Orderdsorder Structure and the Internal Texture of Stilbite

AmeticanMinualogist, Volume70, pages ElH2l, 1985

orderdsorder structure and the internal texture of stilbite

Mzumro Axrzuru,c,ND HIno$il KoNNo lnstitute of Minetalogy, Petrology, anil Economic Geology Faculty of Science,Tohoku (Jniuetsity, Sendai 980, Japan

Abstrrct

stilbite crystals from three localities (Eyi Ju, North Korea, Bomeki, Japan and Poona, India) consist of various growth sectors.With regard to X-rays, the {001} sector is ortho- homoge- rhom'bic,whereas the {110} sectoris monoclinig though the crystalsare chemically neous.Optical study shows that stilbite is triclinic throughout the crystal. The relationship betweenthe orthorhombic and monoclinic symmetriesmay be explainedby orderdisorder of tetrahedron rotations which occur during gowth. Small deviations from monoclinic symmetry may be due to Al/Si ordering.The sectortwinning is explainedby growth ordering. Introduction whereasthe c-axis-directionsare not the samethroughout the crystaldue to reflectiontwinning parallelto (001)' Stilbite is a common zeolite; the chemical formula Monoclinic stilbite is bounded by the (010)'(001), (110) is (Na,K),(CaMg)n*rAl,*r*2ySi2s-'-2rO7r' 28HrO with and (T0t) faces,and each face has growth hillocks or stri- a < 4.89 and -2.22 < y < +0.33 (Passagliaet al.' 1978). ations.A sectoris producedby crystal growth on eachface, There is no compositional gap betweenstilbite (a mono- whereasa domain is produced by crystal growth on the clinic sodium-rich phase)and stellerite (an orthorhombic vicinal faceof the growth hillock or striation; thus a sector sodium-poorphase). consistsof many domains. If the crystal symmetry is re- In someminerals, the symmetryas determinedby optics is lower than that determinedby X-ray diffraction. Such an optical property has been called an oPtical anomaly. Langeman(1886) studied the optics of stilbite, and found that it is triclinic. However,stilbite has been consideredto be monoclinic in X-ray work; Breck (1974) reported the spacegroup C2lm, and Slaughter(1970) and Galli (1971) reported the space groups C2 or Cm. Langeman (1886) tion twin plane occursparallel to (010)as well' Orientation observedsectors corresponding to the crystal macro-faces of the vicinal face on the crystal face is controlled by the and recogrfzed the triclinic nature of stilbite, but did not crystal classmmm of the orthorhombic lattice; correspond- observethe surfacegrowth patternsand correspondingdo- ing domains are related by the three refle9ti91 twin oper- mains within the sectors.Based on studies of the relation ations. Optical vibration directionsof the {110} and {101} betweensurface feature and internal texture, Akizuki and sectorslare shown in Figure 1. The {001} sectorsconsist of Sunagawa(1978), Akizuki et al. (1979),Akizuki (1981a'b) fine twin lamellaeparallel to the (001)face, and extinction and Akizuki (1984)have explained the optical properties directions incline only 0.5" to the a-axisin the Eyi Ju and and internal texturesof someminerals by atomic ordering Poona specimens. producedon sidefaces ofgrowth steps,suggesting a gener- Thin sectionscut in various directions were examined al mechanismfor sectorgrowth. Optical observation Both the Bomeki and Eyi Ju specimenswere obtained from the samehand specimensthat were analyzedby wet chemical methods. The Poona stilbites were analyzedby the surfacefeatures and internal texturesare similar' one of the presentwriters (Konno) (Table 1). Some optical (010) the surfaceof stilbite is composedof many observationsof Bomeki stilbite were carried out by Aki- face: small, thin stilbite crystalsgrown parallel to the (010)face' zuki (19E0).Different authors have used different crystal whose form is similar to the macro-crystal'The small stil- settingsfor stilbite, and evenin the usual settingsfor monoclinic stilbite and orthorhombic stellerite,the crystal axes are not comparable(Kerr, 1977).Figure 1 showsthe orientation usedin the presentstudy. In somefigures, the a-axis 1 Throughout this paper, the Milter indicesrefcr to monoclinic parallel to the crystal elongation is oriented in the vertical axes,though the interaxial angle f observedby X-ray methodsis (Passagliaet al'' l97E)' direction, becausethe a-axisis common through the sector, that of the pseudo-orthorhombiclattice 0003-{otx/85/0708-OEI 4$02.00 814 AKIZUKI AND KoNNoi ORDER-DrS1RDERAND'TEXT|RE oF srILBrrE 815

