Canadian Mineralogist Yol.29, pp. 385-390(1991)

THECRYSTAL STRUCTURE AND THERMALEXPANSION OF TUGTUPITE,Nar[AlrBerSirOro]Cl,

ISHMAEL HASSAN* ANDH. DOUGLAS GRUNDY Deportmentof Geologt,McMoster University, Hamilton, Ontario L8S4MI

ABSTRACT INrnooucrtoN

The crystal structure of tugtupite, ideally Tugtupite is tetragonaland has the idealizedfor- Nas[Al2Be2siso2.p,]C12, has been refined in spacegroup .I4 mula Nas(Al2BqSlO2)Cl2 (Dand 1966, Sdensenel to an R index of 0.023for 621 observedreflections meas- ol. l97l). The structure of tugtupite is isotypic with ured on an automated single-crystal four-circle X-ray that of (cubic), as is that of mineralsof the diffractometer using MoKo radiation. The framework 7 helvite group. The tugtupite framework may be (T representsAl3*, Bd+, and Sia+) are fully cations regardedas intermediatein composition betweenthe this order lowers the cubic symmetry of many ordered, and (AloSkOdt hel- sodalite-group minerals to tetragonal symmetry for tugtu- framework of sodalite and that of of tug- pite, a sodalite-groupmi^neral. The "sodalite" cagein tug- vite (Be6Si6O2/12-.The tetragonal symmetry tupite contains[Na0.611r" clusters. Large Si-O-Be angles tupite is the result of Z-cation ordering (Zrepresents occur in this structure; smaller Si-O-Be angles occur in Al3+, Bd+, and Sia+). helvite-group minerals, because the cages. contain Dafi (L966)determined the spacegroup /? for + [(Mn,Fe,Zn)a.S]6 clusters,whose large effective charge tugtupite by utilizing precession and Weissenberg accountsfor the smaller angles.The thermal expansionfor photographs, and the structure w€rsrefined with film tugtupite is modeled using the DtrS program; the expan- data and the isotypic relationship with the sodalite rotatrons of the 704 tetra- sion is controlled mainly by stnrcture. Interest in tugtupite arisesprimarily from hedra, which are mainly causedby the expansion of Na- result from a fra- Cl bonds and nearly constant Na-O bonds. the detailed structural effects that mework that consistsof three different T cations. Keywords: tuglupite, crystal structure, thermal expansion, In this study, we set out to refine the structure of sodalitegroup, helvite group. tugtupite to obtain better structural parametersfor comparison with structural data for sodalite- and SoMMAIRE helvite-group minerals (Hassan & Grundy 1983' 1984,1985, 1989, 1991, Hassan el a/. 1985).This Nous avons affin6 la structure cristalline de la tuglupite, comDarisonleads to an evaluationof the structural dont la formule id6ale est Na6[Al2Be2SisO2a]C12,dans le effects that arise from different 7 catidns, interstitial groupe .I4 jusqu'i un residu R de 0.023 en utilisant spatral cation (Na+, Mn2+, Fd+, Zrf+), and anions (cl-, 621 r€flexions observ€es(diffractombtre automatisd,rayon- * * OH-, H2O). An opportunity also ari$esto test nement Mo,lfa). Les cations 7 de la trame (Al' , Bez et S?, Sia+) sont complbtementordonn€s. C'est ce degrdd'ordre the applicability of the d-p zrbonding model to the qui r€duit la symdtrie cubique de plusieurs membres de la sodalite-groupminerals (e.9., Cruickshank 1961, famille de la sodalite comme la tugtupite d une symdtrie Brown et al. 1969,Gibbs er ol. 1972).The thermal t6tragonale. La cagetypique de la sodalite contient, dans expansionof tugtupite also is rationalizedin terms la tugtupite, un groupement[Nan.gq3+. Les anglesSi- of its crystal structure, and the mechanismof expan- O-Be sont grands dansla tugtupite; ils sont plus petits dans sion is comparedto that in sodalite-groupminerals les mindrauxdu groupe de la helvite parceque le groupe- (Hassan& Grundy 1984). ment [(Mn,Fe,Zn)0.516+de la cagepossbde une charge effectiveplus 6levee.L'expansion thermiquepeut Otrerepro- ExPsnttvlrNTal- duite par le logiciel DlS. L'expansion serait due surtout rotation des tdtrabdres IOa, d€pendlargement de h la eui in inves- l'expansion desliarsons Na-Cl, la longueur desliaisons Na- The specimenof red tugtupite used this O derneurant d peu prbs constante. tigation is from Ilimaussaq, Narssaq Kommune, South Greenland(Royal Ontario Museum #M3nn). (Traduit par la R6daction) The chemicalcomposition (Table l) is taken from Dand (196Obecause both samplesare from the same Mots-clds: tugtupite, structure cristalline, expansion ther- locality; we assumethat the composition of Dand mique, famille de la sodalite, famille de la helvite. (1966) is repre$entative of our sample. Cell parameters,determined by least-squaresrefinement *Present address: lnstitute for Materials Research, of fifteen high-angle reflections automatically cen- McMaster University, Hamilton, Ontario L8S 4Ml. tered on an automated four-circle single-crystal X- 385 386 THE CANADIAN MINERALOGIST ray diffractometer, are presentedin Table l, together Reflectionsallowable in spacegroup /4 (i,e., h -r with other information pertaining to X-ray data col- k + I = 2n) were collected from two octanls of lection and refinement. reciprocal spaceto a maximum 20 of 65o. A total The intensity data were collected from a cleavage of l4l3 intensilieswere measlued to give a data set fragment mounted on a Nicolet P3 diffractometer. of 647unique reflections, of which 621were classed as observed(Table l). The data were correctedfor Lorentz, polarization, background effects, and TABLE1 . oHEMICAL@MposmoHl . cRysrlt oatn2. lt.to sphericalabsorption (Table 1). All crystallographic INFOFMA'IIONON DATACOIIECTION FOR TlJGTUPITE calculationswere made using the XRAY76 Crystal-

