Canadian Mineralogl*t VoL 13, pp.227-243(1975)

A REAPPRAISAL OF THE STRUCTURES OF THE SERPENTINE MINERALS

F. J. WICKS Mineralogl Depa.rtment' Royal Ontario Museum, Toronto, Canada E. J. W. WHITTAKER Departrnent of Geology & Mineralogy, Oxford Unlversity, Oxford, England

ABSIRACT and compression,it is suggestedthat it is the cona' pression of the octahedral sheet that buckles the The three theoretical stacking schemes for tri- Mg plane so that the Mg atoms occupy two posi- octahe&al l:1 layer silicates developed independent- tions at different levels. This disturbance of the ly by fteadman, Z'tpgin' and Bailey all assume structure in turn tilts the tetrahedra in the way ideal hydrogen bonding between successive layers, that is observed. In chrysotile the curvature only with polytypes developed through shifts of :tal3, partly relieves the mismatch, and evidence is pre- !b/3 or zero, and rotations of 180o or zero. The sented for a similar buckling itr this structure, three systems contain 16 distinct polyty?es, and though to a smaller extent, This postulated buckl- Bailey's nomenclature is extended to provide sym- ing explains some hitherto obscure features of the bols for each one. If lizardite is redefined to in- structure. Antigorite appearsto overcompensatethe clude all serpentines with flat-layer structures, these mismatch in the direction of curyature, but it has can be designated as lizardite followed by the ap- an anomalouslythick octahedral sheet which is propriate polytype symbol. still unexplained. The above system cannot be applied to the chry- sotilo structures because the sbifts between the layers show that the normal hydrogen bonding INrnooucrroN position 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 the Because of their frequent sub-microscopis, groctves between, and the upper one over the ridges fine-grained texture and close chemical relation- formed by, the underlying hydroxyl rows. This ships, an adequate classification of the ser- stacking principle leads to a series of polytypes for pentine minerals was delayed until structural which an analogous nomenclature is proposed. information became available, and this was using subscript c to denote their essentially cylin- itself delayed by the unusual curved layersosu- drical character. Thus clinochrysotile, orthochry- perlattices and disordered stackings that occur sotile Zvyagin's clinochrysotile be- and l-layer these minerals. It was not until 1956 that come 2Ma, 2Or"1 ar.d lM"r. The term in chrysotile by Whit- parachrysotile is retain_ed to describe the cylindrical work on the structures of chrysotile structure with a 9.2A fiber axis. The alternating taker (1952, 1953, L954, I955a, b, c, d, l'956a, wave structure of antigorite, with layer inversions b, c, 1957) and by Jagodzinski & Kunze (L954a, and interconnections between successive layers at b, c) and of antigorite by Zussman (1954) and the inversions, destroys the possibility of systematic Kunze (1956, 1958, 1959) was advanced suf- hydrogen bonding and makes the stacking depen- ficiently for a viable classification to be put for- dent on the primary bonding at the interconnections, ward (Whittaker & Zussman 1956). polytypes so that it cannot be discussed in terms of This classification divided the serpentine min- of either kind. into three structural groups based on A review of the substitution in natural and syn- erals rylin- (chrysotile), thetic chrysotiles and lizardites indicates that there drical layers cornrgated layers is a compositional overlap between curved and flat- (antigorite) and flat layers (lizardite). Chryso- layer structures. Thus, as all flat-layer structures tils was further sub-divided into three varieties, caq b€ regarded as polytypes of-lizardite and all clinochrysotile, orthochrysotile and parachryso- oylindrical structures with a 5.3A fiber axis can tile. Antigorite was soon found to occur with be regarded as polyty?es of chrysotile, lizardite and a range of superlattice periods (comrgation chrysotile are polymorphs. Compositional data for wave-lengtls) from 16-110A as well as the parachrysotile poly- are lacking. Antigorite is not a original 43A @rindley et aI. 1958; Chapman & morph of the other serpentines because it has an Zussman 1959; Kunze 1961). Sub-division of (though slightly) different composi- essentially only the flat-layer serpentines was necessitated by tion. of varieties with a 1-layer cell Because the mismatched tetrahedral and octahe- the discovery dral sheets in lizardite are respectively in tension and a 2-layer cell @ucklidge & Zussman 1965), 227 228 THE CANADIAN MINERALOGIST

a 6layer cell (Zussman & Brindley 1957; Zuss- The purpose of the present paper is to discuss man, Brindley & Comer 1957; Olsen 196l; tho differences between the internal structure of Miiller 1963; Krstanovii & Pavlovid 1967), a the layers of the main divisions of the serpen- 6- (pseudo 2-) lryer and a G (pseudo 3-) layer tine minerals, and to relate them to one another cell (Gillery 1959; Bailey & Tyler 1960), a 3- and to other trioctahedral 1:1 layer silicates in layer cell (Coats 1968) and a 9-layer cell (Jah- terms of structure and crystal chemistry, and anbagloo & Znltai 1968)4. It has since become in this connection to develop further the dis- evident from the work of Zvyugn (1967) that a cussion of Olsen (1961) and Radoslovich fourth variety of chrysotile, L-layer clinochryso- (1963b). The relationships of these structural tile, must be added to the three chrysotiles of features to the chemical differences that have Whittaker & Zussman's classification. been treated previously by Faust @ Fahey OnIy two serpentine varieties have come to (1"962r, Page (1958), and Whittaker & Wicks light whose relationship to the three-fold classi- (1970) are also discussed.However, before pro- fication has been in doubt. One h the splintery ceeding to this discussionit is desirable to con- serpentine @ovlen-type) found by Krstanovis sider the relationships of the layer stacking of the & Pavlovii (1964) which they suggestedto have serpentine minerals to the polytype classifica- a modified clinochrysotile structure. More re- tions of Steadman (1964), Ztyagln (1967) and cently Middleton (1974) has investigated this Bailey (1967a, 1969) for 1:1. layer silicates. further and found Povlen-type clinochrysotile to Some sonfwion has been introduced into the be composed of chrysotile-like layers. He has literature by attempts to fit the serpentines into also found a Povlen-type orthochrysotile whose these classifications, which are not in fact di- structure he has interpreted as an intergrowth rectly applicable to the serpentine structures. of chrysotile and lizardite-like layers. The other serpentine variety is an unusual serpentine from THEoRETTcALPor,yrypns the Tilly Foster mine, New York (Aumento 1967,) which may be o very intimate mixture of Three theoretical stacking schemes for tri- clinochrysotile and antigorite. octahedral l:1 layer silicates have been de- veloped in recent years by (i) Steadman (1964), (i) Zvyagin, Mischenko & Shitov (1966) and Zvyagin (1967) and. (iii) Bailey (1967a, t969). *Jahanbagloo & Zo.ltai (1968) analyzed what they The terminology developed by the called a bexagonal Al-serpentine with a 9-layer tri- various auth- gonal structure and a composition of (Mgr.o"Al."n- ors differs and so do their basic assumptions, Fe.sr)(Si!.a7Al.$)Ou(OH)n.However, as this composi- so that the same series of polyty,pes is not de- tion is closer to the amesite end of the lizardite- veloped in each system, although there is con- amesite solid-solution eries this qrecimen will be siderable overlap (Table 1). The full details of called amei{te .9? in this paper. the procedures are given in the original papers

TABLE I. A CMAiISOII OF POLTYPES

Sbatun (1964) ZvFgln (1967) Bailey (1969) [email protected] Layers lnter- h€dml [xlendd SF@ ln Unlt hyer Strlcture Group Struchre S!ructurc ilodifi- Group Poljdse hlley hup CelI Shlfts X@b€r fl@r Nmber Typ€ catlon Xmnclature

hla/3 17 tl 1 Bl[Atil1t4 &2a/3 t8 z 41 Nl a1 P3t 3 a./3 19&20 3&4 3T 3T 3Tt

C@2, 2 al3+r tt l1 I c20 B 20r 2Or n9! n.l 2 at3+r 2&3 equlvalent & 2h 2 al3rr 4 4" 2i2 az ''l ot 5 5 al3+r 5&6 5H 6H 6Ht

P3l0 I ilone I I A]TCIT'IT B3bl3 2&3 | &2 3T 3R 3R P3lc 2 b/3 a 3 2rnzl

P63@ 2 flon*r 9 vIr I D 2ts D 2Ht 2Ht P6: 2 b/3+? l0 & ll ll 2&3 2H 2H2 2HZ k 6 b/3+r 16 v1 l fr 6R 6Rr

86 ilone,r/3 4&5 lfflnitles rlth 6&z P33 ttane,.b/3 6&7 &lley Group C 312

B 6 ltondr t2 & 13 .tflnltles !ilh bl3rr P6" 6 llon+r _ t4 I t5 Bijley Grolp ! 6lz b/3+r

