American Mineralogist, Volume 63, pages 304-310' 1978

Zektzerite, NaLiZrSiuO,u: a silicate with six-tetrahedral-repeatdouble chains

SusnA.rnGHosn nNo Cus'Nc Wa'N Department of GeologicalSciences, Uniuersity of Washington Sealt le. Washinston98 I 95

Abstract

: zektzerite, Nal-iZrSiuo,u, is orthorhombic, space group Cmca, with cell dimensions: a 14.330(2),b : 17.354(2),and c : 10.164(2)4;Z : 8.The crystal structurehas been determinedby the symbolic addition method and refinedby the method of leastsquares to an R factor of 0.040for 2389reflections, measured on an automaticsingle-crystal diffractometer. The crystal structureof zektzeriteis a three-dimensionalframework consistingof (a) edge- sharing Na-polyhedral chains, (b) octahedral-tetrahedralchains, formed by alternating Li tetrahedra andZr octahedrasharing edges, and (c) corrugateddouble-silicate chains with six- tetrahedraf repeat (Seclrser-Doppelkette)and three different four-membered rings. The Li tetrahedron,with an averageLi-O distanceof 1.959A,shows strong angular distortion. The Zr octahedronis nearly regular,with an averageZr-O distanceof 2.0'/4A.The sodium atom occurs in an irregular cavity formed by the corrugation of the silicatedouble chains; it is coordinatedto six atoms at distancesof 2.37-2.6'7A,and four more oxygenatoms at distancesof 3.12-3.23A.The averageSi-O bond lengthswithin the Si(l)' Si(2)' and Si(3) tetrahedraarel.6l4,l.6l6,andl.6l0A.TheSi-O-Si bondanglesinvolvingoxygenslyingon mirror planesaverage 155.7", whereas those within the singlesilicate chain averagel4'7.6". The larger Si-O-Si anglesare associatedwith shorter Si-O bonds. Zektzeriteis isostructural with tuhualite, (Na,K)Fer+Fe3+Si"O,u,and synthetic Na2MgrSiuo,r. Provided the valence balanceand coordination requirementsare satisfied,a largenumber ofsilicatescan crystallize within this structuretype.

Introduction Experimental ground Zektzerite,LiNaZrSi.O15, ? Il€w mineral found in A spherewith a diameter of 0 44 mm was grinder miarolitic cavitiesof the Golden Horn batholith near from a singlecrystal fragment usinga sphere Washington Pass,North Cascades,Washington, oc- (Bond, 1951). The single-crystalsphere was mounted curs in associationwith quartz, , aegirine. on the computer-controlledautomatic single-crystal riebeckite, , , and elpidite. It is X-ray diffractometer (Syntex PT), and the unit-cell least orthorhombic and occurs as translucent colorless dimensions were refined by the method of MoKa stout prisms. Dunn et al. (1977) determinedits opti- squares,using l5 reflectionsmeasured with (Table cal properties, unit-cell dimensions and possible radiation with20 valuesbetween 30o and 40" spacegroups; they also noted the similarity of its cell l). The unit-celldimensions are in good agreement (1977). dimensionsand chemicalcomposition with thoseof with those determinedby Dunn et al. The tuhualite,(Na,K)zFe7+Fe;+Si'rOgo. HrO. The present intensitiesof all reflectionswithin a 20 value of 65" structure determination shows that thesetwo miner- were measuredon the diffractometer,using MoKa from a als are indeedisostructural, containing double silicate radiation monochromatized by reflection chains with six-tetrahedralrepeat (Sechser-Doppel- graphite"single" crystal,and a scintillationcounter. being kette\.1 A variablescan rate was used,the minimum 2"/min (50kV, 12.5mA), Out of a total 2389reflec- 316 were below 3o(1),where o(1) is the stan- 1See [.iebau (1972) for the terminology and a classificationof tions, silicatestructures based on chain types. dard deviation of the measurementof the intensity,1,

