American Mineralogist, Volume 71, pages I5I0-1514, 1986

Al-Si ordering and twinning in edingtonite Mrzurrrro Axrzurr Institute of Mineralogy, Petrology,and Economic Geology, Faculty of Science,Tohoku University, Sendai 980, Japan

Ansrucr An edingtonite crystal from Ice River, British Columbia, Canada,has a clear, euhedral rim and whitish,anhedral core. The rim showsthe m{lt0}, o{lll}, p{l1l}, and c{001} sectorsand a small 2V value (abottt -22\ with some exceptionsin the { I 1 I } sector,and the core showstwinning without sectorsand a large 2Vvalue (about - 52").X-ray diffraction analysis indicated that both the core and the rim are orthorhombic. However, optical observation leads to a different conclusion: the core is orthorhombic and consists of an ordered Al-Si structure, whereasthe rim, especiallythe { I 10} sector,is evidently triclinic and has a partly ordered structure.

INrnooucrroN the core.The coreis characterizedby its white color, which mineral inclusions,and by a large 2V (- 52) and Edingtonite,a rare barium zeolite (BarAlosi6o2o.8H2O), is due to parallel to the growth hastwo polymorphs-orthorhombic, spacegroup P2r2r2 multiple twinning approximately The rim is transparent and shows a small 2V (Galli, 1976),and tetragonal,space group P42m (Mazzi direction. (-22) with someexceptions in the I I I sectors.A sector et al., 1984). According to Hey (1934), orthorhombic t l is area that was produced by crystal growth on each edingtonite from the Bdhlet mine, Sweden,shows 2 Z : an is produced by crystal growth on -54o, and tetragonal edingtonite from Kilpatrick Hill, face, whereasa domain face of the gowth hillock or striation; thus, a Scotland,shows - 15" < 2V < -20. Grice et al. (1984) the vicinal many domains. describedorthorhombic edingtonite(2V: - 62")from Ice sector consistsof internal texture in the (001) thin River, British Columbia, Canada,andMazzi et al. (1984) Figure 3 shows the top portion of the crystal. refined the crystal structure as tetragonaland disordered. section crosscuttingthe clear p{t1l}, o{ll l}, and m{ll}} On the basis of studiesof the relationship betweensur- The sectionconsists of the more less heterogeneousbetween face features and internal textures of some zeolites and sectors,which are or polars. showlarge 2V values(- 52), otherminerals, Akizuki (l98la, l98lb, 1984,1985) and crossed The light areas as small as -22.'the Akizuki and Koilno (1985) interpreted their optical prop- whereasdark areasshow 2Zvalues Z isparallel to the a axis in the erties and internal textures as being produced by atomic optic vibration direction with exceptions,whereas ordering on side facesof growth steps;furthermore, Ak- {l1l} and {001} sectors some variously inclined to the l0] in the izuki suggesteda generalmechanism for the formation of the Z direction is I l0} gowth sectors.These conceptsare applied in this paper {l l0} and {l I l} sectorswithsomeexceptionsinthe{l to edingtonite. sector. Figure 4 showsthe (001) thin section crossingthe core. The core shows twins and a large 2V value, whereasthe Oprrcn oBSERYATToN rim (t I l0) sector)shows a small 2 Z value and no twins. Figure I showsthe crystal form and facesof edingtonite In the core, the optic vibration directions are in (l l0) from Ice River, Canada. The crystal is bounded by the reflection twin relation with eachother, which is in accord p{ I I I }, o{ I I I }, c{00I }, andm{ll 0} forms,and each face with what has been known in edingtonite (Deer et al., has growth hillocks or kinks. The crystal has a whitish 1963). In the rim, however, the Z direction is roughly core and a clear rim. The surfacemicrotopography of the parallel to the I l0] rather than to the a axis, with vari- crystal faceswas studied with reflection-typeinterference ations from placeto place.The Xdirection may be parallel optical microscopy. Thin sections cut in various planes or nearly parallel to the c axis in both the rim and core. and observedbetween crossed polars on a universal stage The internal textures in the sectorsare compared with allowed determination of 2V. which varies with Al-Si or- growth featureson each face: dering. The 2V values range from - l0' to -54", but 2V Thep(1I1) face. The surfaceis composedof thin steps valuesat -22 and -52 aredominant. with several growth centers (Fig. 5). Although the steps Figure 2 showsthe internal texture in a thin section cut near the growth center are symmetrically inclined to the near the crystal center and inclined slightly to the (l l0) (l l0) plane, the direction ofthe stepsvaries with distance face. The crystal consists of an anhedral core and a eu- from the growth center, and the stepsmoving toward the hedral rim. According to a microprobe analysisby Grice bottom from the growth centerare fine and irregular (Fig. et al. (1984), KrO is more abundant in the rim than in 5). 0003-o04x/86/ll r2-r 5 l0$02.00 l510 AKIZUKI: AI.Si ORDERING AND TWINNING IN EDINGTONITE l51l

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Fig.l. Crystalform with the p(l l-I ), o(l I 1),c(00 I ), and m(l lO) facesofedingtonite from Ice River, Canada.From Griceet al. (1984).

