof leucophanite.Actq Crystallogr. 23,255-259. TI{E CRYSTAL STRUCTURE OF LEUCOPHANITE 197 Cueo, G.Y. (1967):Leucophanite,elpidite and narsarsukite from the Desourdyquarry, Mont St-Hilaire, Quebec.Can. Mineral, 9, 286-287, LoursNArsar,S.J. (1970):The crystalstructureof synthetic soda melilite, CaNaAlSi2O?.Z. Kristallogr. 1'3t,3t+321. Gr.rcr,J.D. & Encn, T.S. (1980:The srystalstructure of moydite. Can. Mineral. M,675-678. (1971):Refinementof the crystalstructureof a natural gehlenite,Ca2Al(Al,Si)p7. Can.Mineral. 10, 822-837. Hewruonre, F.C. (1985):Towardsa structuralclassification of minerals: tbe vIdvTz(Do minerals. Am. Mineral. 10, 455473. (1986): Structural hierarchy in vrM*rrrT"O, minerals. Can. Mineral. A, 625-642. Sr"rrnr,J.V. (1953):Reexaminationof the crystalstructure of melilite.Am. Mineral.3E, 643-661. (1977):Enumerationof 4-connected 3-dimensionalnetsand classificationof frameworksilicates. I. Perpendicularlinkagefrom simplehexagonalnet. Am. Mineral, 62, 703-709. & Srvrnn, J.V. (1986): Enumeration of 4-connected3-dimensionalnetsand classificationof framework silicates. 3-dimensionalnets basedon Zacuaruaser,W.H. (1930):On meliphaniteand leuinsertion of 2-sonnectedverticesinto 3-connected cophanite.Z. Kristallogr, 74, 226-229. planenets.Z, Kristallogr,175, 15-30. KrMArA,M. & Onesu, H. (1982):The crystalstructure Neues Jahrb. of synthetic gugiaite, Ca2BeSi2O7. Mineral., Abh. 143,210-222. ReceivedMay 3, 1988,revisedmanuscriptacceptedSe? tember30, 1988.