257

The Canadian MineralogistVol. 32, pp. 257-270 (1994)

STRUCTURAL VARIATIONS IN CHRYSOTILE ASBESTOS FIBERSREVEALED BY SYNCHROTRON X.RAY DIFFRACTION

AND HIGH.RESOLUTION TRANSMISSION ELECTRON MICROSCOPY

BARBARAA. CRESSEY

Department of Geology, {Jniversity of Southampton, Southampton, SO9 sNH, U.K:

GORDONCRESSEY

Department of Mineralogy, Nanral History Museum, Inndon, SW7 5BD, U.K

ROBERTJ. CERNIK

SERC Daresbury laboratory, Warrington, WA4 4AD, U.K.

ABSTRACT

Four samples of chrysotile asbestos have been studied by synchrotron X-ray diffraction. One of these samples exhibitsasymmetrical 00/ diffraction profiles, and also shows characteristics of "Povlen-type" chrysotile in X-ray fber photographs.The 001 peak asymmetry is interpreted as indicating the presence of two layer-spacings, one about 7.3 A, in common withthe other chrysotile samples, and one about 7.2 A. TEM images of the chrysotile with two appaxent layer-spacings show thepresence of fibers with both curved and flat layers in a variety of disordered tubular structffes having only approximatecylindrical symmetry. The flat layers in these fibers may have a smaller interlayer-spacin S Q .2 A) relative to the curved layer-spacing (7.3 A; because of the possibility of increased hydrogen bonding between flat layers that are stacked in register withone another. In this sample, fibers with flat layers parallel to one direction only, joined at each end by curved layers areobserved to occur commonly. Also present are fibers with polygonal-tube cores made up from flat layers, with outer layers thatare curved in a more normal way. This type of structure appears to be the closest that has been observed to the alternativestructural model proposed by Middleton & Whittaker (1976) for'?ovlen-type" chrysotile. ln the same specimen, cylindricalchrysotile fibers that possess 5-fold symmetry have been imaged, in which the layer stacking is apparently in register radiallyat 15 points around the circumference, arising from the &/3 repeat of the hydroxyl layer.

Keywords: chrysotile, "Povlen-type" chrysotile, transmission electron microscopy, hydrogen bonds, asymmetrical diffraction-peak.

Sonanaerc

Quatre dchantillons de chrysotile ont fait l'objet d'une 6tude par diffraction X avec un synchrotron comme source du rayon-nement. Dans un de ces 6chantillons, qui possdde les traits de chrysotile de type '?ovlen" dans des clich6s de fibres, les profilsde diffraction 001 sont asym6triques. L'asym6trie serait due I la pr6sence de deux espacements_ distincts entre les feuillets, und'environ 7.3 A, en commun avec les autres 6chantillons de chrysotile, et l'autre d'environ7.2 A.I*s images prises par micro-scopie 6lectronique par transmission montrent la pr6sence de fibres ayant d la fois des feuillets courbes et plats dans une vari6t6de structures tubulaires d6sordonn6es qui n'ont qu'approximativement la symdtrie cylindrique. Nous pensons que les feuilletsplats dans ces fibres pounaient avoir un espacement inter-feuillet plus petit (7.2 A) que les feuillets courbes (7.3 A) d cause dela possibilit6 accrue de liaisons hydrogdne entre les feuillets plats, empil6s en registre les uns sur les autres. Dans cet 6chan-tillon, les fibres ayant les feuillets plats parallbles b une seule direction etjoints aux deux bouts par des couches courbes sonttrbs r6pandus. Sont aussi pr6sents des fibres ayant un centre tubulairc polygonal A feuillets plats, avec des couches extemescourbes de la fagon plus courante. Ce type de structure semble se rapprocher le plus du moddle structural alternatif propos6 parMiddleton et Whittaker en 1976 pour expliquer le chrysotile de type "Povlen". Dans le mOme 6chantillon, des fibres cylin-driques de chrysotile poss6dant un axe de sym6trie 5 ont 6t6 photographi6s dans lesquels I'empilement des couches serait enregistre de fagon radiaire i 15 points repbres autour de la circonf6rence, cons6quence de la pdriode b/3 de la couche desgroupes hydroxyles.

(Traduit par la R6daction)

Mots-cl6s: chrysotile, chrysotile de type '?ovlen", microscopie 6lectronique par transmission, liaison hydrogdne, pic dediffraction asym6trique.

258 TIIE CANADIAN MINERALOGIST

lNTRoDUcrroN

The cylindrical structure of chrysotile was firstestablished by X-ray diffraction (Whitlaker 1951,1952,1953,1.954,1955a, b, c, d, 1956a,b, c,1957).The tubular morphology of chrysotile was imaged bytransmission electron microscopy (TEM) by variousworkers in the 1940s and 1950s, but clear confirma-tion that the tubular morphology is formed by a cylin-drical lattice was provided by high-resolution TEM(Yada L967,1971). In his micrographs, the 7.3 A(002) lattice fringes show that the sheets are curved assingle or multiple spirals or as concentric cylinders.[The fringes corresponding to individual layers indexas (002) because a two-layer cell (c = 14.6 A) is usual-ly used for clinochrysotilel. Radial 4.5 A (020) fringesalso were resolved; they are generally not straight butzig-zag in direction, as additional unit cells are incor-porated around the circumference with increasingradius of the curved layers. Some of the micrographsalso show some unusual growth-habits, with irregu-larly shaped fibers that seem to have grown in stages.