The {111} sector.A thin section was preparedparallel to the (l I l) face.When conditions changeabruptly during growth, Al-Si ordering and/or chemical composition vary at each growth step, and therefore microzoning corre- spondingto the growth stepsis observedin the thin section parallel to the growth surface. In some minerals whose growth conditions changedabruptly at the latest stageof crystal growth, surface features are not correlated to in-

Fig. 3. Cross-polarizedphotomicrograph ofthe thin section normalto the c axis of edingtonite.The crystalconsists of the mtll}j,p{l1l}, o{lll}, andc{001} sectors. Optical vibration directionsZ and Y areshown. The Z directionis variously in- clinedto [110]on the(001) plane in the ,|'{ll0} ando{1ll} sectors.

ternal texture (Akizuki, 1981a).Figure 6 shows various domainsin the { 11I } sector;the arrow indicatesthe growth center, which correspondsto the summit of the growth hillock. Straight lamellae corresponding to the growth steps radiate from the growth center. The lattice pattern on the upper area suggestsoverlapping of lamellae. The optical extinction directions are in twin relation betweenthe light and dark domains, as the black and white lines show. The

Fig. 2. Cross-polarizedphotomicrograph of the thin section inclined about l0o to (l 10) in the c-axis direction ofedingtonite Fig. 4. Cross-polarizedphotornicrograph of the (001) thin from Ice River. The crystal consists of an anhedral core with section,which showsa dark rim (m{llo} sector)with fine growth twinning and a euhedral rim with m {l l0}, o{ I I I }, and p{ I I I } zoning and a light core with twinning of edingtonite.The optical sectors. vibration directions Z and Y are shown. t5t2 AKIZUK]: AI-Si ORDERING AND TWINNING IN EDINGTONITE

Fig. 5. Reflection interferencephotomicrograph showing Fig. 7. Cross-polarizedphotomicrograph of the {l 1I } sector growthfearures on the (1Il) faceofedingtonite. The u10l di- in the (111)section ofedingtonite. Dashed lines show fine la- rectionis horizontal. mellaecorresponding to the gowth steps.Extinction angles from the vertical direction and 2V vafuesare shown.Crystal orien- tationis the samewith that of Fig. 6. optic vibration directions Z and Y are parallel to the a and b axeswith some exceptions,and the 2V values vary the upper part of Figure 7, show small 2Z values. The from about -20" to about -50". extinction directions are inclined from I 0" to vertical. The Figure 7 shows internal texture in another thin section optic vibration directions Z and X are parallel to the a parallel to the (11 l) face.Domains showing large 2V val- and c axes, respectively, with some exceptions,and are ues correspondto the growth hillocks, and the extinction in twin relation betweenboth the light and dark domains. angles(8") are symmetrically inclined with respectto the The o(111) face. The (lll) face is rough, becauseit vertical (l l0), a pattern that suggeststwinning. Fine la- consistsof numerous, fine kinks. mellae with kinks, which are shown by dashed lines in - The o{ 11 1 } sector.The 2 V value is about 22, and.the Z direcrion is roughly parallel to the I l0]. No lamellae were observedin the sector, though it is finely heteroge- neous. The m(110)face. The (l l0) faceis composedof many small edingtonite crystalsgrown parallel to the (l l0) face. Steps,without a growth center, lie parallel or normal to the c axis on the face of parallel-grown crystals, though no growth stepsare observablein some areas. The m{110} sector. The thin section normal to the c axis revealedfine growth zoning wfih2V: -22' (Figs.3 and 4). The optic vibration direction Z changesfrom the [10] to the a-axis direction in the (001) section,a situ- ation that suggestsmonoclinic or triclinic symmetry. The sectionparallel to the (l l0) facerepresents some domains correspondingto the parallel-grown crystals.The extinc- tion anglesvary from place to place in the (l l0) section, though the anglesare small. The optic vibration direction X is inclined about lo to the c axis in some domains, resulting in sectorialtwinning. Thesedomains are triclin- rc.

X-n-tv ANALYSES Fig. 6. Cross-polarizedphotomicrograph of the {lI l} sector in the (l I l) section of edinglonite. The rim shows the o{ I I I }, Areas showing small 2V values are abundant in the c{001}, and mIl 10} sectors.Extinction anglesfrom the vertical upper (transparent) part of the crystal, whereas areas direction and 2Zvalues are shown. showing large 2V values are dominant in the lower part. AKZUKI: Al-Si ORDERING AND TWINNING IN EDINGTONITE 1513

Fig. 9. Partial crystal structure projected along the c axis of edingtonite. Circles show Ba ions. Water moleculesare omitted.

