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Agrellite, Na(Ca,RE)Rsinorof: a Layer Structure with Silicate Tubes

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Agrellite, Na(Ca,RE)Rsinorof: a Layer Structure with Silicate Tubes American Mineralogist, Volume 64, pages 563-572, 1979 Agrellite, Na(Ca,RE)rSinOroF:a layer structurewith silicatetubes SusRerA,GHosu AND CHE'NG WAN Departmentof GeologicalSciences, Uniuersity of Washington Seattle, Washington98 I 95 Abstract Agrellite, Na(Ca,RE)rSirOr'F,from the regionallymetamorphosed agpaitic peralkaline rocksin villedieuTownship, Qu6bec, is triclinic,space group Pl, with celldimensions: a : 7.7s9(2),b:18.946(3),c:6.986(1)4,a:89.88(2),0=rt6.6s(2),y:94.32(2)";Z:4. The crystalstructure was determinedby the symbolicaddition method and refinedby the method of leastsquares to an .R factor of 0.045,based on 5343reflections measured on an automaticsingle-crystal diffractometer. The averagestandard deviations in Na-O, Ca-O, and Si-O bond lengthsare 0.004,0.003, and 0.003Arespectively. Due to the presenceof pseudo-C-center,the sodiumpolyhedra and the silicatetetrahedra occur in pairs, whoseconfigurations are nearlyidentical. The crystalstructure of agrellite consistsof two differentNaO, polyhedrawhich are distortedcubes, two eachof CaOuF octahedraand cao.F, polyhedra,and two different lsisoro] doublechains. These double chainsare hollow tubes,formed by the polymerizationof two vlasovite-type[SLo,r] single chains,consisting of corner-sharingfour-membered rings. The silicatetubes, whose diameter is definedby a basket-shapedsix-membered ring, run parallelto the c axisand are hexagonally close-packedin the(001) plane. The sodiumatoms occurring in cavitiescross-link these tubes to form sodium silicatelayers parallelto the (010) plane; theselayers alternatewith the calcium polyhedrallayers along the b axis to form a three-dimensionalframework. The averageSi-O bond lengthis l.6194; the non-bridgingSi-O bond lengths(av. 1.5794)are significantlyshorter than the bridging ones(av. 1.632A).The averageNa-o bond length withinthe NaO' polyhedrais2.667A. Withinthe CaOuF octahedra and CaOuF, polyhedra the averageca-o bond lengthsare 2.374 and 2.5954,and the averageCa-F bond lengthsare 2.197and 2.430A,respectively. Along with narsarsukite,fenaksite, litidionite, canasite, and miserite,agrellite belongs to a distinctclass of silicates,whose structures consist of silicate tubes. Introduction ZrSirOz).Gittins et al. (1976)have determined the Agrellite is a recentlydescribed rock-forming sili- chemicalcomposition and crystallographicand opti- cate with the chemicalcomposition: (Nan.o.Ko.or) cal propertiesof agrellite.For the purposesof the (Car.roREo ,r) (Mn, Fe. Sr, Ba, Mg, Sr)o.rn (Siru..rAlo.or) crystal-structuredetermination, the chemicalcompo- Orr.ro(F,,rOHo.r,).It is found in agpaiticperalka- sition of agrellitehas beensimplified to Na(Ca,.ro, line rocksof the Kipawa Complex,Villedieu Town- REo016)SLO,'F. The crystalstructure of agrellitehas beenbriefly reported by Ghoseand Wan (1978a). ship,T6miscaming County, Qu6bec, Canada (Gittins et al., 1976).It occursas prismaticwhite triclinic crystals(up to l0 crn long) in pegmatiticlenses Crystaldata and gneissescomposed principally of