Conadian Mineralogist Vol. 29, pp. 377-383(1991)

THE CRYSTALSTRUCTURE OF BASIC ,IDEALLY Na.[AluSi.OzaJ(OH)2.3H2O

ISHMAEL HASSAN,N.ANDH. DOUGLAS GRTINDY Departmentof Geologlt,McMaster University, Hamilton, Ontario L8S4Ml

Agsrnacr rer et al, 1970,Bresciani Pahor et sl. 1982)and poly- morphic with basic (hydroxyl) sodalite The structure of basic (hydroxyl) cancrinite, Na6[Al6Si6O2i(OH)2,2H2O(Hassan & Grundy Na6[Al6Si6O2al(OH;r.2.311r6,has been refined to a R 1983).The latter compoundis the high-temperature value of 0.047 for 870 unique observedreflections in space polymorph (Anderson 1968).Basic cancrinite also group X-ray-diffraction The P63 using single-crystal data. is isotypic with the mineral tiptopite, Al and Si atoms of the aluminosilicate framework are fully which ordered, as in other cancrinite-group minerals. Each cage K2(Li2.eNa1.7Ca6.7)[Be6P6O2a](OH)2.1.3H2O, contains one [Na.H2O]+ cluster. The Na2 sites in the is the beryllophosphateanalog of basic cancrinite channelsare fully occupiedby Na atoms. The OH group (Peacor et al. 1987). and remaining H2O also occur in the channels. The The structure of cancriniteis composedof AlOn fremework atoms, the [Na.H2O]+ clusters,and the Na2 and SiOatetrahedra, and the Al and Si atoms are sitesdo not directly contribute to the superstructurein basic completelyordered. The tetrahedraare corner-linked cancrinite. The superstructurearises from long-rangeorder to form a framework consistingof chainsof cages of OH- ions and H2O moleculesand vacanciesassociated parallel to the e axis. Thesecages are bounded at the with thesein the channels.The ideal formula for basic can- by parallel six-memberedrings con- crinite is NaslAl6Si6O2al(OH)r.311r9. top and botlom sisting of alternating AlOo and SiOotetrahedra. The Keywords: cancrinite-group minerals, basic cancrinite, 3-fold axis (parallel to the z axis) runs through the , superstructure. center of theserings. The cagesare further linked to form channels bounded by puckered twelve- SoMMAIRE memberedrings of (Al,Si)O4 tetrahedra around the 63 axes(Fig. l). Nous avons affind la structure cristalline d'une cancri- In cancrinite, one interstitial cation site, Nal, is nite hydroxyl6e, et donc basique, Na6[Al6Si6O24] locatedin the cageon the 3-fold axis and is slightly (OH)2.2.8H2O, iusqu'd un rdsidu R de 0.M7 dans le plane rings. groupe displacedfrom the of the six-membered spatial P63, €n utilisant 870 r6flexions uniques (on observ€essur cristal unique (donn€esde diffraction X). Les Each cagecontains a water molecule site 06), atomesAl et Si de la trame aluminosilicat€e sont parfaite- which is disorderedabout the 3-fold axis becauseof ment ordonnes,tout comme dansles aufies membresde hydrogenbonding, and a Na atom, which is located la famille de la cancrinite. Chaque cage contient un on the Nal site, thus forming [Na.H2O]+ clusters. agencement[Na.H2O]r. Les sitesNa2, dans les canaux, In davyne, the cagecontains [Ca.Cl]+ clustersin- sont pleinementoccup6s par le . Le groupe OH et stead of [Na.HzO]+ clusters (Hassan & Grundy les autresmol€cules d'eau setrouvent aussidans les canaux. 1990).Another interstitial cation site, Na2, is located Les atomesde la trame, lesgroupements [Na.H2O]+ et les ne.u the walls of the channels,in closeproximity to pas sites Na2 ne contribuent directement d la surstructure rings. The channelsalso basigue. Celle-ci serait d un agencement the centerof six-membered de la cancrinite groups, ordonn6 i longue €chelledes ions OH-, des mol€cules contain the large anionic e.g., CO3, SOa, d'eau, et des lacunes le long des canarx. La formule chi- and OH, and the remaining cations (e.9., Na, Ca, mique id6ale de la cancrinite basique serait Nas[A16Si6O24] and K) occur on the Na2 site (Grundy & Hassan (oH)2.3H2o. 1982). (Traduit par la Rddaction) Balrler et ol. (1970) determined the structure of a basic cancrinite with chemical formula Mots-cl6s: mindraux de la famille de la cancrinite, cancri- Na7.1a[A16Si7.08O26.r6].4.05HtOusing X-ray data nite basique,structure cristalline, surstructure. obtained from powdered material. They divided the Na2 site into two positions. Oxygenatoms occur in INTRODUCTION the channels,and the presenceof somekind of sili- cate anion also was detected in the channels. Synthetic basic (hydroxyl) cancrinite is isotypic Approximately one H2O occurs in each cage. with cancrinite-group minerals (Jarchow 1965, Bar- BrescianiPahor et al. (1982)refined the structure of a basic cancrinite of chemical formula *Present address: Institute for Materials Research, Nar.aa[AIuSi6.s3O24.M).5.61H2Oto an R value of McMaster University, Hamilton, Ontario L8S 4Ml. 0.034 for 648 observedreflections that have I > 377 378 THE CANADIAN MINERALOGIST CANCRINITE