0003-004x /78 /0304-0304$02.00 304 GHOSE AND I'YAN ZEKTZERITE

Table I Zektzerite: crystal data culation basedon theseatoms followed by difference Fourier synthesisyielded the positionsof one zektzerite, Naliz!si601s: barholith, N. cascades, positions $:1fl:l_lH" and two oxygen atoms. These atomic were refinedby the method of leastsquares. The difference Colorless. translucent DrisN Fourier synthesissubsequently calculated revealed Orthorhonbic, i@ ce1l volune: 2527.61.7)L3 the positionof the atom. a(;) = 14.330(2) Cel1 conteoE: 8[NaLizrSiUOrrl All the atomic positionaland thermalparameters -3 b(L) = ri.3s4(2) D: 2.79ecm" were refined by the method of full-matrix least m- -1 squaresusing the RrtNe program(Finger, 1969). The cti; = 1s.1641r, D: 2.80scn- c- observedstructure factors (Fo's)were weightedby l/ group: Space Clnca U(l4cK0): 15.409 cn-1 6t(F.), whereo(F") is the standarddeviation of Fo,as determinedby the counting statistics.The atomic as derived from the counting statistics.The intensity scatteringfactors for Li, Na, Zr, Si, and O weretaken data werecorrected for Lorentz and polarization fac- from Cromer and Mann (1968),and correctedfor tors. No absorptioncorrections were made, sincethe anomalousdispersion (Cromer and Liberman,1970). linear absorption coefficient of zektzerite for MoKa Three cyclesof refinementusing anisotropic temper- ature factors yielded a final R-factor of 0.040 for all radiationis small (Table I ). reflections.40 strong low-angle reflections(with F,- Determination and refinementof the crystal structure F. > 10.0 were believed to be suffering from ex- Dunn et al. (1977)could not distinguishbetween tinction and were excludedfrom the refinement.The the two possiblespace groups Cmca and C2ca. The refinementconverged at this stage,the averageshift Wilson statisticsof the measuredintensities indicated u.r.error being0.00. The final atomic positionaland the presenceof a centerof symmetry and Cmca as the thermal parametersare listed in Table 2. Observed correct spacegroup. The crystal structure was deter- and calculatedstructure factors are listed in Table 3.'z mined directly by the syinbolic addition method The bond lengthsand angles,and the dimensionsof (Karle and Karle, 1966),using the computer program '?To obtain a copy of this table, order Document AM-78-065 MurrnN (Germain et al., l97l). The first E-map from the BusinessOffice, MineralogicalSociety of America, 1909 showedthe positionsof the sodium,, two K Street,N.W, Washington,D C. 20006 Pleaseremit $l 00 in silicon,and sevenoxygen atoms. Structure-lactor cal- advancefor the microfiche

Table2 Zektzerite:atomrc positional and thermal parameters(standard deviations in parentheses)

Atom B eqt -11R* 8,, R Btz 9u Bzt

Na 0.25000 0. 21648(8) 0.25000 2.10(3) 25L(7) 78(4) 799(L7) 0 2o9(L) 0 -2(20) Li 0.247t2(44) 0. 00000 0. 00000 1.12(8) L52(22) 107(15) r9t (40) 0 o

Zr 0.25000 -0.08805(1 ) 0.25000 0.38(r) s3(1) 29(r) 88(2) 0 -1 (1) 0

s1(1) 0.39091(4) 0.12869(3) 0.0r792(5) 0.s1(1) 59(2) 42(L) r2e(4) -6(2) -9(3) -r(2) si(2) 0.3896s(4) 0.07306(3) 0.31148(5) 0.51(r) 63(2) 41(r) L29(4) -8 (r) 2(3) -7(2) si (3) 0.39072(4) 0.19281(3) 0.s4143(5) 0.53(r) 67(2) 39(1) 137(4) -5(2) 7(3) -e (2)

