Estructuras minerales serpentinas

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CanadianMineralogl*tVoL 13, pp.227-243 1975)

A REAPPRAISAL OF THE STRUCTURES OF THE SERPENTINE MINERALS

F. J. WICKSMineralogl Depa.rtment' Royal Ontario Museum, Toronto, Canada

E. J. W. WHITTAKERDepartrnent of Geology & Mineralogy, Oxford Unlversity, Oxford, England

ABSIRACT

The three theoretical stacking schemes for tri-octahe&al l:1 layer silicates developed independent-ly by fteadman, Z'tpgin' and Bailey all assumeideal hydrogen bonding between successive layers,

with polytypes developed through shifts of :tal3,!b/3 or zero, and rotations of 180o or zero. Thethree systems contain 16 distinct polyty?es, andBailey's nomenclature is extended to provide sym-bols for each one. If lizardite is redefined to in-clude all serpentines with flat-layer structures, thesecan be designated as lizardite followed by the ap-propriate polytype symbol.

The above system cannot be applied to the chry-sotilo structures because the sbifts between thelayers show that the normal hydrogen bondingposition is not achieved. The layer stacking de-pends on tle fact that the basal oxygens form two

sets, differing in e, and tle lower one lies rn thegroctvesbetween, and the upper one over the ridgesformed by, the underlying hydroxyl rows. Thisstacking principle leads to a series of polytypes forwhich an analogous nomenclature is proposed.

using subscript c to denote their essentially cylin-drical character. Thus clinochrysotile, orthochry-sotile and Zvyagin's l-layer clinochrysotile be-come chrysotile 2Ma, 2Or"1 ar.d lM"r. The termparachrysotile is retain_ed o describe the cylindricalstructure with a 9.2A fiber axis. The alternatingwave structure of antigorite, with layer inversionsand interconnections between successive layers at

the inversions, destroys the possibility of systematichydrogen bonding and makes the stacking depen-dent on the primary bonding at the interconnections,so that it cannot be discussed in terms of polytypes

of either kind.

A review of the substitution in natural and syn-thetic chrysotiles and lizardites indicates that thereis a compositional overlap between curved and flat-layer structures. Thus, as all flat-layer structurescaq b€ regarded as polytypes of-lizardite and alloylindrical structures with a 5.3A fiber axis canbe regarded as polyty?es of chrysotile, lizardite andchrysotile are polymorphs. Compositional data for

parachrysotile are lacking. Antigorite is not a poly-morph of the other serpentines because it has an

essentially (though only slightly) different composi-

tion.Because the mismatched tetrahedral and octahe-

dral sheets in lizardite are respectively in tension

and compression,t is suggestedhat it is the cona'pression of the octahedral sheet that buckles theMg plane so that the Mg atoms occupy two posi-tions at different levels. This disturbance of thestructure in turn tilts the tetrahedra in the waythat is observed.In chrysotile the curvature only

partly relieves the mismatch, and evidence s pre-sented for a similar buckling itr this structure,though to a smaller extent, This postulatedbuckl-ing explainssome hitherto obscurefeatures of thestructure.Antigorite appears o overcompensatehemismatchin the direction of curyature,but it hasan anomalously hick octahedralsheet which isstill unexplained.

INrnooucrroN

Because of their frequent sub-microscopis,fine-grained texture and close chemical relation-ships, an adequate classification of the ser-pentine minerals was delayed until structural

information became available, and this was

itself delayed by the unusual curved layersosu-perlattices and disordered stackings that occurin these minerals. It was not until 1956 that

work on the structures of chrysotile by Whit-

taker (1952, 1953, L954, I955a, b, c, d, l'956a,

b, c, 1957)and by Jagodzinski& Kunze (L954a,

b, c) and of antigorite by Zussman (1954) and

Kunze (1956, 1958, 1959) was advancedsuf-ficiently for a viable classification to be put for-ward (Whittaker & Zussman 1956).

This classification divided the serpentine min-

erals into three structural groups basedon rylin-drical layers (chrysotile), cornrgated layers(antigorite) and flat layers (lizardite). Chryso-tils was further sub-divided into three varieties,clinochrysotile, orthochrysotile and parachryso-

tile. Antigorite was soon found to occur witha range of superlattice periods (comrgation

wave-lengtls) from 16-110A as well as theoriginal 43A @rindley et aI. 1958; Chapman &Zussman 1959; Kunze 1961). Sub-division ofthe flat-layer serpentines was necessitated bythe discovery of varieties with a 1-layer celland a 2-layer cell @ucklidge & Zussman 1965),

227



228 THE CANADIAN MINERALOGIST

a 6layer cell (Zussman& Brindley 1957; Zuss-man, Brindley & Comer 1957; Olsen 196l;Miiller 1963; Krstanovii & Pavlovid 1967), a6- (pseudo2-) lryer and a G (pseudo3-) layercell (Gillery 1959; Bailey & Tyler 1960), a 3-layer cell (Coats 1968) and a 9-layer cell (Jah-

anbagloo & Znltai 1968)4. It has since becomeevident from the work of Zvyugn (1967) that afourth variety of chrysotile, L-layer clinochryso-tile, must be added to the three chrysotiles ofWhittaker & Zussman's classification.

OnIy two serpentine varieties have come tolight whose relationship to the three-fold classi-fication has been in doubt. One h the splinteryserpentine @ovlen-type) found by Krstanovis& Pavlovii (1964) which they suggestedo havea modified clinochrysotile structure. More re-cently Middleton (1974) has investigated thisfurther and found Povlen-type clinochrysotile tobe composed of chrysotile-like layers. He hasalso found a Povlen-type orthochrysotile whosestructure he has interpreted as an intergrowthof chrysotile and lizardite-like layers. The otherserpentine variety is an unusual serpentine fromthe Tilly Foster mine, New York (Aumento1967,)which may be o very intimate mixture ofclinochrysotile and antigorite.

*Jahanbagloo & Zo.ltai (1968) analyzed what theycalled a bexagonal Al-serpentine with a 9-layer tri-gonal structure and a composition of (Mgr.o"Al."n-Fe.sr)(Si!.a7Al.$)Ou(OH)n.owever, as this composi-tion is closer to the amesite end of the lizardite-amesite solid-solution eries this qrecimen will becalled amei{te .9? in this paper.

The purposeof the present paper is to discusstho differences between the internal structure ofthe layers of the main divisions of the serpen-tine minerals, and to relate them to one anotherand to other trioctahedral 1:1 layer silicates interms of structure and crystal chemistry, and

in this connection to develop further the dis-cussion of Olsen (1961) and Radoslovich(1963b). The relationships of these structuralfeatures to the chemical differences that havebeen treated previously by Faust @ Fahey(1"962r,Page (1958), and Whittaker & Wicks(1970) are also discussed.However, before pro-ceedingto this discussion t is desirable o con-sider the relationships of the layer stacking of theserpentine minerals to the polytype classifica-tions of Steadman (1964), Ztyagln (1967) andBailey (1967a, 1969) for 1:1. layer silicates.Some sonfwion has been introduced into theliterature by attempts to fit the serpentines intothese classifications,which are not in fact di-rectly applicable to the serpentine structures.

THEoRETTcAL or,yrypns

Three theoretical stacking schemes for tri-octahedral l:1 layer silicates have been de-veloped in recent years by (i) Steadman(1964),(i) Zvyagin, Mischenko & Shitov (1966) andZvyagin (1967) and. (iii) Bailey (1967a, t969).The terminology developed by the various auth-ors differs and so do their basic assumptions,so that the same series of polyty,pes is not de-veloped in each system, although there is con-siderable overlap (Table 1). The full details ofthe procedures are given in the original papers

TABLE . A CMAiISOII OF POLTYPES

Sbatun (1964)

Layers lnter-SF@ ln Unlt hyer Strlcturehup CelI Shlfts X@b€r

B a i l ey (1969)

[xlenddGroup Poljdse hl ley

Xmnclature

ZvFg l n (1967)[email protected]

h€dmlGroup Struchre S!ructurc ilodifi-fl@r Nmber Typ€ catlon

1 7

t 8

1 9 & 2 0

1z

3 & 4

t ll a / 3

& 2 a / 3

P3t 3 a./3

B l [ A t i l 1 t 4

41 Nl a1

3T 3T 3Tt

C@2,

n.l

' ' lo t 5

I

2 & 3

4

5 & 6

2 al3+r

2 at3+r n9!