?2

Frc. 1. Projectionof the structureof cancrinitedown [001],showing the sitenomen- clature, symmetry elementsfor spacegroup P6., large channelsalong the 63axes, and cagesalong the 3-fold axes(Grundy & Hassin 1982,modified by Papike 1988).

3dI). The framework-atompositions and the inter- ted by Barrer et ol. (1970).One position is on the stitial cation siteswere found to be similar to those 63 axis, and the other two positions are off the 63 in the cancrinite stmcture determined by Jarchow axis and have occupancyfactors of 0.35 and 0.33, (1965).The cagecontains 0.59 H2O on the 3-fold respectively.These positions are possiblelocations axis. Three peaks in a channel correspond to O for O atomsof OH- and H2O. Theseresults lead to atoms,which are in similar positionsto thoserepor- an ideal formula of Nar.r[Al6Si6O24XOH)r.5.5H2O for basic cancrinite. A comparisonof the unit-cell contentswith the resultsof Barrer et al. (1970)indi- TABLE1. BASICCANCRINTTE: CHEMICAL COMPOSnONT catesthat agteementbetween sets of crystallographic resultsis betterthan that betweenchemical analyses No of lons.r (BrescianiPahor et ol. 1982).The structuresof basic Odds vtl o/o psr unlt csll cancrinite and tiptopite also were discussedand com- pared by Peacor et al. (1987). $02 8.27 81 6.19 Superstructurereflections are commonly observed Azog 28.93 At 5.81 in cancrinite-groupminerals (Jarchow 1965,Brown N"2O 24.06 Na 7.95 & Cesbron1973, Foit et al. 1913,Grundy & Hassan Hzo 924 HzO 2.77 1982,Hassan & Grundy 1984).Grundy & Hassan oH 2.00 (1982)studied an exampleof cancrinitewith a well- Toral .s50 developedsuperstructure using high-resolutiontrans- Chsmlcal fomula from anal)€ls: missionelectron microscopy (HRTEM) and X-ray- (OHl NaO.0lAlS.ASl6.2O24l 2.0'2.n H20 diffraction analysis.These authors excludedthe pos- Chomlcaliomula ftom rdnmam: sibility that the superstructruearises from periodic (OHl N%tAlosf 6024l 2.o4,'2.ffiH 20 variation in the stacking sequenceof the six- memberedrings. The X-ray refinement of the struc- rwst anal!€lq J. Muysson, McMaslsr Unlvsrslty ture showsthat the carbonategroups occur on the (H2O by Ponffsldgrormetrlc m€thod) 63 axis, with the^ircarbon atoms in two positions, *based on Al+Sl = 12. Cl and C2, 1.2A apart and with an occupancyfac- THE CRYSTAL STRUCTURE OF BASIC CANCRINITE 379 tor of 0.38 for each of the site. The cagecontains (Table l). The number of Na atoms is at the maxi- one [Na.HrO]+ cluster, and the Na2 site contains mum value of 8.0. Six Na atom valenceunits (v.u.) 0.67 Na, 0.25 Ca, and 0.08 vacancies.This corres- satisfy the charge deficiency on the aluminosilicate ponds to tfe omissionof 3 to 4 CO, groups within framework [Al6Si6Oz]6-,and the remaining 2 v.u. the 40.94A period of the supercell.The cancrinite on the two Na atomsare expectedto be satisfiedby superstructurearises from ordering of carbonate- two OH- ions. There is a deficiencyof 4.5 wt.9o in group vacanciesand the resulting ordering of the the analytical total, so that the amount of H2O is cationsand vacancieson the Na2 sites.A model of inferred to be greaterthan that indicated(Table l). the long-rangeorder was establishedand givesa good Single-crystalX-ray precessionphotographs con- match betweencalculated and observedreflections tain diffraction symmetry and lattice extinctions attributed to the superstructure, both in X-ray and compatible with spacegroup P6, (Fig. 2). All the electron diffraction patterns (Grundy & Hassan substructurereflections are sharp. Crystalsof basic 1982).Recently, HRTEM imagingwas used to show cancrinite show superstructurereflections that are the order among the CO.-group vacanciesin cancri- different. No superstructurereflections occur in some nite (Hassan& Buseck 1992). ;the "extra" reflectionsseen are not super- Neither Barrer et al. (1970)nor BrescianiPahor structurereflections because they are not commen- et al. (1982)reported the presenceof superstructure surate with the substructurereflections (Fig. 2a). reflectionsin basiccancrinite. These reflections are These"extra" reflectionsarise from a minor phase. presentin our sample.An accuraterefinement of the A weak one-dimensionalsuperstructure affecting the crystal structure thus was carried out to determine c* axis occursin somecrystals and givesa supercell the origin of the superstructure in basic cancrinite repeatof about 3c*" (Fig. 2b). The superstructure and to checksome of the subtle structural features reflections are streakedparallel to c*. The crystal previously reported for basic cancrinite. structureof both crystalsused to record the preces- sion photographsshown (Fig.l) was refined. The ExpenrusNral DETATLS final structures are identical within the estimated errors, so that the results for only one crystal are Crystalsof basiccancrinite were prepared hydro- reported here. thermally at I kbar total pressure and at 723 K Unit-cell parameters were obtained by least- (Anderson1968). The crystalsare water clear,hexa- squaresrefinement of 26 substructurereflections gonal prisms, elongatedalong the e axis. The che- having 10" s 20s 35o automatically aligned on a mical analysisgives a ratio of Al and Si atoms of four-circle single-crystalX-ray diffractometer using 0.94, which is close to the expectedvalue of 1.0 graphite-monochromatizedMoKcr radiation. The cell

