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Download the Scanned American Mineralogist, Volume 71, pages 1378-1383, 1986 Crystal structure of lisetite, CaNarAloSioO,u Grusnppr Rossr, Ronnnu. Osnnrr C.N.R., Centro di Studio per la Cristallografia Strutturale, ck Dipartimento di Scienze della Terra, Via A. Bassi 4, 27 100 Pavia, ltaly D.q,laoC. Snnrrrr Laboratoire de Min6ralogie, CNRS Unit6 Associ6 no. 286, Mus6um National d'Histoire Naturelle, 6l rue Buffon, 75005 Paris, France Ansrntcr The crystal structureof lisetite CaNarAloSioOt6t a : 8.260, b : 17.086,c : 9.654 A, spacegroup Pbc2,,Z: 4, has been determined through direct methods using 1445 inten- sities measured with an automated X-ray diffractometer. The anisotropic least-squares refinement led to a final discrepancyindex R : 0.026 for the 1450 observed reflections. Lisetite is a tectosilicatewith a framework basedupon the connection of four-membered rings of tetrahedrawhose apical oxygenspoint alternatelyup (U) and down (D) with respect to the (100) plane (i.e., UDUD rings). The ordered distribution of Al and Si among the tetrahedra results in the lack of the symmetry center. Ca and Na are surrounded by seven and six oxygen atoms, respectively,aI averagedistances of 2.53 and 2.48 A and occur in cavities sandwichedbetween layers of four- and eight-memberedrings of tetrahedra.The structure of lisetite is related to that of feldspars,which is also based on four-membered rings of tetrahedra; however, the rings in feldspars are of the UUDD type, and lisetite contains 1.5 more extra-framework cations. Whereaslisetite sharesthe sameAl-Si frame- work with banalsite BaNarAloSioO,u,the latter has a diferent arrangement of the large cations.Other combinations of extra-frameworkcations should be possiblein a tectosilicate family basedupon UDUD rings of tetrahedra. fN.l:nolucltoN (Table l) were determined by least-squaresrefinement The new Ca-Na-Al-Si-O mineral lisetite was described using single-crystalX-ray data obtained from a Philips very briefly in Smith (1984) and Rossi et al. (1985)and PW I 100 automatic diffractometer. Details of the X-ray in some detail in Smith et al. (1986). Lisetite occurs in data collection are given in Table l. The intensities were the retrogrademetamorphosed Na- and Al-rich, K- and corrected for absorption following the semiempirical Mg-poor Liset eclogite pod in Norway, and it is most method of North et al. (1968); no extinction correction probably stable in the highest-pressure part of the was applied. greenschist,amphibolite, and granulite facies. Electron- The spacegtoup determined from systematicabsences microprobe analyses show that it has almost the stoi- is Pbc2,. The nonstandard spacegroup was preferred in chiometric composition CaNarAloSioO,u,i.e., chemically order to take into account the marked Pbcn (standard midway between the composition of anorthite and the setting)pseudosymmetry exhibited by the mineral (Smith pure Na equivalent of nepheline,but it is neither a feldspar et al., 1986).It will be shown that the lack ofthe symmetry nor a feldspathoid. center