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 is -----n? given in Wicks (1969). ? 9---o a; '- For purposes of generating th,e various poly- o; oMs' 'ho i i types, the three systems assume an ideal tetra- -.. hedral sheet with hexagonal symmetry, linked O .,P @t O A through the apical oxygens without distortion to an ideal trioctahedral sheet with trigonal sym- €*, A fr+ tr- ,O metry (Fig. 1). Stacking of successivelayers is a o'fez o '/ o then assumedto occur in such a way that hydro- gen bonding always develops between every ba- of @*,,,,Qo i sal oxygen of a given layer and the outer hy- "ig------o--- o--- o------€ droxyls of the adjacent layer (see Fig. 2 of Bailey 1969). If such layers are stacked verti- cally above* one another such an optimum l. hydrogen-bonding system is developed, and the structure has a 1:1 layer trigonal unit sell. Equivalent hydrogen-bonding systems are de- veioped if successive layers are shifted relative to one another by -a/3** along any of the three possible t-axes of the layer, ot by L bl3 projection the idealized serpentine -+- -t Frc. 1. [001] of along a y-axis, rotated through 60o, 120o or layer with Mg in the II octahedral sites with re' 180o, or are subjected to combinations of such spect to the t-axes, as defined by Bailey (1969). shifts and rotation. The number of alternatives is In the monoclinic chrysotile polytypes it is neces' reduced by the fact that the layer symmetry sary to rev€rse the positive direction of the .r- makes it possible to describe all polytypes pro- axis in order to satisfy the convention that F -r should be obtuse and as small as possible. duced by b 13 shifts in terms of a single y-axis (Fig. 1). It also makes rotations of :t120' equiv- alent to no rotation. and rotations of t60o or 180' irdistinguishable and therefore describable terlayer relationshipsa/3, a/3 * r, b/3, and simply as rotation (r). b/3 + r, with zero shift classified with the ,/3 Zvyagin (1967) and Bailey (1969) have both shifts. Their results are therefore virtually equi- classified the possible polytypes into four ,goups valent, but in comparing them it is important under the further limiting assumptions that: to note that Zvyagn and Bailey denote the (i) combined shifts of the type a/3 and b/3 do groups by the same four letters but in a dif- not occur ferent way (Iable 1). Bailey shows |hat Zvya' (ii) the relationship between every successive gin's ZMt,polytype is equivalent lo 2Or $ven pair of layers involves the same kind of ths full symmetry of the layers. The members of shift, or shift * rotation, although the di- each group are shown not only to share the rection of the shift may change in a syste- same formal relationship but also to give rise matic way to grve multi-layer cells, as. in to readily distinguishable diffraction intensities the mica polytypes. among their strong reflections (see Table 2, p. Both these authors classify the polytypes into 361, Bailey 1969). Individual. members of a four groups depending on the existence of in- group can be distinguished only by differences among their weak reflections (see Table 3, p. 362, Barley t969). 'rThe convention adopted by Steadman and Bailey' The earlier' classification of Steadman (1964) which is followed here, is to regard the outer hy- is also shown in Table 1. Steadman did not di- droxyls to be on the top of the layers, and tbe sili- vide the structures into groups, and he did Zvyagin's discussion the cate net at the bottom. In not consider those based on shiftls of a/3 * r, opposite convention is adopted, but this males no more (enantiomorphic difference to the nomenclature aod has been al- but he did include four lowed for in the comparisons of the different classi- pairs o0 structutes not given by Zvyryn ot fications. Bailey because diiferent kinds of shifts between alternate pairs of layers are involved. In fact *l'There possible in are two sets of octahedral sites Bailey has shown that structures exist which layer, that occupied by Mg in Figure a 1:1 sitcate contain mixed shift sequences so that these (called II by Bailey) and another (D which in' 1 without volves an interchange of the projected positions of structures listed by Steadman are not Mg and the outer hydroxyls. If Mg occupies set f, interest. Steadman'snumbers 4-7 have affinities the appropriate shifts are + a/3. to Bailey's group C and l2-L5 to Bailey's group 230 THE CANADIAN MINERALOGIST

D. Bailey's polytype nomenclature can be ex- ous Yugoslavian localities and selected those tended readily to take account of these, and composedonly of the disordered 1? polytype for other possible polytypes, as indicated in the last detailed structural studies. column of Table 1, and his extended system is Although the structure of the individual lay- used in this paper. Disordered crysials can ers in lizardite departs from the ideal in a num- usually be assip.ed to one of the major groups, ber of ways, (discussedbelow), the stacking of A, B, C or D, Table 1, on the basis of their the layers one on another is based on hydrogen strong reflections. bonding of the ,basaloxygens of ,one layer to the hydroxyls of the layer below in the manner as- Acruer, PoryrypEs sumed in the derivations of the theoretical poly- types. The use of the nomenclaturediscussed Amesite cronstedtite and above is therefore fully justified. The usefulness of the theoretical polytype The theoretical polgype classifications have schemesis illustrated most clearly by the struc- their greatest application in the multi-layer vari- tural studies on amesite and cronstedtite. In eties. These have been frequently classified in amesite, (polytypes 2flr (Oughton 1957) 2Ht terms developed by Gillery (1959) as 6Q)-layer (Steinfink & Brunton 1956) and 6R, (Steadman structures approximating a 2-layer structure, and & Nuttall 1962), all in Bailey's group D, as well as 6(3)-layer structures approximating a 3-layer as a 9T polytype not developed in the theore- structure. Most ,of the multi-layer lizardites de- tisal schemes(Jahanbagloo & Z,oltai 1968) have scribed in the literature have some degreeof dis- been described. In cronsteiltite, polyty,pes lM, order so that their diffraction patterns do not 2Mt and,37r (Group A), polytypes lT and 2T always fit Bailey's calculated diffraction pattern$ (Group C), and polytypas ZHt 2Hz, 6Rg and (Bailey 1969, Table 3, p. 362) and some have 6Il, (Group D) have been found by Steadman structures not included in Bailey's table. How- & Nuttall (1963 and 1964). lt is of interest to evet, the main structural grou,pA, B, C or D can note that the 6Rg and 6Ht polytypes are de- often be determined by the use of Bailey's Ta- veloped only in Steadman's classification, but Ue 2, p. 361, although elements of two struc- the others are found in all three classifications. tural groups may be present in some specimens. It is intere:ting that the frequency of Lizardite occur- rence of the multi-layer lizardite polytypes re- The name lbardite was proposed by Whit- cordeC in the literature is: 2 in @ailey's) group taker & Zussman (1956) to denote a serpentine A, 1 in group B, 1 in group C and 4 in group mineral possessing a single-layer orthohexa- D. When the one- and two-layer lizardites (17 gonal cell and varying degreesof three-dimen- and 2II) are includedo howevern group C be- sional order; howevel, they noted that some spe- comes by far the most frequent group and the cimens did not conform entirely to the single- frequency of occurrence becomes C>D>A>B. layer cell but contained 1800 rotations in some This is the same relationship derived by Bailey kind of random sequense. The flat-layer ser- (1969) from theoretical structure-stability esti- pentines having multi-layer cells have hitherto mations. been referred 16 gimFly as "multi-layer selpen- Bailey (1969) has indicated that further work tines", with an implication that they are to be is in progress on the multi-layer polytypes. regarded as species distinct from lizardite. A simplification would be achieved if all flat- Chrysotile layer serpentineswere termed Tizardite, so that Whittaker (1953, L956a, b, c) has defined the l-layer and Z-layet lizardites and the three types of chrysotile: clinochrysotile, ortho- multi-layer varietbs could all be regarded as chrysotile, and parachrysotile. Clinochrysotile polytypes. This nomenclature is therefore fol- has a Z-layer monoclinic cell (disregarding lowed in the remainder of this paper. The prob- the cylindrical nature of the structure) with no lems raised in relation to the definitions of poly- rotation between the layers. Orthochrysofile and morphism and polytypism in the presence of parachrysotile have 2Jayer orthorhombic cells small chemical differences are discussedbelow. with rotations of 180o between the layers. In Detailed studies on the type lizardite from clinochrysotile and ortlochrysotile the x-crys- Kennack Cove, Cornwall, England, by Ruck- tallographic axis is parallel to the fiber axis (the lidge & Zussman (1965) indicated that the crys- cylinder axis), whereas in parachrysotile the y- tals were composed of domains equivalent to axis is parallel to the fiber axis. The cylindrical the 17 and 2H polytypes (disregarding the nature of clinochrysotile -+- and orthochryiotile has b/3 disorder). Krstanovi6 (1968) has exam- been confirmed by electron microscope observa- ined a number of lizardite samples from vari- 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) are stacked in such a way that the projecting suggested that parachrysotile might be in the O"s fit into the underlying gtooves and the form of curved crystallites on the surface of withdrawn Oanslie approximately over the OH clinochrysotilo fibrils, but Middleton (1974) has rows. This arrangement involves a shift of shown that it must contain at least some cylin- 0.4A (approximatrly a/ L3) away from the posi- drical component. tiou of normal hydrogen bonding, assumed in Some confusion has developed in the liter- the polytype schemes and found in many 1: L ature as a result of the attempt to assign the layer structures. This shift, overlooked in general chrysotile structures to polytypes in the various discussi,onsof the chrysotile structures, eliminates theoretical schemes.These schemesall make the chrysotile from any discussions of theoretical fundamental assumption that each layer is polytypes based on normal hydrogen bonding, stacked on the previous layer so that normal and leads to a need for a different polytype no- hydrogen bonding is developed (Fig. 2, Bailey menclature for chrysotile as discussed below. 1969). However, in chrysotiles successivelayers The consequences of ignoring this unique are stacked in such a manner that the normal stacking artangement are illustrated by the con- hydrogen bonding does not take place (Whit- fusion existing over the assig,nmentof the stand- taker 1953). The continuously changing register ard polyty.pes to clinochrysotile and orthochry- between adjacent layers in the circumferential sotile. Steadman and Bailey consider clinochry- direction inhibits systematic hydrogen bonding sotile to be a distorted 1T polytype but Zvyagin for all the basal oxygens, and not merely for con:iders it to be a distorted lM. Similarly, t/g of tfum as assumed by Bailey (1969). Steadman and Zvyagin consider orthochrysotile The structural refinements of both clino- to be a distorted 2H, but Bailey considen it to chrysotile (Whittaker 1956a) and orthochry- be a distorted 2Or. T\is confusion is brought sotile (Whittaker 1956b) indicate that the basal about by failing to appreciate that the interlayer oxygens Or and Oz ore separated in the radial shifts in chrysotile, although small, are signi- direction by 0.2A, with Or projecting from the ficant and in the opposite direction to the shift layer structure and O, withdrawn into it. The in the standard polytypes. Also, the total lack cylindrical curvature of the structure aligns the of order along the y (circumferential) direction outer OH groups so that they form rows with destroys the regular hydrogen bonding and re- grooves between them running around the cir- duces the effective repeat unit along x 1o a/2. cumference of the structure. Successive lavers Under these conditions it is possible to choose