Oddo Un % Cofl @rd6rnsr Mb€lall@ lographic Programs (Stewart 1976).

Ar2% 11.15 Al 2.03 a(& &640(1) STRUCTURE RSTTNBN4SNT 902 51.58 Sl 7.s c(A) 8,873(1) B4 5.40 Bs 2.@ v 63) Initially, the positional parametersand isotropic N%o 6.52 Na 7.e DerEfry€lc. Gmj Y K2O o.12 K 0.02 Cryelsbs (m) o.iu,o.,o temperature-factorsof Dand were used, including MgO O.m Mg 0.05 xo26 those for (fully ordered)Al, Be, and Si atorns,and g'7.ao 1.$ fr(m-]) e.5o atomic scattering factors for neutral atoms were s _0.s? s 0.@ gR o^ taken from Cromer & Mann (1968).A full-matrix 101.58 lvladnm 29 650 least-squaresrefinement was made by varying the o-ct,s l,9g 0< h,k,t3 13 atomic positions, isotropic temperature-factors,and Total .g!9 Tolalm. ot inemltlss No.of@huci*doE A7

Chqnlel Fomula No. otnonsq!tu lFol>3d lFl el rABtE 4. rN'rERAToMrcDrsrANcEs (A), ANGLES AND u8€d h retr|mnl Fhot'R 0.023 e ), VALENCESUMS (v.u.) FOF IUGTUPm N%t&Bs23lso2ilCt2 FbEl 4{ 0.030