2 al3rr

5 al3+r

t t 1 c 2 0

4"

5H

B 20r

equlvalent & 2h

2i2

6H

2Or

az6Ht

A ] T C I T ' I T

3T 3R 3R

2 r n z l

I

2 & 3

a

I

| & 23

P3l0 I ilone

B 3 b l 3

P3lc 2 b/3

D 2ts D 2Ht 2Ht

2H 2H2 2HZ

fr 6R 6Rr

v I r

l l

v1

9

l 0 & l

1 6

I

2 & 3

l

P63@ 2 flon*r

P6: 2 b/3+?

k 6 b/3+r

8 6P 3 3

i l o n e , r / 3 4 & 5t t a n e , . b / 3 6 & 7

l f f l n i t l es r l th

&lley Group C

6&z31 2

B 6 ltondr

bl3rrP6" 6 llon+r

_b/3+r

t 2 & 1 3

t 4 I t 5

. t f l n l t l es ! i l h

Bijley Grolp ! 6lz

liere fu strlciure nders according b stedfun or zvyagln arc glven on one llne, they corrcspond to enanfiomrohs



A REAPPRAISAL OF THE STRUCTURES OF THE SERPENTINE MINERALS

and a Cetailed comparison of the methods isgiven in Wicks (1969).

For purposesof generatingth,evarious poly-types, the three systems assume an ideal tetra-hedral sheet with hexagonal symmetry, linkedthrough the apical oxygens without distortion to

an ideal trioctahedral sheet with trigonal sym-metry (Fig. 1). Stacking of successiveayers isthen assumedo occur in such a way that hydro-gen bonding always develops between every ba-sal oxygen of a given layer and the outer hy-droxyls of the adjacent layer (see Fig. 2 ofBailey 1969). If such layers are stacked verti-cally above* one another such an optimumhydrogen-bonding system is developed, and thestructure has a 1:1 layer trigonal unit sell.Equivalent hydrogen-bonding systems are de-veioped if successive ayers are shifted relativeto one another by -a/3** along any of thethree possiblet-axes of the layer, ot by L bl3

along a y-axis,rotated through -+-60o, -t 120o or180o, or are subjected o combinations of suchshifts and rotation. The number of alternatives is

reduced by the fact that the layer symmetrymakes it possible to describe all polytypes pro-

duced by -r b13 shifts in terms of a singley-axis(Fig. 1). It also makes rotations of :t120' equiv-alent to no rotation. and rotations of t60o or180' irdistinguishable and therefore describable

simply as rotation (r).Zvyagin (1967) and Bailey (1969) have bothclassified the possible polytypes into four ,goupsunder the further limiting assumptions hat:(i) combined shifts of the type a/3 and b/3 do

not occur(ii) the relationship between every successive

pair of layers involves the same kind ofshift, or shift * rotation, although the di-rection of the shift may change in a syste-matic way to grve multi-layer cells, as. inthe mica polytypes.

Both these authors classify the polytypes intofour groups depending on the existence of in-

'rThe convention adopted by Steadman and Bailey'

which is followed here, is to regard the outer hy-

droxyls to be on the top of the layers, and tbe sili-

cate net at the bottom. In Zvyagin's discussion the

opposite convention is adopted, but this males no

difference to the nomenclature aod has been al-

lowed for in the comparisons of the different classi-

fications.

*l'There are two possible sets of octahedral sites ina 1:1 sitcate layer, that occupied by Mg in Figure

1 (called II by Bailey) and another (D which in'

volves an interchange of the projected positions of

Mg and the outer hydroxyls. If Mg occupies set f,

the appropriate shifts are + a/3.

? 9---o a; -----n?'-i o ; oMs ' ' h o

i

O .,P@t O-..

A

fr+ €*, ,OAatr- o'fez o '/ o

"i of @*,,,Qo ig--- -- -o---o---o-------€

l.

Frc. 1. [001] projection of the idealized serpentinelayer with Mg in the II octahedralsites with re'spectto the t-axes, as defined by Bailey (1969).

In the monoclinic chrysotilepolytypes t is neces'sary to rev€rse the positive direction of the .r-axis in order to satisfy the convention that Fshould be obtuse and as small as possible.

terlayer relationshipsa/3, a/3 * r, b/3, and

b/3 + r, with zero shift classifiedwith the ,/3shifts. Their results are therefore virtually equi-

valent, but in comparing them it is important

to note that Zvyagn and Bailey denote thegroups by the same four letters but in a dif-

ferent way (Iable 1). Bailey shows |hat Zvya'gin's ZMt,polytype is equivalent lo 2Or $venths full symmetry of the layers. The members of

each group are shown not only to share the

same formal relationship but also to give rise

to readily distinguishable diffraction intensitiesamong their strong reflections (see Table 2, p.

361, Bailey 1969). Individual. members of agroup can be distinguished only by differencesamong their weak reflections (see Table 3, p.

362, Barley t969).The earlier' classification of Steadman(1964)

is also shown in Table 1. Steadman did not di-

vide the structures into groups, and he didnot consider those based on shiftls of a/3 * r,but he did include four more (enantiomorphicpairs o0 structutes not given by Zvyryn otBailey because diiferent kinds of shifts betweenalternate pairs of layers are involved. In fact

Bailey has shown that structures exist whichcontain mixed shift sequences so that thesestructures listed by Steadman are not withoutinterest. Steadman'snumbers 4-7 have affinitiesto Bailey's group C and l2-L5 to Bailey's group




D. Bailey's polytype nomenclature can be ex-tended readily to take account of these, andother possiblepolytypes, as indicated in the lastcolumn of Table 1, and his extended system isused in this paper. Disordered crysials canusually be assip.ed to one of the major groups,A, B, C or D, Table 1, on the basis of theirstrong reflections.

Acruer, PoryrypEs

Amesite and cronstedtite

The usefulness of the theoretical polytypeschemess illustrated most clearly by the struc-tural studies on amesite and cronstedtite. Inamesite, (polytypes 2flr (Oughton 1957) 2Ht(Steinfink & Brunton 1956) and 6R, (Steadman

& Nuttall 1962), all in Bailey's group D, as wellas a 9T polytype not developed in the theore-tisal schemes Jahanbagloo& Z,oltai 1968) havebeen described. In cronsteiltite, polyty,pes lM,2Mt and,37r (Group A), polytypes lT and 2T(Group C), and polytypas ZHt 2Hz, 6Rg and6Il, (Group D) have been found by Steadman& Nuttall (1963 and 1964). lt is of interest tonote that the 6Rg and 6Ht polytypes are de-veloped only in Steadman's classification, butthe others are found in all three classifications.

Lizardite

The name lbardite was proposed by Whit-taker & Zussman (1956) to denote a serpentinemineral possessing a single-layer orthohexa-gonal cell and varying degreesof three-dimen-sional order; howevel, they noted that somespe-cimens did not conform entirely to the single-layer cell but contained 1800 rotations in somekind of random sequense. The flat-layer ser-pentines having multi-layer cells have hithertobeen referred 16 gimFly as "multi-layer selpen-tines", with an implication that they are to be

regarded as species distinct from lizardite. Asimplification would be achieved if all flat-layer serpentineswere termed Tizardite, so thatthe l-layer and Z-layet lizardites and themulti-layer varietbs could all be regarded aspolytypes. This nomenclature is therefore fol-lowed in the remainder of this paper. The prob-lems raised n relation to the definitions of poly-morphism and polytypism in the presence ofsmall chemical differences are discussedbelow.

Detailed studies on the type lizardite fromKennack Cove, Cornwall, England, by Ruck-lidge & Zussman (1965) indicated that the crys-tals were composed of domains equivalent tothe 17 and 2H polytypes (disregarding the-+- b/3 disorder). Krstanovi6 (1968) has exam-ined a number of lizardite samples from vari-

ous Yugoslavian localities and selected thosecomposedonly of the disordered1? polytype fordetailed structural studies.

Although the structure of the individual lay-ers in lizardite departs rom the ideal in a num-ber of ways, (discussed

below), the stacking ofthe layers one on another is basedon hydrogenbonding of the ,basaloxygens of ,onelayer to thehydroxyls of the layer below in the manner as-sumed in the derivations of the theoretical poly-types. The use of the nomenclaturediscussedabove is therefore fully justified.