o(1) o.3707o(r2) 0.21970(8) -0.00542(16) L.2L(2) 222(8) 50(4) 28?G4) s (s) -2e(e) -4(6) 0(2) 0.32801(10) 0.07851(8) -0.07887(14) 0.73(2) 9e(6) s6(4) r72(LL) -15(4) -27(7) -4(s) 0(3) 0. 36177(11) 0. 11174(9 ) 0.17054(rs) L.02(2) 136(7) 102(s) r72(r2) 7(5) -3 (7) 3s(6) 0(4) 0.32978(10)-0. 00240(8 ) 0.34095(14) 0.79(2) L02(6) s3(4) 2L7(r2) -27 (4) 2(7) -10(s) 0(s) 0.36064(11) 0.14017(8) 0.4159r(15) 0.93(2) L36(7) 64(4) 22r(L3) -1 (4) 0(7) -50(6) 0(6) 0. 32984(1r) 0.1740s(8) o.66829(L4) 0.87(2) 121(6) 7L(4) 184(12) -16(4) s2(8) -3(s) 0(7) 0.50000 0. 11001(rs) -0.00132(24) r.33(4) 56(e) 170(8) 362(2L) 0 0 -31(1r) 0(8) 0.50000 0.0s683(13) o.3L7sr(24) 1.1s (3) 41(8) 115(7) 4L9(22) 0 0 -1(10) 0(9) 0.50000 0. 18146(14) 0.5708r(2s) 1.34(4) 72(9) 148(8) 4OO(22) 0 0 25(10)

T. Equrvalenc tsocropac t, calcuiated from anisotropic temperatur^e factors 3J *Form of the (x105;: )1".i1 anisotropic temperature factor exp t r, L ;F; iJ

u-u J-L 306 GHOSE AND WAN: ZEKTZERITE

Table4 Zektzerite:interatomic distances (A) andangles (') (standarddeviations in parentheses)