Table l' chemical analyses,refraclive indices (a, p, y) and crystal correlate to the large m and small ag(ll0) faces, and extend sizesof stilbite --F through fine growth zoning parallel to the (110) face. ',, ,i,, 'n:'::l':';;":::1T'?"n"i#'fi:f:"':"r'$i"i: ,,'r, g.ro:th on the (010) face,and in yicin- Ar2o3 15.4r 16.13 ,r.rl the repetition of tw; al faces inclined rioz nd ,u' symmetrically with respect to the (010) face. The Fe2o3 nd dd vicinal face is distinctly observed in both poona Mno nd nd and Eyi Ju specimens (Fig. 5), and the face angle is about 9o. The striations are simple and fine in both poona and Mco 0.06 nd nd Eyi Ju specimens, whereas they are somewhat rough in the cao 7.s7 7.7s 7.s4 Bomeki specimen. Na2o o'24 o.42 1.08 {001} sector: the (010) thin section shows fine twinning Kzo o'os 0.03 0.r0 parallel to (001).The extinction in the (010) section, +nzo whici 18'54 !2'as 16.23 inclines 0.5" to the poona -Hzo a-axis, is sharp in both ana evi s'8e 1.e3 Ju specimens, whereas it varies within 4" in the Bomeki

T0161 100.34 99.89 99.55

0 I.485 ! I.488 L.492 B r.4sz g j r.+so 1.soo o I r.495 1.498 1.502

o ! 1.489 u i r.rse i 6 t t.4gt indicated in Figure 6; the orientation in other sectorsis similar to this. cryatal 17ot) face: the (101)face occurs on the specimensfrom size(cm) r.rxu.)xl 3x0.5x2 1x0.3x0'3 Bomeki and Eyi Ju, but not on the specimenfrom poona. The (101) face of Bomeki stilbite is divided into several 1. Eyi Ju sttlbite (Kozu er al,, 1937) (Nao. azKo. oI ) (Caq. oqltgo. oq)A1e. a oSTzz,qsotz.29.5Ezo

2. Poona stilbite (Nao. aeKo. oz)Car . g gAl g. oe Slz z. og O7 2. 29.96H2O

3. Bomeki (Tantguchl stilblre and Abe, lggl) (11O)m (Nar . ooKo. oe)Car . ozAle. ge Slz gg Oz 28,9gHzO e. z. )o

bite crystal consistsof two large vicinal faces with sym_ growth T"1l"ul stepsparallel to m and E(110) faces(i.ig. 2). No growth centeris observedon the surface,suggesting a two-dimensionalgrowth mechanism. {010} sector: in thin section, the symmetrical vicinal facesproduce two correspondingdomains, whose extinc_ tion directions incline about 1.5. (poona specimen)to the =(OO1)o a-axis, and are in (001) reflection twin relation with each

