American Mineralogist, Volume 73, pages 1434-1439, 1988

Epistilbite: Symmetry and twinning Mrzunrro Axrzuxr Institute of Mineralogy, Petrology,and Economic Geology, Faculty of Science,Tohoku University, Sendai 980, Japan Hrnorsucu Nrsnroo Hiruzen ResearchInstitute, Okayama University of Science,Kamifukuda, Maniwa-Gun, Okayama 717-06, Japan

Ansrn-q.cr Two kinds of sectoraltwinning are observedin epistilbite: one is that of a structure with topologic symmetry C2/m, and,the other is due to (Al,Si) ordering. Two-dimensional atomic arrangementsare similar on the (001) and (l0I) surfacesof epistilbite. Of the two kinds of atomic arrangements,the one that is presumably more stable will develop on both crystal surfaces;precession photographs have confirmed that this is the (l 01) arrange- ment. The (101) face is normal to the (010) mirror plane in c2/m symmetry, and therefore the two symmetry sites are equivalent on the (101) growth surface,resulting in an (Al,Si)- disordered,monoclinic structure in the { 101} sectors.Since the (l 10) facesare inclined to the mirror plane, the two symmetry sites in the crystal are not equivalent on the surface, resulting in an (Al,Si)-ordered, triclinic structure in the four {ll0} sectors, which are related by both the (100) and (010) sectoralrwin planes.

INrnooucrroN Oprrcal oBSERvATToNS Epistilbite,Car'(Na,KXAl6Sir8Oo8)'l6H,O, is azeolite Epistilbite crystalsfrom two localities(Fossarfell, Be- that is found in cavities of volcanic rock. The composi- rufiord, Iceland;Kuroiwa, Niigata Prefecture,Japan) were tion of the framework is nearly constant, and the ratio observed under reflection interference and transmission Ca: (Na * K) ranges from 9 to 2 (Gottardi and Galli, polarized optical microscopes.Crystal forms of epistilbite 1985). The most common habit of epistilbite crystals are similar in the specimensfrom the two localities (Fig. possessesthe simple combination of {ll0}, {001}, and l). A (100) twin is indicaredby the labeleddashed line {0ll} with small {010}. (100) twinning is common in on the crystal form, which looks like a single (i.e., un- the crystal. twinned) crystal; that is, the (100) twin plane is coincident Theepistilbitestructureisrelatedtothatofmordenite. with the boundary betweenthe two (101) faces.Speci- Kerr (1964) suggesteda pseudo-orthorhombicstructural menssrudiedconsistof(101),(ll0),(010),and(011)faces, model for the epistilbite framework from X-ray powder and correspondinggrowth sectorsare observed between data. Perrotta (1967) refined the structure in the space crossedpolars. Crystal faces,especially (101-)and (ll0) group C2/m. The tetrahedra that are coordinated by the faces, consist of small parallel-grown crystals, and the (Ca,Na) atoms on the mirror plane are preferentially oc- correspondingdomains are observedbetween crossed po- cupied by Al atoms. Slaughterand Kane (1969) and Al- lars as well. The optical orientation of each sector was berti et al' (1985) explained the epistilbite structure as determined within l' by the crystal morphology, (010) noncentrosymmetricwith spacegroup C2. Galli and Ri- cleavageplane, and the (100) and (010) twins in rhe sec- naldi (1974) describedchemical compositions, crystallo- tions, with some exceptions. The optic axial angle was graphic data, and densities of 13 epistilbite crystals. measuredwithin l" by both conoscopicand orthoscopic On the basisof studiesof the relationship betweensur- methods on a universal stage,though the error may be face features and internal textures of some and slightly larger than l" in some sectors showing diffuse otherminerals, AkizukJ (l98la, l98lb, 1984,1985, 1986, extinction. 1987a, 1987b, 1987c),Akizuki and Sunagawa(1978), Akizuki and Konno (1985)and Akizuki et;1. (1979)in- Epistilbite from Fossarfell,Iceland terpreted their optical properties and sectoral twinnings The {101} sector. Figures 2a and 2b show growth pat- as being produced by atomic ordering on side faces of terns on the (101) face and growth sectorsand domains growth steps,which are inclined to the crystal face. Fur- in the sectionnormal to the c axis. The (10I; faceconsists thermore, Akizuki (1987b) suggesteda general mecha- of small parallel-grown crystals with circular-growth hil- nism for the formation ofgrowth sectors.These concepts locks, which are separatedfrom each other by straight are applied in this paper to epistilbite. steps. The {l0I} sector consists of small domains. Al- 0003-o04x/88/rll2-r434$02.00 1434 AKIZUKI AND NISHIDO: SYMMETRY AND TWINNING OF EPISTILBITE 1435