'Ohemlcalarulysb trom Da|p (lS). AlO4Tstrah€dron B€O4 TstEh€dron lSpae qrcuprA; Z - 1; Radmon/Mshrctrdtr - Mo/C; Mot{r - 0.71069A Af-o3 4\ 1.748'(21 Bo-O2 4x 1.601(2) -:11rol -lr"l)p Fol;'\ - Ew(lFol- tFcl)2/:wlFol2l%,w - r. o3o3 4x 2.845(3) Q2-O2 4x 2.633(3) ln-Bas€d on Al + Bo + Sl = 120 2x 2a72p't 2x 2f,29) Mean L& Mean ?,6 os.Al€34x 109.0(1) O2-B+O2 4x 107.7(1) 2x 110-5(11 2x 113.2(1') TABLE2. POSMONALOCCIJPANCY, AND ISOTROPTC IHERMAL Moan 1495 Msan l_095 pnnauEreRs62xro1 SlO4Tofah€dron !,laCoordlrdlon Aom Sne Oe. x ubo sf€1 1.u4(4 Na€l 2.707(11 €1* 1.U7(2) 2.370(3) A 2(d) 1.0 o 1/2 3/4 76(41 -o2 1.581(2) -o2 2.3orl(3) Bo 2(cl 1.0 o 1/2 114 98(14) €3 1.6@(2) -o2. 2.603(3) sl s(s) r.0 0.0127(1, 0.2533(1) 0.4958(r) 74(1', Moan l.620 o3 2.306(3) 01 8(s) 1.0 0.1504(3) 0.1341t(2) 0.u17(2) 1A@'l o1€1r 2.dt2(31 ol-Nao 115.2(1) '1.0 02 8(sl 0.un(21 0.0385(3) 0.6488(2) 119(4) a2 2.587(31 a2 110.6(1) @ 8(s) 1.0 o.4a(4 0.1486(2) 0.132(3) 120(4) €s 2.640(3) -o3 98.0(1) Na 8(S) 1.0 0.15dt(2) 0..t972(2' 0.1818(2) 188(3) o1.€2 2.637(3) o2-Na-ct 12,-8(11 O 2(a) 1.000 o 234(3) 03 2.601(3) -o3 se.5(1) o2-o3 2.723F) olNa-cl 106.1(1) Mean zW BdiglngAngles ol€t€1r 108.6(1) sr€l€r 140.8(2) T BLEs. ANrsorRoptcTEMpERAluRFFAcronsl6 ro! 42 106.7(1) SlO2-Be 143.6(1) 03 108.5(1) sr€$Ar 135.30) Alom U1.t u2 ugg utz u.t3 ua o1.€to2 109.5(1) T-Tdlslanc6 o3 106.0(1) sl-Ar 3.100(1) AI 73(5) 7S 74(81 0 00 o2€l€3 117.2(21 sr+r 3.105(11 BS 86(18) 86 ra(34) 0 00 Msan l_09,4 slBs 3.051(1) sl 71(31 63(3) 86(3) -2(21 -2(3) "3(3) ol 126(e) 116(e) 128{e) 8n 10(8) 1c4 Bond-t,algncssums (v.u.) Al=4x0.7@ =3.076 02 107(8) 119(8) 132(e) -3(6) -35(8) 4(8) Bg=4x0.5@ =2.035 @ 112(81 114(8) 135(e) -1(6) 6f4 38(s) Sl - 0.947+0.940+1.18+1.041 = 4.051 Na zffjrI 160(61 1$(6) 7(5) 24(5) 12(5) tla = 0.25+0.216+0.269f0.115$0.201= 1.041 2n(41 244Q1 0 00 Ol = 0.947+0.91O10.216 .2.l(rll 02 - 1.123{0.509+0.259+0.115 = 2.@6 03 = '|.041+0.769+0.20 1 tUU- up1-zr2i.lr.,tl+upf +Uof +zu.,ltk+2uftht+2uztlll THE CRYSTAL STRUCTURE OF TUGTUPITE 387 , l-or' c

Frc. l. Stereoscopicprojection of the framework of tugtupite showingthe "sodalite" cageand the ordering of the Tcations. The Si and oxygenatoms are unlabeled. a scalefactor. The refinementof the structural model r-Cl "- was constrainedto the idealizedchemical formula I and resultedin an R index of 3.090 (Table 1). The C isotropic temperature-factorswere converted to anisotropic forms, and further cyclesof refinement, with variations in all the refinable parameters, resultedin convergenceto a final R index of 2.3t/0. The occupancyand temperaturefactors for both Na and Cl atoms were refined, and occupancyvalues of 1.00(1)were obtained for both atoms, which agree with the ideal chemical formula and are not signifi- cantly different from the chemical data of Dand (1966).A final difference-Fouriermap was found to be featureless.The interatomic distancesand isotropic temperature-factorsfor the atoms in the structure show nothing unusual, and the refinement was consideredcomplete. The atomic positions and isotropic temperature- factors are given in Table 2, anisotropictemperature- factors are listed in Table 3, and interatomic dis- o1 tancesand anglesare presentedin Table 4. A copy r of the structure factors is available at a nominal o2 charge from Depository of Unpublished Data, CISTI, National ResearchCouncil, Ottawa, Ontario KIA OS2.