FIc. 2. [00] zone X-ray precessionphotographs of basiccancrinite. (a) No well-definedsuperstruclure reflections are present.Some of the "extra" reflectionsarise from a minor phasebecause they are not commensuratewith the substructure reflections. O) Weak superstmcture reflections (e.9., arrow) give rise to a supercell u/ith 3cmc. Such reflectionsare streakedparallel to C (seenon negativebut not on print). MoKo radiation, Zr frlter; (a) p=20', @) p=25'. Note that (a) and (b) are printed at different magnifications. 380 THE CANADIAN MINERALOGIST

TABLE2. CRYSTALDATA AND DATA€OLLECNONINFORMATION All crystallographiccalculations were made using tlte XRAY76 CrystallographicSystem (Stewart 1976). a (A) 12.@(2, c (A) 5.15e(1) SrnucrunB RnrrNsMENr V (A3) 716.9 SpacoGroup P% positional parameters 'I The initial and isotropic z temperature-factorsused were those of cancrinite DenettyCalc. (q/cm| 2,27 (Grundy & Hassan1982). Atomic scatteringfactors Cnatalstse (mm) 0.20x0.20x0.23 for neutral atomswere taken from Cromer & Mann p(cm{) AA (1968).Initially, no oxygenatoms corresponding to pR 0.16 OH and H2O were placed in the channels.Refine- Radlstlon,/Monochrorutor Mo/Q ment of this structural model using the full-matrix MoKc(A) 0.710@ least-squaresmethod, unit weights, and a variable Totalno.ofl mn overall scale-factor progressedrapidly. Isotropic No. ot unlquo rsflecilons 959 temperature-factorswere convertedto anisotropic No. ol nonoqutu l Fol > sr I Fl B7o forms, and the structural model convergedto an R FlnarR->( lFol-lFcl )ltlFol o.w7 index of 0.061.The resultingstructural geometryis - Fd fu=r = pi{ 1| r.l | Fcl',2/t\tlFol2ft 0.057 comparable to that in cancrinite (Grundy & Hassan 1982). Difference-Fourier sections for a channel show electron-densitypeaks in positions similar to the oxygenatoms of the carbonategroup in cancri- parameters and other information pertinent to X- nite, as well as a position on the 63 axis. ray data collection and refinement are given in Table atomswere placed on thesesites, andtheir occupancy 2. All reflectionintensities were measured from two factors were adjusted to match the results of the che- asyrnmetricuni* (h,k>0; t /> 0) out to a maximum mical analysis.The refinement convergedto an R N of. 65o using a Syntex P2, diffractometer. The index of 0.047. Difference-Fouriersections