Ir l-r'- i-T-'l \

o.oss +x +x 1M 1T 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 standard - polytypes.-All I values are calculated on a basis of a 5.344 and c - 7.32L. 232 THE CANADIAN MINERALOGIST

alternate origins for a which leads to further dershift respectively. In the known structure of sources of confusion. clinochrysotile, which we now propose to de- In order to distinguish the polytypes that arise note 2h1[a, I is about 0.1A and overshift and in the chrysotile structure from those applicable undershift occur in successivelayers, with the to flat-layer structures, it is proposed to use a result that F has the samevalue (93.3') as if no subscript c (for cylindrical) immediately follow- undershifl or overshift occurred, but of course ing the main symbol (e.g, ZMa, where 2 = the the cell contains two layers. number of layers in the cell, M = monoclinic, The stacking of ideal layers with a rotation - c cylindrical and 1.: polytype number), and of 180' between successivelayers produces a to proceed to derive a number of expected poly- separation of. a/ 6 between the rows of hydroryls tyTres. from one layer to the next. In order to produce In both the ortho. and clinochrysotile struc- orthochrysotile with Or keyed into the groove$ tures described by Whittaker the Or atoms not between the rows of hydroxyls in the layer be- only project out of the layer in the radial direc- low there must be a shift, from the ideal stack- tion, but are dis,placedby O.1OAfrom their ideal ing position, of 0.4 :tDA in the opposite direc- position in the rdirection, i.e. parallel to the tion to the shift necessaryto produce Bailey's cylinder axis. In the discussion Lrelow, we sug- 2Or disordered polytype. This shift reduces the gest a mechanism for the production of this a/ 6 separation between hydroryl rows in suc- displacement which is an inevitable result of the cessive layers by O.4 18A. Either undershift compressive stress in the octahedral sheet of the or overshift in both layers would be compatible serpentine structure. If it is accepted that the with the orthorhombic character; however, the displacement always occurs, then a numbet of structures would be different and would give possible polytypes can be predicted. significantly different 20/ intensities. The known As stated above, for a simple stacking of structuro of orthochrysotile has undershift in ideal layers with Or keyed into the grooves be- both layers and we denote it 2Ora, The theore- tween rows of hydroxyls in the layer below, tically possible structure with overshift in both there would be a shift of the rows of hydroxyls layers may be denoted zor"a. Strucfuires with on top of one layer with respect to those on alternate layers rotated by 180" but with alter- the one below of approximately 0.aA, which is nating undershift and overshift can also.be hy- in the opposite direction from the shift in Bai pothesized, but they would be monoclinic with ley's and Zvyagin's lM disordered polytype I = 90.4o for a value of E = 0.1A, and would (Fig. 2). If this situation were repeated in every constitute an enantiomorphic pair. layer, a 1M" structure would result with F = It is now imFortant to consider the effect of 93.3'. If however Or is displaced by I in the over- and undershift in possible lJayer struc- r-direction relative to the upper parts of its own tures. For I - 0.1A these would have F = layer, then because it is keyed into the grooye 94.1" and 92.5o respectively, and we denote of the layer below, the shift between one layer tlese as LMa anrd lMea respectively @9. 3) and the next will be changed to 0.4 t8A. When Zvyagn (1967) has published electron diffrac- this shift is greater than and less than O.4A we tion patterns of a l-layer dinochrysotile fibril propose to call the situation over:hift and un- with p = 1O6.5owhich he suggestsis the (dis- ordered) ordinary IM polfipe. However, if lM occurs, it would be expected to have F = 103.7'. Because of the complete circumfer- ential disorder in chrysotile, it is always possibtre to choose an alternative F-angle due to fhe effec- tive a/2 repeat along r (see Fig. 2). The alter- native value for our LMa polytype is 106.4', in excellent agreement with Zvyaginos obserya- tion. We therefore conclude that his material has the 1M* stackingnand a F-angle that would be expressedmore conventionally at 94.2'. F:i The high-psolution h1 electron micrographs of HHmchrysotile fibers sectioned perpendicular to L-Y the fiber axis (Yada 1967) ,gtve firrther support 1Md 1Mc2 2M"1 20116 to the existence of a L-layer clinochrysotile. The doublo spiral and concentric circular structures Frc. 3. Stacking relationships among the chrysotile could possess a Z-layer clinochrysotile stflrcture polytpes LMos tfuIa" ?*74 and 2Oro1, as defined by Whittaker (1956a) or an ortho- A REAPPRAISAL OF THE STRUCTURES OF THE SERPENTINE MINERALS 233

Frc. 4. Equi-inclination fiber photograph of chrysotile D" from Krantz Kop, Natal. chrysotilo structure (Whittaker 1956b). How- possible to include parachrysotile in the scheme eyer, multispiral structures with an odd nunr- without introducing an additional symbol to ber of layers, and those w,ith numerous single- denote the orientation of the layer axes. It is layer dislocations, cannot pos$essthe 2-layer suggested that the name parachrysotile should clinochrysotile structure. The 2-layer struc- therefore continue to be used as in the past. Thus ture cannot form because the alternate positive chrysotile is a generic name for all the varieties; and negative displacementof layers with respect specific polytypes with * parallel to the cylinder to one another would be put out of sequence axis can be unambiguously denoted as chryso- by each extra layer added by dislocation, or by tile with a polytype symbol, and material with y the odd number of layers in multispiral struc- parallel to the cylinder 'axis can be unambig- tures. However, if the displacement is in the uously denoted by parachrysotile, to which a same direction in each layer so that the L-layer system of polytype symbols may also be added clinochrysotile structure (Whittaker 1,956a; at some time in the fufure should this prove nec- Zvyagin 1967) develops, these multilayer spiral essary. It is clear that parachrysotile must be re' and single-layer dislocation growths would pre- garded as a polymorph, and not a polytype, of sent no problem. trollowing the same reasoning, chrysotile. orthochrysotilo or parachrysotile could not have Antigorite multispiral growths with odd numbers of lay- ,ers, or possessa high number of single-layer Zvyagin (L967) and Bailey (1969) have classi- dislocations. fied antigorito as a distorted 17 polytype. In addition to the three chrysotile polytypes However, the curvature of the structure has the 2M* 2Or"t and tMa that have now been ob- samo effect as in chrysotile in precluding syste- served, in many specimens there is evidence matic hydrogen bonding between successivelay- from the greater breadths of. the 2N reflections ers, and the control on the lay'er stacking is relative to those of the 00/ reflections that mis- exerted by the direct interconnections between takes in the layer stacking may occur within a successivelayers on the placesa-0 and a- single fibril. In the extreme case there may be Vz where the corrugations invert. Furthermore, no discernible regularity in the stacking, and the existenceof the superlattice, and the chang- the 2Ol reflections are smeared into virtually ing relationship between successivelayers over continuous streaks along the layer lines. This the half wavelength of the corrugated structure, disordered stacking is conveniently denoted as change the diffraction pattern fundamentally chrysotile D". An r+ay photograph of such a from that calculated for the theoretical 1T specimen from Krantz Kop, Natal (Whittaker polytype. Thus there is little value in discuss- 1956d) is shown in Figure 4. ing antigorite in terms of the th,eoreticalpoly- With the introduction of an adequate poly- types. type nomenclature it is no tronger necessaryto use the prefixes ortho- and clino. for chrysotile, Cunvrrcal Rsl-atloNsrilPs AMoNc firE as these can be more meaningfully replaced by SrnPpNrnqe MnqeRALs the appropriate polytype symbol. However, all Chrysotile and lizardite the polytypes discussedabove have the r-direc- tion parallel to the cylinder axis, and it is not It is generally accepted that the layer curva- 234 THE CANADIAN MINERALOGIST