DIscussloN

The tugtupite structurc

The cell parametersobtained for the red tugtupite qsedin this investigation[a = 8.640(l), c = 8.873(l) Al ditfer from thoseoltained by Dand (1966)[a = 8.538(4),c : 3.817(4)Al, wlrich arewell outsidethe pnges in a (8.634- 8.643A; and c (8.867- 8.870 A) given by Sdrensenet al. (1971).Results of the Frc. 2. Na atom coordinations: (a) five-fold coordination presentrefinement land the data of Sdrensenel a/. in tugtupite; (b) four-fold coordination in sodalite. 388 THE CANADIAN MINERALOGIST

(1971)ldo not indicatethat the sampleused in this six-memberedrings have an ordered:urangement of study has a significantly different composition from Z atoms consistingof one Al, one Be, and four Si the sample studied by Darrd (1966), despite the atoms; the Be and Al atoms are diametrically oppo- apparent differences in cell parameters. In fact, the site eachother in the rings (Fig. l). The cagescon- atomic positions axenot significantly different from tain Na and Cl atoms; the Cl atoms are at the corners those of Dand (1966);however, significant differ- and center of the unit cell. The final low R index can encesoccur in bond lengths becauseof the cell be taken to confirm the assumedfull order of the parameters used by Dand (1960. The R index three I cations Al, Be, and Si. obtained by Dand (1966)was 8.990 basedon film The Na atom is five-fold coordinatedby one Cl data, isotropic temperature-factors,and CuKc radi- and four O atomsin tugtupite (Fig. 2a), in contrast ation. to its four-fold coordination in sodalite (Fig. 2b). The isotropic temperature-factorsobtained by However, the Na-Cl bond length Q.707 A) in tug- Dand (1966) are small becausethe crystal was pro- tupite is shorter than that in sodaliteQ.736 A). Ot bably not small enough(= 0.3 mm in diameter)for the four oxygenatoms bonded to sodium iqtugtu- absorption to be negligible (p : 86 cm-r, CuKa: pite, threehave a meanbond-distance (2.356 A) simi- Dand 1966).The crystal usedin this study is smaller, lar to that in sodalite(a353 A). The fourth and the absorption is quite low (Table l). Further- atom is further (2.603A), but it is within bonding more, the isotropic temperature-factorsobtained for distancein tugtupite becausein sodalite^theother tugtupite in this study (Table 2) are comparableto oxygenatoms are much further (3.087A). those of sodalite-and helvite-groupminerals (Has- The BeO, and AlOa tetrahedra in tugtupite are san& Grundy 1983,1984, 1985), which suggeststhat closeto bet'g regular, wit[ Be-O and Al-O distances our temperaturefactors are realistic. of 1.631A and 1.748A, respectively.The SiOa The general structural features of tugtupite are tetrahedraare distorted. In the pure Si-O-Si link- similar to those of sodalite.The framework is charac- age,the Si-O dispncesare l.644A.and 1.647 A; this terized by four-memberedrings in the facesof the {istanceis 1.581Awith a Be linkage,and it is 1.609 unit cell, and these rings are linked to form six- A with^an Al linkage. The mean Si-O distanceis memberedrings about the cornersof the unit cell 1.620 A. Therefore, individual Si-O bond length (Fig. l). The four-memberedrings in the facesnor- doesvary with the other cationsin the Si-O-Ilink- mal to the a axeshave an ordereddisposition of Z age, but this effect is not observedfor the mean Si-O atoms consistingof two Si, one Al, and one Be atom, bond lengths(see below). The satisfactoryvalence- whereasthe four-memberedrings