Cylindrite: the Relation Between Its Cylindrical Shape and Modulated Structure

American Mineralogist, Volume 77, pages 758-764, 1992

Cylindrite: The relation between its cylindrical shape and modulated structure

Su Wa.nc, PorBn R. Busrcr Departments of Geology and Chemistry, Arizona State University, Tempe, Arizona 85287, U.S.A.

AnsrnLcr Cylindrite, a lead, tin, antimony, iron sulfosalt having a distinctive cylindrical morphology and a composite structure, is characterizedby incommensurate structural modulations. It contains two types of sheets,and these have pseudohexagonal(H) and pseudotetragonal (T) structures with distinct lattice dimensions. The structure changes systematically from core to periphery of the crystals. The b and c axes of the H and T sheetsare almost parallel to each other near the crystal cores. Further from the cores the angular divergencebetween the axes of the two types of sheetsincreases (i.e., the sheets are increasingly rotated relative to one another), and the wavelength of the structural modulation decreases.The crystal accommodatesthe dimensional misfit between the H and T sheetsby a combination of this divergence,the variation of the structural modu- Iation, and the curving ofthe layers.

INrnooucrroN exactly parallel to one another. The two types of sheets Crystals of almost all minerals form with flat faces,and are untrsual in that they have diferent structures and these faces characteristicallygrow at s tl as incommensurate dimensions par- another. Cylindrite belongs to a select ng. They stack along the a* direction in that typically displays an anomalous n HT. plied by its name, it forms in crystals Lc complications arise where crystals ders. Although highly unusual, tubul more units having diferent structures adoptedby a few other minerals, such€ llmann, I 968; Buseckand Veblen, 1988; 1967,1971;VeblenandBuseck,1979). 990; Petriceket al., l99l), and modu- andlibelo, 1987),andtochilinite(Tor ly result (Cowley, 1979; Buseck and 1983; Zolensky and Mackinnon, l98t lhe structural complexity increasesin vidual crystalsof theseminerals are ge degreeofdimensional mismatch among ic in dimensions.Because oftheir smal its. For comparison, in the serpentine to obtain good diffraction patterns anr a relatively small mismatch (Table 2). ferent parts and orientations of the cr1 sheetsin serpentineare accommodated the relation betweentheir morphologie lys: (l) a curled structure is adopted, as tures. Cylindrite, in striking contrast, I wave struclureoccurs, as in antigorite; severalmillimeters in diameter, and th are replaced by small cations, as in li- opportunity for understanding the st ite, there is a larger structural mismatch with cylindrical morphology. r serpentine-type mechanisms occur in Makovicky (lg7l, lg74) examined cylindrite using combination. In the b direction, the layerscurl into cyl- scanning electron microscopy (SEM) and X-ray diffracl inders as in, chrysotile, but no fixed dimensional relation- tion and showed that it forms in concentric, curved lay- ship was observed between the two sheets.In the c di- ers. The regular cylindrical structure is distorted within rection, approximately parallel to the axis of the cylinders, the cylinder coresso that it assumeshorseshoelike forms high-resolution transmission electron microscopy (Fig. I,arrow)orotherirregularshapes.Makovicky(1974) @RTEM).images show that the dimensionsof the two proposedthat the mineral consistsof two typesof'rtru.- sheetsare in the approximateproportionl3c,/l2cn(b,,c, tural sheetsthat alternate regularly, and he called them and.bn,cn r€presentthe unit lengths ofthe b and c axes pseudotetragonal(T) and pseudohexagonal(H), reflecting of the T.and H sheets,respectively); they produce a match ih"it ."tp""iive symmetries. The H iheet, with compol by forming modulatedintergrowths, as in antigorite (Wang sition MSr, essentiallyrepresents an octahedral(Sno*,F;)S, andf(uo' l99l; Wang, 1988;Williams and Hyde, 1988). sheet with the berndtite (brucite-like) structure; the T The cylindrite modulations give rise to satellite reflec- sheet,with composition (Pb,Sb,Snr")S,repiesents two PbS tions in selected-areaelectron diffraction (SAED) pat- layerswith the galenastructure. The lattice parametersof terns, and the vector q, which describesthe modulation, the two sheets,both of which are A centered,are given is oriented parallel to thesereflections in reciprocal space. in Table l; in spite of being intergrowri, their axesare not If a diffraction vector, H. is described in terms of either 0003-004x/92l0708-0758$02.00 758 WANG AND BUSECK: CYLINDRITE-SHAPE AND STRUCTURE 759