The Na ?oLghedron lhe Si(1) Tetrdhedron - - si(1) - o(2) 109.92(8) Na -- 0(3) 2,554(2)(x2) 9(l)-Na-0(3') s9.24(B) si(1) 0(1) 1.623(2) 0(1) - - si(1) - 0(3) 105.s7(8) Na - 0(5) 2.667(2)(x2) 0(3)-Na-0(s) 58.24(5) (x2) si(1) 0(2) 1.593(2) o(1) - - si(1) - o(7) 110.60(1r) Na - 0(6) 2.368(2)(x2) 0(3)-Na-0(5') 79.60(5)(x2) si(1) 0(3) 1.533(2) 0(1) - - sI(1) - 0(3) 1l-0.08(8) Na - 0(1) 3.r2o(2)(x2) 0(3)-tla-0(6) 99.04(s)(x2) si(1) 0(7) 1.608(1) o(2) - - 0(7) 111.38(10) Na- 0(1') 3.225(2)(x2) 0(s)-Na-0(6) 109.4s(s)(x2) Mean I.614 0(2) si(1) - si(1) - 0(7) 109.12(11) Meanof 6 2.530 0(6)-Na-0(6') 73.31(8) 0(3) L09.45 Meanof 10 2.787 0(1)-Na-0(1r) 109.76(5)(x2) o(1) - o(2) 2.633(2) Mean 0(1)-Na-0(3) 53.21 (4) (x2) o(1) - o(3) 2.593(2) - 0(3) - 0(3') 3.587(3) 0(1)-Na-0(5) 101.86(5)(x2) o(r) 0(7) 2.651(2) - 0(3) - 0(5) 2.542(2)(x2) 0(1)-Na-0(6) 54.97(4) (x2) o(2) 0(3) 2.644(2) - 0(3) - 0(5') 3.342(2)(x2) 0(1')-Na-0(5) 50.52(4)(x2) o(2) o(7) 2.645(2) - 0(3) - 0(5) 3.746(2)(x2) 0(1')-Na-0(6) 14 .67(5) (x2) 0(3) 0(7) 2.647(2) 0(5) - 0(6) 4.II4(2)(x2) Meas 2,636 o(6) - o(6') 2.821(3) 0(1) - 0(1r) 3.5I8(3)(x2) The Si(2) Tet"ahedran 0(1) - 0(3) 2.593(2)(x2) - si(2) - o(4) 11r.79(8) 0(t) - 0(5) 3.t04(2)(x2) si(2)-0(3) 1632(2) 0(3) - - o(s) 102.44(8) 0(1) - 0(6) 2.6re(2)(x2) si(2) - 0(4) 1.594(2) o(3) si(2) - - - 0(8) 110.27(11) 0(1') - 0(s) 2.564(2)(xz) si(2) o(5) 1.630(2) 0(3) si(2) - si(2) - 0(8) 1.607(r) o(4) - si(2) - o(s) 10e.11(8) 0(1') 0(6) 1.460(2)(xz) - Mean I.6L6 o(4) - si(2) 0(8) 112.24(10) - si(2) - 0(8) 110.s7(10) :he L1 lefrahe@on 0(s) o(3) - o(4) 2.611(2) Mean 109.40 Li - 0(2) 1.950(4)(x2) 0(2)-Li-0(2') 0(3) - 0(5) 2.542(2) Li - 0(4) 1.957(4) (x2) 0(2)-Li-0(4) o(3) - o(8) 2.658(2) Mean 1.959 0(2)-Li-0(4'|) o(4) - o(5) 2.626(2) o(4)-Li-o(4') o(4) - o(8) 2.659(2) - 0(2) - 0(2') 3.161(3) llean o(s) o(8) 2.66r(2) 0(2) - 0(4) 3.596(2)(x2) Mean 2.636 0(2) - 0(4') 2.742(2)(x2) 0(4) - 0(4') 3.234(3) The si(3) TetPalrcdron Mean 3.179 si(3) - o(r) r.617(2) o(r) - si(3) - o(5) 104.44(e) - si(3) - 0(s) 1.627(2) o(L) - si(3) o(6) 109.4e(8) The zr )ctahedran - si(3) - 0(6) 1.s90(2) o(1) - si(3) o(9) 110 06(11) - - o(6) 112.0s(8) zr-0(2) 2014(I)(x2) 0(2)-zt-0(2t) 17O.84(7) si(3) - 0(9) 1.606(1) o(s) si(3) - - 0(9) 109.58(11) zr - o(4) 2.091(1)(x2) o(2)-zE-0(4) 91.u (s)(x2) Mean 1.610 o(5) si(3) - si(3) - o(9) 111.03(1r) zt - 0(6) 2.056(1) (x2) 0(2)-zr-0(4') 82.36(5)(x2) 0(6) - 109.44 Mean 2.074 0(2)-zr-0(6) 95.56(6) (x2) 0(1) 0(s) 2.564(2) Mean 0(2) - zr - 0(6') 91.10(6)(x2) 0(1) - 0(6) 2.61e(2) - 0(2) - 0(4) 2.973(2)(x2) 0(4)-zr-0(4') 49.37(8) o(1) o(9) 2.64r(2) 0(2) - 0(4') 2.742(2)(xz) 0(4)-zr-0(6) 9r.90(6)(x2) 0(5) - o(6) 2.668(2) - 0(2) - 0(6) 3.05e(2)(x2) 0(4)-zr-0(6r) r77 . s9(6) (x2) o(s) o(9) 2.642(2) - 0(2) - 0(6') 2.948(2)(x2) 0(6)-zr-0(6') 86.90(8) 0(6) o(9) 2.635(2) o(4) - 0(4,) 2.940(3) Mean of 12 90.03 Mean 2.628 0(4) - 0(5) 2.e80(2)(x2) 0(6) - 0(6') 2.827(3) Cation - Cation distffices Si-0-Sidngles Mean 2.93I - si(1) - si(1') 3.1265(11) si(1) - 0(r) si(3) 147.04(11) - si (1) - si(2) 3.1361(8) si(1) - o(3) si(2) L47.11(11) - si(r) - si(3) 3.1068(7) si(2) - 0(5) si(3) 147.60(11) - si(2) - si(2') 3.1625(11) si(1) - 0(7) si(1')152.77(17) - s1(2)- si(3) 3.1275(7) si(2) - o(8) si(2')1593s(17) - si(3) - si(3'i) 3.1319(1r) si(3) - o(9) si(3')154.26(17) zt 2-9654(2)(x2) Li - O(2) - zr 94.58(12) t-i - si(1) 3.044(4)(x2) Li - o(4) - zr 94.16(12) Zr- si(2) 3.020(4) Zt- Si(1) 3.4626(5)(x2) Zr- si(2') 3.4946(6)(x2) Zr- s1(3) 3.4445(6)(x2)