X-ray and electron diffraction patterns produced bysplintery or lath-like varieties of chrysotile, commonlyknown as "Povlen-type" chrysotile, exhibit dffierencesfrom the patterns produced by fibers of "normal"(silky) chrysotile (Eckhardt 1956, Zussman et al.1957, Krstanovic & Pavlovic 1964,1967). Thesepat0erns show a series of sharp i&/ reflections on i oddlayer lines instead of, or superimposed on, the hk}reflections with diffrrse tails characteristic of the cylin-drical wrapping of the structural layers in chrysotile.Middleton & Whittaker (1976) studied diffractionpatterns produced by "Povlen-type" chrysotile indetail and concluded that the extra reflections resultedfrom a polygonal tubular structure, possibly with acore of normal, cylindrical chrysotile. An alternativearrangement ofa polygonal core surrounded by cylin-drical material also was proposed, but considered lesslikely to occur.

By using transmission electron microscopy,Cressey & Zussman (1976) observed fibers ofpolygo-nal se4rentine of the kind postulated by Middleton &Whittaker. These polygonal fibers are composed ofsectors of flat layers arranged to form polygonalprisms, either with or without a core of cylindricalchrysotile. All the polygonal fibers observed in cross-section using TEM have unusually large fiber dia-meters (up to 2000 A) compared with normalchrysotile (generally 200 - 500 A in diameter)."Povlen-type" diffraction effects also have beenobserved in single -chrysotile fibers of "normal dia-meter" (up to 200 A) seen longitudinally in the TEM(Middleton 1974),bat until now cross-sections ofnormal-diameter'?ov1en-type" fibers have never beenobserved. This may be because of the difficultiesinvolved in obtaining thin, electron-transparent cross-sections of fibers by ion-beam thinning or other

methods. More examples of similar polygonal fibers,and more complex structures with flat layers stackedin sectors, have been observed in subsequent TEMstudies (Cressey 1979, Wei & Shaoying 1984, Mellini1986, Yada & Wei 1987, Mitchell & Putnis 1988).Other, irregular structures involving curled layerspassing laterally into flat layers have been recorded,e.8., by Veblen & Buseck (1979) and Cressey &Hutchison (1984).

In this study, we show how modem high-resolutiondiffraction and imaging techniques (synchrotron X-raydiffraction and transmission electron microscopy)have provided new evidence on the nature of curved-layer and flat-layer structures in fibrous serpentines.

E)cERMEl.rrAL

We have investigated four samples of chrysotileasbestos from the collection of the Natural HistoryMuseum (Table 1) by X-ray diffraction using theSynchrotron Radiation Source (SRS) at the SERCDaresbury Laboratory, Warrington, U.K. Thesesamples all consist of bundles of fibers up to 4 cmlong which, in hand specimen, appear similar to manyother (silky) chrysotile fibers, except that specimen8M1929,t974 is more splintery than most.

Synchrotron X-radiation is tuneable to a chosenwavelength and is delivered as a monochromatic,high-intensity, well-collimated, near-parallel beam,and thus instrumental peak broadening is minimized.Consequently, although chrysotile specimens producepeaks that are broadened because of the fine size ofthe fibrils (generally about 150 A in diameter;, resolu-tion of the diffraction pattern is suffrciently good toenable observation of subtle structural variations in thediffraction profiles.

Bundles of asbestiform chrysotile fibers 2G-30 mmlong and about 0.5 mm in diameter were mountedhorizontally on the axis of the goniometer of Station9.1 of the SRS (Cernik & Bushnell-Wye 1991). With acount time of I second per step and a step size of0.01' 20, diffraction data were recorded inDebye-Sclerrer geometry from the stationary bundlesof fibers. In this orientation of the cylindrical lattice,only 001 and 0ft0 reflections are observed. Thusrecorded, the X-ray diffraction pattern of one of thechrysotile specimens (8M1929,1974) shows a marked

TABTJ 1. SAMPLESOFCHRYSOULESTT]DIED

EMLn9Jn48M1924,429E,!},[7r29,2fiBMl974,l18

Nil D€spesaadun -ine, Zvishavane, ZnbabryeDuros' mino, Zvishapsle, ZimbabweSherbc&, Western AustralisCaesiat, Briti$ Cohmbia

STRUCTURAL VARIATIONS IN CHRYSOTILE ASBESTOS FIBERS 259

asymmetry of 00/ peaks. To investigate further thesignificance of this unusual asymmetry in the peak, wehave examined this specimen by high-resolution TEM.Samples were vacuum-impregnated with epoxy resin,and then cross-sections of these fiber bundles were cutand prepared for TEM by ion-beam thinning. Thinnedsamples were examined in a JEOL JEM 2000FXTEM, operating at an accelerating voltage of 200 kV.