The theoretical polgype classifications havetheir greatest application in the multi-layer vari-eties. These have been frequently classified interms developed by Gillery (1959) as 6Q)-layerstructures approximating a 2-layer structure, and

as 6(3)-layer structures approximating a 3-layerstructure. Most ,of the multi-layer lizardites de-scribed n the literature have some degreeof dis-order so that their diffraction patterns do notalwaysfit Bailey's calculateddiffraction pattern$(Bailey 1969, Table 3, p. 362) and some havestructures not included in Bailey's table. How-evet, the main structural grou,pA, B, C or D canoften be determined by the use of Bailey's Ta-Ue 2, p. 361, although elements of two struc-tural groups may be present n some specimens.

It is intere:ting that the frequency of occur-

rence of the multi-layer lizardite polytypes re-cordeC n the literature is: 2 in @ailey's)groupA, 1 in group B, 1 in group C and 4 in groupD. When the one- and two-layer lizardites (17and 2II) are includedo howevern group C be-comes by far the most frequent group and thefrequency of occurrence becomesC>D>A>B.This is the same relationship derived by Bailey(1969) from theoretical structure-stability esti-mations.

Bailey (1969) has indicated that further workis in progress on the multi-layer polytypes.

Chrysotile

Whittaker (1953, L956a, b, c) has definedthree types of chrysotile: clinochrysotile, ortho-chrysotile, and parachrysotile. Clinochrysotilehas a Z-layer monoclinic cell (disregardingthe cylindrical nature of the structure) with norotation betweenthe layers. Orthochrysofile andparachrysotile have 2Jayer orthorhombic cellswith rotations of 180o between the layers. Inclinochrysotile and ortlochrysotile the x-crys-tallographic

axisis parallel

to the fiber axis(the

cylinder axis), whereas n parachrysotile the y-axis is parallel to the fiber axis. The cylindricalnature of clinochrysotile and orthochryiotile hasbeen confirmed by electron microscopeobserva-tion by Yada (1967), but the nature of para-



A REAPPRAISAL OF THE STRUCTURES OF THE SERPENTINE MINERALS 231

chrysotile is not as well-understood.Yada (1971)suggested hat parachrysotile might be in theform of curved crystallites on the surface ofclinochrysotilo fibrils, but Middleton (1974) hasshown that it must contain at least some cylin-

dricalcomponent.

Some confusion has developed in the liter-ature as a result of the attempt to assign thechrysotile structures to polytypes in the varioustheoretical schemes.These schemesall make thefundamental assumption that each layer isstacked on the previous layer so that normalhydrogen bonding is developed (Fig. 2, Bailey1969). However, in chrysotiles successiveayersare stacked in such a manner that the normalhydrogen bonding does not take place (Whit-

taker 1953). The continuously changing register

between adjacent layers in the circumferentialdirection inhibits systematic hydrogen bondingfor all the basal oxygens, and not merely fort/g of tfum as assumed by Bailey (1969).

The structural refinements of both clino-chrysotile (Whittaker 1956a) and orthochry-sotile (Whittaker 1956b) indicate that the basaloxygens Or and Oz ore separated n the radialdirection by 0.2A, with Or projecting from thelayer structure and O, withdrawn into it. Thecylindrical curvature of the structure aligns theouter OH groups so that they form rows with

grooves between them running around the cir-cumference of the structure. Successive lavers

are stacked in such a way that the projectingO"s fit into the underlying gtooves and thewithdrawn Oans ie approximately over the OHrows. This arrangement involves a shift of0.4A (approximatrly a/ L3) away from the posi-

tiou of normal hydrogen bonding, assumed nthe polytype schemes and found in many 1: Llayer structures. This shift, overlooked in generaldiscussi,ons f the chrysotile structures, eliminateschrysotile from any discussions of theoreticalpolytypes based on normal hydrogen bonding,and leads to a need for a different polytype no-menclature for chrysotile as discussed below.

The consequences of ignoring this uniquestacking artangement are illustrated by the con-fusion existing over the assig,nmentof the stand-ard polyty.pes to clinochrysotile and orthochry-

sotile. Steadman and Bailey consider clinochry-sotile to be a distorted 1T polytype but Zvyagincon:iders it to be a distorted lM. Similarly,Steadman and Zvyagin consider orthochrysotileto be a distorted 2H, but Bailey considen it tobe a distorted 2Or. T\is confusion is broughtabout by failing to appreciate that the interlayershifts in chrysotile, although small, are signi-ficant and in the opposite direction to the shiftin the standard polytypes. Also, the total lackof order along the y (circumferential) directiondestroys the regular hydrogen bonding and re-

duces the effective repeat unit along x 1o a/2.Under these conditions it is possible to choose

I r

l -r '-i-T-'l

+x

1M 1T

o.oss+x

2Mct

Frc. 2. Stacking relationships between chrysotile 2M"1 anld disordered l7

and LM polytypes in the [010] projection. The standard orientation of

2|4"1 reverses the polarity of the r-axis in com,parison to the standardpolytypes.-All I values are calculated on a basis of a - 5.344 and

c - 7.32L.

\




alternate origins for a which leads to furthersources of confusion.

In order to distinguish the polytypes that arisein the chrysotile structure from those applicableto flat-layer structures, it is proposed to use asubscript

c(for

cylindrical) immediately follow-ing the main symbol (e.g, ZMa, where 2 = thenumber of layers in the cell, M = monoclinic,c - cylindrical and 1. polytype number), andto proceed to derive a number of expected poly-tyTres.

In both the ortho. and clinochrysotile struc-tures described by Whittaker the Or atoms notonly project out of the layer in the radial direc-tion, but are dis,placed y O.1OA rom their idealposition in the rdirection, i.e. parallel to thecylinder axis. In the discussion Lrelow, we sug-

gest a mechanism for the production of thisdisplacement which is an inevitable result of thecompressivestressin the octahedral sheet of theserpentine structure. If it is accepted that thedisplacement always occurs, then a numbet ofpossible polytypes can be predicted.

As stated above, for a simple stacking ofideal layers with Or keyed into the grooves be-tween rows of hydroxyls in the layer below,there would be a shift of the rows of hydroxylson top of one layer with respect to those onthe one below of approximately 0.aA, which is

in the opposite direction from the shift in Bailey's and Zvyagin's lM disordered polytype(Fig. 2). If this situation were repeated in everylayer, a 1M" structure would result with F =93.3'. If however Or is displaced by I in ther-direction relative to the upper parts of its ownlayer, then because it is keyed into the grooyeof the layer below, the shift between one layer

and the next will be changed to 0.4 t8A. Whenthis shift is greater than and less than O.4A wepropose to call the situation over:hift and un-

F:iHHm-Yh11Md 1Mc2 2M"1 20116

Frc. 3. Stacking relationshipsamong the chrysotilepolytpes LMos tfuIa" ?*74 and 2Oro1,

dershift respectively. In the known structure ofclinochrysotile, which we now propose to de-note 2h1[a, I is about 0.1A and overshift andundershift occur in successiveayers, with theresult that F has the samevalue (93.3') as if no

undershifl or overshift occurred, but of coursethe cell contains two layers.The stacking of ideal layers with a rotation

of 180' between successive ayers produces aseparation of. a/ 6 between the rows of hydrorylsfrom one layer to the next. In order to produceorthochrysotile with Or keyed into the groove$between the rows of hydroxyls in the layer be-low there must be a shift, from the ideal stack-ing position, of 0.4 :tDA in the opposite direc-tion to the shift necessary o produce Bailey's2Or disordered polytype. This shift reduces the

a/ 6 separation between hydroryl rows in suc-cessive layers by O.4 18A. Either undershiftor overshift in both layers would be compatiblewith the orthorhombic character; however, thestructures would be different and would givesignificantly different 20/ intensities. The knownstructuro of orthochrysotile has undershift inboth layers and we denote it 2Ora, The theore-tically possible structure with overshift in bothlayers may be denoted zor"a. Strucfuires withalternate layers rotated by 180" but with alter-nating undershift and overshift can also.be hy-

pothesized,but they would be monoclinic withI = 90.4o for a value of E = 0.1A, and wouldconstitute an enantiomorphic pair.