albite, mi- A smallcleavage fragment of agrellitewas ground crocline, aegirine-augite,alkali amphibole, with to a spherewith a diameterof 0.25mm (Bond, l95l), or without eudialyteand nepheline,and sometimes which was mounted on the computer-controlled hiortdahliteand other membersof the wiihlerite single-crystalX-ray diffractometer(Syntex Pl). The group, mosandrite,britholite, miserite,vlasovite, unit-cell dimensionswere determinedby least- calcite,fluorite, clinohumite, norbergite, zircon, biot- squaresrefinement of the 20 values(between 30 and ite, phlogopite,galena, and an unnamedmineral (Ca 40o) measuredfrom 15 reflections,using mono- 0003-004x/79/0506-0563$02.00 563 564 GHOSE AND II/AN: AGRELLITE chromaticMoKa radiation(Table l). The dimen- Table l. Agrellite: crystal data sionsof the triclinic unit cell are in good agreement with thosedetermined by Gittins et al. (1976),except Agrelli!e, Na(Cr,R.E. )2S14O1OF that we havechosen the anglea as acuterather than V1l1edieu Tomship, T6miscaming County, Qu6bec, Canada obtuse,to maintaina right-handedaxial system. An colorless transparenE prisms, N.M.N.H. # 134022 1(z) test of the measuredintensities indicated the Triclinic, I centricspace group P7, which was subsequentlycon- o(L), t.tsg(z) Space group: P 7 firmed by the crystal-structuredetermination. b(i.)' ra.s+e(:) cel1 volume 1,i3; r sr+.ztl) .@y, o.seolr) cel1 contenc: 4[Na(ca,R.E.)2si4o1oF] -? Collectionof the intensitydata o("): s9.88(2) Dt 2.902ecn' -1- The X-ray intensity data were collectedfrom the B("), rro.os(z) Da: 2.887 c cn single-crystalsphere on the SyntexPT diffractometer .((")t s4.34(2) p(Mo,(o): r-8.20 cm-l by the 0-20 method, using MoKa radiation mono- chromatizedby reflectionfrom a graphite "single" crystaland a scintillationcounter. The variable-scan The combinationof 5 symbolsyielded 32 solutions. method was used,the minimum scanrate bein9 2o/ Four E-maps appearedequally probable,out of min (50 kV, l2 mA). All reflectionswithin 20 : 60" whichone showed a geometricallybetter view of the were measured,a total of 5343reflections, out of structure.From this E-map,positions of 4 Ca, 7 Si, I which 1436were below 3o(1),where o(1) is the stan- F, and 14 O atoms were determined.Two cyclesof dard deviationof the intensity1, asdetermined from structure-factorcalculations followed by difference the counting statistics.For all reflectionsless than Fourier synthesesyielded the positions of all the 0.7o(I),the intensitywas set to 0.7o(1),regardless of missingNa, Si, O, and F atoms.A few cyclesof whetherthe measuredintensity was positive or nega- refinementusing the isotropic temperaturefactors tive. The measuredintensities were corrected for reducedthe R factor to 0.086from an initial valueof Lorentz, polarization,and absorptionfactors. 0.40. The isotropictemperature factor for Ca(lA) turned out to be negative,indicating partial occu- Determinationand refinementof the structure pancy of this site by the rare-earthatoms. A differ- The crystal structurehas been determinedby the enceFourier synthesis also indicated positive electron symbolicaddition method (Karle and Karle, 1966), densitiesat the Ca sites.Hence, a site-occupancy using the computer program MurrlN (Germainel refinementof the calciumand rare-earthatoms was al., l97l). 