Y a

Fig. l. Generalcrystal form of epistilbite.The crystalshows (100)twinning between crossed polars without exception, though theform is single.D(010), e(101), rr(1 l0), z(01l) though the growth pattern on the surface suggeststhat each domain has the same crystallographic orientation, each domain shows wa\ry optical extinction, that is, the optical vibration direction Iis not always parallel to the D axis, suggestinga small deviation from monoclinic symmetry. Figure 3 showsa composition plane of a (100) twin that correspondsto the boundary between the two {101} sectors.The compositionplane, which is inclined to the (100) plane, is a fine comblike plane. Except for this comblike twin plane, no twins are observed within the {l0I} sectors.The extinction directionswith respect to the c axis are differentbetween the two {l0l} sectors, though their crystal orientations are related by the (100) twinning: one is about l0o, and the other is less than 5'. The (010) sections,which were variously inclined, as is cleady seenon a universal stage,show lamellae approx- imately normal to the (101) face,and endsof the lamellae correspondto the parallel-growncrystals. The optic axial angle,which was measuredin the (100) section, is 2V, : 46'. The {110} sector.The { 110} sectorhas small domains, by the (100) twinning (Fig' 2b). some of which are related Fig. 2. Interferencereflection optical photomicrographof Fine growth bands parallel to the (l 10) face develop grorvthfeatures on the (l0I) face(a), and cross-polarizedpho- through the domains in the thin section normal to the c tomicrographof the thin sectioncut normal to the c axis (b)' axis. The I vibration directions are approximately sym- The sectionconsists of the {tol} (centraland lower portions) metrically inclined about 2'to the b axis in the { I l0} and and {ll0} (upperportion) sectors. The {l0I} sectoris divided growthbands par- {110} sectors, though they vary slightly from place to into smallsectors, and the { I 10}sectors have place, suggestingtriclinic symmetry. Therefore, the (010) allelto the (110)face. The extinctiondirections of the {110} (010)plane. The b axis is "mirror plane" is really a twin plane. Thus, the four tri- sectorsare inclinedabout 2oto the horizontal.Epistilbite from Fossarfell,Iceland. clinic {ll0} sectorsare related by the (100) and (010) twins (Fig. 2b). Figures4a and 4b show the growth pattern on the (1 10) B sector correspondsto the (ftkO) face, and the C sector face and the correspondinggrowth domains in the { 110} to the (l0i) face.Both the A and B sectorsmay be tri- sector,which are observedin the (l l0) section.The (110) clinic, and the C sector may be monoclinic. stria- face consists of curved growth steps and parallel-grown The {010} sector.The (010) face consistsoffine the epistilbite crystalswith vicinal faces.Figure 4b showsdo- tions parallel to the c axis. The thin sectionnormal to parallel b mains correspondingto the vicinal faces of one parallel- c axis shows fine polysynthetic twinning to the grown crystal. The optic orientations of the three small axis in 1t1e{010} sector, which is correlated to the stria- domains (A, B, and C) are shown by stereographicpro- tions on the (010) face. The symmetry is reduced slightly jections in Figure 5. The thin section was cut in the di- from the monoclinic orientation. rection parallel to the (ll0) face, and the crystal orien- Epistilbite from Kuroiwa, JaPan tation was confirmed by the (010) .The optical of many orientation of each sector, however, varies within about The {10i} sector.The (101) surfaceconsists flat with some 3obecause ofthe diftrse extinction. The A sector corre- parallel-grown crystals, whose surface is (010) between spondsto the growth face parallel to the (l l0) face' The simple steps.In the (101) and sections 1436 AKIZUKI AND NISHIDO: SYMMETRY AND TWINNING OF EPISTILBITE