at this diffracfometer was operatingin the 0 - 20 scanmode, stage showed no significant features exqeptfor a with A scanrange of (MoKo1 - 0.85') to (MoKo2 small amount of residual density (1.3 e/A3) on the + 0.85') and variable scan rates of 3 to 29.3' 63axis at (0, 0, 0.97). Site occupanciesand tempe- min-r, dependingon the intensity of a preliminary rature factors for all the interstitial ions were refi- scan. The two standardreflections measuredafter ned, while the framework atom parameterswere held every 50 reflectionsdid not change.The data were invarianl (Table 3). The resulting occupancy-factors corrected for Lorentz, polarization, background give a chemicalformula that agreeswell with that effects,and sphericalabsorption (Table 2). Equiva- obtained from the chemicalanalysis (Table l). The lent reflectionswere averagedto produce a unique final structural parameters are presentedin Tables data-set. Refinementswere canied out using the 3, 4, and 5. A table of structurefactors is available resultingstructure-factors for which lF"l > 3olFl. at a nominal chargefrom the Depository of Unpub-

TABLE3.ATOinC (!ORDI|.IAIES,I9OIROP|O, ANDAI{SOIROPIC'THERMA!eam".rErEnS 6&rO1

Aom ano 8n9 r y z U.q Urt u2 ulg utz utg ua @

FErmri< aldB 01 6c 1.0 o .Ml4't .1@4(4) .666e00) 176 104(16) 52(20) r73(r8) r'16(16) "2(1s) s06) 02 8c 1,0 o .fi94(4) .504(4) .?"6906) 236 N{14 14r(16) 36S(2s) 10604) 54(26) 4o(a, Ot 6c 1.0 o .@7(4',t .3!'3r(4) .0619(s) 170 172|Jq e(^l 741?01 12004 3706) 44t14 04 6c 1.0 o .3rs(4) 38nF) .0421(e) b7 e81' a7(211 101(20) 197(1s) 1104 28(18) Sl 6c t.0 sl .@(o 416fl .m 8A 8s(5) s(6) 7e(6) 55(4) "il(8) 3C4 AI 6c 1.0 AI .070{r) .4r&'(r) .7616(0 80 7r(6) 8q6) e4(o 3s(5) €(8) 11(e) Irnddalffi 061 6c o.17 o .614{2n .10/d7(al .c/r4(5s) 214{55) O52 0c 0.17 o .064r(q .10s@) .s8zc/4) 238(681 05 2a 034 o 0 o .@l(al) 38r(59) OO 8c 0.g] o .6r62e0 30s7(42) .6e|]6(601 687 687(162)663(16) 471(20) 232(2n) 56(101) s(16, Nal 2b 1.0 trla 219 1lt .r2at06) 334 214 218 66/(40) 109f4 0 r{a2 6c 1.0 NA .rc0(3) 266s(3) .286r(s) 4r0 a8(14) 56e(20) 3rS(24) 288{14) -S(r0) @09)

.adsdropictEntpodehdor - oq1-zr2131-f 31-1 qstalh1tr1)l THE CRYSTAL STRUCTURE OF BASIC CANCRINITE lished Data, CISTI, National ResearchCouncil of TABI€ 4. FFAMEWORKIN'IERATOMIC DISTANCES, ANGI€S, AND Canada, Ottawa, Ontario KIA 0S2. BONDVALENCES