It is now imFortant to consider the effect ofover- and undershift in possible lJayer struc-tures. For I - 0.1A these would have F =94.1" and 92.5o respectively, and we denotetlese as LMa anrd lMea respectively @9. 3)Zvyagn (1967) has published electron diffrac-tion patterns of a l-layer dinochrysotile fibrilwith p = 1O6.5owhich he suggests s the (dis-ordered) ordinary IM polfipe. However, iflM occurs, it would be expected to have F =103.7'. Because of the complete circumfer-ential disorder in chrysotile, it is always possibtreto choosean alternative F-angle due to fhe effec-tive a/2 repeat along r (seeFig. 2). The alter-native value for our LMa polytype is 106.4',in excellent agreement with Zvyaginos obserya-tion. We therefore conclude that his materialhas the 1M* stackingnand a F-angle hat wouldbe expressedmore conventionally at 94.2'.

The high-psolution electron micrographs of

chrysotile fibers sectioned perpendicular tothe fiber axis (Yada 1967) ,gtvefirrther supportto the existence of a L-layer clinochrysotile. Thedoublo spiral and concentric circular structurescould possessa Z-layer clinochrysotile stflrctureas defined by Whittaker (1956a) or an ortho-




Frc. 4. Equi-inclination fiber photograph of chrysotile D" from Krantz Kop, Natal.

chrysotilo structure(Whittaker

1956b). How-eyer, multispiral structures with an odd nunr-ber of layers, and those w,ith numerous single-layer dislocations, cannot pos$ess he 2-layerclinochrysotile structure. The 2-layer struc-ture cannot form because he alternate positiveand negativedisplacementof layers with respectto one another would be put out of sequenceby each extra layer added by dislocation, or bythe odd number of layers in multispiral struc-tures. However, if the displacement is in thesame direction in each layer so that the L-layer

clinochrysotile structure (Whittaker 1,956a;Zvyagin 1967) develops, hese multilayer spiraland single-layerdislocation growths would pre-sent no problem. trollowing the same reasoning,orthochrysotilo or parachrysotilecould not havemultispiral growths with odd numbers of lay-,ers, or possessa high number of single-layerdislocations.

In addition to the three chrysotile polytypes2M* 2Or"t and tMa that have now been ob-served, in many specimens there is evidencefrom the greater breadths of. the 2N reflections

relative to those of the 00/ reflections that mis-takes in the layer stacking may occur within asingle fibril. In the extreme case there may beno discernible regularity in the stacking, andthe 2Ol reflections are smeared into virtuallycontinuous streaks along the layer lines. Thisdisordered stacking is conveniently denoted aschrysotile D". An r+ay photograph of such aspecimen from Krantz Kop, Natal (Whittaker1956d) s shown in Figure 4.

With the introduction of an adequate poly-

type nomenclature it is no tronger necessary o

use the prefixes ortho- and clino. for chrysotile,as these can be more meaningfully replaced bythe appropriate polytype symbol. However, allthe polytypes discussedabove have the r-direc-tion parallel to the cylinder axis, and it is not

possible o include parachrysotile in the scheme

without introducing an additional symbol todenote the orientation of the layer axes. It is

suggestedthat the name parachrysotile should

therefore continue to be usedas n the past.Thus

chrysotile is a genericname for all the varieties;

specific polytypeswith * parallel to the cylinder

axis can be unambiguously denoted as chryso-

tile with a polytype symbol, and material with y

parallel to the cylinder 'axis can be unambig-

uously denoted by parachrysotile, to which a

system of polytype symbols may also be added

at some time in the fufure should thisprove

nec-essary. It is clear that parachrysotile must be re'

garded as a polymorph, and not a polytype, of

chrysotile.

Antigorite

Zvyagin (L967) and Bailey (1969) have classi-

fied antigorito as a distorted 17 polytype.

However, the curvature of the structure has the

samoeffect as in chrysotile in precluding syste-

matic hydrogen bonding betweensuccessiveay-

ers, and the control on the lay'er stacking is

exerted by the direct interconnections betweensuccessiveayers on the placesa-0 and a-

Vz where the corrugations invert. Furthermore,

the existenceof the superlattice,and the chang-

ing relationship between successiveayers over

the half wavelengthof the corrugated structure,

change the diffraction pattern fundamentallyfrom that calculated for the theoretical 1Tpolytype. Thus there is little value in discuss-

ing antigorite in terms of the th,eoreticalpoly-

types.

Cunvrrcal Rsl-atloNsrilPs AMoNc firESrnPpNrnqe MnqeRALs

Chrysotile and lizardite

It is generally accepted that the layer curva-




TABLE . AI'10UNTSF TRMIEI{T SUBSTITUTIONN CHRYSOTILESNDLIZAR0ITE5 (1956) that natural chry.otile contains up to29 ! L.9 (A1zo"+ Fe,Os),Gillery sugg=sGdc mrrsitional break between chrysotile and:i.:.a:dite t x = 0.2. Olsen (1961) howed thati'rindley & von Knorring's (1954) 6-layer lizar-d i t es ad x :0 .11 and : , 0 .12 ,

andhe pro -posedon the basis of thesevalues,and of theanalyses of Kalousek & Muttart (1,957), thatthe assumed reak s at x :0.10. Subsequently,Radoslovich (1963b) concluded that ,.the highvalue, .r - O.25,was more acceptablo,'.Morerecently Chernosky (1975) has synthesizedchry-sotile from r = O.O o O.25 and lizardite fromr : O.0 to 2.A, demonstratingthat there is nocompositional break.

It is to be noted that even if a compositionalbr,eakdid exist at x - O.25, there would still

be a rangeof composition rom x : O.25 O.75in which b".tlbt* and yet flat layers are formed.It follows that there must be a mechanism ofmisfit relief other than curvature, and it is,therefore, perf,ectly feasible for alternativemechanismsof misfit relief to be operative overa given range of composition. Table 2 showsthe value of .r for the 15 specim,ens iscussedby Whittaker & Wicks (1970) and among thesespecimens t may be seen that lizardites occurdown to r = O.08 and chrysotiles up to .r :0.14. In view of the small number of specimens

studied, it would be surprising if the range ofoverlap were not appreciably greater than this.

The evidence from Gill,ery's and Chernoskyoswork suggests that multi-layer lizardites arefavoured by high values of x, and the highestvalue of ,r in Table 2 correspondsto a speci-men of this type. Howevero even if this trendexists, there is again no clear-cut distinctionbetween the composition ranges of 1- and 2-layer lizardites and multi-layer lizardites.

The possibility that systematic compositionaldifferences might exist among serpentine min-erals and their varieties has led some authorsto question the propriety of regarding them aspolymorphs or polytypes. Within the range oftheir overlapping compositions now demon-sttat'ed, one need clearly have no inhibitionsabout this. Furthermore, it would be absurd todefine polymorphism or polytypism so narrowlyin respectof chemical identity that one part ofan isomorphous series was regarded as poly-morphic or polytypic *i1h 1s:pect to anotherstructure, whereas another part was not. It

would seem almost inevitable that two polymor-phic structures should have somewhat differentcapacities for accepting isomorphous replace-ment and therefore that close study should re-veal incomplete equivalence in their chemicalcompositions. Equally, it would seem highly

Averaqe

9.01

0.03

0.035

0.045

0.08

0.09

Averaqe

0 . 1

0 . 4

0 . 1 6

0.20

0.23

SampleNo.*

s- 2

c -6

L- 5

L- 3

L- 2

Sample o. -

c-4

i-zL- 4

0.10 c-7 0 .35 S-3

0.105 s- l

C = c h r y s o t J l e , L . l - o r Z - l a y e r ' l i z a r d l t e , S = n u l f i - t a y e r l l z a r d l t e

Se e able 3. page 1034, {ht t taker & Utcks (1970) .

ture that occurs n both chrysotile and antigoriteis due to the misfit between

the octahedral andtetrahedral sheets with boor).b*t, and neitherchrysotile nor antigorite would be expected toform if the composition were such as to marebo"t brut.lowever, if curvature is not the onlyconceivable way in which misfit might be ac-commodated when the composition is such asto make one expect bu") bat, then the converseassumption that lizardites cannot form underthese circumstancesneed not be true. Flat-layersffuctures are expected when 6o* = bet, ofwhen Do"t(6u, accommodation n the latter case

depending mainly on tetrahedral rotations @a-doslovich & Norrish 1962; Radoslovich 1962).The data available from analvtical Cwhitta-

ker & Wicks 197O)and synthesiss.tuOlesCher-nosky 1971, 1975) suggest that luardite andchrysotile can indeed both form in overlappingcomposition ranges corresponding to bo"t)b*",and that the quest for a break in the composi-tion range between chrysotile and lizardite pur-sued by Gillery $959), Olsen (1961) and Ra-doslovich (1963b) is fruitless.