the thermal ellipsoidswith their standarddeviations hedral chains: (a) an edge-sharingNa polyhedral were calculatedusing the program ERnon (Finger, chain; (b) an octahedral-tetrahedralchain, formed 1972,private communication).The bond lengthsand by alternating LiO4 tetrahedra and 2106 octahedra anglesare listed in Table 4 and the dimensionsof sharing edges;and (c) a corrugatedsilicate double thermal ellipsoidsin Table 5. The averagestandard chain, with six-tetrahedralrepeat (Sechser-Doppel- deviation in Li-O, Na-O, Zr-O. and Si-O bond kette) (Figs. I a,b,c). lengthsare 0.004,0.002, 0.001, 0.002A and in O-Li- polyhedral chain O, O-Na-O, O-Zr-O, and O-Si-O angles0.20, 0.05, The l',la 0.06,0.09' respectively. The sodium atoms occur within cylindricalchan- nels,formed by the corrugation of the silicatedouble Description of the structure chains(Fig. 2). Of the ten Na-O bondswithin 3.25A, The crystalstructure of zektzeriteis a three-dimen- six are comparativelyshort, ranging from 2.3684,to sional framework,consisting of three typesof poly- 2.66'7A, forming a highly distorted octahedron; GHOSE AND IYAN' ZEKTZERITE

Table 5 Zektzerite:thermal ellipsoids (standard deviations rn with alternatingLiOo tetrahedralandZrOu octahedra parentheses ) (Fig. lb). Within the chain the Li-Zr distanceis (av. Angle ( ) vith respect to 2.965A.The Li-O distances 1.9594)within the Atom Axis mpl*tude (A) +a +b +c LiOo tetrahedron show very little variation, whereas the O-Li-O angles show a large variation, ranging rL 0.109 90 0 90 r2 o.r29 32(7) 90 58(7) from 88.9oto 133.3'. The Zr octahedronis nearly o.226 r22(2) 90 32(2) regular, the Zr-O distancesvarying from 2.056 to rI 0.102 90 88(17) 2(r7) 2.091A (av. 2.074A), and the O-Zr-O angles from ?z o.t26 0 90 90 !3 0. 128 90 2(r7) 92(r7) 82.4" to 95.6'. It shares one edge [0(6)-0(6') TI 0.066 90 0 90 2.827A1with an adjacentNa polyhedron. r2 0.067 84(9) 90 174(9) T3 (5) 0.075 u4 90 95(5) The corrugated double-silicate chain with six-tetrahe- rT 0. 073 r39(11) 58(10) 67(8) dral repeot T2 0.082 80(22) 42(22') 130(18) r3 0. 085 sL(27 (29) ) 65(26) 49 The most interesting part of the structure is the TI 0,012 r26(10) 40(e) 75(7) corrugated silicate chain with six-tetrahedralrepeat r2 0.081 r22(15) 97(11) 147(ls) 0.088 r28 (2O) L29<20) 62(2O) parallel to the c axis. Such a single silicatechain is si(3) P1 0.074 r04(9) 25(9) 70(6) connectedto an identicalchain acrossa mirror-plane r2 0.081 r4r (18) 89(13) 129(18) r3 0. 090 125(10) r1)(/) 46(8) by sharing corners, giving rise to a double-silicate chain (Sec/zser-Doppelkette).These double chains 0(1) T1 0.087 88(2) 3(4) 88(4) r2 0.121 76(5) 89(4) 166(5) contain three different four-membered tetrahedral 0.154 166(4 88(2) 104(4) ) rings(Fig. lc). ?I 0. 081 126(19) 52(20) 58(21 ) r2 0.095 8s(15) 46 (1r) 135(12 ) 0. 111 144(11) 112(10) 1-r7(10) The t h ree-di me ns ional framewo rk