TABLE1. Chemicalanalyses and unit-cellparameters of epis- tilbitecrystals

Fossarfell(wt%) Kuroiwa(wt%) sio, 5779 58.22 Alro3 17.62 | /.JC FerO. 002 0.02 Mgo 0.02 0.01 CaO 8.21 8.46 Na.O 1.39 0.59 K"O 0.06 0.03 H,O(+) 12.21 11.92 H,O (-) 3.10 3.68 Total 10042 100.28 B.E- +1.9o/" +5.8V" Cationson the basisot 48 oxygens Si 17.67 17.82 AI 6.35 6.26 Fe 0.00 0.00 Mg 0.01 0.00 Ca 2.69 2.77 NA 0.82 0.35 K 0.02 '12.45 0.01 H,O(+) 12.17 H.o(-) 3.16 3.76 a (A) s.089(2) e.0s9(2) b (A) 17769(4) 17.765(3) c (A) 10.237(2) 10.239(2) Bf) 124.57(2) 124.s8(21 v(A.) 1361(1) 1363(1 ) Nofe: Numbers in parenthesesare esd's in the last decimal olace 'Balance error

ties are similar in the unit-cell parameters,though the a Fig. 3. Cross-polarizedphotomicrograph of { l0I } sectorsrn cell edgeis slightly longer in the Kuroiwa specimenthan the(010) thin section.The crystalhas two (l0I) facesand two in the Fossarfell specimens.In powder-diffraction pro- corresponding{ I 0I } sectors,which are related by the(l 00)twin- files, no splitting of peaks was observed in either speci- ning with comblikecomposition plane. The c axis is vertical. men; Epistilbitefrom Fossarfell,Iceland. that is, no deviations from monoclinic symmetry were found in the crystalsin the present X-ray analyses. Because of pseudo-orthorhombic symmetry, atomic crossedpolars, the {101} sectoris seento consistof the arrangementsalong both the (001) and (101) facesare domains conesponding to the parallel-grown crystals on similar. It is not clear from morphological observation the surface. The optical extinction is sharp, and the I which face appearson the surface of the (100) twinned parallel direction is to the 6 axis, suggestingmonoclinic crystal. A single crystal with a natural face [either (001) symmetry. The comblike twin plane is observedbetween or (l0l)l at the crystal edge,which was obtained from the two {101} sectorsas well. the thick (010) section, was observed by a precession The {110} sector.The (ll0) section shows a texture camera.This observationshows that (101) faces,not (001) similar to that of the crystal from Fossarfell.The optical faces,appear on the surfacein the structural setting. vibration direction I is inclined about 2 to the D axis, suggestingtriclinic symmetry as well. DrscussroN The {010} sector.Growth featureson the (010) faceand Low symmetry in showing so-called optical internal textures in the {010} sector are similar to those anomalies had not been detected by X-ray analysis for of the crystal from Fossarfell. many years, and therefore its origin was attributed to crystal strain. The low symmetries, however, have been Crrpnrrc,tI, AND X-RAy ANALysES observed by recent X-ray and neutron analysesof some Wet-chemical analysesand unit-cell parametersof the minerals with optical anomalies such as garnet (Tak6uchi specimensused for the study are indicated in Table l. et al., 1982),topaz (Akizuki et al., 1979; Pariseet al., No differencesin chemical composition were observed 1980), (Mazzi and Galli, 1978),stilbite (Galli, betweengrowth sectorsby electron-microprobeanalysis. l97l; Akizuki and Konno, 1985),chabazite (Mazzi and The unit-cell parameters were derived by least-squares Galli, 1983),and edingtonite(Akizuki, 1986).Their in- refinement of powder-diffraction data correctedby using ternal textures, which are observed between crossedpo- synthetic fluorophlogopite (Nl3S 675) as an internal lars, are correlatedwith the surfacegrowth features.How- standard. Also, both the specimensfrom the two locali- ever, yugawaralite (Akizuki, 1987b), (Artioli AKIZUKI AND NISHIDO: SYMMETRY AND TWINNING OF EPISTILBITE r437