Fig. 1. Schematicsketch of a (010) section of Eyi Ju stilbite. This showsgrowth sectorsand the relation betweencrvstal orien- tations of monoclinic stilbite (m) and orthorhombic siellerite(o). The (fi)l)- and (-t0l)- are normal to the plane of the figure, whereas(110)- is striationsparallel to the c-axis. inclined to the plane ofthe figure.The {ll0} and sectors,which are indicated by solid {1 10} sector:the sectionnormal to the a-axisshows four {T0l} and dasheAhnes, are optically symm€tricalwith eachother. The equivalent sectors,as shown by Langeman (lgg6). {001} sectorsconsist of {110} the fine reflectiontwins parallel ro (001). Figure 4 depictsthe internal Optic vibration directions texture of thJ{ll0} sector which are symmetrical with observed respect to the twin plane are repre_ in the (010)section. Roughly vertical,ihici stripes sentedon the figure. TEXTURE OF STILBITE 816 AKIZUKI AND KONNO: ORDER-DISORDERAND

s€ctorconsists Table 2. Na + K content, interaxial an$e p, extinction angles sectorwas not observed,because this {010} p and2\ valuesof stilbite of various kinds of domains. The interaxial angles are

2V (Na + K)20 sector B extLnction extinction x a- (010) b- (101) (vt. Z) (to.5") (Jo.s") (+1")

{oro } 44-45' € {oor} eo.os" 0.5' 43-45 'a o'29 {uo} go.3'l 1-t 2-2.5 43 {r-01} 90.os 0.5 32-33 the anglesshown in Table 2 are averagevalues' { 010} 42-44 Table 3 shows sodium and potassiumcontents and the 8 36-42 E {oor} 90.05 0.5 interaxial anglep in the {001} and {110} sectorsofthe two 6 o'45 go.49 J)-44 {1r.oJ 3-4 Poona specimens(A and B). Sodium and potassium con- 23 {101} I tents arJ the same in both sectorswithin error, and elec- the specimensfrom {010} 2.5-3 tron microprobe analysesshow that through 5 JO three localities are chemically homogeneous I {oor} 90.0s 0.5-r.5 the o'87 et al. (1978)suggested that E {110} so.4s 3.5-5 2.3-3 40-43 the sectors.Although Passaglia the stilbite crystals {101} stilbite is chemically heterogeneous, studied by us are chemicallyhomogeneous, and structur- 32-34 { o1o} ally heterogeneous.Table 2 showsthat the interaxial angle 30 } {oor} eo.4s <4 sodium content decreases,and that the E 1'18 B'an$e decreases as the {r1o} 90.62 35-36 of the p of the {001} sectoris smaller than that {110} { 10u I 22

(010) a-(0L0) qtd b-(101) are exti.nctin aqles fm 4-ei8 on md frcm b-mLs on (101) ,ectiil,' respeetiuely'

blocks without growth hillocks, whereasthe face of Eyi Ju stilbite consists of one or two blocks that have growth hillocks with two curved vicinal faces.

comblike pattern with each other (Fig' 1)' The section normal to the a-axisshows a rhombic {101} sector(Lange- man, 1886,Fig.27), in which some indistinct domains are observed. X'raY and chemical enalyses separatedfrom The {001}, {110} and {101} sectorswere the thi& sectionsnormal to the a-axis or parallel to (010)' and the 204 and 2d diffraction peaks (in the pseudo-

Fig. 2. Reflection interferencephotomicrograph of small stilbite angles p grown parallel to eachother on (010)'The surfaceconsists -20roz),-calculat.d the pseudo-orthorhombic interaxial oyr,i"l, faces with symmetrical steps parallel to m and were ftom the equation (90' + 2/66L: fl oi t*o ni"inul p-angle the 10).The e-axisis vertical.Poona stilbite' t 0.05")ofPassaglia et al. (1978).The of {010} 1q(f AKIZUKI AND KoNNo: 2RDER-DIiORDERAND TExruRE oF srrLBrrE 8r7

