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 sym- metry 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-axis in 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 mon- oclinic stilbite and orthorhombic stellerite,the crystal axes are not comparable(Kerr, 1977).Figure 1 showsthe orien- tation 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$e p, extinction angles sectorwas not observed,because this {010} p and2\ valuesof stilbite of various kinds of domains. The interaxial angles are 2V (Na + K)20 sector B extLnction extinction x a- (010) b- (101) (vt. Z) (to.5") (Jo.s") (+1") {oro } 44-45' € {oor} eo.os" 0.5' 43-45 'a o'29 {uo} go.3'l 1-t 2-2.5 43 {r-01} 90.os 0.5 32-33 the anglesshown in Table 2 are averagevalues' { 010} 42-44 Table 3 shows sodium and potassiumcontents and the 8 36-42 E {oor} 90.05 0.5 interaxial anglep in the {001} and {110} sectorsofthe two 6 o'45 go.49 J)-44 {1r.oJ 3-4 Poona specimens(A and B). Sodium and potassium con- 23 {101} I tents arJ the same in both sectorswithin error, and elec- the specimensfrom {010} 2.5-3 tron microprobe analysesshow that through 5 JO three localities are chemically homogeneous I {oor} 90.0s 0.5-r.5 the o'87 et al. (1978)suggested that E {110} so.4s 3.5-5 2.3-3 40-43 the sectors.Although Passaglia the stilbite crystals {101} stilbite is chemically heterogeneous, studied by us are chemically homogeneous,and structur- 32-34 { o1o} ally heterogeneous.Table 2 showsthat the interaxial angle 30 } {oor} eo.4s <4 sodium content decreases,and that the E 1'18 B'an$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-axis showsa 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.