X-N,qy frFFRaqrpN

The 004 diffraction profiles from the four samplesof chrysotile recorded using synchrotron radiation areshown in Figure 1. Clearly, one of these samples(8M1929,1974) shows distinct asymmetry of the004 peak. Analysis of this profile, using a peak decon-volution procedure, suggests that two layer-spacings(7.31 A and7.2l A) may be present. Subsequently,0012 and 0016 profiles also were recorded; these showt}le same aslmmetry with befter resolution. This asym-metry of 001 peaks was not observed in patterns fromany ofthe other chrysotile samples. In addition, a con-ventional X-ray photograph (recorded on film in a

cylindrical camera using CuKa radiation) of thechrysotile sample with two apparent layer-spacingsshows characteristics of ooPovlen-type" chrysotile, l. e.,additional sharp reflections appear superimposed onthe diffuse tail of the 130 reflection. The other sampleswith single 00/ peaks in their synchrotron X-ray dif-fraction patterns (corresponding to a layer-spacing ofapproximately 7.3 A) have fiber photogaphs typicalof normal chrysotile.

Figure 2 shows the asymmetrical diffraction pro-files, peak profile fits and their residuals for the 004,0012 and 0016 reflections, recorded from chrysotile8M1929,1974 us,ing synchrotron radiation. A pseudo-Voigt function was used to model the peaks. Clearly,two peaks are required to fit the main peak and theshoulder region on the high-angle side, and these arecentered about values that yield 7.3 I A and 7 .21 A forthe layer-spacing dom.

The profile fitting in Figure 2 was constrained byallowing only two peaks to contibute to the fit; thisprovides a simple interpretation in terms of twodistinct layer-spacings. However, the real situationmay be more complex, as variations in layer-spacings

73L A hyer spacing

7.21, A layer spacing

16.5 L7.A 17.5 18.0

2e( l :1 .1019A)Frc. 1. Profiles of the 0M X-ray diffraction peak recorded from stationary samples of

chrysotile fibers using synchrotron radiation. (A) 8M1929,1974:. (B) 8M1929,429;(C) BM1974,118; (D) BM1929,250.

260 THE CANADIAN MINERALOGIST

100

004

17.0 18.0 66.0 67.0

2e t I= 1 .1019A) z O ( ) = 1 . 1 0 1 9 A ) 2o 1) = l.m3 A)Ftc. 2. Synchrotron X-ray diffraction profiles recorded from chrysotile 8M1929,1974, together with profile fits (solid lines)

and their residuals. In each case, two peaks have been used to fit the main peak and the shoulder. Note the relative decreasein the peak-height intensity of the shoulder with the increasing order of reflection. lntensities relative to the 0M peak areindicated on the vertical scale.

could occur, depending on the exact structure of theserpentine. Therefore, we also have performed fitswith up to twelve peaks with unconstrained widths. Byfitting with multiple peaks, it is apparent that the finestructure of the profiles can be modeled in more detail,suggesting that it may be appropriate to model theseprofiles with peaks that extend over a range ofd-values corresponding to layer-spacings between7.2 A and 7.4 A. lt. is also apparent that a greaterspread of d-values conributes to the shoulder regionthan to the main peak. Modelling with multiple peaksstill awaits quantitative analysis of peak positions andtheir widths, and further work on this is in progress.

However, a qualitative assessment of relative peak-widths can be made. It is evident that the relativepeak-height of the shoulder component (representingthe 7.21 A layer-spacing) decreases almost linearlywith 1./d, and this reduction in peak height is likely tobe due to peak broadening. Both peak components(7.31 A and 7 .21 A; in the profile are broad, owing tothe small size of the crystallites, but size-broadening isindependent of the order of a reflection. However,peak broadening that arises from microstrain doesdepend on the order of a reflection, with peak breadthincreasing as l/d increases. This has been expressedby Langford et al. (1988) as p = 4V124, where B is the

integral breadtl, and 7 denotes the microstrain. Wetherefore draw the conclusion that the serpeltinestructure with the smaller layer-spacing (7.21 A) issignificantly more sffained.

