Complex Polytypism: Relationships Between Serpentine Structural

American Mineralogist, Volume 80, pages 1I 16-1131, 1995 Complex polytypism: Relationshipsbetween serpentine structural characteristicsand deformation Jrr,r.r.lNF. B.lNrrnr,o,Sruncrs W. B.llr,rvrt Wrrrrarr W. B,lRKEn Department of Geology and Geophysics,University of Wisconsin-Madison, 1215 West Dayton Street, Madison, Wisconsin 53706, U.S.A. Ronrnr C. Sernn II Bureau of Topographic and Geologic Survey, Department of Environmental Resources, Harrisburg, Pennsylvania 17 I 05-8453, U.S.A. AssrRAcr Serpentinite from Woods Chrome Mine, l,ancaster County, Pennsylvania, consists of planar serpentine,randomly interstratified serpentine-chlorite,a seriesofphases basedon regular interstratification ofserpentine and chlorite, minor chlorite, polygonal serpentine, and antigorite. The serpentinemineralogy is complex, including structureswith long-range order in (l) the octahedralcation sequenceand (2) the sequenceofdisplacements between adjacent 1:l layers. In addition to previously described (but generally less common) 2I, 2Ht,2H2,6R,, and 6Rrlizardite polytypes,the assemblagecontains planar serpentines with long-rangeorder in octahedral cation sequencesbut with random b/3 (and possibly a/3) displacementsbetween adjacent layers. By comparison with calculated electron diffraction intensities,we identifiedthree- (I,I,[), four- (I,I,[,II and I,I,I,II), five- (I,II,I,II,II), six- (II,I,I,I,II,II), seven-(I,II,I,II,II,II,II and I,II,I,II,I,II,II), and nine-layeroctahedral sequences.In addition, the assemblageincludes rare three- and four-layer serpentineswith regular stacking; in some casesthe stacking is nonstandard, insofar as zero and +b/3 displacementsoccur together. By comparison between electron diffraction intensities and calculated patterns, we identified a threeJayer regular stacking sequencethat involves 0,0,-b/3 displacementsbetween adjacent layers (a : 98, 0 : 90",.y : 90'), a fourJayer monoclinic sequencewith 0,-bl3,0,+b/3 (a : 90o,A :90", ? : 90"; I,I,II,II), and three four-layertriclinic sequenceswith 0,-bl3,-b/3,-b/3 (a : 90o,0 :9C', ^y: 90"),0,0,0,-bl3 (a:96",0:90, ? : 90'),and -bl3,-b/3,-b/3,+b/3(": 96",B:90',7: 9g'; displacementsbetween adjacent layers. Planar layer silicatesexhibit macroscopicpreferred orientation. The strong lineation in the foliation defined by the silicate sheetsparallels either a or b, suggestingserpentine crystals were rotated, recrystallized, or both during sheardeformation. We suggestthat for crystalswith b parallel to the lineation, deformation induced regular layer displacements.For crystals with a parallel to the lineation, periodic displacementsof OH planesmay have promoted development of regular octahedralcation sequences. Two-layer (I,II) and three-layer (I,I,II) serpentinesand randomly interstratified serpentine-chlorite contain frequent but nonperiodic planar defectsperpendicular to a* and the pseudo-a* axesthat are interpreted to displacepolytypic sequences.These defects predate chloritization and, in some cases,appear to serve as sites for chlorite nucleation. Layer silicates are crosscut by late-stagepolygonized two-layer serpentine with disordered or regular stacking. In some caseswith regular stacking, enantiomorphic 6R, segmentswith a common c-axis direction parallel to their boundary are separatedby sectors with 2H, stacking. Polygonized serpentinesare nucleated on steps at the surfacesoflayer silicates and may have developed at a late stageby recrystallization of curved serpentine. Inrnooucrrox origin. They are common products of hydrothermal al- Structural and compositional details of layer silicate teration of periodotite, lherzolite, harzburgite, etc., in assemblagesmay provide insights into the petrogenetic which primary mafic silicates are converted to assem- history of the rocks that contain them. Serpentinemin- blagesthat include chlorite, brucite, lizardite, antigorite, erals characterize greenschistfacies rocks of ultramafic chrysotile, and polygonal serpentine.They form in low- temperaturereactions near the Earth's surface.Serpentin- t DeceasedNovember 30, 1994. ization of ultramafic rocks associatedwith spreading zones 0003-004x/95/l l l 2-l l 16$02.00 1116 BANFIELD ET AL.: SERPENTINE POLYTYPISM AND DEFORMATION tt17 profoundly changestheir geophysical(magnetic suscep- related to II by a 180"rotation. Structural variations arise tibilities and seismic velocities), isotopic, and chemical when layer sequencesinvolve combinations of type I and characteristics. Serpentine minerals are constituents of II octahedra.The secondsource ofpolytypic variation is chrysotile asbestos,and thus they are also of interest be- the positioning of adjacent planar layers in one of three causeofconcern over their effectson human health. ways to ensure H bonding. The sixfold rings