in the facesnor- sum calculations(Brown & Altermatt 1985)for the mal to the c axis consistof Si atomsonly. As a result cations and anions in tugtupite also confirm that the of the linkage of the four-memberedrings, all the three Z cations, Al, Be, and Si, are completely ordered (Table 4). TABLE5. COMPARISONOF FRAMEWORKGEOMETRY tNsoME soDAUTE€noup urnenars' Comparison of framework geometry in sodqlite- group minerals Tugt Hslvte OH€odasoda Grard M€anValu€s m@ Tugtupite, helvite-groupminerals, sodalite, and SlO4Totrah€dron basic (hydroxy) sodalite provide an excellent oppor- 9lo 1.620 1.6A 1.616 1.620 r.621 tunity to comparestructural effectsthat arisefrom o€ 2.w 2.@ 2.AB 2.W 2.W dissimilarcations (Si4*, Al3*, Bd*) on the Zsites AlO4 Tglrah€dron and very different interstitial cations (Na+, Mn2+, Al€ 1.78 1.74 1.742 1.744 Z*+,F&+) and anions(Cl-, S2-,OH-, H2O).The o-o 2.8V 2.U7 2.V4 2,W pertinent bond-dislances and angles of the frame- BeO4Tsllahedron work are comparedin Table 5. Be€ 1.631 1.604 r.633 The bond model of Brown et al. (1969) indicates o€ 2.063 2.668 2.666 that the Si-O bond lengh is affected by the Tcation T-T Dkfiances forming the Si-O-Zlinkage. This effect is observed sl.Al 3.102 3.14tt 3.140 3.12A for individual Si-O bonds in tugtupite (Iable 4), but sl-Be 3.051 2.91 LC76 not in the other minerals (cl, helvite and sodalite, Bddglng Angles Table 5). The meanSi-O distancesfor the minerals sl€.Al 136.3 13&7 138.2 197,4 consideredin Table 5 are nearly constant despitethe sl-GBs 143.8 125.0 13rL3 fact that the Iatoms are different. The meanAl-O sl-o€l 140.8 140.8 bond lengths also are identical and do not vary sig- nificantly among the minerals,as are the mean Be- tTu6 = qggplt"to"*oda - baslc 0t)|droxy)eodallts; O bond lengths (Table 5). Consequently,for the $oda - eodalhe;'M€anrralu6 br hdvtegmup mhro|alo(sL gfuchrrs rsffnsmsnls). sodalite-groupminerals, the mean Z-O distancesare THE CRYSTAL STRUCTURE OF TUGTUPITE 389 constant, and such distances are not affected by is modeledusing the DZS structure-modelingpro- different interstitial atoms or different Z cations. eram (Villiger & Meier 1969)and thermal expansion Among tlte mineralsconsidered (Iable 5), the prin- data of Henderson & Taylor (1977). The high- cipal difference is in the Z-O-Zbridging angles.The temperature structure modeling assumesthat the Si-O-f (Z : Si, Al) anglesare nearlyequal, but the structureswould adjust to a rise in temperatureby main differenceoccurs in the Si-O-Be anglesin the rotation of ?Oa tetrahedral units as in sodalite helvite-groupminerals and in tugtupite (Table 5). (Hassan& Grundy 1984),Cell parametersat the var- The value of the Z-O-f angle has been proposed ious temperatureswere fixed to the valuesobtained to be a qualitative measureof the degreeof d-p r experimentallyby Henderson& Taylor (1977),and bonding in the silicate framework (Cruickshank the structureswere fitted to theseparameters. As in 1961,Brown et sl.1969, Gibbs er al.1972). A T- sodalite,the Na-O distancesare expectedto change O-Ibond angleof l8o conespondsto maximumd-p little, and the geometry of the ZOo tetrahedra is zrinteractions. Both Al and Si have 3d orbitals avail- expectedto remain relatively constant. The starting able for suchinteractions. However, Be atomshave positionalcoordinates are taken from this work; the no available orbitals of the correct symmetry to coordinatesfor the Na site are adjustedso that the