TABLE2. AI'10UNTS0F TRMIEI{T SUBSTITUTIONIN CHRYSOTILESAND LIZAR0ITE5 (1956) that natural chry.otile contains up to 29 ! L.9 (A1zo" + Fe,Os),Gillery sugg=sGda Averaqex SampleNo.- Averaqex SampleNo.* c mrrsitional break between chrysotile and 9.01 0.1I :i.:.a:diteat x = 0.2. Olsen (1961) ihowed that 0.03 c-4 s-2 i'rindley & von Knorring's (1954) 6-layer lizar- 0.035 0.l4 c-6 diteshad x:0.11 andr:,0.12, andhe pro- 0.045 i-z 0.16 L-5 posed on the basis of these values, and of the 0.08 L-4 0.20 L-3 analyses of Kalousek & Muttart (1,957), that 0.09 0.23 L-2 the assumedbreak is at x :0.10. Subsequently, ,.the 0.10 c-7 0.35 Radoslovich (1963b) concluded that high S-3 - 0.105 s-l value, .r O.25, was more acceptablo,'. More recently Chernosky (1975) has synthesizedchry- sotile = = = from r O.Oto O.25 and lizardite from C chrysotJle, L. l- or Z-layer'lizardlte, S nulfi-tayer llzardlte : See lable 3. page 1034, {htttaker & Utcks (1970). r O.0 to 2.A, demonstrating that there is no compositional break. It is to be noted that even if a compositional ture that occurs in both chrysotile and antigorite br,eak did exist at x - O.25, there would still is due to the misfit between the octahedral and be a range of compositionfrom x : O.25-O.75 tetrahedral sheets with boor).b*t, and neither in which b".tlbt* and yet flat layers are formed. chrysotile nor antigorite would be expected to It follows that there must be a mechanism of form if the composition were such as to mare misfit relief other than curvature, and it is, bo"t l brut.flowever, if curvature is not the only therefore, perf,ectly feasible for alternative conceivable way in which misfit might be ac- mechanismsof misfit relief to be operative over commodated when the composition is such as a given range of composition. Table 2 shows to make one expect bu") bat, then the converse the value of .r for the 15 specim,ensdiscussed assumption that lizardites cannot form under by Whittaker & Wicks (1970) and among these these circumstancesneed not be true. Flat-layer specimensit may be seen that lizardites occur sffuctures are expected when 6o* = bet, of down to r = O.08 and chrysotiles up to .r : when Do"t(6u, accommodation in the latter case 0.14. In view of the small number of specimens depending mainly on tetrahedral rotations @a- studied, it would be surprising if the range of doslovich & Norrish 1962; Radoslovich 1962). overlap were not appreciably greater than this. The data available from analvtical Cwhitta- The evidence from Gill,ery's and Chernoskyos ker & Wicks 197O) and synthesis (Cher- s.tuOles work suggests that multi-layer lizardites are nosky 1971, 1975) suggest that luardite and favoured by high values of x, and the highest chrysotile can indeed both form in overlapping value of ,r in Table 2 corresponds to a speci- composition ranges corresponding to bo"t)b*", men of this type. Howevero even if this trend and that the quest for a break in the composi- exists, there is again no clear-cut distinction tion range between chrysotile and lizardite pur- between the composition ranges of 1- and 2- sued by Gillery (1961) $959), Olsen and Ra- layer lizardites and multi-layer lizardites. doslovich (1963b) is fruitless. The possibility that systematic compositional Radoslovich (1962) has shown that the for- differences might exist among serpentine min- bo"t= but when r:O,75. This limit will be erals and their varieties has led some authors mula (Mgu-"A1")(Si4"AL)Om(OH)r*gives rise to to question the propriety of regarding them as raised by the presence of octahedral Fez+ re- polymorphs or polytypes. Within the range of placing Mg or octahedral Fea+ replacing Al, their overlapping compositions now demon- and will be lowered presence by the of tetra- sttat'ed, one need clearly have no inhibitions hedral Fe3+ replacing Al. Therefore, it would be about this. Furthermore, it would be absurd to expected, and seemsto be a fact, that chrvso- define polymorphism or polytypism so narrowly tiles can accept more Fer+ than lizardite.'On in respect of chemical identity that one part of the basis of his synthesiswork gave which lizar- an isomorphous series was regarded as poly- dite and no chrysotile .at x :0.25 and above, morphic or polytypic *i1h 1s:pect to another and becauseof the statement by Nagy & Faust structure, whereas another part was not. It would seem almost inevitable that two polymor- * phic Although the simple formula, corresponding to structures should have somewhat different one structural formula unit, is used elsewhere in capacities for accepting isomorphous replace- this paper a double formula is used in this section ment and therefore that close study should re- because it has been used as the basis of the dis- veal incomplete equivalence in their chemical cussions quoted from the literature. compositions. Equally, it would seem highly A REAPPRAISAL OF THE STRUCTURES OF THE SERPENTINE MINBRALS 235 probable that the oeculredce of (possibly partly structure. and the content of these ions often ordered) isomorphous substitution should modi- seems to brhigher 'in ffitigodt€ -than in chry- fy the relative stability of different polytypes sotile (W'icks & Whittaker 1970). of a structure, and therefore that close study should reveal compositional differences (at least .on a statistical basis) between polytypes. The Srrucruner VenreuoNs appeal to small compositional differences to The available Jtructural and chemical data deny the existence of polymorphic or polytypic relationships therefore seems to be a profitless Radoslovich (L962) has pointed out that the exercise. calculated b-parameters of the ostahedral and In view of the above discussion it is suggested tetrahedral sheets of most trioctahedral 1:1 that all ,flat-layer serpentines,whether t- ot 2- or layer silicates rarely match, so that adjustments multi-layered ty,pes,should be regarded as poly- in tle ideal atomic positions must occur. The ty.pes of fizardite; that l-layer clino-, 2-layer cli- nature of these adjustments is fairly well under- no- and orthochrysotile should be regarded as stood when ib*r,l bt*, but not when D*t ) bt t. polltypes of chrysotile; and that lizardite, chry- In this section we therefore seek to elucidate sotile and parachrysotile should be regarded as the adustments and distortions involved in this polymorphs becausetheir structures involve dif- latter case. The available data are rather inade- ferent processesof misfit relief, and their differ- quate, being confined to the results of two-di- ent stacking arrangements are consequencesof mensional refinements of the structures of 'this and not merely simple alternatives such as lnardite L? (Krstanovic 1968), chrysotile 2M"r ,occur between polytypes. (clinochrysotile, Whittaker L956a), chrysotile 2Or.' (otthochrysotile, Whittaker 1956b), and Antigorite antigorite (Kunze 1956, 1958, 1961). The lack refinements is unforfunate, The analysesdiscussed by Whittaker & Wicks of three-dimensional but is an inevitable result of the disorder present (1970) indicate that antigorites have a higher chrysotile, and the complexity SiOz content and a lower MgO and IfrO+ con- in lizardite and of the antigorite structure has led to a similar tent than the other serpentine minerals. This Despite the uncertainties in calcula- .difference in cqmposition is expected as a re- limitation. tions bond lengths and angles that arise from sult of the alternating wave structure which re- of these limitations. the trends in the variations 'duces the Mg and OH content relative to Si positions are indicated at the points of inversion, as discussedby Zuss- from the ideal atomic man (1954) and Kunze (1956; 1958, 1961). clearly. In to the calculations possible we Antigorito is therefore not a polymorph of the order make y coordinates throughout, and also .other serpentines in the strict sens€, but is a assumeideal assumptions and phase of essentially (though only slightly) dif- make the following additional Whittaker's Fourier maps give fer,entcomposition. The departure of the compo- approximations. single weighted mean position of O, and OHr sition from the ideal MgaSirOs(OlI)4must vary a he suggestedthat these atoms may be at with the wavelength of the corrugations in the and As this structure and antigorites which differ from one tw'o levels separatedby 0.224 along ze. supported by the more recent re- another in this way ar,e therefore neither poly- assumption is of lizardite 1T by Krstanovic, in which morphs nor polytypes; they are related in a si- finement by 0.4A, milar way to one another (and to the ideal corn- he found O; and OHr to be separated preferred coordinates of Og and position) as the integral memb,ersof nonstoichio- Whittaker's z OHr used in the following discussion.Whit- metric oxide series are related to one another are preferred .r coordinate is also used for and to the stoichiometric compound. This has taker's Or. In the antigorite refinement no difference been fully demonstratedby Kunze (1961, Ta- the Os and ble 5, p. 239). was assumed or detected between between the Mgr and Mgr It follows from the discussion of chrysotile OHr coordinates or coor.dinates, the Mg and Mg, octahedra are and lizardite that high-alumina antigorite with so be in this structure. Each b*"4bet is not to be expected,as curving is taken to identical has tetrahedra and 8 octahedra, essential to the antigorite structure, and none half-supercell 9 different bond lengths and has been found. As shown by Whittaker & each with slightly Wicks (197O), F"'* substitution in antigorite may be more extensive than in the other ser- *The standard 1:1 layer orientation of the x, y pentines. This would be expected to raise the and z axes is used throughout this paper. In the permitted Al * Fe'+ content before the flat original structure descriptions (Whittaker 1953, Iizardite structure supervenes over the curved 1956a, b) x and z are reversed. 236 TTIE CANADIAN MINERALOGIST