m

Fig. 4. Interferencereflection optical photomicrograph of growth featureson the (l 10) face (a), and cross-polarizedphotomi- crogiaph of the thin section parallel to the (l l0) face (b). The c axis is vertical. Three small sectors(A, B, C), which may correspond to the three vicinal faces,(1 l0), (rm), and (101), are represented.Epistilbite from Fossarfell,Iceland.

et al., 1985;Akizuki, 1987a),and (Akizuki, The twinning of structure with symmetry A/m 1985), which show clearly lower symmetries between Figure 6 shows a partial crystal structure projected along crossedpolars, show no evidenceof the lower symmetry the D axis of epistilbite. If the composition plane between by X-ray diffraction. Also, the secondgoup of the min- the { 101} sectorsis inclined to the (100) plane, strain will erals showsthe internal texturesthat are correlatedto the occur along the boundary. However, the inclination of surfacegrowth features,and therefore, their internal tex- the comblike composition plane is small, reducing the tures and optical properties were explained according to strain. Some domains in the { I l0} sectorsare related by Akizuki's theory of sectoraltwinning. The atomic order- the (100) twinning. The two-dimensional atomic arrange- ing, which reducesthe symmetry in X-ray analysis,may ments are similar on the (110) and 11t0) faces,because be too little in such minerals for detection by X-ray dif- of the pseudo-orthorhombic symmetry. If the (1 l0) face fraction. Although epistilbite crystals used for the study of the parallel-growncrystal is produced on the (l l0) face Co not show low symmetry by X-ray analysis,they have of the host, the (100) twin will result. Sucha relation does optical characteristicssimilar to those of garnet, topaz, not occur in the {101} sector,and thereforeno twins are and other zeolites.Therefore, the symmetry and twinning observedbetween the domainsin the {l0l} sector. of epistilbite seemedexplicable by Akizuki's theory as well. Although very fine growth sectors with different due to (Al'Si) ordering atomic orderingswere observedin adularia and The twinning with electron microscopy (Akizuki and Sunagawa,1978; The tetrahedral sites that are directly coordinated with Akizuki, 1987c),epistilbite is homogeneousunder a low- cations such as Ca and Na are preferentially occupied magnification electron microscope.Two kinds of sectoral with Al atoms. The crystal symmetry is attributed to twins are observedin epistilbite: one is that of structure charge balance between Al or Si atoms and the cations with symmetry C2/m, and the other is due to the (Al,Si) on crystal faces.Ifthe tetrahedra are produced after the ordering. cations on the gowth step, the tetrahedra will be pref- 1438 AKIZUKI AND NISHIDO: SYMMETRY AND TWINNING OF EPISTILBITE

lpm

(2Vx= 38") j.\

niol t

Fig. 6. A partial crystalstructure of epistilbiteprojected onto the(010) plane. Pseudomirror plane (pm) is verticalin thepseu- do-orthorhombicstructure. T(Al,Si) sites (solid circles), oxygen atoms(open circles), and Ca atoms (stars) are represented. Num- bersgive the heightsof the atomsin thousandthsof a cell edge. Watermolecules are omitted. (Modified from Slaughterand Kane, 1969).