As this chrysotile specimen is known to possess"Povlen-type" characteristics, it is likely that curvedand flat layer structures are present (Middleton &Whittaker 1976, Cressey & Zussman 1976). Theobserved strain-broadening is consistent withthe hypothesis that the structure of flat-layer serpen-tine is more strained than that of a curved-layer struc-ture. In chrysotile, structural curvature enables partialcompensation for the misfit between the sheets ofoctahedra and tetrahedra. Presumably, no suchcompensation occurs in flat-layer serpentine, unlesssubstantial Al3+ and Fe3+ substitutions are present inthe octahedrally and tetrahedrally coordinated sites.Energy-dispersion X-ray analysis of chrysoti le8M1929,1974 in the SEM indicates the presence ofonly 1.5 tntclo Fe and negligible Al. Structure refine-ments by Mellini (1982) and by Mellini & Zanazzi(1987) show that in lizardite (flat layers) the layer oftetrahedra is distor0ed to ditrigonal symmetry by rota-tion of tetrahedra to facilitate increased interlayerhydrogen bonding, even though an undistorted hexag-onal arrangement of the layer of tetrahedra would be

STRUCTURAL VARIATIONS IN CHRYSOTILE ASBESTOS FIBERS 261

more likely to reduce the intralayer misfit strain withthe layer ofoctahedra. Chisholrn (1992)has discussedthe effects of intralayer and interlayer strain on thetotal strain associated with curved and flat layers inserpentine; from this assessment of strain, it can beappreciated that unless the fiber radius of chrysotile iseither very small or very large, a curved structureis likely 10 possess less total strain than a flat-layerstructure. There is, however, no clear indication inpublished cell parameters that samples of lizarditegenerally have smaller interlayer spacings thansamples of cbrysotile.

Elrc"rnox MrcRoscopy

Having seen features of '?ovlen-type" chrysotile insingle-crystal-type diffraction patterns from specimennumber 8M1929,1974, and evidence of both 7.31 A

and the unexpected 7.2I A layer-spacings in the syn-chrotron XRD pattems, we have sought explanationsfor tfiese features by high-resolution TEM. Our TEMobservations of cross-sections of these fibers revealtubes made up of curved and flat layers in disorderedcylindrical structures, with only approximate rotation-al symmetry. We have not, however, found any large,polygonal fibers with clearly defined boundariesbetween sectors of flat plates, such as those seenpreviously in specimens of "Povlen-type" chrysotile.

Fibers that most closely resemble the'?ovlen-type"chrysotile images previously published have cores offlat or gently curving layers. Curved sheets lacksmooth, continuous cylindrical curvature but areananged with discontinuously curved sections which,like the flat sheets, form approximately polygonaltubular cores. Examples are shown in Figure 3. Thelattice fringes are, unfortunately, a little faint because

Frc. 3. TEM image ofchrysotile 8M1,929,1,97 4with the beam parallel tothe fiber axes, showing(002) lattice fringes. Notethat several fibers havepolygonal cores made upof flat layers, with outerlayers that are moresmoothly curved,

262

Ftc. 4. Fiber sections record-ed with the beam nearlyparallel to the fiber axes.(A). These fibers haveelongate cross-sectionswith straight (002) latticefringes parallel to theelongation. (020) fringesare visible in curvedsections of some fibers.(B). A fiber from (A)showing (002) and (020)fringes in opposite quad-rants, both parallel to theplane containing the {iberaxis and the beam. (C). Afiber from (A) showing(002) and (020) fringesthat are not in exactlyopposite quadrants, andthe two fringe sets are notparallel to one another.

TIIE CANADIAN MINERALOGIST

these fibers are in a very thin area of the specimen.Ion-beam thinning always produces a damaged, amor-phous surface-layer. Adjacent to the edges of the holesproduced by ion-beam thinning, the very thin parts ofthe specimen will therefore consist almost entirelyof amorphous material. The outer layers of the fibersin Figure 3 are particularly indistinct because theyhave largely become amorphous, either during ion-beam thinning or during exposure to the electronbeam. Howevero the outer layers, where visible,appear to be curved in a more normal way. Thesestructures seem to be the closest that have beenobserved to the alternative model of tle structure pro-posed by Middleton & Whittaker (1976) for "Povlen-type" chrysotile. We have found them occurring onlyvery infrequently in the electron-transparent parts ofthis specimen.

We have, however, observed another structureinvolving flat and curved sheets in a fibrous habit inthis specimen. In the thinned areas examined, it occurswith fairly high frequency. Fibers of this type consistof flat layers parallel to one direction only, joinedcoherently at each end by curved layers, resembling

263

the appearance of an oval running track. A typical areacontaining such elongate cross-sections of fibers isshown in Figure 4. It is not possible to account forsuch elongation by assuming that the fiber axis isinclined to the electron beam; a tilt of 60' would benecessary to produce a ratio of minor to major axis ofl:2, as is observed here. In any case, such tilt wouldextinguish the (002) fringes on the long sides andleave them on t}re curved ends, which is the oppositeof what we observe.