0(3) 0.088 9s(5) 112(4) 22(4) Each octahedral-tetrahedralchain is connectedto y2 0.118 14(r2) 79(1r) 81(7) 0.130 r03(11) 25(7) 7O(4) four adjacent silicatedouble chains by sharing cor- ners. Likewise, each silicate double chain is con- o(4) rI 0. 075 r24 (7 ) 35(7) 84(5) r2 0. 106 109(16) 96(11) 160(14) nectedto four adjacentoctahedral-tetrahedral chains ?3 0.115 140(r9) r24 (L7) 7r (15) by corner-sharing.The Na polyhedralchains are con- 0(5) II 0.078 91(4) 39(6) 51(6) r2 0.1i9 3(28) 92(18) 87(22) nectedto a pair of silicatedouble chainson the one 0. 123 93(28) 129(5) 39(s) side and an octahedral-tetrahedralchain on the other 0 (6) !l 0.082 54(10) 104(9 ) 39( 10) by sharing polyhedral edgesand corners. A three- r2 0.102 82(8) 156(8 ) 113(7) 0.725 L43(7 ) r09(6) 60(6) dimensional ffamework structure is formed in this

TT 0.075 180 90 90 fashion (Fig. 2). The two good cleavages{100} and r2 0.134 90 109(7) L6r(7 ) o,L64 90 re(7) r09(7) {010}break the cation-oxygenbonds,leaving the sili- cate double chains intact (cf. amphiboles). 0(8) rI 0.065 r80 90 90 rz 0.133 90 r79 (L2) 9r(r2) r3 0.148 90 91(12) 1(12) Anisotropic thermal vibration 0(e) rI 0. 087 r80 90 90 The vibration of the zirconium and the !2 0. 139 90 s5(14) 145(14) thermal r3 0.155 90 35(23) ss (23) silicon atoms are nearlyisotropic, whereas that of the lithium atom can be representedby a slightly oblate spheroid(Fig. lb). Commensuratewith its environ- within the octahedron, the O-Na-O angles range ment, the thermal vibration of the sodium atom is from 58.2oto I17.6'. Four longerNa-O bondsrang- strongly anisotropic,the vibration ellipsoidbeing a ing from 3.1204 to 3.225A completethe Na poly- strongly prolate spheroid (Fig. la). The oxygen hedron. Each Na polyhedron sharestwo opposite atoms are mildly anisotropic, except those bridging edges[O(l)-O(l')] with two adjacentNa polyhedra oxygenson mirror planes, which are fairly strongly to form a chain parallel to the c axis (Fig. la). anisotropic,the maximum vibration directionbeing normal to the Si-O-Si plane (Fig. 1c). The octahedral-tetrahedralchain The Li tetrahedron shares two opposite edges Comparisonwith the tuhualite structure and the tuhual- tyPe [O(2)-O(4') 2.742A1with two Zr ocr.ahedraon either ite structure side, thereby forming a chain parallel to the c axis, Zektzerite is isostructural with tuhualite (Merlino, 108 GHOSE AND WAN: ZEKTZERITE

-rr,

, ,"Ort"rite: stereoscopicviews of the three structuralcomponents: (a) the Na-polyhedralchain; (b) the octahedral-tetrahedral chain, formed by alternatingLi tetrahedraand Zr octahedra;and (c) the corrugaled silicatedouble chain with six-tetrahedralrepeat