results from the ordered distribution ofSi and Al The aims of this paper are to describe the srructure among the tetrahedra of the silicate framework. determination and refinementby single-crystalX-ray dif- The crystal structurewas determinedby meansof direct fraction carried out on the co-type crystal G20lb7-LT2 methods (program MULrAN78,Main et al., 1978). The and to compare the lisetite structurewith that of feldspars least-squaresisotropic refinement convergedat an R fac- and the rare minerals banalsite BaNarAloSioO,u(Haga, tor of 0.05. The site occupancyrefinement of the nonte- 1973) and stronalsite SrNarAloSioO,u(A. Kato, pers. trahedral cationic sites was carried out without chemical comm.). constraints and by assumingthat they were occupied by Ca or Na and/or werevacant. The final discrepancyfactor, DnrnrurrNl.TroN AND REFINEMENT oF THE after the anisotropic refinement, was 0.026 for the 1445 CRYSTAL STRUCTURE reflectionswith 1 > 3o(1)and 0.050 for all the measured The co-typecrystal, a small fragment oflisetite detached reflections. from an uncovered petrographic thin section, was used The troubles that arosein the simultaneousrefinement for the crystal-structureanalysis. The lattice parameters of parametersbelonging to pairs of atoms relatedby pseu- 0003{04xl86/I I I 2-t 378$02.00 I 378 ROSSI ET AL.: CRYSTAL STRUCTURE OF LISETITE t379 Table1. X-raycrystal and refinement data Table 2. Site occupancies,atomic coordinates ( x 104)and equivalent isotropic temperature factors a(4) 8.260(1) b(s) 17 086(1) xla ylb B4 c (A) 9.654(1) 2500(0) 0 66(2) Unit-cellvolume (43) 1362.5 Ca Ca 0.92 Na 0.08 42(21 4115(1) NaA Na 0.94 Ca 0.06 5352(2) 3461(1) 4147(2)1.73(5) Spacegroup Pbc2, NaB Na 0.95 4671(2) 3508(1) 811(2) 1.27(s) Crystaldimensions (mm) 0.26x0.16x0.07 T1A Ar 0.84 Si 0.16 6688(1) 907(1) 604(1) 0.43(2) Radiation MoKc T2A Ar 0 11 Si 0.89 8174(1) 760) 3074(1) 0.53(2) Scan mode @ T3A Al 0.88 Si 0 12 6969(1) 1525(1) 4557(1)0.s2(2) Scan width (") 2.0 T4A Ar 0.10 si 0.90 8040(1) 2432(1) 2063(1) 0.48(2) d range (') 2-30 T1B Ar 0.08 si 0.92 3263(1) 866(1) 4416(1) 0.57(2) 4441 T2B At 0.91 Si 0.09 1722(1) 146(1) 1905(1)0.54(2) Measured reflections 0.58(2) Independentreflections 2096 T3B Al 0.12 si 0.88 2994(1) 1s54(1) 306(1) 2815(1)0.s7(2) Observedretlections. 1445 T4B Ar 0 92 Si 0.08 1915(2) 2365(1) 01A 7908(4) 1672(2) 1091(3)1.04(7) F (sym)"' 0.036 02A 6807(4) 180(2) 1864(3)0.63(6) B (obs) 0.026 o3A 4727(41 1126(2) 121(3) 0.86(6) (all) F 0.050 04A 7438(4) 645(2) 1032(3) 0.73(6) 0.89(6) 'Only the reflectionswith / > 3o(/)were considered o5A 8402(4) 852(2) 3997(3) 6223(3) 0.89(6) as observed. o6A 7624(4) 1805(2) 6934(4) 2326(2) 3434(3) 1 15(7) -- P (sym) : l-)/) l, wnere t : (tn^,+ t^F,y2 o7A ) {/n, 01B 20s2(4) 1564(2) 3878(3) 0.75(6) o2B 3178(4) 179(2) 3259(3) 0.88(6) 03B 5088(4) 1108(2) 4791(411.23(7) 04B 2583(4) 615(2) 5921(3) 0.88(6) 05B 1611(4) 980(2) 8es(3) 0.87(6) dosymmetry elements (high conelations, physically im- 06B 2441(4) 17s7(2) -1257(3) 0.70(6) possible thermal parameters,departures from the pseu- 07B 3125(4) 230s(2) 1347(3)0.68(6) o8 