LC Fig. 8. X-ray diffractionpeaks of(A) the upperportion (trans- DrscussroN parent)and (B) the lower portion (whitish)of the edingtonite X-ray analysis reveals that the area showing large 2V peaks, crystal.The two whichare shownby the arrows,corre- values (- 52') consistsofAl-Si ordered structure and that spondto thoseindicated by stars. the area showing small 2V values (-22") has partly or- dered structure.Through X-ray analysis,edingtonite from These parts were separated,and two diffraction patterns the B6hlet mine, Sweden,was interpreted as tetragonal from 32 to 33'and from 40.5 to 41.5" 20 werc recorded by Taylor and Jackson (1933) but as orthorhombic by by X-ray diffractometer using CuKa radiation, a scanning Galli (1976). Also, edingtonite from Ice River, Canada, speedof 7roper minute, and a chart speedI cm per degree. was interpreted as orthorhombic by Grice et al. (198a) The two scanscovered the intervals for the l3l and 3l I but as tetragonalby Mazzi et al. (1984). The edingtonite and the 141 and 4l l, respectively,which are unequivo- crystaldiscussed in this paperis interpretedas orthorhom- cally indexable pairs (Fie. 8). The peaks split into four, bic through X-ray analysis. However, the optical obser- exceptfor a broad peak (A) between32 and33 20.The vations lead to a different conclusion. Although the core weak peaks with arrows in the upper portion (A) corre- is orthorhombic and ordered,the rim, especiallythe { I l0} spond to the strong peakswith stars in the lower portion sector,evidently is triclinic, not tetragonal,in spite ofits (B). disordered structure, becauseno optical vibration direc- Table I showslattice spacingsofthe peaksrepresented tions are parallel to the crystal axes.This suggeststhat Al in Figure 8, which are comparedwith thoseof orthorhom- and Si occupanciesare slightly different among the T2 bic and tetragonaledingtonite crystals (Mazzi et al., 1984). tetrahedra (a, b, c, and d in Figs. 9 and 10). It should be phenomenon, The lattice spacingsofthe outsidetwo peaksare consistent noted that adularia showsa similar i.e., the with those of orthorhombic edingtonite. Also, the inside {110} sector consists of a triclinic, disordered structure two peakssuggest the orthorhombic symmetry, and their (Akizuki and Sunagawa,1978). averagelattice spacingsare similar to those oftetragonal Figure 9 showsa partial crystal structure ofedingtonite. edingtonite.The upper portion showingdominantly small 2Vvalues has strong inside peaks,whereas the lower por- tion showing dominantly large 2 Zvalues has strong out- side peaks. The single-crystalX-ray precessionmethod revealed no deviation from tetragonal symmetry in the crystals showing small 2trlvalues.

Table 1. Latticespacings (A) ot edingtonite Orthorhombic Tetragonal

2.761 131 2.763 2.755 2.753 2.754 2.750 2.742 311 2.741 2.203 2.203 141 2.201 projection tetra- 2.195 2.195 2.192 Fig. 10. A clinographic of the edingtonite 2.189 2.186 hedral chain, in which tetrahedra with Al atoms are shadowed. 2.183 2.183 411 2.181 Growth step is drawn at the bottom. l514 AKIZUKI: AI-Si ORDERING AND TWINNING IN EDINGTONITE