influence only on the Si(2) and Si(5) sites during growth, and thesewill be preferentiallyoccupied by aluminum ions. The Si(5)site sharestwo oxygenswith an adjacentsodium ion, whereasthe Si(2) site is linked to one such oxygen. This might suggestthe aluminum occupancyof Si(5)to be greater than that of Si(2).Galli and Alberti (1975)suggest_ ed that the Si(5) site is preferentially occupied by alu_ minum. The principal differencebetween the orthorhombic and monoclinic structures involves the tetrahedra directly linked to the sodium ions (Fig. 8); theseare rotated in a clockwise manner in the monoclinic structure relative to their configuration in the orthorhombic structure. Galli and Alberti (1975)suggest that the causeof the rotation is repulsion betweensodium and calcium ions. Consequently it is necessaryto consider the effect of sodium on the growth of stilbite. In stilbite, the two sodium ions bond to two O(2) oxygensand some HrO molecules,and not to O(l) oxygens(Fig. 8). Thus the immediatesurroundings of sodium are differenton the (110)and (l1l) faces,that are equivalentin orthorhombic stellerite.The local chargebal_ ance will be diflerent on the two faces of stilbite, and a Fig. 3. Cross polariznd photomicrograph of a thin section more balanced atomic arrangementmay develop during parallel to (010).The domains(m and m) correspondto rhe vicinal growth. If the clockwise-rotatedstructure facesof the small crystalson (010), shown in Figure as shown in Fig. 2. The a_axis 8 is produced is vertical.Poona stilbite. on the m(110) face, the counter-clockwise rotated structurewill occur on the symmetricalm(ll0) face, resulting in monoclinic, twinned {110} sectors(Fig. 3). Be_ causethe tetrahedraSi(5a) and Si(5b)have equivalentenvi_ ronments with respect (001) sector. According to the chemical formulae of the three to calcium ions on the face of the orthorhombic specimens (Table 1), the substitution of sodium structure,the clockwiseand counter- (+potassium)for calciumis negligible.

Discussion

in X-ray studies,the true symmetry of the (average)unit cell is triclinic. Correlation betweensurface gowth features and internal textures suggeststhat the internal textures were produced during growth. The optical properties and internal texture may be interpreted by the atomic order_ disorder growth sector mechanismsuggested by Akizuki (1981a)for analcime.

Orthorhombic-monoclinic structures In the crystal structure of stilbite (Fig. g), calcium ions are near the middle of the channels;they are coordinated by eight water moleculesand do not bond to the frame_

Fig. 4. Cross polarized photomicrograph of m{1t0} sector in thin section parallel to (010). The m and m{ll0} faces'and corre- HrO moleculesat the crystal surface, will have a direct sponding domains are seen. The a-axis is veriical. poona stilbite. 818 AKIZUKIANDKoNNo:2RDER_DISaRDERANDTEXTUREoFSTILBITE

the a'axis.The (001)face, which is normal to the Fig. 5. Crosspolarized photomicrograph of a {001} sectorin thin sectionnormal to the kink' The thick arrows show contact paper, kinks at some placesshown bythin arrows, and the s€ctor twin plane correspondsto planesof parallel growth crystals.The b-axisis horizontal.Poona stilbite.

(Galli, 1971).If orthorhombic cellswithout sodium are produced at growth steps,the following monoclinic cells may rotate either clockwiseor counter-clockwise'Thus, in the of the monoclinic cells {001} sector,the rotation direction b""oa.. more disorderedas the numbr of orthorhombic p cells (without sodium) increases.The interaxial angle of the pseudo-orthorhombiclattice closesto 90" in all sectors as the sodium content decreases,and furthermorethe angle that of the B of the {001} sector becomessmaller than {110}' sectors in thesame crYstal. Theseideal symmetriesare in agreementwith those observedby X-ray diffraction and optical microscopy' Stel- lerite is orthorhombic. The diffraction pattern of a whole stilbite crystal shows a mixture of monoclinic and orthor- phases.If the powder pattern is made using a to the a-axis. hombic AKIZUKI AND KoNNo: ORDER-DIiDRDERAND TExruRE oF srILBr.rE El9