Radoslovich (1962) has shown that the for-

bo"t= but when r:O,75. This limit wil l bemula (Mgu-"A1")(Si4"AL)Om(OH)r* ives rise toraised by the presence of octahedral Fez+ re-placing Mg or octahedral Fea+ replacing Al,and will be lowered by the presenceof tetra-hedral Fe3+replacing Al. Therefore, it would beexpected,and seems o be a fact, that chrvso-tiles can accept more Fer+ than lizardite.'Onthe basisof his synthesiswork which gave lizar-dite and no chrysotile .at x :0.25 and above,and becauseof the statementby Nagy & Faust

* Although the simple formula, corresponding toone structural formula unit, is used elsewhere inthis paper a double formula is used in this sectionbecause t has been used as the basis of the dis-cussions quoted from the literature.



probable that the oeculredce of (possibly partlyordered) isomorphous substitution should modi-fy the relative stability of different polytypesof a structure, and therefore that close studyshould reveal compositional differences (at least.on a statistical basis) between polytypes. Theappeal to small compositional differences todeny the existence of polymorphic or polytypicrelationships therefore seems to be a profitlessexercise.

In view of the above discussion it is suggestedthat all ,flat-layerserpentines,whether t- ot 2- ormulti-layered ty,pes,should be regarded as poly-ty.pes of fizardite; that l-layer clino-, 2-layer cli-no- and orthochrysotile should be regarded aspolltypes of chrysotile; and that lizardite, chry-sotile and parachrysotile should be regarded as

polymorphs because heir structures involve dif-ferent processes f misfit relief, and their differ-ent stacking arrangements are consequences f'this and not merely simple alternativessuch as,occur between polytypes.

Antigorite

The analysesdiscussed y Whittaker & Wicks(1970) indicate that antigorites have a higherSiOz content and a lower MgO and IfrO+ con-tent than the other serpentine minerals. This.difference in cqmposition is expected as a re-

sult of the alternating wave structure which re-'duces the Mg and OH content relative to Siat the points of inversion, as discussedby Zuss-man (1954) and Kunze (1956; 1958, 1961).Antigorito is therefore not a polymorph of the.other serpentines n the strict sens€,but is aphase of essentially (though only slightly) dif-fer,entcomposition. The departure of the compo-sition from the ideal MgaSirOs(OlI)4must varywith the wavelength of the corrugations in thestructure and antigorites which differ from oneanother in this way ar,e therefore neither poly-morphs nor polytypes; they are related in a si-milar way to one another (and to the ideal corn-position) as the integral memb,ers f nonstoichio-metric oxide series are related to one anotherand to the stoichiometric compound. This hasbeen fully demonstrated y Kunze (1961, Ta-ble 5, p. 239).

It follows from the discussion of chrysotileand lizardite that high-alumina antigorite withb*"4bet is not to be expected,as curving isessential to the antigorite structure, and none

has been found. As shown by Whittaker &Wicks (197O), F"'* substitution in antigoritemay be more extensive than in the other ser-pentines. This would be expected to raise thepermitted Al * Fe'+ content before the flatIizardite structure supervenesover the curved

235

structure. and the content of these ions oftenseems to brhigher 'in ffitigodt€ -than in chry-sotile (W'icks & Whittaker 1970).

Srrucruner VenreuoNs

The available Jtructural and chemical data

Radoslovich (L962) has pointed out that thecalculated b-parameters of the ostahedral andtetrahedral sheets of most trioctahedral 1:1layer silicates rarely match, so that adjustmentsin tle ideal atomic positions must occur. Thenature of these adjustments is fairly well under-stood when ib*r, bt*, but not when D*t ) bt t.In this section we therefore seek to elucidatethe adustments and distortions involved in thislatter case. The available data are rather inade-quate, being confined to the results of two-di-mensional refinements of the structures oflnardite L? (Krstanovic 1968), chrysotile 2M"r(clinochrysotile, Whittaker L956a), chrysotile2Or.' (otthochrysotile, Whittaker 1956b), andantigorite (Kunze 1956, 1958, 1961). The lackof three-dimensional efinements is unforfunate,but is an inevitable result of the disorder presentin lizardite and chrysotile, and the complexityof the antigorite structure has led to a similarlimitation. Despite the uncertainties in calcula-

tions of bond lengths and angles that arise fromthese limitations. the trends in the variationsfrom the ideal atomic positions are indicatedclearly.

In order to make the calculationspossibleweassume deal y coordinates hroughout, and alsomake the following additional assumptions andapproximations. Whittaker's Fourier maps give

a single weighted mean position of O, and OHrand he suggested hat these atoms may be attw'o levels separatedby 0.224 along ze. As thisassumption s supported by the more recent re-

finement of lizardite 1T by Krstanovic, in whichhe found O; and OHr to be separatedby 0.4A,Whittaker's preferred z coordinates of Og andOHr are used in the following discussion.Whit-taker's preferred .r coordinate is also used forOr. In the antigorite refinement no differencewas assumed or detected between the Os andOHr coordinates or between the Mgr and Mgrcoor.dinates, o the Mg and Mg, octahedraaretaken to be identical in this structure. Eachhalf-supercellhas 9 tetrahedra and 8 octahedra,each with slightly different bond lengths and

*The standard 1:1 layer orientation of the x, y

and z axes is used throughout this paper. In the

original structure descriptions (Whittaker 1953,

1956a, b) x and z are reversed.

A REAPPRAISAL OF THE STRUCTURES OF THE SERPENTINE MINBRALS



236 TTIE CANADIAN MINERALOGIST

bond angles d,ependingon their position. Theresults for one average tetrahedron and oneaverageoctahedronare presented.

With regard to chemical composition of thema.terialswhose structure is to be discussed,no

analysis has been given for the lizardite LT re-fined by Krstanovi6 (1968) but he states "thatthe total amount of cations other than Si andMg was found to be less lhan L% by weight"(Krstanovii 1968,p. 165),so it can be assumedit is very near the ideal compositionMg"SizOa(oH)".

The chemical compositions of the chrysotiles2M"t a\d 2Ora vsed for crystal structure deter-minations by Vithittaker (I956a, b) are unfor-tunately not knsqla, but the survey of serpentinechemical compositions by Whittaker & Wicks

(1970) indicates that chrysotiles generally havelow substitution, so that it is not unreasonableto assume hat this is true of the samplesstu-died by Whittaker.

The antigorite used by Kunze (L956, 1958,1961) has been analyzed (Zussman1954) andhas tle formula (M&.erFe2+.orFe3+.ogAl.oa)(Sir.sr-Al.o')Ou(OII)a.

Distortions in lizardite

The dimensions of an uncompressedoctahe-dral sheet are the result of the balance betweenthese orces-" (I) cation-cation repulsion acrossshared octahedral edges,(II) anion-anion repul-sion along shared edg,es, nd (III) cation-anionbonds within octahedra." (Radoslovich 1963a,p.80). Of these hree forces, the first is consideredto be the most significant. The divalent cationsrepel each other, causingthe anion to move in-wards along the sheetnormal to produce a thin-ning of the octahedral sheet (Baitey L967b). If.trivalent cations substitute for the divalent ca-tions the forces of repulsion will be stronger.

The ideal Mg-octahedral sheetwould have i

= 8.78A and a thicknessof 2.43A (Iable 3);both shared and unshared edges would have alength of 2.974, and all O-Mg-O angles wouldbe 90'. Ilowever, in brucite the cation-cationrepulsion between the Mg atoms extends b to

9.434 and. hins the sheet to 2.llA. shorteninethe shared octahedral edges o 2.79A by draw-ing the OH in along the z dir,ection,and length-ening the unshared ones to 3.144. The corres-ponding O-Mg-O angles become 83.3' and96.8o respectively. n this discussion he actualbrucite sheetrather than the ideal brucite sheetwill be used as a model of the unconstrainedoctahodral sheet.

When 6,.t {b* it is well-established hat themain adjustments take place in the tetra-hedral sheet by rneans of tetrahedral rotations

(Radoslovich & Norrish 1962; Radoslovich1962). T\e octahedral sheet is therefore likelyto be under very little stress,and the availabledata on minerals of this type are of interest asa point of reference.