351 reflectionswith E values)1.8 were carried out, using the full-matrix least-squarespro- usedfor the phasedetermination. The phasesof the gram RnrNr (Finger, 1969).The scatteringfactor for following three reflectionswere chosen to definethe the rare-earthatoms was approximated by averaging origin: the scatteringfactors of the rare-earthatoms in the chemicafanalysis reported by Gittins et ql. (1976). hk I E Phaseangle The site-occupancyrefinement indicated most of the l -13 0 2.98 00 rare-earthatoms to be concentratedin the Ca(lA) 4 -19 2 2.88 00 site;the temperaturefactor for Ca(lA) becameposi- 40 -5 2.74 00 tive. The R factor at this stagewas 0.076. In addition, the phaseof the reflection408 (E : Sincethe number of variableparameters was too 2.58)was found to be 0o, usingthe ), relationship. large,the final least-squaresrefinement of the struc- The phasesof the following five reflectionswere rep- ture using anisotropictemperature factors was car- resentedby symbols: riedout by theprogram Cnvr-se (Stewart et al.,1972) in blocks,one block being assigned to eachatom. The Final atomic scatteringfactors for Na, Ca, RE, Si, O, and hkl E phaseangle F weretaken from Cromer and Mann (1968)and 2 0-4 4.01 00 corrected for anomalous dispersion(Cromer and 4 t2-4 3.21 1800 Liberman, 1970). The observed structure factors 890 2.55 00 (.Fo's)were weighted by the formula l/o2(Fo), where 6-60 2.37 1800 o(Fo) is the standarddeviation of the measurement 7 4-4 2.34 00 of .Fo, as determinedfrom the counting statistics. GHOSE AND llAN: AGRELLITE 565 Table 2. Agrellite:positional and thermalparameters and siteoccupancies (with standarddeviations in parentheses) u22 *33 '12 *13 u^^ Atom ,LL ZJ ca(1A) 0.99796(r4) 0.215s4( s) 0.99620(15) 1,16(3) 16s( 5) e3( 4) 90( 3) -o( 4) 72( 3) 2s< 3) Ca(1B) 0.54L76(L2) 0.2846L( 4) 0.01966(13) 0.92(3) r42( 4) 76( 4) 83( 4) 3( 3) 66( 3) 18( 3) ca (2A) 0.4552r(r2) 0.72007( 4) 0.48010(13) 0.94(3) 133( 4) 85( 4) 8e( 4) 1( 3) 71( 3) 2r( 3) ca (28) 0.0015s(13) 0.78166(5) 0.s0022(14) L.27(3) re4( 4) r0r( 4) L24( 4) 14( 3) 108( 3) 27( 3) Na(A) 0,23539(28) 0.99102(1r) 0.86794 (34) 2.49(7) rsl( 9) 287(L2) 286(11) s( 8) 80( 8) 2e( e) Na (B) 0.26L04(29) 0.50234(r2) 0.13576(36) 2.84(8) 139( 9) 370(13) 33r(12) -3( e) 74( e) 40(10) s1(1A) 0.20870(16) 0.93103(6) 0.35569(17) 0.81(3) 106( s) 6e(s) 76( s) 8( 4) 51( 4) 27( 4) si(18) 0.30739(16) o.s6807(6) 0.6ss42(18) 0.79(3) 10s( 5) 70(5) 83( 5) L2( 4) 54( 4) 28( 4) si(2A) 0.48573(16) 0.877e5(6) o.2t317(r7) 0.80(4) 108(s) 82( s) 63( s) -1( 4) s6( 4) 20( 4) si(2B) o.02460(16) 0.61933(6) 0.79391(r7) o.75(4) 93( s) 78(s) 74( 5) -o( 4) 46( 4) 18( 4) si(3A) 0.16705(16) 0.08998( 6) 0.33394(17) 0.78(3) 92( s) 77(s) 14( 5) s( 4) 48( 4) 23( 4) si(38) o.67404(76) o.s9o22(6) 0.34109(17) 0.7s(3) 87(5) 70(s) 8s( s) 6( 4) 45( 4) 2r( 4) si(4A) 0.48619(16) 0.8777r( 6) 0.71341(r8) 0.80(4) 107( s) 7s( 5) 68( 5) -4< 4) ss( 4) 2r( 4) si(4B) 0.02139(16) 0.61974( 6) 0.23430(18) 0,72(4) 94( 5) 81(s) 67( 5) 2( 4) s0( 4) 23( 4) F(A) 0.7607(4) 0.76r0(2) 0.1268(s) 2.7(L) 170(14) 243(r7) 423(L9) -2(L2) 117(13) 82(14) F(B) 0.2353(4) 0.24sL(2) 0.3s9s(s) 2.5(r) 180(14) 223(l-6) 356(r7 ) -3 (r2) 69(13) 3(r3) o(1A) 0.3493(4) 0.
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