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2 I dimeter of cvlindd 1L0 (rm) 4oo 3oo 2oo too Q roo zoo 3oo 4@ I cylindrite entu Fig.2. The distribution ofSb acrossa cylindrite cylinder.

Fig. I . SEM imageof cylindrite showingthe structurein the transparent foils oriented parallel to (100) were prepared coreof a cylindricalcrystal. for study by repeatedcleaving (Hirsch el al., 1977). For observation ofcylinder crosssections, slabs having about a l-mm thicknesswere glued betweentwo Si wafers with the H or T lattice parameters,then H : hr* + kb* + /c* epoxy resin to make a sandwich. These sandwicheswere + mq(h, k, l, m are integers),where q : + qbb* + QoN* then ground and polished perpendicular to the cylinder q,e* (q", qo, and are coefficients;de Wolffet al., 198l). 4" axis, thereby obtaining specimens30 pm thick with ori- In cylindrite, eo, eu and q, do not have fixed rational entation normal to the { 100} cleavageplanes. They were values. Transmission electron microscope (TEM) obser- ion thinned using a Gatan 600 ion mill. A cooling stage vations showed that the orientational relations between was used to minimize damageduring ion milling. the axes ofthe H and T sheetsand between these sheets Secondary electron images and quantitative analyses and the vector q are variable (Wang and Kuo, l99l), were obtained with a JEOL JXA-8600 electron micro- although the details ofthe variations were unclear. probe using a l0-nA beam current. Standard calibration The dimensional mismatch in the b direction, which is procedureswere employed using galenafor Pb and S and parallel to the tangent ofthe cylinders, is larger than that the metals for Sn, Sb, and Fe, respectively.The electron- in the c direction. flowever, no structural modulations transparent foils were examined in an Akashi EM-002B have been observedin the b direction. Specificquestions TEM operated at 200 kV with a LaB, filament and examined in this paper are whether there are structural equipped with a t l0' double-tilt, side-entry high-reso- differencesbetween cylindrite fragments having large vs. lution goniometer stage.The spherical aberration coeffi- small radii of curvature, and how and why the relation cient, C", of the objective lens is 0.4 mm. Heating exper- betweenthese sheetsand the modulation vector q varies. iments were performed using a Philips EM 400T TEM SppcrunNs AND ExpERTMENTALDETAILS at an acceleratingpotential of 120 kV. A cylindrical crystal with a 6-mm diameter, from Poo- Onsnnv^LtroNs po, Bolivia, was used for TEM study. In order to contrast Microprobe analyseswere performed acrossthe width the structural differencesbetween sheetsthat are close to of a 2-mm cylinder in order to determine whether com- the core of the crystal and those far from the center, frag- positional variations coincide with distancefrom the core. ments from the innermost, medial, and outermost parts of the cylinder were used for TEM observation. Electron- TABLe2. Comparisonbetween the dimensions of thetwo types of sheetsin the serpentineand cylindritestructures TABLE1, Latticeparameters of thesheets in thecylindrite strue ture Seroentineminerals a