produce interactions of this type with the oxygen three shorter Na-O distancesare identical to those atom coordinating it. Therefore, a small Si-O-Be in sodalite. Structurei were calculatedat various tem- angle in helvite may be expectedas a result of the peraturesin the range of 20 to 905oC,but herethe smaller contribution to the bonding of the frame- structural parametersare given only for the struc- work (Holloway et ol. 1972);however, such a large Si-O-Be angle in tugtupite is surprising, but no sim- ple explanation can arise from the d-p r bonding model. 200 c qEo c The T-O-T anglesin sodalite-typematerials are Alom ! yz v probably controlled mainly by the interstitial ions. At0 112 3/4 o rl2 3/4 The greater the effective chargeof the cageclusters, B€0 1/2 1/4 o 1/2 the smaller the I-O-Z angle. The cageclusters in gt 0.0127 0.5s4 0.4s69 0.0168 02532 0.4ts9 sodalite and tugtupite are and basic 01 0.1495 0.1334 0.4124 0.1534 0.t@ 0.,1630 [Naa.Cl]3+ g2 0.34dt o.va 0.6601 (hydroxy) 0.971 0.0@ 0.64€8 sodalite contains [Na4.OH.H2O]3' C)3 0.4?57 0.1487 0.1378 o.4& 0.t495 0.13S2 clusters.These clusters all havethe samenet charge M 0.1m8 0.1918 0.1834 0.1818 02167 0.2050 (3 + valenceunits). However, in the helvite-group q0 00 000 minerals,the [(Mn, Fe, Zn)o.S]6+clusters are highly .Cdl - - chargedand have smaller sizes;thus their effective Enr6e6: e - &4(), c - 8.87t A; "a 8.769, c &9764 chargeis much strongerthan in the other minerals. Therefore, the [(Mn, Fe, Zn)0.$]6+ clustersstrongiy attract the oxygenatoms in the Si-O-Be linkagesand TAALE7. COMPARISONOF D(PERIMENTAL STRUCruRES give rise to small Si-O-Be anglesin helvite-group ANDMODELED minerals comparedto Si-O-Z anglesin the other Thb noc 9050c minerals, in particular, the Si-O-Be anglesin tug- Work tupite (Table 5). At€3 4x 1.748(2) 1.78 1.78 Be€2 4x 1.631(2) 1.681 1.6|' Thermal expansion of tugtupite st€r 1.W(21 1.il2 1.643 oli f .il7(2) 1.64{! 1.64{t Henderson& Taylor (lW1) obtaineddata on ther- o2 1.581(2) 1.580 1.580 mal expansionof a red tugtupite from Kvanefjeld, .o3 1.608 Iftmaussaq,Greenland over the rangeof 20 to 905'C. 1.6@(2) 1.607 Thesedata indicate smooth variations in response Mean l-@ 1.618 1.619 to thermal expansion. The expansionis anisotropic, grol€l 140.8(2) 141.4 146.6 being greater along the a axesthan along the c axis, Sl02-B€ lrt3.6(1) 143.8 149.1 as indicated by the decreasein c/a ratio with increas- sto3-Al 135.3(r) 135.4 [email protected] ing temperature. A combination of tilting of the NaOl 2.370(3) e355 2.355 IOo tetrahedra, the presenceof three types of Z 42 2.303(3) 2.355 2As5 atoms, and the distortion of the SiO4 tetrahedra €3 2.S(3) 2.355 2.355 does not allow unambiguous interpretation of the Mean 235C 2355 2.355 thermal expansionof tugtupite and its anisotropy in .o2' 25$ 2.87 terms of the crystal structure(Henderson & Taylor 2.ffir(3) 1977). Na-O 2.707 2.76 3.088 In this paper the thermal expansionof tugtupite 390 THE CANADIAN MINERALOGIST turesat 20 and 905oC(Table 6). Unit weightsin the DaNo,M. (1966):The crystalstructure of tugtupite- DIS refinementwere found to be appropriate.The a newmineral, NasAl2BqSi3Oz(Cl,S)2. Acta Crys- 20oC modeled structure is almost identical to the tallogr. 