Stnucrunr Dnscnrprrorq et.tl Dtscusslor.r Slo4tsfahodrcn Alo4tetrahodron Tlol r.oz6)A r.ooov.u. T2o1 r.ztz(o)A o.z$v.u. ..o2 -O2 Framework 1.615C4 1.019 1.718(5) 0.781 .o3 1.626(9) 0.989 -@ 1.727(5',1o.7u -o4 pn7 The geometryof the aluminosilicateframework €4 1.620(51 l,@5 1.747m (Table 4) is similar to that in cancrinile (Grundy & Msan 1.621 4.4.4 Moan l:74 Hassan1982) and to that obtained for basiccancri- Edg€6oftotrhedm (A) nite by BrescianiPahor et al. (1982).The average o1o2 2.612(51 o1o2 2.749(81 2.81751]- Al-O and Si-O distancesand the valence sums for 03 2.62301) o3 the framework cations indicate completeordering of 04 2.656(8) "o4 2.754(111 2.693(9) o2o3 2.888f4 the Al and Si atoms. The valence sums for the fra- o2o3 .o4 -o4 2.897(6) mework O atoms also are satisfactory (Table 4; 2.67A01\t 2.n2n Brown & Shannon 1973). o3€4 2.616fn vJ44 M@n z,ffi Men 2,@ AnglosIn tsfahodra (o Coge ) o1-T1-o2 107.6(3) o1-T2O2 106.3(2) 107.7(3) €3 109.7(3) The cagesin basic cancrinite, cancrinite, and vish- "o3 €4 110.0(3) €4 107.5(4) nevitecontain one cluster.The Nal site [Na.HrO]+ o2-T1o3 112.4(S'l o2-T2-o3 113.9(3) on the 3-fold axis is fully occupiedby a Na atom. 111.8(3) o4 113.4(4) The bond-valencesum about Nal is 1.0 v.u., as "o4 o3.T1€4 107.4(4t o&T2o4 105.8(2) expected(Table 5). The 06 sireis fully occupiedby M€an 1(p.5 M€an an oxygenatom of the water molecule,but this site l@![ Bruqlnq Angl6(o Bord-valenaeo(lna torlramqflo* O atom is 3-fold because hydro- ) disorderedabout the axis of TlOl-T2 148.5(4) 01 = 0.783+1.000+0.065+0.161- [email protected] gen bonding, as in other cancrinite-groupminerals T'|02-T2 149.2(3) 02 = 0.78111.019+0.179 = 1.979 (Grundy Hassan1982, Hassan Grundy 1984). & & T1OC.T2 135.9(4) Og = 0.704f0.9$a0.189 = 1.942 However, BrescianiPahor (1982)placed el al. the T1.O4-f2 135.9(31 04 - 0.727r'[email protected] - 1.914 06 siteon the 3-fold axis,with an occupancyfactor of 0.59.

Channel

The Na2 sites in the channels are fully occupied by Na atoms, and thesesites are similar to thosegiven by BrescianiPahor et ol. (1982),but the O atom sites 051 and O52 located in the channelsare slightly dif- ferent and have occupancy-faclorsthat are one-half TABLE6. CATION.ANIONDISTANCEg AND BOND VAIENCES FOR INTERS'TTNAL those given by Bresciani PaIrcr et al. (1982, Table ATOMS 3). The 05 site on the 63 axis has an occupancy- factor identical to that given by BrescianiPahor et TrlgoElblpyamld ocrah€dnl ql. (1982),but the 05 position is different, although Na2Ql Z.ISZOA O.tete)v.u. Ne2.o1 LenA 0.161(2rv.u. related. The two 05 positions are related by a dis- "o3 2.394(8) 0.18s(3) .o3 2.394(8) 0.18s(3) placement along the e axis from the plane of the 04 2.4S(6) 0.r&(2) €4 2408(6) 0.1S2(2) o51a zds(30) o.11oc4 -c'rlf 26o900) 0.105(15) triangular array of Na2 sites, whereasa similar dis- .@A placement €slo 2.689(64 o:n(ls, 2.6'10(35) oto2n in the oppositedirection occursin the spe- 2.823(sr) 0.14i(ro) (1982); 0610 2.646(s4) o.ios0) osf cimen of Bresciani Pahor et al. thus the -o3 2.579n o.o8'/(o) 03 2.e7Sn o.(E'/(o) Na2-O5 distances are similar in both specimens 04 2.913(11) 0.06s(0) .o4 2913(11) o.o€s(0) (Table 5). The Na atom on the Na2 site could be in -o5 2.S(6) o.os7(ol "o5 2.ffi(6) 0.057(01 either a trigonal bipyramidal coordination by M@n 2.655 tl,Sg Mm 2.653 tl,@ bonding to oxygen atoms on the O5l positions, or tl,alOl x3 2.8s4(5) 0.195(0) Bordrlm 8m tor qygsB an octahedral coordination by bonding to the O ..O2x3 2.417(51 0.587(2) 05 - 3x0.057 - 0.171v.[ atomson the O52 positions(Table 5). Similar coor- o6a 2.ss(26) o.trs(g) O51- 0.12+0.109+0110= 0.340 dinations occur in cancrinite (seeFig. 7 in Grundy "060 z.Bss(26) orq)(12) O52- 0.10510.102+0141- 0.348 & Hassan1982). In both coordinations,the bond- Mo 2.6s0 t-0,90 valencesum about Na2 is closeto the expectedvalue of I v.u. (Table5). a-abovo: b-b€!fr, 382 THE CANADIAN MINERALOGIST