H sheet T sheet Si-O sheet 0.53nm 0.92nm Brucite sheet 0.54 0.94(3 x 0.313) a 1.173nm 1.176 nm Difference 0.01 0.02 b 0.367 0.579 0.632 0.581 Cylindrile' b d 91' 91' 91" s2 T sheet 0.579nm 0.581nm "Y 91" 95' H sheet 0.367 0.632 Difference 0.212 0.051 Note: a, b, c arc lrcm Makovicky (1974)and a, are from Wang and B, I * Kuo(1991). Values for b and c are from Makovicky (1974). 760 WANG AND BUSECK: CYLINDRITE_SHAPE AND STRUCTURE

H sheet exterior T sheet

core Fig. 3. Some [00] SAED patterns ofcylindrite. (a), (b), (c) Patternsofthe H sheets;(d), (e), (f) patterns ofthe T sheets.(a), (d) The outside edgeof a cylinder; (b), (e) the middle of a cylinder; (c), (0 close to the cylinder core. The modulation vector is denoted bv q. WANG AND BUSECK: CYLINDRITE_SHAPE AND STRUCTURE 761

Fig.4. Some [100] HRTEM images of cylindrite. The imagescorrespond to the respectiveSAED patterns of Figure 3.

Concentrations of Fe, Sn, Sb, Pb, and S were measured giving a total of 100.15. Sb rangesfrom 12.03wt0/o at the with wavelength-dispersivespectrometers at 18 evenly cylinder center to 11.39 at lhe periphery. Becausethe spacedpoints at 50-pm intervals. The averagesof these range is small and becausewe did not observe comple- microprobe analyses,corresponding to the points in Fig- mentary, systematicvariations in the other elements,the ure 2, show the following weight percentvalues: S : 24.68, indicated zoning should be consideredtentative. Fe:2.70,5n:27.37, Sb: 11.68,and Pb:33.72, There is a small angle betweenthe a axesof the H and 762 WANG AND BUSECK:CYLINDRITE_SHAPE AND STRUCTURE

Fig. 5. Two [00] HRTEM imagesof the H sheets(a, b) and sketches(c, d) showing the crystal modulations. (a) Exterior and (b) close to the core of a cylindrical crystal. Sketchesc and d correspondto imagesa and b, respectively.The d representsthe angle betweenq and the c axis of the H lattice. The wavelengthsof the modulations are (b) longer where q is parallel to c than (a) where it is not.

T sheets,and so we were able to obtain separated[100] SAED patterns and HRTEM images for the two types of sheets.The direction of the modulation vector, q, that the two sheets have in common is determined by the relative orientations of the sheets.The vector q is consis- tent with a vector extending from the 002 reflection from the H sheetsto 002 arising from the T sheets.Therefore this modulation can be used for determining the relative orientations and rotations ofthe sheets. SAED and HRTEM studies show that the lattice parameters of the H and T sheetsare constant across the width of a cylindrical crystal, although the relative rotations ofthe two sheetsand thus the orientation ofq change with radial position. The b* and c* axesin SAED patterns and the b and c axes in HRTEM images of the H and T sheetsare closerin direction to eachother at the interiors than at the exteriors of the cylinders (Figs. 3, 4). The vector q also shifts progressivelytoward c ofboth ofthe sheetsnear the cylinder cores, so that the c* axes of the H and T sheetsand q are roughly parallel near the cylinder cores (Figs. 3c, 3ffor reciprocal spaceand Figs. 4c, 4f for real space).The curvature of the cylindrite layers and the angular relation between the two types of sheets are directly correlated. q Fig. 6. A [001] TEM image of a region far from the core of The vector changesin both magnitude and direction the cylinder. Rather than being smoothly curved, the sheetsare with radial position. The modulation wavelengths near kinked. Regions b,, br, and b. are domains having slightly dif- the cylinder cores are longer than those near the outsides ferent orientations. of the crystals (Fig. 5). The H and T sheets retain the WANG AND BUSECK: CYLINDRITE-SHAPE AND STRUCTURE 763 proportion l3c,/l2cn in the c direction. Where q is parallel to the c axes ofthe H and T sheets,the wavelength of the modulation is l2cn and 13c,.Where the angle between q and the c axis of the H lattice is d, or the angle between q and the c axis of the T lattice is {, the wavelength of the modulation is lql : l2cncosd: l3c,cos{. In some incommensurate modulated structures (e.g., 7-NarCOr; van Aalst et al., 1976),the orientations of the satellite peaks changerelative to the major substructure reflectionswhen the crystal is heated.The effectis similar to the swing in q that can be observed in Figure 3. In order to determine whether the changesin cylindrite occur as a function of temperature, the crystalswere heated in the electronmicroscope. The sampledecomposed when heated to -550 "C (l 'Clmin), but q and the relation between the H and T sheetsremained constant up to the decomposition temperature. Therefore, we conclude that the changesshown in Figures 3 and 4 are not a temperature effect. The smooth curvatures of the H and T sheetsare in- temrpted near both the exteriors and cores of the cylinders. Where the radius is large, the sheetsform flat segments (Fig. 6), similar to the structures of Povlen-type chrysotile(Middleton and Whittaker, 1976)and the phases shown by Barbier et al. (1985). Smoothly curved sheets also do not form where the radius of curvature is very small, although the limiting radial value is uncertain. Fig- ure 7, for example, shows that the smooth arcs of the layersdisappear where the radius of curvature is lessthan - 16 nm (bottom part ofthe figure).