20,812-816. measuredstructure in this paper, exceptfor a slight shift in the Na-siteposition 6). With increase Grnas,G.V., Haun, M.M., LoursNatrur,S.J., Ban- Qable rrrr, L.S. (1972): in temperature,the ZOo geometry remains nearly & Yow, H. Correlationsbetween Si-O bondlength, Si-O-Si angleand bond overlap constant, and three Na-O distancesalso are cons- populations calculated using extended Hiickel tant, but the Na-O2*,Slistancebecomes plogressively molecular orbital theory. Arn. Mineral. 57, shorter,from 2.596 Aat2}oCto2.487 A at 905"C 1578-1613. as the 5-fold coordinatedNa site becomesmore regu- lar (Table 7). Major changesoccur in the bridging HesseN,I. & GnuNDv,H.D. (1983):Structure of basic angles,which increaseas the tetrahedrarotate with sodalite,Na6A16Si6O2a(OH)t 2}l16. Acta Crystal- thermal expansion,and also tbe length of the Na; logr. C39,3-5. Cl bond increasesfrom2.707 Ax20'C to 3.088A & (1984):The at 905oC(Table 7). The latter value is identical to - crystal structuresof sodalite-groupminerals. Acta Crystallogr.840, Gl3. that modeled for sodalite at 905oC (Hassan & Grundy 1984).Therefore, the mechanismof ther- & - (1985):The crystal structuresof mal expansionof the tugtupite structure involves the helvite-group minerals, (Mn,Fe,Zn)sIBe6Si6O2]52. expansionof the Na-Cl bond lengths,which forces Am. Mineral. 70. 186-192, the Na atoms toward the centerof the six-membered rings. Becausethe Na-O bond lengths are nearly & - (1989):The structrue of nosean, constant, this mechanismcauses the tetrahedrato ideally Nas[Al6Si6O2a]SOa.H2(). Can. Mineral. 27, rotate. An analogousmechanism of thermal expan- 165-172. sion operatesin the sodalite-groupminerals (Has- & -(1991): The crystalstructure of san & Grundy 1984). at 293 and 153K, Can. Mineral. 29. 123-130. ACKNOWLEDGEMENTS -, PrrrnsoN,R.C. & GnuNov,H.D. (1985):The structureof lazurite,ideally NqCa2(A16Si6O24)S2, We thank Dr. Fred J. Wicks of the Royal Ontario a memberof the sodalitegrotp. Acta Crystallogr. Museumfor providing the tugtupite samples.Dr. J. c41,827-832. Dave Embury, McMaster University is thanked for his support and encouragement.We also thank the HENnensoN,C.M.B. & Taylon, D. (1977):The ther- referees,Drs. R.B. Fergusonand R.C. Peterson, mal expansionof tugtupite. Mineral, Mag 47, and 130-131. Dr. R.F. Martin for useful comments.This work was supportedby grants from NSERC of Canada. Horrowav, W.M. Jn.,Gronoauo, T.J. & PeeconD.R. (1972):Refinement of the crystalstructure of hel- REFERENcES vite, Mn4(BeSiO4)3S.Acta Crystallogr. 828, ll4-117. Bnowr,G. E., Gmns,G. V. & Rrane,P.H. (1969):The natureand the variationofthe Si-O andAl-O bonds SonnsrN,H., DeNqM. & PrTrnsnr, O.V. (1971):On in frameworksilicates. Am. Mineral.54, 10&1061. the mineralogy and paragenesisof tugtupite Na6Al2Be2SisO24(C1,S)2.Medd. Grdnl and f El ( I 3). BnowN,I.D. & Alrsnvarr, D. (1985):Bond-valence parametersobtained from a systematicanalysis of SrEwanr,J.M. (1976):The XRAY 16 system.Com- the inorganiccrystal structure database. Acta Crys- puter ScienceCenter, Univ. Marylond, CollegePark, tallogr, B/l, ?A4-2f17. Maryland, Tech. Rept. TR-446. Cnounn,D.T. & MaNN,J.B. (1968): X-ray scattering Vrr-r-rcnn,H. & Memn,W.M. (1969):DLS - AFortran factors computedfrom numericalHartree-Fock programfor the leastsquares refinement of inter- wavefunctions. Acta Crystallogr.AU, 321-324. atomic distances.Inst. Kristallogr.Petrogr. Eidg. Tech. Hochschule,Sonneggtrasse 5, Zurich. CnurcrsuaNx,D.W.J. (1961):The role of 3d-orbitals in ri'-bondsbetween (a) , phosphorus,sulphur or chlorineand (b) orygenor nitrogen,J.Chem. Soc. ReceivedJuly 14, 1990, revised manuscriptaccepted 53, 5486-5504. October26, 1990.