In the area shown in Figure 4, (002) fringes (con-centric or spiral) are generally less clear around thecurved parts of fiber sections. The flat layers seem tosuffer damage under the electron beam less quicklythan the curved parts. Commonly, (020) fringes (fromcenter to circumference) are visible in the curvedsections instead. If there is a slight misalignment ofthe fiber axis with respect to the electron beamn thenthe (020) and (002) fringes would tend to persist inalternate quadrants, and both sets of fringes wouldbe parallel to the plane containing the fiber axis andthe beam. Generally, this appears to be the case inFigure 4 (e.9., Fig. 4B), where the axes of elongation

STRUCTTIRAL VARTATIONS IN CHRYSOTILE ASBESTOS FIBERS

o02

>--e----e-r---C-a ->-+-J-r-<

a a a a l -

a t a - a - ' - '

a a - a a - a a . t I a t

a - t - ' a t - t - a

a aa

aa

a

a

FIc. 5. A cylindrical array of latrice points (after Whittaker 1955b) joined in such a wayas to represent the lanice fringes shown in Figure 48. Note, however, that in Figure 4the (002) fringes are mostly straight, not curved as here, because the fiber is oval incross-section. not circular.

F==rt - a - a t a t a

264 TIIE CANADIAN MINERALOGIST

happen to be roughly parallel to the plane containingthe fiber axes and the beam.

It should be noted that (020) fringes are not trulyradial, or they would diverge from the center withadditional unit cells added around the circumferenceas the radius of the curved layer increases, givitrgthe zig-zag appearance shown by Yada (1971). InFigure 4, the (020) fringes are generally straight andequally spaced over considerable areas, to a fairdegree of approximation, though there are some depar-tures. Whittaker (1955b) used a mask for opticaldiffraction in which the dots lined up over consider-able areas. In Figure 5, a section of Whittaker's cylin-drical mask is reproduced, with lattice points joined upin such a way as to resemble the distribution of (020)and (002) fringes that we observe. In Figure 4, somesections (e.g., Fig.4C) show clear (020) and (002)fringes that are not strictly in alternate quadrants, and

(020) fringes are not parallel to (002) fringes. Thissuggests that the position and orientation ofthe planeswhere the points of the circular (or distorted circular)lattice line up with each otler need not be in alternatequadrants. It seems reasonable that they could easilybe offset by up to 36o from the obvious positions, i.e.nthe angle between the clear (002) and (020) fringescould be anything up to this angle, and not constantfrom center to circumference, as appears to be the casein some of the fiber sections shown here.

Not all fibers are elongate in cross-section with flatlayen. Some are elongate but consist of curved sheetswith variable radii of curvature, particularly in thecore regions, and others are more nearly circular insection. Multilayer spirals also are found fairly com-monly. A few examples are shown in Figure 6.Boundaries between fibers commonly are indistinct,with spaces between fibers filled by amorphous or

Ftc. 6. Fiber sections exhi-bit ing various growthirregularities, includingvariable radii of curvatureand multi-layer spirals.

2[H

poorly crystalline material, or curved sheets. Centralvoids in fibers are commonly filled with similarmaterial, and some fibers have very irregularly shapedbut continuous sheets in the core regions, which maybridge the central holes. Whittaker (1957) suggestedthat such infilling would be likely, and would explainthe observed measurements of density. It is notpossible to know how much of the material with noapparent structure was originally amorphous and howmuch has become so because of damage in the elec-tron beam or during specimen preparation.

In parts of the specimen that are thicker, a combina-tion of strong diffraction-contrast and a degree ofdamage under the electron beam shows a pattern thatindicates the existence of five-fold symmetry. Thisfrst demonstration of five-fold symmetry in a nafuralcrystalline material has been reported elsewhere(Cressey & Whittaker 1993). Because this observation

265

has implications concerning the nature of the inter-layer bonding in this specimen, it is also describedhere.

Where fibers are exactly parallel to the beam, dif-fraction contrast makes them appear completely dark.With increased exposure to the electron beam, thedark contrast breaks into radial bands, with the diffrac-tion contrast being lost from the outside edge fust andextending toward the center with time. Figure 7 showsexamples of this behavior. In this areq fibers are cir-cular in section. Dark diffraction-contrast remainswhere (002) fringes are clearly visible and fades wherethe fringes disappear as these areas become amor-phous under the beam. There are almost always15 radial bands; the number 15 is significant. Aspointed out by Whittaker (1954),2n d*, is approxi-mately equal to 5b, so that ifnvo successive layers arein step with one another somewhere, then they will be

STRUCTI]RAL VARIATIONS IN CHRYSOTILE ASBESTOS FIBERS

ftc. 7. Sections of fiben withtheir axes exactly parallelto the beam. Strong dif-fraction-contrast. whichinitially made the fibersections appear complete-ly dark, has broken into15 radial bands after aperiod of exposure to tleelectron beam has causedsome of the structure tobecome amomhous.

266 TT{E CANADIAN MINERALOGIST

a

O l

a

a

r a

a

. . \ . . .r i " '. . f .

; . ' \ ' . .. o a t . .