115(7)-221(1) 2457(8) 0.92(6) dosymmetryless than the e.s.d.,etc.; seeDollase, 1970, o9 103(7) 2641(1) 2464(8) 0.88(6) for nepheline),were overcomeby refining alternately only Note. Atom pairs related by the pseudosymmetryhave the same label way, refinement ''A" one half of the atoms at once. In this the plusan extra and "B," respectively(e.9.' T1A and T1B). Numbersin reachedconvergence easily, and in the final set ofatomic oarenthesesare e.s.d.'s referredto the last digit. parameters(Tables 2 and 3), refined simultaneouslyin the last cycle, only two coordinatesshow departuresfrom the PDcnpseudosymmetry that are less than five times their e.s.d.'s. Half-ionized scattering factors for Al, Si, and O, and fully ionized scatteringfactors for Ca and Na, were used Table3. Anisotropicthermal parameters in the refinement (International Tablesfor X-ray Crys- Atom 4"" 4"" P23 tallogtaphy, 1974).The observedand calculatedstructure 9,, factors are compared in Table 4.' ca 23(1) 5(1) 20(1) 1(1) 4(1) 1(1) NaA 39(3) 17(1) 58(3) 4(1) -1(2) -2(1) NaB 30(3) 13(1) 40(2) -2(1) -12(2) 10(1) DnscnrprroN AND DIScussIoN oF THE srRUcruRE T1A 14(21 s(1) 9(1) 0(1) -1(1) 1(1) The tectosilicateframework r2A. 20(1) 5(1) 13(1) 1(1) 1(1) 0(1) r3A 12(1) 5(1) 16(1) 0(1) 3(1) 0(1) The basic structural unit of lisetite is a four-membered r4A 16(1) 4(1) 13(1) 1(1) 30) 1(1) 16(1) 1(1) 0(1) 0(1) ring of tetrahedra lying in the (100) plane (Fig. l). The T1B 22(1\ 4(1) r2B 14(1) 5(1) 18(1) 0(1) 3(1) 0(1) apical oxygens of the tetrahedra point upward (U) and r3B 22(1) 6(1) 12(1) 0(1) 3(1) 2(1) r4B 24(2) 4(1 1s(1) 0(1) -7(1) 2(1) downward (D) alternately with respectto the plane of the ) -4(1) o1A 56(5) 6(1) 25(3) 1(2) 2(3) ring, following the schemeUDUD, which is one of the o2A 28(4) 6(1) 13(3) 4(2) -4(3) 0(1) four possibleways to build up a four-membered ring (Smith o3A 23(4) 11(1) 17(3) 0(2) -3(3) 3(1) -3(2) and Rinaldi, 1962). The connection of the rings along o4A 23(4) 9(1) 13(3) 4(3) 3(1) o5A 28(4) 8(1) 26(3) 0(2) 8(3) 1(1) [00] through the apical oxygensproduces chains oftet- 06A 29(41 e(1) 22(3) -2(2) 0(3) 3(1) rahedra (Fig. 1) similar to those that have been found 07A 56(4) 7(1) 30(3) 2(2) 10(3) 1(1) 17(3) 1(21 4(3) 1(1) isolatedin the mineral narsarsukiteNao(TiO),SirO,o (Pya- olB 32(41 6(1) o2B 29(4\ 8(1) 24(3) 0(2) 0(3) 1(1) tenko and Pudovkina, 1960).The lateral linking ofthese o3B 34(4) 11(1) 41(4) 0(2) 6(3) -8(2) chains gives rise to a three-dimensional framework of o4B 43(4) 7(11 17(3) 0(2) 6(3) 1(1) osB 34(4) 8(1 21(3) 3(2) -1(3) 0(1) ) -1(3) tetrahedra.Thus lisetite is a tectosilicateand exhibits the 068 20(4) 7(1) 20(3) 2(2) 2(1) same framework type as that found in banalsite Ba- o7B 26(4) 5(1) 20(3) -2(2) 5(3) -2(1) 08 2s(4) 7(1) 32(3) -2(2) 13(3) 0(3) -8(3) -1(3) -' 09 20(41 7(1) 35(3) 0(2) To obt in a copyof rable 4, order DocumentAM-86-316 Notei The anisotropicthermal parametersare defined by fromthe Business Ofrce, Mineralogical Society ofAmerica, 1625 exp - (h20! + k2F22+ l21s + 2hk?p + 2hl7,s + 2kl1a).
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