1969) and a synthetic orthorhombic phase tionshipsamong phasesisostructural with tuhualite NarMgrSi.Or,(Cradwick and Taylor, 1972).In tu- can be shown in terms of the cation sites: hualiteand in NarMgrSiuOru,the octahedral-tetrahe- M(t) M(2) M(3) M(4) dral chainsconsist of alternatingFe2+ tetrahedra and Fe3+octahedra, and alternating Mg tetrahedra and Coordination Mg octahedrarespectively. The angular distortion number9l046 within the Fe2+and Mg'+ tetrahedrarespectively in Na2M92SizOrc Na+(l) Na+(2)Mg'*(l) Mg'*(2) these two structures are closely comparable to the angulardistortion found within the Li tetrahedronin Tuhualite. zektzerite.The coordination of Na [specificallyNa(2) (Na,K)Fe'z+Fe'+Si.O,u in NarMgrSiuOruland the configurationof the silicate .0.5H,O3 I Na+,K+ Fe2+ Fe8+ double chains are closelycomparable in all three Zektzerite, structures.In NarMgrSiuOruthere is an additional LiNaZrSi.O'u I Na+ Li+ Zra+ Na-position[Na(l):x : 0.0000,y : 0.4099,z : 0.25681,which is vacant in tuhualiteand zektzerite. where I indicatesa vacancy. This Na site is nine-coordinated,with three short It is clear that provided the coordinationand total Na-O bonds rangingfrom 2.50 to 2.63A,and three valence balance requirements are satisfied,a large pairs ol long Na-O bonds ranging from 2.75 to number of combinationsof differentcations can pro- 3.06,4.The NaO, polyhedronis a tri-cappedtrigonal ducethe samestructure type. Some possible examples prism, the threecloser occurring at the cen- of other compounds of the tuhualite structure type ters of the prism faces.Because of the larger average are: cation-oxygendistance (2.81A), this is more site suit- 3The infrared spectraof tuhualite indicatethe absenceof water able for potassium.In tuhualiteK would presumably mofeculesin the structure(G R. Rossman,1977, private commu- nrefer this site over Na. The crvstal-chemicalrela- nication). GHOSE AND WAN' ZEKTZERITE 309

c I 0

O-b

Fig 2 A view of one-half of the zektzeritestructure projected along [00]; the secondhalf is obtained by reflectionacross a mirror plane(at r : t/z\,passing through the oxygenatoms, O(7), O(8), and O(9), therebydoubling the silicatechain (l Fig. I in Merlino, r969).

NarZnMgSiuO,u Narl-iFe3+Si.O,,a bonds can be classifiedinto threetypes: (A) oxygens bonded to one Si * Zr -| Li (or Na), (B) oxygens NarZn FeSi.O,, NarLiCr3+SiuOru bondedto two Si only, (C) oxygensbonded to two Si NarBeMgSi.O,, NarLiVs+SiuO,u * Na. The Si-O bondsinvolving oxygens of (A), (B), and (C) typesaverage 1.593 (+ 0.003)4, 1.607(+ NaKFe'z+Fe2+Si6O15NaMgFe3+Si.Oru 0.001)A,and 1.627(+ 0.015)Arespectively. NaLiTi'+SiuOru. The larger degreeof variation of the Si-O bond lengthsinvolving oxygenatoms of the (C)-type can Discussion be further rationalized on the basisof the length of the Na-O bond; when the Na-O bond is longer,the The Si-O bond lengths and the ^Si-O-Si angles in Si-O bond is shorter and uiceuersa. The shortestSi- zektzerite O bond tsi(3)-0(6), l.590Al involves the most The averageSi-O bond lengthswithin Si(l), Si(2), charge-deficientoxygen atom [0(6), e.s.v.sum 1.77], and Si(3) tetrahedraare 1.614,1.616, and l.6l0A a phenomenonfirst noted by Merlino (1969)in tu- respectively.The variation of Si-O bond lengths hualite. within each silicate tetrahedron reflects bond The Si-O-Si bond anglesfall into two groups: strengths receivedby the oxygen atoms from other thoseinvolving oxygens lying on mirror planesaver- cations,in additionto the contributionfrom silicon. age 155.7o(+ 4.2"), whereasthe other threewithin On the basisof the oxygen coordination the Si-O the single silicate chain average 147.6" (+ 0.6').

a This compound has been synthesizedduring hydrothermal These anglesare all larger than the averageSi-O-Si growth of quartz crystalsat Bell TelephoneLaboratories (K. Nas- angle(140') f,oundby Liebau(1961) in a largenum- sauand G. R. Rossman,1977,privatecommunication) ber of silicates.The larger Si-O-Si anglesare associ- 3r0 GHOSE AND WAN' ZEKTZERITE