I O ,

. A.ra a

a aa a

a a aa a a

a aa ao oa aa a

aa

a

a - a - a - a j a - aa - a - a - a - a - aO+O+a a a a a a

a a a a a aa a a a a

aa a a

aa aa a

a a a a

a aa

o a. o

7'o.a

t o .

in step five times around the circumference of thefiber. If all the layers are in step on a radial plane, thenthey are all in step five times. In Figure 8, we repro-duce Whittaker's mask, marking with open circlesthose lattice points that line up radially. On the mask,the lining-up occurs ten times around the circum-ference because the lattice points are sepuatedby blTto allow for the effect of projection of the centeredunit-cell down a. [This is, of course, why we alwaysobserve (020) fringesl. If the structure were more sus-ceptible to decomposition under the electron beamwhere the layers are out of step, then one could pro-duce an effect similar to that in Figure 7, but with onlyfive spokes. However, the hydroxyl side of the serpen-

a ao o

aa

a l. o

a a

aa

a t t a '

. a t '

a a aa a a ^

v a' l 6 o r l l t o

o o ^. " o o

l o o o

t a ^

" o r

o l l r o

l a a a o !

FIo. 8. A cylindrical array of lanice points (after Whitraker 1955b), representing a pro-jection of ttre chrysotile lattice in cross-section. The points are sepaxated cicumferen-tially by b/2 to allow for the effect in projection of the centered unit-cell down a, andthose marked with open circles are rows of points that exactly line up radially, ientirnes in projection. Note that alternate rows of open circles represent lanice points atheight zero and a/2, so the overall symmetry is five-fold. Ticks with circumferentialspacing of l/3, to represent the repeat of the hydroxyl side of the serpentine layer, areshown for successive serpentine layers in one of the five sectors. Solid lines representthe 15 radial positions (arising from the bl3 repeat of the hydroxyl groups) wherelocal sirnilarity of structure between the hydroxyl groups of one layer and the basaloxygen atoms of the next layer outward could allow hydrogen bonding to occurbetween successive layers,

aa l

aa

aI

a

l f

tine layer has a repeat of only bl3, so there is localsimilarity of the arrangement of the basal atoms ofoxygen of one layer with respect to hydroxyl groupsof the next inward 15 times per circumference. These15 positions are marked on the diagram in Figure 8.The areas between these 15 spokes, where layers areout of step and so hydrogen bonding between layers isnot possible, could well produce 15 radial bandswhere decomposition under electron bombardmenttakes place more readily, as seen in Figure 7.

Chisholm (1.992) has pointed out tlat polygonalserpentine almost always has 15 sectors (or, less com-monly, 30), because minimal misfit occurs only forpolygonal microstructures with 15 or 30 sectors,

a

depending on the exact cell dimensions of the flatlayers. A close inspection of Figure 7 suggests thatpossibly, in some fiber cross-sections, the (002) layersare not smooth, continuous curves. In the bands whereless decomposition has occurred (i.e., those regionswhere successive layers are probably more nearly instep with one another), the layers may seem to beflatter. (Care must be taken not to mistake confiastreversals along a layer for discontinuities of curva-ture). Possibly, under the right geological conditions,fibers such as these could recrystallize to form large,well-developed polygonal fibers similar to ttrose seenin previous TEM studies, with 15 sectors, as discussedby Chisholm. Such recrystallization would increasethe extent to which sheets could be stacked in registerwith one another. For large-radius fibers, this wouldbe a much more stable amangement, as pointed out byChisholm (1992).

The specimen in which Cressey & Zussman (1976)originally found a high proportion of polygonal fibers

267

comes from a serpentinite that has undergone progrademetamorphism after the initial serpentinization. Thepolygonal f iber shown in Cressey & Zussman(1976,Fig.4) appears to have overgrown earlier-formed cylindrical fibers of chrysotile. A high-resolution micrograph of another polygonal fiber fromthe same specimen, shown in Figure 9, shows thatlayers are mostly coherent across sector boundaries.This micrograph also shows that in the cylindricalcore. the sheets seem to be somewhat disordered incomparison with the outer, flat layers. These observa-tions are in accord with our proposed mechanism forthe formation of polygonal fibers by recrystallization,involving an adjustment of the interlayer stackingfrom a disordered to a more ordered arrangement. Thefiber section is not a complete polygon, and in themissing parts where the sheets are not constrained,they tend to curl. These curved sections have smallradii of curvature, similar to the normal, cylindricalfibers.

STRUCTURAL VARTATIONS IN CHRYSOTILE ASBESTOS FIBERS

200 &:rs*

Figure 94

268 THE CANADIAN MINERALOGIST

Fto. 9. High-resolution TEM image of a polygonal serpentine fibeg specimen 18501 (see Cressey & Zussman 1976). (A) Thewhole fiber section and (B) enlarged deail from part of (A), showing flat layers passing coherently across sector bound-aries and continuirg lalerally into curved sheets. Note also the degree of disorder between layers in curved sheets near thefiber center.