the same mechanism as those of growth twins in many minerals such as quartz and spinel. In the Ice River eding- tonite, gowth mechanisms differed in the core and rim, and the core might have dissolved slightly before the rim formed, as implied by the anhedral shape of the core. Crystal structures of natrolite and edingtonite are sim- ilar. Since all the T2 tetrahedra are equivalent with respect to Al and Si on the pyramidal face of edingtonite (Fig. I lA), the twinning and disordering are easily produced by simple exchange of Al and Si on the growth surface of edingtonite. However, such exchange is difficult in na- Fig. I l. Orderedcrystal structures projected along the c axes trolite during growth, because the orientations of T2 tetra- (A) of edingtoniteand (B) natrolite.Tetrahedra with starsare hedra are not equivalent with respect to Al and Si on the occupiedby Al ions and othertetrahedra without starshave Si growth surface (Fig. I lB), though they are equivalent in ions.Water molecules are omitted. the three-dimensional structure. In twinned natrolite, the framework of one part is rotated about the normal to ( I l0) The steps(L and R) of the vicinal faceson the (lll), with respect to the other (Pabst, l97l). Natrolite with a which are symmetrical with respectto the (l l0) plane, are disordered structure may be produced during extremely projected on the (001) plane. In ordered edingtonite, T2 high speed crystal growth. Thus, both ordered and dis- tetrahedra, whose oxygensare directly coordinated with ordered structures are commonly observed in natural Ba ions, are alternately occupied with Si and Al ions, edingtonite in spite of the mineral's rarity, whereas the becauseof the aluminum avoidance rule forbidding Al- disordered structure is rare in natural natrolite, which is O-Al bonds. On the contrary, Tl tetrahedra,whose oxy- a common zeolite. gens are not directly coordinated by Ba ions, will pref- AcrNowr,nucMENTS erentially be occupiedwith Si. The orderedstructures that are produced on the L and R steps will be symmetrical I would like to thank ProfessorJoseph R. Smyth, University for critical reading and corrections to the manu- with respectto the (ll0) plane. If the T2(b) T2(c) of Colorado, and script. Also, I am grateful to ProfessorSatosi Kanisawa, Tohoku tetrahedraare occupiedby Al ions during growth on step University, for his help with the X-ray work. R, the T2(a) and T2(d) tetrahedrawill be occupiedby Al ions on step L, and the result is twinning. In the ortho- RnrnnnNcr:s rhombic, twinned crystals, the a axis of one component Akizuki, Mizuhiko. (198la) Origin of optical variation in chaba- coincideswith the b axis ofthe other, and vice versa; the zite.Lithos. 14. 17-21. - c axis is common. Some domains in the { l1 I } sectorare ( 198lb) Origin of optical variation in analcime.American composed of the partially ordered structure, which may Mineralogist, 66, 403-409. - (1984) variations in grossular-andradite growth Origin ofoptical be due to a high rate or a rough step. garnet. American Mineralogist, 69, 328-338. AIso, a partially orderedstructure occurs in the {ll0} -(1985) The origin of sector twinning in harmotome. sector. Figure l0 shows an ordered crystal structure of American Mineralogist, 70, 822-828. edingtonite. Growth stepsnormal to the c axis move from Akizuki, Mizuhiko, and Konno, Hiroshi. (1985) Order-disorder AVSi and the internal texture of stilbite. American Miner- bottom to top in the structure. Although the T2O) tet- of alogist, 70, 8 1,1-82I . rahedron bonds with a tetrahedron under the paper and Akizuki, Mizuhiko, and Sunagawa,Ichiro. (1978) Study of the Tl, the T2(a) tetrahedron coordinates with only the Tl sector structure in adularia by means of optical microscopy, on the growth step. If the Tl tetrahedron of the bottom infra-red absorption, and electron microscopy. Mineralogical is occupied by Si, the T2(a) tetrahedron can be occupied Magazine,42, 453-462. Deer, W.A., Howie, R.A., and Zussman,J. (1963) Rock-forming by Si or Al. If one of the T2(a) and T2(b) retrahedra or minerals: Volume 4, Framework silicates.Wiley, New York. both are occupiedwith Al, the T2(c) and T2(d) tetrahedra Galli, Ermanno. (1976) Crystal structurerefinement of edington- will be occupied with Si, though they are shown with Al ite. Acta Crystallographica,B32, 1623-1 627 . in Figure 10. Thus, Al and Si are distributed randomly Grice, J.D., Gault, R.A., and Ansell, H.G. (1984) Edingtonite: occulrences.Canadian Mineralogist, through the T2 tetrahedra,but Si will more The first two Canadian be abundant 22,253-258. than Al, becauseof the aluminum avoidancerule. There- Hey, M.H. (1934) Studieson the zeolites.VI. Edingtonite. Min- fore, becauseof total charge balance, the Tl tetrahedra eralogicalMagazine, 23, 483-494. are occupied not only by Si but also by Al, and a dis- Mazzi,Fiorenzo, Galli, Ermanno, and Gottardi, Glauco. (1984) orderedstructure results(Mazzietal., 1984).If the growth Crystal structure refinements of two tetragonal edingtonites. Neues Jahrbuch fiir Mineralogie, Monatschefte,37 3-382. stepsare symmetrically inclined to the c axis on the (l l0) Pabst, A. (1971) Natrolite from the Green River Formation, face,a triclinic, twinned structure will occur in the {l l0} Colorado, showingan intergrowth akin to twinning. American sector. The (l I l) face is very rough, and therefore the Mineralogist,56, 560-569. crystal structure has long-rangedisorder as well. Taylor, W.H., and Jackson, R. (1933) The crystal structure of edingtonite. Zeitschrift fiir Kristallographie, 86, 53-64. The twins in the { l1 I } sector are correlated with the growth hillock and vary as the growth patterns changeon Mlrvuscnrpr REcETVEDJuve 26, 1985 the surface.The twins in the core. however. result from Mnxuscrusr AccEprEDAucusr 21, 1986