In amesite 2llz (Steinfink & Brunton 1956),which is assumed o have a compos^ition loseto MgdlXSiAl)O5(OII)n, b is 9.2O4 and thecalculated b*' is 9.774, suggesting that thereis not much stretching of the octahedral sheet(Radoslovich L962). T\e M-M distances ars3.07A (where M =,MerAl) and the sheet is2.o24 thick (Iable 3). The O and OH &ro co-planar, as are the hy'droxyls of the OH plane,and the Mg and A1 atoms occupy a singleplaneequidistant from the O,OH and OH planes.TheM-O and II[-O}J bond lengths are 2.04.A. Theshared edges of the octahedra are 2.694 andunsharedones 3.074. The amesite 9? structure(Jahanbagloo& Znkai 1968) is less aluminousin the tetrahedral sheet but the octahedral sheetis similar to amesite except that it also has asmall amount of iron of undetermined valence,

andpossibly

some vacancies o maintain charge

TABLE . INTERATOI4ICTSTtrCES NDAXGLIS

bn d Lengt h , A ng l es e t c .Brucit€ Bruci& Af,esiteIdeal observed 2HZ

hesixe Lizodl te Chrysot l le Chrysotj le Ant igor i te9T lT 20rct 2fl"t

calcllated roct

ca l c ! l a t ed r tet

ocAiedral sheet thiclness

u'-i l at 30' to t

il-fi parallel 6 y

AveraS€\-0H I

Averaqe M--OH {

ktrahedral shee! thlckness

I - T a t 3 0 c t o 4

I- l para l l e l b ,

Averag€ l-0

Tetrahdra rctation

- 9 . 1 7 9 . 1 9- 9 . 5 6 9 . 3 5

9 . 4 3 9 . 2 0 9 , t 7 1

2 . l r 2 . 0 2 2 . 0 4

3 . t 4 3 . 0 7 3 . 0 6

3 . l4 3 . O 7 3 . 0 6

2 . 1 0 2 . M 2 . 0 4

- 2 . 2 9 2 . 2 6- 3 . 0 6 3 . 0 6- 3 . 0 6 3 . 0 6- 1 . 6 8 I . 6 6- 1 2 . 3 "

2.08 2.44

3 . 0 9 3 . I

3 . 0 6 3 . 0 €

2 . a 6 )) 2 . 1 7

2 , 0 6 )

2 . 2 6 2 . 2 2

3 . 0 8 3 . 0 2

3.07 3. f f i

1 . 6 4 1 . 6 4

9 . 4 5 9 . 4 5 9 . 4 5 9 . 2 9

9 . 1 5 9 . r 5 9 . t 5 9 . r 5

9 . 1 8 6 9 . 2 9 . 2 9 . 2 3

8,78

2-43

2 . 9 3

2.93

2 . 1 0

2 . 2 0

3,08

3.08

2 , 0 3

2 . 1 3

2 . 1 5

3 . 0 7

t . 6 l

2 . m

3.08

3.06

2.06

2 . 4 6

2 . 1 3

3 . 8

3 . 0 7

1 62

'uls occupid by lg ri th or l l thoux lessef A l , Fez{or Fe3+

*T ls occupi€d y Sl { i th or ul tholt Al ,




balance. Tlne b""" calculated with Radoslovich'sformula is 9.19A assumingall ferric and 9.20Aassumingall ferrous iron. The D is 9.17A whichmay suggest that the octahedral sheet may beslighrly -compressed but it is more likely that

D is smaller than 9.19A because of the vacan-cies. The octahedral sheet is 2.05A thick buiin all other aspects h,e amesite 9T structure issimilar to the amesite 2Hz structure. The cron-stedtite 77, 3T and 6Ra (Steadman& Nuttall1963) all show similar features.

The lizardite LT (Krstanovii 1968) representsthe extreme case of b*t) bw with maximumcompressionalong both the x and y axes. Theoctahedral sheet thickness of 2.2O4 is @partfrom th,eanomalousvalue for the antigorite dis-cussedbelow) tle thickest observedamong tri-

octahedral L: L silicates (Table 3) and amongtrioctahedral 2:L Tayer silicates @ailey L967b),The extreme compression buckles the plane oftho rnagnesium cations, forcing them to occupytwo strusturallydistinct positions having z co-ordinates differing by O.4A. The Mgi remain inan almost central position 1.O4A from the aver-age position of the OsOHl plane and 1.16Afrom_theOHTOFLplane, and the Mgz atoms axe1.45A from the OgOHr plane and 0.75A fromthe OHzOIIg plane. This buckling of the Mgplane appears to be the key to understanding

all tle remaining adjustments.As a direct result of the buckling, the aver-

age Mgi-Mgr distance is increased from 3.O6Ato 3.O9A (compare Mg-Mg distancesof 3.14Ain brucite, Table 3), although there is littlecorresponding increase in MgrMgz distances.

It would be expected that as a result of thedepression of Mgr relative to Mgz, the threeOHr and OHe hydroxyls would be displaced to-waxds a point in their own plane having the rand y coordinates of Mg. Their y coordinatesare unknown, and the displacements of their xcoordinates are not significant in view of quotederors. The displacementsare, however, in theright direction on average.

There is a significant difference of about0.3A betweenthe z ooordinatesof OHr and Os,with OHr being the closer to the Mg atoms. Itis suggestedhat this arises rom a repulsion ofthe Si atoms (carrying their attached Og atomswith them) by the d,epressedMgr atoms. The re-sulting depression of the Os atoms then perrnitsan upward shift of OHr. The average unshared

edgesof the octahedra(and

the correspondingangles at Mg) are smaller than those found inbrucite, and the average shared edges(and cor-responding angles)are larger than those foundin brucite. These differences aro clearly the re-sult of the compressionof the oc.tahedral heet.

In spite of the buckling of the Mg plane, the

extent of the compression of the octahedralsheet is rather small, and the tetrahedral sheetis therefore under considerable ension. Rados-lovich (1962) calculated the mismatch along b

for lizardites to be 0.224 but since Krstanovic'sspecimen s nearly a pure Mg-lizardite, the mis-

match must be near a maximum of O.3A (Table

3), the difference between brucite and the idealtetrahedral sheet (Brindley 1967). The sheet is

stretch,ed and thinned to 2.L5A. The siliconsmaintain a near-hexagonalarray but the^ Si-Si

distancesare increasedto 3.07 and 3.064 (Ta-

ble 3).It is important to emphasize at this point

that although tle lizardite is described as poly-

type lT, because this correctly describes the

nature of the stacking, t is in fact orthorhombicand only pseudo-trigonal,and an explanation ofthis fact is needed.

The loss of equivalancebetween Mgr and Mgzthat ,arisesfrom the buckling of the Mg plane

does not destroy the trigonal symmetry of thearrangement of Mg, but it increases the sizeof tle trigonal unit and 'lvhen combined withthe tetrahedral sheet the symmetry is lost. Thegreater proximity of Mgi to Si compared withMgr will lead to unbalanced repulsion betweenMgr and Si which will tend to rotate alternate e-

trahedra about 11301 na lTfOl axes.This shouldbe manifested in a relative positive shift of the xcoordinatesof Si, Or and Oz relative to Og,and aseparation between th,e z coordinates of Or andOz with Or going negative and O, positive. Theobserveddifference in z between Or and Oz is

0.4OA which is just significant ,Q.Sa). The cor-respondingshifts of Si, Or and Oz relative to Os

would be about 0.2A. which is not observed,butas this value coresponds only to about 1.1o this

is not surprising.The length of the 7-O, bond in lizardite

(where T = Si) is appreciably shorter than inamesite (where T = Sio.oAlo.r). he observed

length of 1.56 :t 0.05A in trtzarditedoes notdiffer significantly from the expected value of

1.62lt. Thus, part of the apparent thinning ofthe tetrahedral sheet of lizardite relative to ame-site Cfable 3) is due to this compositional effect.Tho stretching effect may be expected o show

most clearly in the thicknessof the Si- (meanOr,

Oz) part of the sheet which is only 0.50A com-pared with 0.58A in amesite (which falls to

b.SeA wnen corrected for change of Z occu-pancy). The ?-Or and 7-O, bond lengths cannotbe calculated as reliably as ?-O; in absence ofy coordinates, but based on idealized y co-

ordinates, he valuesobtained give a satisfactory

average ?-O bond length of 1.614 Clable 3).