Where fiben are slightly tilted with respect to theelecton beam, then the (020) and (002) fringes tend topersist in alternate quadrants, as described above.Such a degree of misalignment gives rise to four radialdark bands of diffraction contrasto replacing the com-plete darkness or many (generally 15) bands seenwhere alignment of the fiber axis with the beam ismore perfect. With slightly more misalignment, orgreater damage under the electron beam, or both, the(020) fringes tend to disappear, leaving only the (002)fringes and one pair of dark diffraction-contrast bandsat 180o. This pattern of diffraction-contrast behaviorwith tilt was noted by Cressey & Zussman (1976),though at tle time there was no evidence to explainwhy it happens.

DIscussIoN

From the split (00/) reflections observed in thesynchrotron diffraction profi le of specimen8ML929,L974, two layer-spacings appear to bepresent. One is 7.31 A, about the value normally foundfor chrysotile, and the other is about 7.21 A. Electronmicroscopy suggests that the most obvious differencebetween this and other chrysotile specimens is thepresence of substantial numbers of fibers with flatlayers in this specimen. It appears that the flat layenin many of the fibers have a smaller interlayer-spacingbecause of the hydrogen bonding developed betweenflat layers that are stacked in register with one another.In cylindrical layers of chrysotile, micrographs such as

STRUCTT.JRAL VARTATIONS IN CHRYSOTILE ASBESTOS FIBERS 269

that shown by Yada (l97l,Fig. 14) illustrate fiben inwhich layers are stacked completely randomly, out ofregister with one another. In the cylindrical fibersshown in this paper, the layer stacking is apparently inregister at fifteen points around the circumference, butat all other positions the layers must be stacked outof register with one another. In cylindrical fibers,therefore, hydrogen bonding between layers is notgenerally possible. Published values for the layer-spacings of chrysotile and lizardite, the flat-layervariety of serpentine, suggest that there is a range ofvalues from a little above 7.2 A le.S., 7.233 A for

"alizardite reported by Mellini (1982)l to over 7.3 A.However, there is no clear pattern of lizardite tendingtoward the lower value and chrysotile the higher.Lizardite commonly has a significant degree of dis-order in its structure, and commonly contains more AIand Fe than chrysotile.

When we study ion-thinned specimens by TEM, itis inevitable that we can only image a tiny proportionof the material in any specimen. This is especially truefor sections of asbestos fibers embedded in resin; thevast majority of the areas thinned to electron trans-parency consist of the resin. Therefore, we can neverbe sure of the extent to which our TEM observationsare representative of the whole specimen. X-raydiffraction, however, gives us information averagedover a much larger volume. We cannot say with confi-dence that in specimen 8M1929,1974, the proportionof flat layers relative to curved layers seen in the TEMcorresponds with the ratio of 7.21 A to 7.31 A layer-spacing detected by X-ray diffraction, but the com-parison seems reasonable, as far as we can judge. Itwould be interesting to carry out synchrotron X-raydiffraction experiments on fiber bundles consistingentirely of well-formed polygonal serpentine if suffi-ciently large bundles of long, parallel fibers could befound.

In specimen 8M1929,197 4, the elongate-sectionedfibers tend to be aligned with the axes of elongation allin roughly the same direction. Possibly these fiberssuffered directional pressure during growth, or laterdeformation while still in the host rock. The possi-bility of deformation during specimen preparationmust be considered, but it seems more likely that suchprocesses would cause brittle fracture rather thanplastic deformation. We have seen no evidence ofbroken fibers.

Our findings show, yet again, that the serpentinegroup of minerals can form in numerous complexstructural arrangements that may seem difficult toclassify. Imaging by high-resolution TEM shows that,on a fine scale, the structtues commonly consist ofmixtures or intermediate forms. Layers commonlypass coherently from one structure type to anotherover a few unit cells. Nevertheless, all the observedvariations can be described as consisting of compo-nents of the three known structure types: cylindrical

chrysotile, flat-layer lizardite or alternating-waveantigorite. A single fiber may therefore consist of bothchrysotile and lizardite. Transitions from one structuretype to another occur as the best solution to thepr-blems caused by interlayer and intralayer sgain .atihe time of crystallization (or recrystallization) inresponse to the local physicochemical conditionsaffecting each group of atoms. Most serpentinites havebeen formed under disequilibrium conditions, soperhaps it should not seem surprising to find thatio -any composite or intermediate structures exist.

ACIC.IOWI.EDGEIVIEI.ITS

We thank Dr. Eric Whittaker and Prof. JackZussman for helpful comments on our observations,kindly sharing with us their considerable wisdomand experience of the serpentine minerals. Thereferees are also thanked for their comments andsuggestions. Mr. Tony Wighton, Natural HistoryMuseum, is thanked for producing excellent-qualifydemountable polished thin sections of chrysotile. Weacknowledge funding by a SERC glant to carry out thesynchrotron X-ray diffraction experiments.

REFERENCES

CERNIK, R.J. & BusuNrLL-WYE, G. (1991): The use ofDebye-Scherrer geometry for high resolution powderdiffractton. Mater. Sci. ForurnT9'\z' 455462.