ated with shorter Si-O bonds, in agreementwith the tuhualite, and ProfessorCharles T Prewitt, State University of molecularorbital calculationson silicatechain fras- New York at Stony Brook, Dr. Ronald C. Rouse, University of Michigan, and Dr. PeteJ Dunn, Smithsonian ments(Tossell and Gibbs, 1977). Institutron,Wash- ington, D C for reviews of the paper. This researchhas been supported in part by the NSF grant EAR 76-13373(Geochemis- Planarity of the corrugated single-silicqte chain try). The corrugated single-silicate chain has the References sequenceo( I )-Si(1)-0(3)-Si(2)-O(5)-Si(3)-O( l')- Si(l')-O(3')-Si(2')-O(5')-Si(3')-,the oxygen atoms Bond,W L (1951) Makingsmallspheres Reu Sci 1nstr.,22,344- being the bridging ones within the single-silicate 315 chain (Fig. lc). It is approximatelyplanar, the plane Cradwick, M E and H. F. W. Taylor (1972)The crystalstructure of NarMgrSiuor".Acta Cr)tstallogr.,828, 3583-3587. being nearly parallel to the (100) plane.The mini- Cromer, D T and D. Liberman (1970) Relativisticcalculation of mum and maximum deviationsof the bridgingoxy- anomalous scattering factors for X-rays. "/. Chem Phys., 53, gen atoms from the least-squaresplane definedby the l89r-1898 siliconatoms only are 0.284 and 0.454 respectively. - nnd J. B. Mann (1968) X-ray scatteringfactors computed The degreeof the corrugation of the chain is in- from numericalHartree-Fock wave functions.Acta Crystallogr, A24,32t-324 dicatedby the ratio of the unit length of the corru- Dunn, P J, R C. Rouse,B Cannon and J A Nelen (1977) gated chain us. the chain length when fully stretched Zeklzerite: a new lithium sodium zirconium silicaterelated to : out, which is l0.l64116.804 0.60. tuhuafite and the osumilite group. Am Mineral ,62, 416-420. Finger, L W (1969) Determinationofcation distribution by least The planarity of the three crystallographically-distinct squares refinement of single crystal X-ray data Carnegie Inst. four-membered silicate rings Wash Year Book. 2. 216-217. Germarn,G , P Main and M M Woolfson (1971)The appli- Within the double-silicatechains, three different cation of phaserelationships to complex'structuresIII The rings can be discerned,which are: optimum use of phase relationships Acta Crystallogr.,A27, 368-376 (r ) si(l )-o(7)-si(1')-o(3)-si(2)-o(8)-si(2, )-o(3, ) Karle,J and L L Karle (1966)The symbolicaddition procedure (2)Si(2)-O(8 for phase determination for centrosymmetric and non-cen- )-si(2')-O(s )-Si(3 )-o(e)-si(3' )-o(5, ) trosymmetric crystals ,4cr4 Crystallogr , 2l , 849-859. (3)Si( r)-o(7)-si(t')-o(1 )-si(3)-o(e)-si(3')-o( t,) Liebau, F. ( l96l ) Untersuchungeniiber die Grdsse des Si-O-Si- VafenzwinkelsAtta Crystallogr, 14, | 103-1109. The average deviations of the bridging oxygen - (1972) Crystal chemistry of silicon In K H. Wedepohl, atoms from the least-squaresplanes passedthrough Ed., Handbook ol Geochemistry Vol II,z3, l4-A-l-14-A-32, the siliconatoms only for rings(l), (2), and (3) are Springer-Verlag,Berlin Merlino, S ( 1969)Tuhualite crystalstructure. Science, 166, 1399- 0.11, 0.23, and 0.294 respectively.The acuteangles t40l betweenthe rings(1 ) and (2), (2) and (3), and (3) and Tossell,J A. and G V. Gibbs (1977) Predictionof 7'-O-Iangles ( I ) are 59.6", 52.8", and 6'7.7" respectively. from molecular orbital studieson corner-sharingmineral frag- ments (abstr ) Trans Am Geophys Union, 58, 522. Acknowledgments It is a pleasureto thank RussellBoggs, Seattle, Washington for the zektzeritecrystals, Professor George R Rossman,California Manuscript receiued,Juty 15, 1977, accepted lnstitute of Technologyfor discussionson the crystalchemistry of Jor publication, Nouember I, 1977.