Curvature and distortions in chrysotile

Curvature of the layers has often been givenas a means whereby misfit between sheets ofdifferent sizes can be accomm6dated withoutstress. For any given composition there is only

one ideal radius of curvature, and thus in anyindividual chrysotile fiber there is only one layerthat has the strain of misnnlstr completely re-lieved along the circumference of the curve. Forthe layers within fhis radius there will be over-compensation and for those outside there will beunder-compensation.

Whittaker (1957) calculated the ideal radiusfor chrysotile to be 88A and poiuted out thatthe residual stress increases more slowly out-wards than inwards. Yada (1967, I97I) ob-served that the minimum inner radius of chry-

sotile is 35 - 404 and that the normal maximumouter radius varies from 135- 14OA, but ex-ceptional fibers have outer radii from 90 to2204. It is clear from these figures that theaverage structure in chrysotile fibers must usual-ly correspondto material lying outside the ideal88A radius of curvature, and thus have mis-matches only partly relieved by the curvature.Furthermore, since the curvature occurs aboutthe x-axis, relief can only occur in the y-direc-tion and the mismatch along the r-axis is com-pletely unsompeDsated y the curvature. There-

fore, in chrysotile (excluding parachrysotile)there is a v€ry strong compressionalstress inthe x-direction in the octahedral sheetand thereis also a lesser compressionalstress in the y-direction in the average octahedral sheet.

Radoslovich (1962) calculated a mismatch of0.12A for chrysotile, but the basis of this is notclear and theoretically it could be as high as0.3A. Thus, one would expect the averagecom-pression n the octahedral sheetto be somewhatmore than half of that in lizardite. In absenceof precisely-known compositions for Whittaker's2Ma and. 2Ora structtres and in view of theuncertainties attached to some of their de-tailed parameters, the best test of this conclu-sion is to compare the mean of tleir relevantparameterswith those of lizardite and amesite(seeTable 3). The most important would seemto ,be the octahedral sheet thickness of chrvso-tile at 2.08A against 2.2OA for lizardite and2.O24 for amesite{', he mean tetrahedral sheer

* For the comparison of sheet thicknesses o be in-

dicative of distortions in the sheets. the thicknessesshould be corrected for the direct eifect of the sizeof the ions. Therefore the thickness quoted for ame-site would be more comparable with those of theserpentines f corrected in this way to 2.08-4. octa-hedral) and 2.204 (tetrahedral).

thickness of. 2.194 against 2.154 for lizarditeand,2.294 for amesite;departure of the mean es6e1dinz.te f Mg from the centre of tho octahe-dral sheetof chrysotile is 0.07A cbmpared with0.21A in lizardite and zero in amesite; he dif-ference in

zcoordinates of Or

andOz is prob-

ably about 0.2A compared with O.4A in fizarditeand zero in amesite, and the corresponding dif-fetence for O' and OHr is also estimated atabout 0.2A compared with O.27A in fizarditeand zero in amesite. In the discussion of lizar-dite we have shown that all fhese parameterscan be interpreted as arising either from theextension of the tetrahedral sheet or directlyfrom the complession of the octahedral sheet,or indirectly from the buckling of the Mg planethat arises fro.m that compression. No evidense

has been given for such a buckling in chrysotiie,but it has not hitherto been looked for. As twophenomena are present in chrysotile which,when they occur in hzardrte, we have inter-preted as due to this buckling, it seems verylikely that it also oscurs, though to a more lim-ited extent, in chrysotile.

Curvature and distortion in antigorite

The antigorite structure refined by Kunze(1958) has boa = 9,294, calculatedaccordingto the formula of Radoslovich (1.962),and has

b*t = 9.L54 because of tle small amount ofsubstitution of Al. The mismatch is thereforeonly 0.14A. The superlattice epeat s 43.3A analthe radius of curvature varies from 724 at oneinversion to 50A at the next inversion, with anaverage of 61A (Kunze 1958), the curvaturebeing along the r-direction instead of along they-direction as in chrysotile.

Because every layer in the structure of anti-gorite is able to adopt the same radius of curva-ture, it might be expected hat the radius adoptedwould be such as to relax the stressescomDlete-ly in the r-direction. However, the calculatedradius of curyature for a mismatch of O.14A isabout 190A, so that the curvature is substan-tially greater than would be expected,and mustintroduce a tension in the .r-direction in theoctahedral sheet at least as great as the com-pression in the y-direction. Such a balancingof tension and compression s perhaps not un-reasonable,and its reality can be argued fromthe greater Mg-Mg distances(3.114) at 30oto the .r'axis compared with those (3.O8A) pa-

rallel to the y-axis. The most surprising anomalyis the thickness of the octahedral sheet. whichis greater than that in the other serpentines,whereas it would be expected to be le:s. Thisanomaly reoeives a contribution from the un-usually long Mg-(O,OH) bonds (2.17A) found




by Kunze, but it is also due to unusually large(O,OH)-Mg-OH angles at shared octahedraledges,which would normally be interpreted asdue to extreme compression of th,e octahedralsheet.

Becauseof the excessivecuryaxure it would

be expected hat the tetrahedral sheetwould beunder tension along y but (unusually for a ser-pentine) und,er compression along .r. This re-ceives support from the Si-Si distances whichare 3.08A parallel to y and 3.O2A at 30o to x.These values are to be compared with thesituation in lizardite and chrysotile where thereis tension in both directions and the Si-Si dis-stancesdo not differ from one another by morethan 0.01A. The thickness of the tetrahedralsheet is not significantly different from that inthe other serpentines,and there is no evidence

for any difference in e coordinates of Or andOg. There is thut no evidencefor tilting of thetetrahedra, and consequently there is not evenany indirect evidence for buckling of the Mg$heet.

Curvature and distortion in parachrysotile

Little detail is known of the structure of para-chrysotilg but some predictions can be basedon the variations found in the other structures.Parachrysotile and antigorite both curve alongthe .r-axis so that the major compressivestressin the octahedral sheet occurs along the y-axis.flowever, as in the other chrysotiles, the curv-ing of the structure will only partly relieve themismatch, so that there will still be significantcompressivestressalong the x-axis in the octa-hedral sheet of parachrysotile. Therefore theconditions are unlike antigorite and similar tothe other chrysotiles except that the major andminor stressdirectionsare reversed. t is logical,therefore, to expect the variations from theideal in parachrysotile o be similu to, althoughnot id,entical

with, the variations found in theother cbrysotiles. In particular one would ex-pect buckling of the Mg sheet, with the resul-tant displacementof Or to a lower e coordinatethan Oz. This is consistent with the fact thatWhittaker (1956c) found it nec€ssary o postu-late this samedistortion as in the other chrvso-tiles in order to interpret the stacking in pira-chrysotile.

OvEnsnrrr ANDUNDERSHTrTN Cnnvsorn r

Por-vrypBs

In the discussionof chrysotile polyrypesaboveit was pointed out that overshift or undershiftwould occur if Or were dis'placed rom its ideal

position relative to the upper part of its ownlayer. In the original discussion of these dis-placements,Whittaker (L956a,b) discu:sed hemin terms of a displacement of Or relative to thewhole of the remainder of the layer includingO:, and found weak evidence that this existed.

Ifowever, the processwould be equally effectiveif Oz (and ev,en Si) partook of the same dis-placement relative to the octahedral layer. Thediscussionof the effects of buckling of the Mgsheet now provides us with a mechanism forproducing a displacemento8, of this kind. If 8 istaken as positive when the displacementof Oris in the direction away from the nearest Mgatom in its own layer, then the tilting of the te-trahedra about ax,es brough the Oa atoms dueto repulstion of Si by Mgr leads to a positivedisplacement I amounting to about 0.2A in

lizardite, which applies equaff to Or and Ozisince the relative z-displacementsof Or and Oqin chrysotile are about 0.2A compared with0.4A in Izardite, we may expect a correspond-ing value of I of about *0.1A. This would leaddirectly to the observed overshift in chrysotile7Ma. and in alternate layers of 2.JVIa,and to theobserved undershift (becauseof the 180' rota-tions between ayers) n2Or"'. We can, however,offer no explanation of the observedundershiftin the alternate lavers of 2M"r.