CrilsHou\4, l.E. (1992): The number of sectors in polygonalserpentine. Can. Mineral. 30, 355-365.

Cnssssv, B.A. (1979): Electron microscopy of serpentinitetextures. Can. Mineral. 17, 7 4l-756.

& HrncilsoN, J.L. (1984): Microstructure of anti-gorite revealed by high resolution electron microscopy.Inst. Pltys. Conf., Ser. 68, 409412.

& Wnrrrarsn, E.J.W. (1993): Five-fold symmetryin chrysotile asbestos revealed by uansmission electronmicroscopy . Mineral. Mag. 57 ,729-732.

& ZussrraaN, J. (1976): Electron microscopic studiesof serpentinites. Can. Mineral. 14,307'3I3.

ECKHARDT, F.-J. (1956): Rd,ntgenographische Unter-suchungen am Schweizerit. Neues Jahrb. Mineral'Moruttsh,3243.

KnsreNovtd, I. & PAvLovlC, S. (1964): X-ray study ofchrysotile. Ant Mineral. 49, 17 69-177 1.

& - (1967): X-ray study of sixJayer ortho-serpentine. Am. Mineral 52, 87 1-876.

LeNcrono, J.I . , Dnlusz, R., pr KstrsnR' T.H. &MttrnvrEunn, E.J. (1988): Profile analysis for microcrys-talline properties by the Fourier and other methods. AzstJ. Phys.4l,173-187.

270 THE CANADIAN MINERAIOGIST

MELLINTT, M. (1982): The crystal structure of lizardite lT:hydrogen bonds and polytypism. Am. Mineral. 67,587-598.

(1986): Chrysotile and polygonal serpenrine fromthe Balangero serpentinite. M ineral. Mag. 50, 301-305.

& Zayezzt, P.-F. (1987): Crystal structures of li-zardite-lT andlizardite-2HI from Coli. Italv. An.Mineral. 72,943-948.

MIDDLEToN, A.P. (1974): Crystallographic and Mineral-ogical Aspects of Serpentine. D.Phil. thesis, OxfordUniv., Oxford, U.K.

& Wurrrerrn, E.J.W. (1976): The structure ofPovlen-type chrysotiTe. Can. Mineral. 14, 301-306.

Mncurl4 R.H. & Punrs, A. (1988): Polygonal serpentinein segregation-textured kimberlite. Can. Mineral. 26,991-997.

VEBLEN, D.R. & Busncr<, P.R. (1979): Serpentine rninerals:intergtowths and new combination stmctures. Science206. 1398-1400.

Wst, L. & SueovrNc, J. (1984): The discovery of Povlen-type hydroxyl-chrysoti le and i ts mineralogy. ActaMineral. Sinica2. I l l - l16.

WHrrrarcn, E.J.W. (1951): An orthorhombic variety ofchrysotile. Acla Crystallogr. 4, 187-188.

(1952): The unir cel l of chrysoti le. AcraC rystalLogr. 5, 143-144,

(1953): The structure of chrysoti le. AcraCrystallogr. 6, 7 47 -i 48.

(1954): The diffraction of X-rays by a cylindricalIattice.I. Acta Crystallogr. 7,827 -832.

(1955a): The diffraction ofX-rays by a cylindricallatt\ce. II. Acta C rystallo g r. 8, 261-265.

(1955b): The diffraction ofX-rays by a cylindricall^ttice. m. Acta Crystallogr. 8, 265-27 1.

(1955c): The diffraction of X-rays by a cylindricall^ttice. IV. Acta Crystallogr. 8, 726-729.

(1955d): The classification of cylindrical lattices.Acta C ry stallogr. 8, 57 1 -57 4.

_ (1956a): The structure of chrysotile. II. Clino-chry sotile. Acta C rystallo gr. 9, 855-862.

(1956b): The structure of chrysotile. III. Ortho-chry sotile. Acta Crystallogr. 9, 862-864,

(1956c): The structure of chrysotile. IV. Para-chrysotile. Acra Crystallogr. 9, 865-867.

(1957): The structure of chrysotile. V. Diffusereflections and fibre textlure. Acta Crystallogr. 10, 149-156.

Yaoe, K. (1967): Study of chrysotile asbestos by a high reso-lut ion electron microscope. Acta Crystal logr. 23,704-707.

(1971): Study of microstructure of chrysotileasbestos by high resolution electron microscopy. AcraC ry sra I I ogr. M7. 659 -664.

& War, L. (1987): Polygonal microstructures ofPovlen chrysotile observed by high resolution electronmicroscopy. Euroclay 87 (Sevilla, Spain), Abst.

ZussMAN, J., BnrNnrsv, G.W. & Cousn, J.J. (1957): Electrondiffraction studies of serpentine minerals. Am. Mineral.42, 133-153.

Received Apil 7, 1993, revised manuscript accepted Augu.st22. 1993.