can exactly Although proceduresfor distinguishing standard poly- superimposealong c (zero shift) or be shifted by a/3 along is types of l: I layer silicates, micas, and chlorites are well the three pseudohexagonala axes (the senseof shift established(see Bailey, 1988a),the significanceofthe de- determined by whether cations in the underlying layers velopment of one layer silicate polytype in place of an- are type I or II) or +b/3 along the three b axes. polytype other is poorly understood. In general,factors considered Various assumptions were made in all deri- to influence polytype development include (l) the chem- vations to limit the number of possible structures-Bai- istry, temperature, and pressureduring gowth; (2) crys- ley's (1969) derivation ofthe 12 standard l:1 layer sili- tal-chemical details, including structural distortions of in- cate poly"typeswas achievedby excluding structureswith dividual layers; and (3) the growth mechanism shifts along both a and b and those with both zero and (particularly ifassociated with spiral growth on screwdis- +b/3 or +a/3 mixed in the same crystal. In the vast locations;Baronnet, I 975). majority of studies, serpentine polytypes identified by Experimentalists have attempted to define P",o-I sta- standard powder XRD techniquesare reported to be one bility fields for mica polytypes (e.9.,Yoder and Eugster, of the l2 standardpolytypes (Bailey' 1969),I Zbeing the 1955), and observational data have been used to argue most common. However, structures containing more (e.g', that specificpolytypes develop in reaction seriesthat cor- complex sequencesdo form under some conditions relate with increasing temperature (e.9., type I chlorites the 6R, polytype reportedby Steadmanand Nuttall, 1962, are replacedby the IIDb polytype during metamorphism; and the 6I' polytype inferred for Unst serpentineby Hall seeBrown and Bailey, 1962;Karpova, 19691,Curtis et al., er.al., lg76). Results presentedhere illustrate that, under 1985;for other discussionson chlorite seeHayes, 1970; some circumstances,these polytypes may be important Velde, 1965;Walker and Thompson,1990; Walker, 1993; constituents of assemblagesof l: I layer silicates' de Caritat et al., 1993). Other experimental studies on Polytypes based on +b/3 displacementsand charac- micas suggestthat the degreeof supersaturationmay in- terized by long-period regular stacking must involve more fluence stacking (Amouric and Baronnet, 1983). How- complex sequencesof displacements,including sequences (simply re- ever, results such as those of Bigi and Brigatti (1994), that intermix zero and tbl3 displacements -). which reveal numerous complex biotite polytypes in a ferred to below as 0, +, or Possiblesequences for all t/3 single assemblage,suggest that factors other than tem- three- and four-layer structures (excluding reg.ulat (1995)' perature, pressure,and saturation state must also influ- displacements)are listed by Bailey and Banfield ence polytypism. who describe the procedure to identify three- or four- O'Hanley (1991) describedvariations in serpentine- layer polytypes (regular stacking involving zero ot b/3 : mineral (lizardite, antigorite, chrysotile, and polygonal shifts, regardlessof slant of octahedra, 0 90"). Their (or serpentine)distribution with proximity to fault zonesand method involves recognition of the high-intensity the importance of faults and fractures in controlling the missing) 02/ reflections, regardlessof whether the [100]' (1995) accessof fluids neededin olivine + enstatite + serpen- [1 l0], or IT0] zone is obtained.Bailev and Banfield with tine reactions. However, a direct connection between also calculateddiffraction data for l: I layer silicates (up complex polytypism and deformation was not proposed. long-period order in the octahedral cation sequence Some corroborating evidence for such a link might be to trlr"n layers) and provided criteria on the basis of20/ inferred from microbeam X-ray diffraction (XRD) cam- intensity data to identify each sequenceuniquely. Here the era results of Wicks and Whittaker (1977), who reported we use results from this theoretical study to decipher polytypes' the common l7 polytype in pseudomorphic and nonpseu- detailed structuresof previously undescribed ser- domorphic serpentinesbut multilayer polytypes in slick- Lizardite, antigorite, and chrysotile are varieties of to ensided veins. Although it has been shown that mechan- pentine distinguished by the mechanismsthey employ ical grinding during sample preparation for XRD relieve strain associatedwith lateral misfit between the (e.g., experiments can change the chlorite polytype (Shirozu, tetrahedral and larger octahedral sheets Pauling, 1963), relationships between deformation and polytyp- 1930). Although polygonal serpentine(described

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