Cnvsrnl CHsN4rceLeNl END-MsN4ssnFoRMur-a SupsnsrnucruREIN BASICCANcRINITE

The presentstructural model doesnot allow an The origin of the superstructurereflections in basic unambiguousdistinction to be made betweenOH- cancrinite can be deducedby the elimination of parts and H2O in the channels.However, O5l, O52, and of the structure that do not directly contribute to the 05 are all possiblesites for the O atoms of these supercellformation. Both cancriniteand basiccan- gxoups.The OH group shouldoccur at a shorterdis- crinite have a similar framework geometry, in which tance to the Na2 site than that of HrO. Therefore, the Al and Si atoms are fully ordered, so that their the longer Na2-O5 distance indicates that 05 structuresdo not contain stackingfaults (seeHas- correspondsto H2O, and O51 and O52 to OH san & Grundy 1990).Each cagein both structures (Table 5). The occupancyof thesesites by OH and contain one Na[.H2O]* cluster. Neither the frame- H2O also is supported by the bond-valencesums works nor the cageclusters contribute to the super- about these sites, becauseboth O5l and O52 are structure. The Na2 site does not show positional more valence-enhancedthan 05 (Table 5). These disorderin either structure. Becausethe Na2 site is results give a chemical formula, Na6[Al6SiuOrn] fully occupiedby Na atoms in basic cancrinite, it (OH)2.04.2.68H20,that agreeswell with the results doesnot contributeto the formation of a supercell. of the chemical analysis,thus indicating that the site The weak superstructure reflections in basic cancri- assignmentsare essentiallycorrect. In comparison, nite arise from ordering of OH and H2O and vacan- the chemical formula given by BrescianiPahor et ol. cies associatedwith thesein the channels. (1982)is Na7.6a[Al6Si6O24].5.98(H2O,OH). From the precessionphotograph, the spacingof the superstructurereflections givesa supercellrepeat (Fig. Deducingan ideal end-memberformula from the of approximately 3c*o" 2b). In the cell with structural model for basic cancrinite requiresan eva- water moleculesonly, the chargeon t}te Na2 sitemust positions luation of O-O distancesfor the O atomsof OH and be balanced through the 03 and 04 (Table H2O in the channel.There are six equivalentposi- 5), which causessome correlation betweenthe tions for O5l or O52 per cell, and eachset of posi- degreeof order of OH and H2O in adjacent chan- produces tions is arrangedin two trigonal planes.Two equi- nels;this order the weak superstructurein (Fig. valent positions occur for 05. For any half of a unit somecrystals of basic cancrinite 2b). The X-ray cell, occupancyof a O51 position allows simulta- scatteringfactors for OH and HrO are small, and neousoccupancy of a O52 position in that half of thus the superstructurereflections in basic cancri- the cell, but excludesoccupancy of the 05 position nite are weak comparedto those in cancriniteand ard vice versa.This result permits, by exclusionof vishnevite.The variable developmentof the super- presence the 05 position, full occupancyof (a) four positions structurereflections indicates the of subtle changesin the H2O content and degreeof order of [2 O5l and 2 O52)per cell, or (b) with only the 05 sites,two positions per cell, or (c) three positions per OH and H2O in the channelsof basic cancrinite. Nonrandom distribution of thesespecies also cause cell [one 05l and one O52 in one-half a cell and 05 in the other half of the celll. streakingin the superstructurereflections along the c* direction (Fig. 2b).