CoNcr,usroNs The basic structures of the two types of constituent sheetsof cylindrite remain constant, independentof their radial positions in the crystal. However, their changing interrelationships with radial position give rise to both the incommensurate modulations and the unusual mor- Fig.7. A HRTEM imageshowing the transitionfrom phology of cylindrite. Theserelationships change with [00U the a smooth(top) to kinkedarc (bottom) near the cylindercore. radial position of the sheetswithin the crystal, i.e., with the amount of curvature of the sheets.Thus, the rotations AcxNowr.BocMENTs of the sheetsrelative to one another increasefrom core to periphery of the cylindrical crystals, as does the devi- We thank J.C. Clark for help with electron microprobe analysis and K. Tomeoka, S.A. Kissin, and T. McCormick for helpful reviews. Electron ation of q, the modulation vector, from c in the respective microscopy was performed at the Facility for High Resolution Electron sheets. Microscopy at Arizona State University. The ASU HREM facility is sup- Sheet curvatures are smooth except at maximum and ported by NSF and ASU, and this study was supported by NSF grant minimum radial values, although the limiting radii seem EAR-8708529. The electronmicroprobe was obtained through NSF gra.nt EAR-840816E. to vary among different crystals, perhaps in responseto compositional differences.For minimal curvature (max- RrrnnBxcns crrED imum radius), the crystals develop planar segments, Barbier, Hiraga, K., Otero-Diaz, L.C., White, T.J., Williams, T.B., whereasfor maximum curvature, near the crystal cores, J., and Hyde, B.G. (1985) Electron microscopestudies of some inorganic and the sheetsdevelop somewhat contorted shapesthat may mineral oxide and sulphide systems.Ultramicroscopy, 18, 2ll-234. also display kinking. The wavelengthsof the modulations Belkin, H.E., and Libelo, E.L. (1987) Fibers and cylinders of cryptomel- in our sample decreasefrom core to periphery of the crys- ane-hollandite in Permian bedded salt, Palo Duro Basin, Texas. Amer- tal. There are clearly a variety of related chemical and ican Mineralogist, 72, 12ll-1224. Buseck,P.R., and Cowley, J.M. (1983) Modulated and interyrowth struc- structural featuresthat combine to causeformation of the tures in minerals and electron microscope methods for their study. unusual cylindrical shape. American Mineralogist, 68, I 8-40. 764 WANG AND BUSECK: CYLINDRITE_SHAPE AND STRUCTURE

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