bond angles d,ependingon their position. The = 8.78A and a thicknessof 2.43A (Iable 3); results for one average tetrahedron and one both shared and unshared edges would have a averageoctahedron are presented. length of 2.974, and all O-Mg-O angles would With regard to chemical composition of the be 90'. Ilowever, in brucite the cation-cation ma.terialswhose structure is to be discussed,no repulsion between the Mg atoms extends b to analysis has been given for the lizardite LT re- 9.434 and.thins the sheet to 2.llA. shortenine fined by Krstanovi6 (1968) but he states "that the shared octahedral edgesto 2.79A by draw- the total amount of cations other than Si and ing the OH in along the z dir,ection,and length- Mg was found to be less lhan L% by weight" ening the unshared ones to 3.144. The corres- (Krstanovii 1968, p. 165),so it can be assumed ponding O-Mg-O angles become 83.3' and it is very near the ideal composition Mg"SizOa 96.8o respectively. In this discussionthe actual (oH)". brucite sheet rather than the ideal brucite sheet The chemical compositions of the chrysotiles will be used as a model of the unconstrained 2M"t a\d 2Ora vsed for crystal structure deter- octahodral sheet. minations by Vithittaker (I956a, b) are unfor- When 6,.t {b* it is well-establishedthat the tunately not knsqla, but the survey of serpentine main adjustments take place in the tetra- chemical compositions by Whittaker & Wicks hedral sheet by rneans of tetrahedral rotations (1970) indicates that chrysotiles generally have (Radoslovich & Norrish 1962; Radoslovich low substitution, so that it is not unreasonable 1962). T\e octahedral sheet is therefore likely to assume that this is true of the samples stu- to be under very little stress, and the available died by Whittaker. data on minerals of this type are of interest as The antigorite used by Kunze (L956, 1958, a point of reference. 1961) has been analyzed (Zussman 1954) and In amesite 2llz (Steinfink & Brunton 1956), has tle formula (M&.erFe2+.orFe3+.ogAl.oa)(Sir.sr-which is assumed to have a compos^itionclose Al.o')Ou(OII)a. to MgdlXSiAl)O5(OII)n, b is 9.2O4 and the calculated b*' is 9.774, suggesting that there Distortions in lizardite is not much stretching of the octahedral sheet The dimensions of an uncompressedoctahe- (Radoslovich L962). T\e M-M distances ars dral sheet are the result of the balance between 3.07A (where M =,MerAl) and the sheet is these forces-" (I) cation-cation repulsion across 2.o24 thick (Iable 3). The O and OH &ro co- shared octahedral edges, (II) anion-anion repul- planar, as are the hy'droxyls of the OH plane, sion along shared edg,es,and (III) cation-anion and the Mg and A1 atoms occupy a single plane bonds within octahedra." (Radoslovich 1963a,p. equidistant from the O,OH and OH planes. The 80). Of these three forces, the first is considered M-O and II[-O}J bond lengths are 2.04.A. The to be the most significant. The divalent cations shared edges of the octahedra are 2.694 and repel each other, causing the anion to move in- unshared ones 3.074. The amesite 9? structure wards along the sheet normal to produce a thin- (Jahanbagloo & Znkai 1968) is less aluminous ning of the octahedral sheet (Baitey L967b). If. in the tetrahedral sheet but the octahedral sheet trivalent cations substitute for the divalent ca- is similar to amesite except that it also has a tions the forces of repulsion will be stronger. small amount of iron of undetermined valence, The ideal Mg-octahedral sheet would have i and possibly some vacanciesto maintain charge

TABLE3. INTERATOI4ICDTSTtrCES AND AXGLIS

Brucit€ Bruci& Af,esite hesixe Lizodlte Chrysotlle Chrysotjle Antigorite bnd Length, Angles etc. Ideal observed 2HZ 9T lT 20rct 2fl"t

calcllated roct 8,78 - 9.17 9.19 9.45 9.45 9.45 9.29 calc!lated rtet - 9.56 9.35 9.15 9.r5 9.t5 9.r5 9.43 9.20 9,t71 9.186 9.2 9.2 9.23

ocAiedral sheet thiclness 2-43 2.lr 2.02 2.04 2.20 2.m 2.08 2.44 u'-il at 30' to t 2.93 3.t4 3.07 3.06 3,08 3.08 3.09 3.Il il-fi parallel 6 y 2.93 3.l4 3.O7 3.06 3.08 3.06 3.06 3.0€ AveraS€ \-0H I 2,03 2.06 2.a6 ) Averaqe M--OH { 2.10 2.10 2.M 2.04 ) 2.17 2.13 2.46 2,06 )

ktrahedral shee! thlckness - 2.29 2.26 2.15 2.13 2.26 2.22 I -Tat30c to4 - 3.06 3.06 3.07 3.8 3.08 3.02 I-l parallel b , - 3.06 3.06 3.07 3.07 3.ffi Averag€ l-0 - 1.68 I .66 t.6l 1-62 1.64 1.64 Tetrahdra rctation - 12.3" 'u ls occupid by ilg rith or llthoux lessef Al, Fez{or Fe3+ *T ls occupi€dby Sl {ith or ultholt Al, A REAPPRAISAL OF THE STRUCTURES OF THE SERPENTINE MINERALS 237 balance. Tlne b""" calculated with Radoslovich's In spite of the buckling of the Mg plane, the formula is 9.19A assuming all ferric and 9.20A extent of the compression of the octahedral assumingall ferrous iron. The D is 9.17A which sheet is rather small, and the tetrahedral sheet may suggest that the octahedral sheet may be is therefore under considerable tension. Rados- slighrly -compressed but it is more likely that lovich (1962) calculated the mismatch along b D is smaller than 9.19A because of the vacan- for lizardites to be 0.224 but since Krstanovic's cies. The octahedral sheet is 2.05A thick bui specimen is nearly a pure Mg-lizardite, the mis- in all other aspectsth,e amesite 9T structure is match must be near a maximum of O.3A (Table similar to the amesite 2Hz structure. The cron- 3), the difference between brucite and the ideal stedtite 77, 3T and 6Ra (Steadman & Nuttall tetrahedral sheet (Brindley 1967). The sheet is 1963) all show similar features. stretch,ed and thinned to 2.L5A. The silicons The lizardite LT (Krstanovii 1968) represents maintain a near-hexagonal array but the^ Si-Si the extreme case of b*t) bw with maximum distances are increased to 3.07 and 3.064 (Ta- compression along both the x and y axes. The ble 3). point octahedral sheet thickness of 2.2O4 is @part It is important to emphasize at this from th,e anomalous value for the antigorite dis- that although tle lizardite is described as poly- cussed below) tle thickest observed among tri- type lT, because this correctly describes the octahedral L: L silicates (Table 3) and among nature of the stacking, it is in fact orthorhombic trioctahedral 2:L Tayer silicates @ailey L967b), and only pseudo-trigonal,and an explanation of The extreme compression buckles the plane of this fact is needed. tho rnagnesium cations, forcing them to occupy The loss of equivalancebetween Mgr and Mgz two strusturallydistinct positions having z co- that ,arises from the buckling of the Mg plane ordinates differing by O.4A. The Mgi remain in does not destroy the trigonal symmetry of the an almost central position 1.O4A from the aver- arrangement of Mg, but it increases the size 'lvhen age position of the OsOHl plane and 1.16A of tle trigonal unit and combined with from_the OHTOFL plane, and the Mgz atoms axe the tetrahedral sheet the symmetry is lost. The 1.45A from the OgOHr plane and 0.75A from greater proximity of Mgi to Si compared with the OHzOIIg plane. This buckling of the Mg Mgr will lead to unbalanced repulsion between plane appears to be the key to understanding Mgr and Si which will tend to rotate alternate te- all tle remaining adjustments. trahedra about 11301ana lTfOl axes.This should As a direct result of the buckling, the aver- be manifested in a relative positive shift of the x age Mgi-Mgr distance is increased from 3.O6A coordinatesof Si, Or and Oz relative to Og, and a to 3.O9A (compare Mg-Mg distances of 3.14A separation between th,e z coordinates of Or and in brucite, Table 3), although there is little Oz with Or going negative and O, positive. The corresponding increase in MgrMgz distances. observed difference in z between Or and Oz is It would be expected that as a result of the 0.4OA which is just significant ,Q.Sa). The cor- depression of Mgr relative to Mgz, the three responding shifts of Si, Or and Oz relative to Os OHr and OHe hydroxyls would be displaced to- would be about 0.2A. which is not observed,but waxds a point in their own plane having the r as this value coresponds only to about 1.1o this and y coordinates of Mg. Their y coordinates is not surprising. are unknown, and the displacements of their x The length of the 7-O, bond in lizardite coordinates are not significant in view of quoted (where T = Si) is appreciably shorter than in erors. The displacementsare, however, in the amesite (where T = Sio.oAlo.r).The observed right direction on average. length of 1.56 :t 0.05A in trtzardite does not There is a significant difference of about differ significantly from the expected value of 0.3A between the z ooordinates of OHr and Os, 1.62lt. Thus, part of the apparent thinning of with OHr being the closer to the Mg atoms. It the tetrahedral sheet of lizardite relative to ame- is suggestedthat this arises from a repulsion of site Cfable 3) is due to this compositional effect. the Si atoms (carrying their attached Og atoms Tho stretching effect may be expected to show with them) by the d,epressedMgr atoms. The re- most clearly in the thicknessof the Si- (mean Or, sulting depression of the Os atoms then perrnits Oz) part of the sheet which is only 0.50A com- an upward shift of OHr. The average unshared pared with 0.58A in amesite (which falls to edges of the octahedra (and the corresponding b.SeA wnen corrected for change of Z occu- angles at Mg) are smaller than those found in pancy). The ?-Or and 7-O, bond lengths cannot brucite, and the average shared edges (and cor- be calculated as reliably as ?-O; in absence of responding angles) are larger than those found y coordinates, but based on idealized y co- in brucite. These differences aro clearly the re- ordinates, the values obtained give a satisfactory sult of the compression of the oc.tahedralsheet. average ?-O bond length of 1.614 Clable 3). 238 THE CANADIAN MINERALOGIST