Full occupancyof four positions as in (a) requi- CoNCLUSIoNS res 2OH- for charge.balanceand also 2H2O, which seemsan unlikely model becauseH2O is not expec- Basic cancrinite has an aluminosilicate framework ted to be in such close coordination with the Na2 in which the Al and Si atomsare fully ordered.The cation position. On average,only lwo O5x (where framework atoms, the cage clusters, [Na'HtO]+, xequals 1 or 2) positionsper cell are expectedto be and the Na2 site, which is fully occupied by Na filled with OH. Becauseof the remotenessof the 05 atoms, do not directly contribute to the satellite position from the Na2 position, 05 contains H2O reflections in basic cancrinite. Thesereflections arise rather than OH. Therefore,05 doesnot contribute from ordering of OH, H2O, and vacanciesin the in any major way to chargebalance in the channel channels.The ideal end-memberformula for basic (Table 5). For a full complementof 6 Na per chan- cancriniteis Nas[Al6Si6O24](OH)2.3H2O. nel, there must be 2 OH on the O5x positions, and the extra O observedmust be H2O on the 05 posi- ACKNOWLEDGEMENTS tion [case(c)],which is confirmedby the refinement. Therefore, an ideal end-memberformula for basic Financial assistancewas provided by NSERC of cancrinite is Nas[Al6Si6O24](OH)2.3H2O,whereas Canada.Dr. J. Dave Embury, McMaster University, BrescianiPahor et al. (1982)suggested an ideal end- is thanked for his support and encouragement.We memberformula Nar.,[AluSiuO2](O]I) 1.r.5H2O for alsothank Dr. Paul B. Moore, an anonymousrefe- their specimen. ree, and Robert F. Martin, for useful comments. THE CRYSTAL STRUCTURE OF BASIC CANCRINITE 383

REFERENCES HassaN,I. & Busncr,P.R. (1992):The origin of the superstructurein cancrinite.Can. Mineral. (in press). ANDERSoN,P.A.M. (1968): Phase Relations of the Hydroxyl Feldspathoids in the system & Gnuunv, H.D. (1983):Structure of basic NaAlSiOo-NaOH-H2O.Ph.D. thesis, McMaster sodalite, Na6Al6Si6O2a(OH)2-2H2O. Acta Crystal- Univ., Hamilton, Ontario. logr. C39,3-5.

BennER,R.M., Cor-E,J.F. & Vrr-r-rrcnn,H. (1970): & - (1984): The character of the Chemistryof soil minerals.VII. Synthesis,proper- cancrinite-vishnevitesolid-solution series. Caz. ties, and crystal structuresof salt-filled cancrinites. Mineral, 22,333-3N. J. Chem,Soc. A, 1523-1531. & - (1990):Structure of davyneand impli- BnEscnu PaHon,N., Cerr.tcants,M., NanorN,G. & cations for stacking faults. Caz. Mineral. 2J, Raroaccro,L. (1982):Structure of a basiccancri- 34t-349. nite, Acta Crystallogr,B3E, 893-895. JasncHow,O. (1965):Atomanordnung und Struktur- BnowN, I.D. & SuarNoN,R.D. (1973): Empirical verfeinerung von Cancrinit. Z. Kristallogr. 122, bond-strength- bond-lengthcurves for oxides.,4cla 407422. Crystallogr.429, 266-282, PAnrKE,J.J. (1988):Chemistry of rock-formingsilica- Bnowx,W.L. & CEsnnou,F. (1973):Sur les surstruc- tes:multiple-chain, sheet, and frameworkstructu- turesdes cancrinites. C. R. Acad. Sci.Paris276D. res. Rey. Geophys.26, 407-444. t4. Pracon,D.R., Rousr,R.C. & Amr, JuNc-Ho(1987): Cnounn,D.T. & MarN, J.B. (1968):X-ray scattering Crystalstructrue of tiptopite, a frameworkberyl- factors computedfrom numericalHartree-Fock lophosphateisotypic with basic cancriute. Am. wavefunctions. Acta Crystallogr.A2/.,32l-3A. Mineral. 72.816-820.

Fotr, F.F. Jn., Precon, D.R. & HErNnrcH,E.W. SrEwARr,J.M. (1976):The XRAY 76 system.Compu- (1973):Cancrinite with a new superstructurefrom ter ScienceCenter, Univ. Maryland,College Park, Bancroft, Ontario. Can. Mineral. 11. 940-951. Maryland. Tech. Rept. TR446. GnuNov,H.D. & HassnN,I. (1982):The crystalstruc- ture of a carbonate-richcancrinite. Can. Mineral, ReceivedJuly 14, 1990, revised manuscriptaccepted 20,239-25t. October26, 1990.