erentially occupied by Al ions. On the other hand, if the tetrahedra form prior to the addition of cations during growth, the tetrahedra will be occupied by Si ions. If the growth step is norrnal to a mirror plane, the two sym- metric sites will be equivalent on the step surface, and the mirror plane will be maintained, resulting in a dis- ordered structure. Conversely, if the step inclines to the 34' Y* mirror plane, the two symmetric sitesin a crystal will not be equivalent on the step surface,and therefore ordering will occur and the symmetry will be reduced.In this case, the mirror plane will changeinto a twin plane. Thus, the (Al,Si) ordering may differ from sectorto sector(Akizuki, 1987b). The optical properties of the epistilbite crystals can be explained by this concept. Figure 7 is a partial crystal structure projected onto the plane normal to the a axis. According to Perrotta (1967) and Slaughter and Kane (1969), the TA sites, which are directly coordinated by Ca and minor Na ions, are preferentially occupied with Al atoms, rather than the TB and TC sites. If the crystal growson the (101)face, normal to the (010)mirror plane, the mirror plane will be maintained, and the twofold axis will disappearduring growth; that is, the TA-l and TA-2 sites and the T'A-l and T'A-2 sites will be equivalent with each other with respectto the Ca ion on the surface, whereasthe TA-l and T'A-l sites will not be equivalent with each other. This result means that in the Kuroiwa

ts

Fig. 5. Stereographicprojections of the optical orientations of A, B, and C domains shown in Fig. 4b. Optic wibration di- rections(X, Y, Z), opticaxes(stars), and a centerofthe projection (arrow) are shown. AKIZUKI AND NISHIDO: SYMMETRY AND TWINNING OF EPISTILBITE t439

therefore the (Al,Si) ordering varies along the steps, showinga wavy extinction in the {l0I} sector;that is, the structure deviates slightly from the monoclinic sym- metry. Thus, epistilbite consistsof crystal structureswith the spacegroups Cl, C2, and Cm, all of which are con- sistent with the piezoelectriceffect observedin this min- eral (Bond, 1943;Ventriglia, 1953).