DtscusstoN LNp CoNcrustoNs

The various lizardites and multi-layer ortho-hexagonal serpentinescan now be visualized asa single lizardite family with structural varia-tions, and polytypes, varying with chemicalcomposition. At the pure magnesiumend of thecomposition range the lizardite structure is un-der extreme compression n the octahedralsheetand extreme stretching in the tetrahedral sheet,producing buckling of the Mg plane, downwarp-

ing of Or and uplifting of Oz, and reducing thelayer symmetry to orthorhombic. These distor-tions may be the reason that the simple 1? or2fl stacking fypes,usually with considerablydis-ordered ! D/3 shifts, are the only ones knownto form under natural conditions. A synthetic6=layer polytype has been produced hydro-thermally by Jasmund & Sylla (1971, 7972).

The substitution of Al, as is well-known, re-lieves the misfit, first to produce bo.t = but a\dthen 6o"t bt o on the way towards the amesitecomposition. With the reduction in the mis-

match, not only do the internal structural de-formationsbecomesmaller and smaller until withb*t l bt"t only simple tetrahedral rotations oc-cur, but also the number of possible polytypesincreases ignificantly because he better- formed




structurcs have more stacking possibilities thanthe deformed structures.

The substitution of Fe'+ for Mg and Si hasthe same effect as Al substitution, but is lesseffective in the octahedral sheet and more effec-tive in the tetrahedral sheet. Ferric-rich selpen-

tines have been recorded (L2 and I-3 of Whit-taker & Wicks 197O; ferrian lizardrte, Ping-Wen& Che 1968), but they do not reach lh€ bo* =6t"t condition, let alone approach the ferric-ironanalogue of amesite. However, it is theoreticallypossible for (Mg,Fe'+XSiFes+)O5(OH)ao exist.With greater and greater ferric substitution itio presumablypossible or the lizardite structureto acceptmore and mo e ferrous iron (seeS-3,Fig. 6 Whittaker & Wicks 1970) until the cron-stedtite composition, (Fez'?+Fe3+)(SiFe'+)Os(OIDais reached.

The substitution of trivalent cations Ni3+,Co'+, Mn3+ and Cr8+ for Mg would also reducethe mismatch, but extensivesubstitution wouldrequire the substitution of Al or Fe3+ in the te-trahedral sheet to balance the charges.

Probably the only suitable divalent cationsmaller than Mg is Ni'*, and it seems hat thiscan substitute freely for Mg. The nickel ana-logue of lizardite, nepouite (Maksimovic 1973),is known and intermediate members betweenlizardite and nepouite exist (Springer 1974) sug-

gesting a solid-solution series. f the b parame-ter of 4Ni(OH),.NiOOH = 9.274 (Jambor &Boyle 1964) can be taken as D for the octahedralsheet of nepouite, it can be seen that bo"r} b-"for the entire series.This suggests hat the poly-types will be limited to disordered IT or 2Hunder natural conditions. The nepouite (ROMnumber M18475) noted in Springer (1974) isthe 1T polf'type. A synthetic 6-layer polytypehas been produced hydrothermally by Jasmund& Sylla (1971, 1972).

It would be expected hat the substitution of

Fe'+, Mnz+, Co'+o Znz+, and Cf* would beextremely limited because hey would increasethe mismatch in a structure alreadv near thelimits of structural adjustment. The survey oflizardite compositions(Whittaker & Wicks 1970)indicates that these have very low Fe'+ con-tents. Becauseof this, it would be expected hatno lizar'dite analogue containing large amountsof these cations would exist. However, greena-lite (Steadman& Youell 1958) seems o be theFe"+-analogue and caryopilite* @eacor et aI.1974) seems to be the Mn-analogue, although

* Kato (1963) has shown that ektropite - caryopi-Iite, and that bementite is structurally similar to thefriedelite group of minerals.

neither is understood well. In ferrous hydroxide,Fg(OH),, b is9,72A so the mismatch in a Fe!+-lizardite would be 0.57A, and in pyrochroite,

Mn(OH),, b is 9.974 so the misrnatch n a Mn-llr:ar:dite would be O.82A (Donnay & Ondik1973). These seemtoo large to be overcomeby

the variations found in the Mg-lizardite 1Tstructure. Perhaps there are vacancies in theoctahedral sheets of these minerals, with sometrivalent ions to maintain charge balance, thatholp to decrease the mismatch. In kellyite, theMn-analogire of amesite (Peacor et al. 1974),and in zinalsite, the Zn-analogue of amesite(Chukhrov & Petrovskaia I97l) the A1 substi-tution produces a condition of bo"t bot.

A pure magnesium chrysotile has a fullystretched tetrahedral sheet and a highly but notcompletely compressed octahedral sheet. Thuschrysotile could be expected o accept sorne A1or Fe3+ substitution in both sheets, but thecompositional imit would be exp,ectedo be con-siderably less than that corresponding to D*t =

bat sinco significant mismatch is needed o pro-

duce curving. The substitution of trivalent ca-tions such as Ni"+, Coe+, Mn3+ and Cf+ (to-

gether with balancing tetrahedral Al or Fee+)

would ,be restdcted by the same limitations.The substitution of Ni'+ for Mg would re-

duce the mismatch slightly and allow a larger

"ideaf' radius of curyature to develop. The Nianalogueof chrysotile ZM"t (Wcoruite) has beendescrib,edby Faust et al. (1969) and it would

appear that a solid solution exists between the

two end-members.The substitution of divalent cations larger

than Mg is limited by the amount of curvingthe structure will accept.Noll et al. (1958) haves.ynthesizedCo-chrysotile, but attempts to syn-thesize Z* and Mn-chrysotile have failed (Roy

& Roy 1954). The observed diameters of the

Co-chrysotile tube w,ere smaller than the dia-

meter of synthetic Mg- and Ni-chrysotiles, aswould be expectedbecauseCo2+ would have asmaller radius of 'oideal" curvature than the

other two. An ionic radius larger than Co!+may well be too large to allow the chrysotilestructure to form.

The antigorite structure, like chrysotile, is

neoessarily limited to compositions producing

b*r} b*t. Although Fe2+ tends to dominateoyer F,ee+ n the analyzed specimensrevigwedby Whittaker & Wicks (I97O), there is no ob-vious reason why appr,eciableFes+ should not

be present, and the compositions noted maysimply reflect a reducing environment of forma'tion. Substitutions generally would be expectedto be subject to limitations similar to those inchrysotile, although in fact they seem to be less



A REAPPRA]SAL OF THE STRUCTURES OF TIIE SERPBNTINE MINERALS

TABLE. TRIoCTAHEDMLil LAYERILICATES

241

SERPENTINE ROUP

Serpent lne Nl -Serpent lne F e -S e rp e nt l ne M n -S e rp e nt J ne Z n -S e rD e nt J ne C o- S er p en t in e

ant igor l te

chrysotl l e

parachrysoti l e'I

I zardi te

7

synthEt lc

??

? ? ? ?

pecoral te ? ? ?

? 1 ? 7

nepoui te greenal i te caryopi l i te ?

Subst l tut lon of Al in l /3 of the octahedral s i tes and l/2 of the tetrahedral s l tes produces boct.btet

an d elJnlnates curved s tructures such as chrysot lle, antJgorJte, pecoraJte, etc .

? ber thler lne kel lyl te zlnalsJte'lanes l te

SubstJ tutJon of Fe - Jn l /3 of the octahedral s l tes and l/ 2 of the tetrahedral s l tes also produces

boct t btet '

? c r o n s t e d t l t e ? 7 ?

rigorous. Ferroan antigoriteshave been reportedby Frondel (1962) andDietrich (1972).Completesubstit'ution by Ni'+ would be expected to bepossible, and a specimen with a high Ni con-tent has been reported by Faust (1966). Be-causeof the crosslinking between the layers inantigorite, it is not expected to be able to formdifferent polytypes,

Tho strucfures and chemistry of the serpen-tine group of minerals discussed above, to-gether

with some related trioctahedral 111 laversilicates, are summarized in Table 4.

ACKNowl,EDGMENTS

Thanks is extended to Mr. J. Mulock, ROMArt Department, who drafted Figures I,2 and 3and the ROM Photography Department forphotographing the illustrations.

RBpnnrNcrs

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Manuscript received February 1975.

Estructuras minerales serpentinas

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