Curvature and distortions in chrysotile thickness of. 2.194 against 2.154 for lizardite Curvature of the layers has often been given and,2.294 for amesite; departure of the mean e as a means whereby misfit between sheets of s6e1dinz.teof Mg from the centre of tho octahe- different sizes can be accomm6dated without dral sheet of chrysotile is 0.07A cbmpared with stress. For any given composition there is only 0.21A in lizardite and zero in amesite; the dif- one ideal radius of curvature, and thus in any ference in z coordinates of Or and Oz is prob- individual chrysotile fiber there is only one layer ably about 0.2A compared with O.4A in fizardite that has the strain of misnnlstr completely re- and zero in amesite, and the corresponding dif- lieved along the circumference of the curve. For fetence for O' and OHr is also estimated at the layers within fhis radius there will be over- about 0.2A compared with O.27A in fizardite compensation and for those outside there will be and zero in amesite. In the discussion of lizar- under-compensation. dite we have shown that all fhese parameters Whittaker (1957) calculated the ideal radius can be interpreted as arising either from the for chrysotile to be 88A and poiuted out that extension of the tetrahedral sheet or directly the residual stress increases more slowly out- from the complession of the octahedral sheet, wards than inwards. Yada (1967, I97I) ob- or indirectly from the buckling of the Mg plane served that the minimum inner radius of chry- that arises fro.m that compression. No evidense sotile is 35 - 404 and that the normal maximum has been given for such a buckling in chrysotiie, outer radius varies from 135 - 14OA, but ex- but it has not hitherto been looked for. As two ceptional fibers have outer radii from 90 to phenomena are present in chrysotile which, 2204. It is clear from these figures that the when they occur in hzardrte, we have inter- average structure in chrysotile fibers must usual- preted as due to this buckling, it seems very ly correspond to material lying outside the ideal likely that it also oscurs, though to a more lim- 88A radius of curvature, and thus have mis- ited extent, in chrysotile. matches only partly relieved by the curvature. Curvature Furthermore, since the curvature occurs about and distortion in antigorite the x-axis, relief can only occur in the y-direc- The antigorite structure refined by Kunze tion and the mismatch along the r-axis is com- (1958) has boa = 9,294, calculated according pletely unsompeDsatedby the curvature. There- to the formula of Radoslovich (1.962),and has fore, in chrysotile (excluding parachrysotile) b*t = 9.L54 because of tle small amount of there is a v€ry strong compressional stress in substitution of Al. The mismatch is therefore the x-direction in the octahedral sheet and there only 0.14A. The superlatticerepeat is 43.3A anal is also a lesser compressional stress in the y- the radius of curvature varies from 724 at one direction in the average octahedral sheet. inversion to 50A at the next inversion, with an Radoslovich (1962) calculated a mismatch of average of 61A (Kunze 1958), the curvature 0.12A for chrysotile, but the basis of this is not being along the r-direction instead of along the clear and theoretically it could be as high as y-direction as in chrysotile. 0.3A. Thus, one would expect the averagecom- Because every layer in the structure of anti- pression in the octahedral sheet to be somewhat gorite is able to adopt the same radius of curva- more than half of that in lizardite. In absence ture, it might be expectedthat the radius adopted of precisely-known compositions for Whittaker's would be such as to relax the stressescomDlete- 2Ma and. 2Ora structtres and in view of the ly in the r-direction. However, the calculated uncertainties attached to some of their de- radius of curyature for a mismatch of O.14A is tailed parameters, the best test of this conclu- about 190A, so that the curvature is substan- sion is to compare the mean of tleir relevant tially greater than would be expected, and must parameters with those of lizardite and amesite introduce a tension in the .r-direction in the (see Table 3). The most important would seem octahedral sheet at least as great as the com- to ,be the octahedral sheet thickness of chrvso- pression in the y-direction. Such a balancing tile at 2.08A against 2.2OA for lizardite and of tension and compression is perhaps not un- 2.O24 for amesite{', the mean tetrahedral sheer reasonable, and its reality can be argued from the greater Mg-Mg distances (3.114) at 30o * to the .r'axis compared with those (3.O8A) pa- For the comparison of sheet thicknesses to be in- dicative of distortions in the sheets. the thicknesses rallel to the y-axis. The most surprising anomaly should be corrected for the direct eifect of the size is the thickness of the octahedral sheet. which of the ions. Therefore the thickness quoted for ame- is greater than that in the other serpentines, site would be more comparable with those of the whereas it would be expected to be le:s. This serpentines if corrected in this way to 2.08-4. (octa- anomaly reoeives a contribution from the un- hedral) and 2.204 (tetrahedral). usually long Mg-(O,OH) bonds (2.17A) found A REAPPRAISAL OF THE STRUCTURES OF THE SERPENTINE MINERALS 239 by Kunze, but it is also due to unusually large position relative to the upper part of its own (O,OH)-Mg-OH angles at shared octahedral layer. In the original discussion of these dis- edges,which would normally be interpreted as placements,Whittaker (L956a,b) discu:sedthem due to extreme compression of th,e octahedral in terms of a displacement of Or relative to the sheet. whole of the remainder of the layer including Because of the excessivecuryaxure it would O:, and found weak evidence that this existed. be expected that the tetrahedral sheet would be Ifowever, the processwould be equally effective under tension along y but (unusually for a ser- if Oz (and ev,en Si) partook of the same dis- pentine) und,er compression along .r. This re- placement relative to the octahedral layer. The ceives support from the Si-Si distances which discussionof the effects of buckling of the Mg are 3.08A parallel to y and 3.O2A at 30o to x. sheet now provides us with a mechanism for These values are to be compared with the producing a displacemento8, of this kind. If 8 is situation in lizardite and chrysotile where there taken as positive when the displacement of Or is tension in both directions and the Si-Si dis- is in the direction away from the nearest Mg stancesdo not differ from one another by more atom in its own layer, then the tilting of the te- than 0.01A. The thickness of the tetrahedral trahedra about ax,es tbrough the Oa atoms due sheet is not significantly different from that in to repulstion of Si by Mgr leads to a positive the other serpentines,and there is no evidence displacement I amounting to about 0.2A in for any difference in e coordinates of Or and lizardite, which applies equaff to Or and Ozi Og. There is thut no evidence for tilting of the since the relative z-displacementsof Or and Oq tetrahedra, and consequently there is not even in chrysotile are about 0.2A compared with any indirect evidence for buckling of the Mg 0.4A in Izardite, we may expect a correspond- $heet. ing value of I of about *0.1A. This would lead directly to the observed overshift in chrysotile Curvature and distortion in parachrysotile 7Ma. and in alternate layers of 2.JVIa,and to the Little detail is known of the structure of para- observed undershift (becauseof the 180' rota- chrysotilg but some predictions can be based tions between layers) in2Or"'. We can, however, on the variations found in the other structures. offer no explanation of the observed undershift Parachrysotile and antigorite both curve along in the alternate lavers of 2M"r. the .r-axis so that the major compressivestress in the octahedral sheet occurs along the y-axis. DtscusstoN LNp CoNcrustoNs flowever, as in the other chrysotiles, the curv- ing of the structure partly will only relieve the The various lizardites and multi-layer ortho- mismatch, so that there will still be significant hexagonal serpentinescan now be visualized as compressive stress along the x-axis in the octa- a single lizardite family with structural varia- parachrysotile. hedral sheet of Therefore the tions, and polytypes, varying with chemical conditions are unlike antigorite and similar to composition. At the pure magnesium end of the the other chrysotiles except that the major and composition range the lizardite structure is un- minor stress directions are reversed.It is logical, der extreme compressionin the octahedral sheet therefore, expect to the variations from the and extreme stretching in the tetrahedral sheet, ideal parachrysotile in to be similu to, although producing buckling of the Mg plane, downwarp- not id,entical with, the variations found in the ing of Or and uplifting of Oz, and reducing the other particular cbrysotiles. In one would ex- layer symmetry to orthorhombic. These distor- pect buckling of the Mg sheet, with the resul- tions may be the reason that the simple 1? or tant displacement of Or to a lower e coordinate 2fl stacking fypes, usually with considerablydis- than Oz. This is consistent with the fact that ordered ! D/3 shifts, are the only ones known Whittaker (1956c) found it nec€ssaryto postu- to form under natural conditions. A synthetic late this same distortion as in the other chrvso- 6=layer polytype has been produced hydro- tiles in order to interpret the stacking in pira- thermally by Jasmund & Sylla (1971, 7972). chrysotile. The substitution of Al, as is well-known, re- lieves the misfit, first to produce bo.t = but a\d then 6o"t( bt o on the way towards the amesite OvEnsnrrr AND UNDERSHTrTrN Cnnvsorn r composition. With the reduction in the mis- Por-vrypBs match, not only do the internal structural de- formations becomesmaller and smaller until with In the discussionof chrysotile polyrypes above b*t l bt"t only simple tetrahedral rotations oc- it was pointed out that overshift or undershift cur, but also the number of possible polytypes would occur if Or were dis'placedfrom its ideal increasessignificantly becausethe better- formed 240 THE CANADIAN MINERALOGIST structurcs have more stacking possibilities than neither is understood well. In ferrous hydroxide, the deformed structures. Fg(OH),, b is9,72A so the mismatch in a Fe!+- The substitution of Fe'+ for Mg and Si has lizardite would be 0.57A, and in pyrochroite, the same effect as Al substitution, but is less Mn(OH),, b is 9.974 so the misrnatch in a Mn- effective in the octahedral sheet and more effec- llr:ar:dite would be O.82A (Donnay & Ondik tive in the tetrahedral sheet. Ferric-rich selpen- 1973). These seem too large to be overcome by tines have been recorded (L2 and I-3 of Whit- the variations found in the Mg-lizardite 1T taker & Wicks 197O; ferrian lizardrte, Ping-Wen structure. Perhaps there are vacancies in the & Che 1968), but they do not reach lh€ bo* = octahedral sheets of these minerals, with some 6t"t condition, let alone approach the ferric-iron trivalent ions to maintain charge balance, that analogue of amesite. However, it is theoretically holp to decrease the mismatch. In kellyite, the possible for (Mg,Fe'+XSiFes+)O5(OH)ato exist. Mn-analogire of amesite (Peacor et al. 1974), With greater and greater ferric substitution it and in zinalsite, the Zn-analogue of amesite io presumably possible for the lizardite structure (Chukhrov & Petrovskaia I97l) the A1 substi- to accept more and mo e ferrous iron (see S-3, tution produces a condition of bo"t( bot. Fig. 6 Whittaker & Wicks 1970) until the cron- A pure magnesium chrysotile has a fully stedtite composition, (Fez'?+Fe3+)(SiFe'+)Os(OIDastretched tetrahedral sheet and a highly but not is reached. completely compressed octahedral sheet. Thus The substitution of trivalent cations Ni3+, chrysotile could be expected to accept sorne A1 Co'+, Mn3+ and Cr8+ for Mg would also reduce or Fe3+ substitution in both sheets, but the the mismatch, but extensive substitution would compositionallimit would be exp,ectedto be con- require the substitution of Al or Fe3+ in the te- siderably less than that corresponding to D*t = trahedral sheet to balance the charges. bat sinco significant mismatch is needed to pro- Probably the only suitable divalent cation duce curving. The substitution of trivalent ca- smaller than Mg is Ni'*, and it seemsthat this tions such as Ni"+, Coe+, Mn3+ and Cf+ (to- can substitute freely for Mg. The nickel ana- gether with balancing tetrahedral Al or Fee+) logue of lizardite, nepouite (Maksimovic 1973), would ,be restdcted by the same limitations. is known and intermediate members between The substitution of Ni'+ for Mg would re- lizardite and nepouite exist (Springer 1974) sug- duce the mismatch slightly and allow a larger gesting a solid-solution series. If the b parame- "ideaf' radius of curyature to develop. The Ni ter of 4Ni(OH),.NiOOH = 9.274 (Jambor & analogue of chrysotile ZM"t (Wcoruite) has been Boyle 1964) can be taken as D for the octahedral describ,edby Faust et al. (1969) and it would sheet of nepouite, it can be seen that bo"r} b-" appear that a solid solution exists between the for the entire series.This suggeststhat the poly- two end-members. types will be limited to disordered IT or 2H The substitution of divalent cations larger under natural conditions. The nepouite (ROM than Mg is limited by the amount of curving number M18475) noted in Springer (1974) is the structure will accept. Noll et al. (1958) have the 1T polf'type. A synthetic 6-layer polytype s.ynthesizedCo-chrysotile, but attempts to syn- has been produced hydrothermally by Jasmund thesize Z* and Mn-chrysotile have failed (Roy & Sylla (1971, 1972). & Roy 1954). The observed diameters of the It would be expected that the substitution of Co-chrysotile tube w,ere smaller than the dia- Fe'+, Mnz+, Co'+o Znz+, and Cf* would be meter of synthetic Mg- and Ni-chrysotiles, as extremely limited because they would increase would be expected becauseCo2+ would have a the mismatch in a structure alreadv near the smaller radius of 'oideal" curvature than the limits of structural adjustment. The survey of other two. An ionic radius larger than Co!+ lizardite compositions (Whittaker & Wicks 1970) may well be too large to allow the chrysotile indicates that these have very low Fe'+ con- structure to form. tents. Becauseof this, it would be expectedthat The antigorite structure, like chrysotile, is no lizar'dite analogue containing large amounts neoessarily limited to compositions producing of these cations would exist. However, greena- b*r} b*t. Although Fe2+ tends to dominate lite (Steadman& Youell 1958) seemsto be the oyer F,ee+in the analyzed specimens revigwed Fe"+-analogue and caryopilite* @eacor et aI. by Whittaker & Wicks (I97O), there is no ob- 1974) seems to be the Mn-analogue, although vious reason why appr,eciableFes+ should not be present, and the compositions noted may simply reflect a reducing environment of forma' * Kato (1963) has shown that ektropite - caryopi- tion. Substitutions generally would be expected Iite, and that bementite is structurally similar to the to be subject to limitations similar to those in friedelite group of minerals. chrysotile, although in fact they seem to be less A REAPPRA]SAL OF THE STRUCTURES OF TIIE SERPBNTINE MINERALS 241