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Akizuki, M. (1981a) Origin of optical variation of analcime. Amencan Mineralogist, 66, 403409. -(1981b) Origin of optical variation of chabazite.Lithos' 14, 17- 2l -(1984) Origin of optical variations in grossular-andraditegarnets. sinB American Mineralogist, 69, 328-338. -(1935) The origin of sector twinning of harmotome. American Mineralogist, 70, 822-828. - (1986) Al-Si ordering and twinning in edingtonite.American Min- eralogist,71, I 5l0-l 514. -(1987a) Crystal symmetry and order-disorder structure of brew- sterite.American Mineralogist, 72, 645-648. Fig. 7. Crystalstructure of epistilbiteprojected along the 4 -(1987b) An explanation of optical variation in yugawaralite.Min- axis.Mirror plane(m) is vertical.Side face of growthstep, which eralogicalMagazine, 51, 615-620. is inclinedto the b axis,moves in the directionof the arrow. -(1987c) Al,Si order and the internal texture of prehnite. Canadian T(Al,Si)sites (solid circles), oxygen atoms (open circles), and Ca Mineralogist, 25, 707-7 16. the in- atoms(stars) are represented.Numbers give the heightsof the Akizuki, M., and Konno, H. (1985) Order-disorder of Al/Si and American Mineralogist, T0, 8lrt-821. atomsin thousandthsof a cell edge.Water molecules are omit- ternal texture of stilbite. M, and Sunagawa,I. (1978) Study of the sector structure in (Modified Perrotta.1967.) Akizuki, ted. from adularia by means of optical microscopy, infra-red absorption, and electron microscopy.Mineralogical Magazine,42' 453462 Akizuki, M., Hampar, M.S., and Zussman,J. (1979) An explanation of specimen with the flat domains on the (101) face, the anomalous optical properties of topaz. Mineralogical Magazine, 43, {101} sector consists of the monoclinic structure with 237-24t. spacegroup Cm. Two morphologically symmetric {l0l} Alberti, A, Galli, E., and Vezzalini, G. (1985) Epistilbite: An acentric Zeitschrift fiiLrIkistallographie, 173' 257- sectorsshow different 2V values, suggestingthat the de- with domain structure. 265. gree of (Al,Si) ordering is different between the two. Arrioli, G., Smith, J.V., and Kvic, A (ISSS) Multiple hydrogen positions If the groMh surfbce or step, which is inclined to the in the zeolite brewsterite,(Sro*Btuor)ALSi6O,6 5HrO. Acta Crystallo- mirror plane, moves in the direction shown by arrow in graphrca,C4l, 492497. Figure 7, the four TA-1, TA-2, T'A-1 and T'A-2 sites Bond, W.L. (1943) A survey for piezo-electnc materials. Bell Technical 22, 145-152 will not be equivalent with respectto the Ca ion on the System Journal, Galli, E. (1971) Refinementof the crystal structureof stilbite. Acta Crys- surface:that is, both the mirror plane and twofold axis tallographica.826. 833-84l. will disappear in the crystal, resulting in an (Al,Si)-dis- Galli. E., and Rinaldi, R. (1974) The crystal chemistry of epistilbite. ordered structure. This reasoningsuggests that the space AmericanMineralogist, 59, 1055-1061. (1985) zeolites, minerals and rocks, group is the noncentrosymmetric,triclinic Cl in the { I 10} Gottardi, G., and Galli, E. Natural Berlin, Tokyo. (010) twofold axis 18. Springer-Verlag, sectors.Thus, the minor and the [010] Kerr, I.S. (1964) Structure ofepistilbite. Nature, 202, 589 in the monoclinic structure changeinto a twin plane and Mazzi,F., and Galli, E. (1978) Is eachanalcime different? American Min- twin axis in the epistilbite crystal, respectively:the { I l0} eralogist,63,448460. -(1983) of chabazite.Neues Jahrbuch and {110} sectorsand the {110} and {II0} sectorsare The tetrahedral framework fiir Mineralogie Monatshefte, 461480. relatedby the (100) twin, and the I 10} and { 1l0} sectors { Parise,J.B., Cufl C., and Moore, F.H (l 980) A neutron diffraction study and the {110} and {110} sectorsare relatedby the (010) of topaz: Evidencefor a lower symmetry Mineralogical Magazine,43, twin. Differencesin the optical orientations of the A, B, 943-944. and C sectors,which are shown in Figure 4b, may suggest Penotta, A.J. (196?) The crystal structure of epistilbite. Mineralogical the diferences of (Al,Si) ordering among the sectors. Magazine,36, 480-490. M., and Kane, W.T. (1969) The crystal structure of a disor- (010) plane Slaughter, If the crystal grows on the face, the mirror dered epistilbite. Zeitschrift fur Kristallographie, 130, 68-87. will disappearin the crystal, and the twofold axis will be Tak6uchi, Y., Haga, N., Umizu, S., and Sato, G. (1982) The derivative maintained, resulting in a structure with the spacegroup structure of silicate gamets in grandite. Zeitschrift fiir Kristallographie, C2. Such a {010} sectorconsists ofpolysynthetic sectoral l 58.s3-99. Ventriglia, U (1953) Simmetria della heulandite e piezoelettrici0 di al- of the and sectors,suggesting Cl twinnings {ll0} {Il0} cune zeoliti. Rendiconti della SocietdItaliana di Mineralogia e Petro- symmetry. Some (010) faceshave fine striations parallel logia,9, 268-269. to the c axis, which may be due to a repetition of the (110)and (110) faces.In the epistilbitecrystal from Fos- Mem;scnrrr REcETvEDJurv 13, 1987 sarfell,the growth stepson the (101) face are circular, and MeNUscRrPrAccEPTED Jurv l, 1988