Bllrol

2b' Ct CE Ct lCuKsl Fig. 7. X-ray diffractionpeaks of {001} and {ilO} sectorsin two differentspecimens from PoonaIndia. sector of the large crystal, the pattern will be monoclinig orthorhombic or mixed. No chemicaldifference is observcd between passaglia the {001} and {110} sectors. et al. (1979) l!. noted orthorhombic, monoclinic o or mixed stilbite from o many localities,and suggestedthat the stilbite is chemicallv

Fig. 8. Part of the structure of stilbite projected along the b_ axis. Numbers adjacent to calcium and sodium give the heights of Table 3. Sgdiumand potassium contents & internalangle in the the atoms in thousandthsof a cell edge.Rotations of the-tetra_ {001}and {110}sectors oftwo diff;rentpoona stilbite crystals (A hedra and sodium atoms are shown by curved arrows. Repulsion between sodium and calcium ions is shown by straight arrows. Also, schematic internal texture is shown below. The m and speclEen sector Na2o(wr. Z, KZO(ut. iO B m(l 10)are inclined to the plane of the figure, whereasc and (001) are normal to the plane of figure. Modified from Galli and Alberti A {o0r} 0,44 0.03 90.05" (r97s). { uo } o.4o 0.03 90.490

B { 001} 0.85 o.02 90.05. {u0 } 0.8s 0.02 90.49" heterogeneous.We suggestthat stilbite is chcmicallyhomo- lhe agLee - geneous tr2ezo+ 21zoi are neaswed. to be but structurally heterogeneous. 0.020 md 0.2o in the t001\ md {1L0} eecto?s, ?espec_ ti,ueLy, Triclinic structure though the peaks ae mo?e or Less broad,, especialT.y in the {110} aeeto?a. Itet chmical analu_ Small deviations from monoclinic symmetry may be ex- aea. plained by the following mechanism.According to the ideasof Akizuki (1981a),the Si(2)and S(5) siteseach split 820 AKIZUKI AND KONNO: ORDER-DISORDERAND TEXTURE OF STILBITE

(010),which is a perfect cleavageplane, cuts a minimum number of Si-O-Si and Ca, Na-O bonds (Galli, 1971)' growth splitting occursin the direction normal to the (010) plane during growth, and strain decreases' Conclusion sym- The {T01} and {001} sectorsshow orthorhombic metry d'ue to the disorderedrotation of tetrahedra which o"",r. ott the side facesof stepsformed during growth' The ordered rotation of {110} sector is monoclinic due to the thr tum. tetrahedra.Furthermore, the stilbite structure re- duces to triclinic symmetry, becausean ordered arrange-