TABLE4. TRIoCTAHEDMLlil LAYER SILICATES

SERPENTINEOROUP

Serpentlne Nl-Serpentlne Fe-Serpentlne Mn-SerpentJne Zn-SerDentJne Co-Serpentine

antigorl te ???? 7 chrysotl l e pecoralte ? ? ? synthEtlc parachrysoti l e ?1?7 ? 'I I zardi te nepouite greenalite caryopilite ? ?

Substltutlon of Al in l/3 of the octahedral sites and l/2 of the tetrahedral sltes produces boct.btet and elJnlnates curved structures such as chrysotlle, antJgorJte, pecoraJte, etc.

'l aneslte ? berthlerlne kellylte zlnalsJte

SubstJtutJon of Fe- Jn l/3 of the octahedral sltes and l/2 of the tetrahedral sltes also produces boct t btet'

?cronstedtlte?7?

rigorous. Ferroan antigorites have been reported & voN KNonnnc, O. (1954): A new by Frondel (1962) and Dietrich (1972). Complete variety of antigorite (ortho-antigorite) from substit'ution by Ni'+ would be expected to be Unst, Shetland Islands. Amer. Mineral. 39, 794- possible, and a specimen with a high Ni con- 804. tent has been reported by Faust (1966). Be- Corvrnn, J. J,, Uyeoe, R. & ZussMAN, J. (1958): Electron-optical observations with crys- cause of the crosslinking between the layers in tals of antigorite. Acta Cryst. Ll, 99-102. antigorite, it is not expected to be able to form CHArMAN, J. A. & Zussrran, J. (1959): Further different polytypes, electron-optical observations on crystals of anti- Tho strucfures and chemistry of the serpen- goite. Acta Cryst. L2, 550-552. tine group of minerals discussed above, to- CnenNosrv, J. V. (1971): Minerals of the serpen- gether with some related trioctahedral 111 laver tine group. Carnegi.eInst. Yearbook 70, 153-157. silicates, are summarized in Table 4. (1975)t Aggregate refractive indices and unit cell parameters of synthetic serpentine in the system MgO-AlrOs-SiOr-HlO. Amer. Mlneral, 60, ACKNowl,EDGMENTS 200-208. Cnurnnov, F. V. & Petnovsrere, N. V. (1971): Problems ol Mineral Homogeneity and Inhomo- Thanks is extended to Mr. J. Mulock, ROM geneity. Moskva "Nauka", I92-20I. Qn Russian). Art Department, who drafted Figures I,2 and 3 Coers, C. J. A. (1968): Serpentine minerals from and the ROM Photography Department for Manitoba. Can. Mineral 9.321-347. photographing the illustrations. DIETRTcH, Y. (1972): Ilvait, Ferroantigorit und Greenalith, als Begleiter oxidisch-sulfidischer Ve- rerzungen in den Oberhalbsteiner Serpentiniten. RBpnnrNcrs Schweiz. Mineral. Petrog. Mi.tt. 52, 57-74. DoNNAy, J. D. H. & ONur, H. M. (1973): Crys- Aurvrcxro, F. (1967): A serpentine mineral show- tal Data Determinative Tables. 2, 3rd edit. U.S. ing diverse strain-relief mechanism. Amer. Min- Dept. Com. Natl. Standards and Joint Comm. on eral, 52, 1399-L413. Powder Diffraction Standards. Benrv, S. W. (1967a): Polytypism of layer silicates. FAUsr, G. T. (1966): The hydrous nickel-magrre- Short Course Lecture Notes on Layer Silicates. sium silicates - the garnierite group. Amer. Min- Amer. Geol. Inst. eral. 51, n9-298. (1967b): Crystal structure of layer sili- & Farrpv, I. J. (L962): The serpentine cates. Short Course Lecture Notes on Layer Si- group minerals. U.S. Geol. Surv. Prof. Paper licates. Amer Geol. Inst. 3844, 1-92. (1969): Polytypism of trioctahedral 1:1 -, MesoN, B. & DwoRNu(, E. J. layer silicates. C'lays Clay Minerals L7, 355-371. (1969) : Pecoraite, NiBSi4Ol0(OH)s, the nickel analog & Tvr-m, S. A. (1960): Clay minerals as- of clinochrysotile, formed in the Wolf Creek sociated with the Lake Superior iron ores. ,Ecoz. Science165' 59-60.'. Geol. 55, 150-175. meteorite. (jenkin' Bnrrornl G. W. (1967): Nomenclature, geometry FRoNDEL, C. (1962)t Ferroan antigorite of ideal layers, bonding. Short Course Lecture site). Amer. Mineral. 47, 783:785. Notes on Layer Silicates. Amer. Geol. Inst. GTLLERY,F. H. (1959): The x-ray study of synthetic 242 TIIE CANADIAN MINERALOGIST

Mg-Al sepentines and chlorites. Amer. Mineral. von Silikaten mit rtihrenformig gebauten Pri- 44, 143-t52. markristallen. Koll. Z. L57, l-Ll, JecooztNsrr, H. & KuNzs, G. (1954a): Die Rijll- Or-srN, E. J. (1961): Six-layer ortho-hexagonal ser- chenstruktur des Chrysotils. I Allgemeine Beu- pentine from the Labrador Trough. Amer. Min- gungstheorie und Kleinwinkelstreuung. N. lahrb. eral. 46, 434-438. Mineral. Monat. 95-108. Oucrro\ B. M. (1957): Order-disorder structures & _ (1954b): Die Riillchenstruk- in amesite. Acta Cryst. L0, 692-694. tur des Chrysotils. fI Weitwinkelinterferenzen. Pecr, N. J (1968): Chemical differences among the N. tahrb. Mineral. Monat. 113-130. serpentine "polymorphs". Amer. Mineral. 53" & _ (1954c): Die Riillchenstruk- 201-215. tur des Chrysotils. III Versetzungswatchstum der Pracon, D. R., EssrNE, E, J., SrvrrvroNs,W. B. & Riillchen. N. Jahrb. Mi.neral. Monat. 137-150. BrcELow, W. C. (1974): Kellyite, a new Mn-AI JAHANBAGLoo,I. C. & 7,oa-rN, T. (1968): The crys- member of the serpentine group from Bald Knob, tal structure of a hexagonal N-serynntine. Amer. N.C. Arner. Ml,neral. 60, 1153-1156. Mineral. 55, I+24, Pntc-WrN, C. & CrrB, C. (1968): Ferrolizardite -_ Jarweon, J. L. & Bovr.r, R. W. (1964): A nickel hy- a new variety of the serpentine groap. Internat. droxide mineral from Rock Creek, British Co- Geol. Rev. 10, 905-916. lumbia. Can. Mineral. 8, 116-119. Rloosr.ovrcrr, E. W. (1962): The cell dimensions JesMLrNo,K. & Syr-ra, H. M- (1971): Synthesis of atrd symmetry of layer-lattice silicates. II. Re- Mg- and Ni-antigorite. Contr. Mineral. Petrol. gression relations. Amer. Mineral. 47, 617-6A6. 34, 84-86. (1963a): The cell dimensions and sym- & _..--..---- (1972): Synthesis of Mg- and metry of layer-lattice silicates. IV. Interatomic Ni-antigorite: a correction, Contr, Mineral Pe- forces, Amer. Mineral. 48, 76-99. trol. 84, 346. (1963b): The cell dimensions atrd symme- KALousEK, G. L. & MurrART, L. E. (1957): Studies try of layerJattice silicates. VL Serpentine and on the chrysotile and antigorite components of kaolinite morphology. Amer. Mineral. 48, 368- serpentine. Amer, Mineral. 42, L-22. 378. Kero, T. (1963): New data on so-called bementite. & Nonnrsr, K. (1962): The cell dimen- J. Jap. Mineral. Petrol. Econ. Geol. 49, 93-103 sions and symmetry of layer-lattice silicates. I. (in Japanese). Abstr. Amer. Mineral. 49, 446- Some structural considerations. Amer. Mineral. 447 (t964). 47, 599-616. KnsreNovrb, J. (1968): Crystal structure of single. Rov, D. M. & Rov, R. (1954): An experimental layer lizardite. Z. Krist. 126, 163-169. study of tle formation and properties of synthe- & Pevrovra, S. (1964): X-ray study of tic serpentines and related layer silicate minerals. chrysotile. Amer. Mineral. 49, 1769-1771. Amer. Mineral. 39, 957-975. _ (1967): & X_ray study of 6_ Rucrr-mcr, J. C. & ZussvreN, J. (1965): The crys- layer ortho-serpentine. Amer. Mineral. 52, S7I- tal structure of the serpentine mineral, lizardite 876. MgrSi:Or(OH)a. Acta Cryst. Lg, 381-389. KuNza, G. (1956): Die gewellte Struktur des Anti- SrlrNcEn, G, (1974): Compositional and structural gorits, I. Z. Krist. 108, 82-107. variations in garnierites. Can. MineraL 12, 381- (1958): Die gewellte Struktur des Anti- 388. gorits, II. Z. Krist. LL0, 282-32A. StneoueN, R. (1964): The structure of the triocta- (1959): Fehlordnungen des Antigorits. Z. hedral kaolinite-type silicates. Acta Cryst. L7, Krist. lLL, 190-212. 924-927. (1961): Antigorit. Strukrurtieoretische & NurrALL, P. M. (1962\: The crystal Gru4dlagen und ihre praktische Bedeutung fiir structure of amesite. Acta Cryst. 15, 510-511. die weitere Serpentin-Forschung. Forf,rchr. Min- & - (1963): polymorphism in eral. 39,206-324. cronstedtite. Acta Cryst. 16, 1-8. - Mxsrvrovrc, Z. (1973): The isomorphous series & G964: Further polymorph- lizardite-nepouite. Zapiski V ses. Mineral. O bshch. ism in cronstedtite. Acta Cryst. 17, 404406. LAz, M3-149. (transl. from Russian by M. & Yourr.r,, R. F. (1958): Mineralogy and Fleischer, U.S. Geol. Surv.). crystal structure of greenalite, Nature 181, 45. SrrrNrrNr, H. & BnuNroN, G. (1956): The crystal Mmoratox, A. P. (1974): Crystallographic and structure of amesite. Acta Cryst, S, 487-492. Mineralogical Aspects of Serpentine. D.Phil. thesis, Oxford Univ. WmrrAKrR, E. J. W. (1952): The unit c:11 of chry- satile. Acta Cryst. 5, 143-144. Mijrr-En, P. (1963): 6-layer Serpentin vom Piz (1953)l The structure of chrysotil:. lcla Lunghin, Maloya, Schweiz. N. Jahrb. Mineral. Cryst. 6, 747-748. Abh.700, 101-111. (1954): The diffraction of .:r-rays by a NAcY, B. & Feusr, G. T. (1956): Ser,pentines:na- cylindrical lattice. I. Acta Cryst. 7, 827-832. tural mixtures of chrysotile and antigorite. Amer. (1955a): The diffraction of .r-rays by a Mineral. 41, 817-838. cylindrical lattice. II. Acta Cryst. 8, 26I-264. Nor-r,, W., Krncnrn, H. & Svarnrz, W. (1958): (1955b): The diffraction of x-rays by a Adsorptionsvermdgen und specifische Oberfldche cylindrical lattice. III. Acta Cryst. 8, 265-271. A REAPPRAISAL OF THE STRUCTURES OF THE SERPENTINE MINERALS 243

(1955c): A classification of cylindrical high resolution electron microscope, Acta Cryst. lattices. Acta Cryst. 8, 571-574. 23, 704-707. (1955d): The diffraction of r-rays by a (197t): Study of microstructure of chry- cylindrical lattice. fV. Acta Cryst. 8, 726-729. sotile asbestos by high resolution electron micro- (1956a): The structure of chrysotile. II. scopy. Acta Cryst. 427, 659-664. Clinochrysotile. Acta Cryst. 9, 855-862. Zussuex, J. (1954): Investigation of the crystal (1956b): The structure of chrysotile. III. structure of antigorite. Mineral. Mae. 30, 498- Orthochrysotile. Acta Cryst. 9, 862-864. 512. (1956c): The structure of chrysotile. IV. & BrrNor.ev, G. W. (1957): Serpentines Parachrysotile. Acta Cryst. 9, 865-867. with 6-layer ortho-hexagonal cells. Amer. Min- (1956d): The Structures of Chrysotile eral. 42, 666-670. and Crocidolite. Ph.D. thesis, London Univ. -, & Courn, J. J. (1957): Elec- (1957): The structure of chrysotile. V. tron diffraction studies of serpentine minerals. Diffuse reflections and fibre texture. Acta Cryst. Amer. Mtneral. 42, 133-153. L0, 749-156. Zwacrry B. B. (1967): Electron-Di,ffraction Anal- & Wrcrs, F. J. (1970): Chemical differ- ysis of Clay Mineral StrucTures. R. W' Fair- etrces among the serpentine "polymorphs": a bridge, ed., Plenum Press, N,Y. discussion. Amer. Mineral. 55, 1,025-1047. Mrsncnrmo, K, S. & StrTov, V. A. & ZussueN, J. (1956): The characteriza- (1966): Ordered and disordered polymorphic tion of serp€ntine minerals by x-ray diffraction. varieties of serpentine-type. minerals arrd their Mineral. Mag. 3L, 107-126. diagnosis. Soviet Phys. Cryst. L0, 539-546 (transl. Wtcrs, F. J. (1969): X-Ray and Optical Studies on from Russian). Serpentine Minerals. D.Phil. thesis, Orf,ord Univ. Yeoe, K. (1967): Study of chrysotile asbestosby a Manuscript received February 1975.