into two sitesduring growth. Assuminggrowth to be in the b-direction in Figure 8, the S(5a) tetrahedron forms after the two sodium ions during growth, whereas the S(5b) forms prior to the sodium ions. This suggeststhat the Si(Sa)site will be preferentiallyoccupied by aluminum due Acknowledgments the other hand' to the local effect of the sodium ions. On We thankProfessor F. C. Hawthorne,University of Manitoba' the site is the Si(Sb)site will be occupiedby silicon,because Canada,for valuablesuggestions and Englishcorrection of the produced before the sodium ions. If the crystal gtows on manuscript.Also, we thank ProfessorE Galli, Universityof and Universityof Mani- !0(110),the Si(2a)site will form after the sodium ions Modena,Italy, and Professor R. B. Ferguson, will be occupied by aluminum ions; the Si(2b) site is toba,Canada, for criticalreading ofthe manuscript' formed before the sodium ions and will be occupied by at both the Si(2)and Si(5) silicon. Due to the Al/Si ordering References sites, the C2lm symmetry is reduced to Cl in domains Akizuki, Mizuhiko (1984) Origin of optical variations in corresponding to the symmetrical vicinal faces on (010) grossular-andraditegarnet. American Mineralogist, 69, 328' (Fig.2). 338. partial view of the stilbite structure Figure 9 shows a Akizuki, Mizuhiko (1981a)Origin of optical variation in analcime' tetrahedraare normal to the a-axis; only the Si(2)and Si(5) American Mineralogist, 66' 403-4@- by shown.Growth stepsnormal to the figure are indicated Akizuki, Mizuhiko (1981b)Origin of optical variation in chaba- the letters A and B. If the crystal grows on step A, alu- zite.Lithos, 14,17-21. minum ions will preferentiallyoccupy the Si(2a),Si(2d) and Akizuki. Mizuhiko (1980) Texture of stilbite from Bomeki, the of Si(5b) sites under the influence of the sodium ions, and suburb of Sendai,Japan. Journal of the JapaneseAssociation Economic Geologists, 75' silicon ions will occupy the Si(2b),Si(2c) and Si(5a)sites' Mineralogists, Petrologists, and Also, the aluminum occupancymay be different between 38-43.(In JaPanese). Mizuhiko and Sunagawa Ichiro (1978) Study of the the Si(2a) and Si(2d) sites, becausethe immediate sur- Akizuki, sector structure in adularia by means of optical microscopy' are different with respectto the Na betweenthe roundings infra-red absorption, and electron microscopy' Mineralogical surface,and the diad axis disappearsfrom two site; on the Magazine,42"453462. sector may triclinic C1 and the structure. Thus the {001} Akizuki, Mizuhiko, Hampar, M. S. and Zussman'Jack (1979)An vicinal faces the domains correspondingto the symmetrical explanationof anomalousoptical propertiesof topaz' Mineral- A and B are in a twin relation (Fig. 5). Galli (1971)suggest- ogical Magazine, 43,237 -24L. ed that in stilbite, the Si(5)site splits into two sites,Si(5a) Srect, O. W. (fSZ+)Zeolite Molecular Sieves'John Wiley & Sons' and Si(5b). Inc. in natural stilbite' Neues The sector is similar to the {001} sector,and the eernf, P. (1964) Ionic substitutions {10U Mineralogie,1965' 198-208' structure.is orthorhombic or nearly orthorhombic' The Jahrbuchfiir AKIZUKI AND KONNO: ORDER-DISORDERAND TEXTURE oF STILBITE 821

Galli, Ermanno (1971)Refinement of the crystal structure of stil- Harmotom, Phillipsit und Desmin. Neucs Jahrbuchfiir Miner- bite. Acta Crystallographica,827, 833-84f. alogie,Geologie und Palaeontologie,2, 83-141. Galli, Ermanno and Passaglia,Elio (1973) Stelterite from Vil- Passaglia,Elio, Galli, Ermanno, Leoni, Leonardo and Rossi. lanova Monteleone,Sardinia, Lithos, 6, 83-90. Giuseppe(1978) The crystal chemistryof stilbitesand stellerites. Galli, Ermanno and Alberti, Alberto (1925)The crystal structure Bulletin de Min6ralogie,101, 368-375. of stellerite. Bulletin de la Soci6t6francaise de Min6ralogieet du Slaughter,Maynard (1970)Crystal structure of stilbite. American Cristallographie,98, I l-18. Mineralogist, 55,387-397. Kerr, P. F. (1977) Optical Mineralogy,4th ed. McGraw-Hill Book Taniguchi, Masahiro and Abe Tomohiko (1981) Stilbit€ from Company,New York. Bomeki, Miyagi Prefecture,Japan. Journal of the JapaneseAs- Kozu, Shukusuke, yoshinori Seto,Kunikatsu and Kawano (1932) sociation of Mineralogists, Petrologists and Economic Geol- New occurrence of stellerite from Eyi Ju Mine. Journal of the ogists,76,32+330. JapaneseAssociation of Mineralogists,petrologists, and Eco- nomic Geologists,lE, 5l-5E. (In Japanese) Manuscript receiued, March 27, 1984; Langeman, (1886) Ludwig Beitrdgezur Kenntnissder Mineralien: acceptedfor publication, February 5, 1985.