ClayMinerals (2001)36, 1–14 This paper is based on the first George Brown Lecture delivered at the Clay Minerals Group Meeting held atthe University of Exeter on 6th April 2000. Order-disorderinclaymineralstructures A. PL A N C° O N * ISTO,CNRS-Universite ´d ’Orle´ans,1A ruede la Fe´rollerie,45071 Orle´ans,Cedex 2,France (Received 6April 2000; revised 24 July 2000 ) ABSTRACT:Some recent works dealing with the concept of order-disorder in clay minerals are considered, including those aspects of order-disorder which appeared in the Brindley &Brown (1980) monograph, i.e.disorder in the distribution of cations, disorder in layer stacking, order- disorder in mixed-layer systems and finite crystal size as alattice disorder. Heterogeneity of samples and polymorphous transformations are also considered as other types of disorder. Most of these works emphasize that accurate structural characterization can only be obtained if several techniques are combined (e.g.XRDand IR, EXAFSand Mo¨ssbauer spectroscopies, etc.).Another conclusion is that accurate structural determination provides the key to the genesis of clays. K EYWORDS:order,disorder,p hyllosilicates,clay minera ls,cat iond istribution,h eterogeneity,mixed layer. Clayminerals can be described very simply by the structuralcharacterizati onof clay minerals, the stackingof two kinds of layers: 1:1 layers and 2:1 mechanismsby which they change when they are layers.In fact these layers can have a verywide subjectedto different physical or chemical stresses rangeof chemical compositions, of distributions of andthe correlation between structure and proper- isomorphouscations, and can stack in a lotof ties.The aim is rather to review the most recent differentways, including mixed layering. Their paperspublished on these topics, to show what the structuraltriperiodicity is the exception, in which ‘stateof the art ’ is. casede scriptionin volvestheorder-disorder Theclassical division of order-disorder which concept.On the other hand, these minerals have appearedinth eBrindley& Brown(1 980) theability to evolve, e.g. to change their layer or monographwill be retained here, i.e. disorder in interlayercations, to sustain polymorphous trans- distributionof cations, disorder in layer stacking, formationsby octahed ralcation migratio n,to long-rangeand short- rangeorder, order-disorder in modifythe stacking of the layers, and even to mixed-layersystems, finite crystal size as a lattice changethe type of the layers, from 1:1 into 2:1 for disorder,adding the polymorphous transformations example.The variety of clay minerals induces the andheterogeneity of the sample as two other types varietyin their properties. Accurate determination ofdisorder. ofthe structural characteristics leads to a clearer understandingof,and improv ementsin, th e THEDIFFERENTTYPESOF predictionof the properties of clays. ORDER-DISORDER Theaim of this paper is not to reconsider, in an exhaustiveway, what has been published about the Order-disorder in mixed-layersystems Serpentineto chlorite transformation. It is known thatserpentine (S) transforms into chlorite (C), and *E-mail: Alain.Plancon@univ -orleans.fr viceversa(Baile y,1 988),andthat random # 2001The Mineralogical Society 2 A. Planc° on serpentine-c hloritei nterstratificationsoccur. +a/3tetrahedralshift necessitates the repositioning Regularlyinterstra tifiedserpentine- chloritealso ofO- andOH-coordinati ngoctahedral cations, existssuch as S 1C1 (dozyite),S 2C1, S3C1, S1C2, requiringmovement of the octahedral cations from S1C3 (Banfield et al.,1994;Bailey et al., 1995). It type-IIto type-I positions. Except for the 2:1 layers, wasnot known why these long- period,regularly thestacking and octahedral slants are inherited. interstratifiedvarieties crystallize instead of discrete Conversionof smectit eintokaolin ite. The serpentineor chlorite or a randominterstratificati on existenceof mixed- layerkaolinite- smectiteshas ofthese. Normal c oulombicintera ctionsare beenknown for some time (Sudo & Hayashi, unlikelyto allow structural communicati onon 1956).UsingX-ray diffraction (XRD) theywere suchlarge scales. This problem was clarified by describedas randomly interstratified layers (Schultz Banfield& Bailey(1996) who studied several et al.,1971;Wiewiora, 1971) and as 1- 1ordered specimensfromtheWoodsChr omeMin e, interstratification(Thomas, 1989 ).Althoughthe LancasterCounty, Pennsylvania, by high- resolution directobservat ionof the individu allayers is transmissionelectron microscopy (HRTEM) witha possiblewith HRTEM, nosuch study had been pointresolution of ~0.19 nm. They observed that performedfor interlayered kaolinite-smecti teuntil regularlyinterstratified S -Carefrequently inti- recently,probably because the minerals formed matelyintergro wnwith serpent inesthat have duringa weatheringalteration process are poorly repeatdistances identical to those of the regular crystallizedand difficult to characterize. Amouric interstratification(Fig. 1). For example, dozyite &Olives(1998) reconsidered this problem, using a S1C1 isintimatel yassociatedwiththree- layer low-lightcamera equipped with an yttrium alumi- octahedralorder I,I,II (where II represen tan niumgarnet in order to minimize possible beam octahedralsheet of the serpentine layer rotated by damage.Two typesof smectite crystals were 1808 withrespect to I) .Longerperiod polymorphs observed:S, whichgives images showing only (S2C1, S1C2, S2C2, S3C2 and S1C4, all with b = 908) 1nmfringes and another, S ’ where1.25 nm fringes areeach accompanied by serpentines with equiva- arespaced evenly. In addition to these smectite lent c-axisperiodicities. They apparently form by crystals,mixed- layerkaolinite- smectitewas also theselective growth of Ib chloriteunits from I,II observed.Three samples from the Paris basin were octahedralsequences in long-periodserpentines. For studiedwith 10, 45 and 60 % kaolinlayers. The example.. I IIIIIIIII.. ? .. S S C S S C relativeabundance of S ’ layerscompared to the S ..(S2C1)or .. IIIIIIIIIII.. ? S C C S C ... layersincreases as thepercentage of K increases.In Becausepolytypes differ in their octahedral tilt the 10% and 45% kaolinitesamples the lateral sequencesthey almost certainly have different transitionsof one smectite layer into one kaolinite structuralenergies and should exhibit differential layerare visible in images, the 1 nmsmectite stability,which explains why I,II sequences are fringesprevailing in the 10 % kaolinitesample. In convertedselectively into chlorite. the 60% kaolinitesample, another type of lateral Microscopicstudies are consisten twiththe changeis also observed: the transition of a S ’ layer formationof regular interstratificati onby tetrahe- into a KS’ unit,which can be interpreted as the dralinversion within existing serpentine. Atomic partialintercal ationof one kaolinite layer in resolutionimages reveal that the tetrahedral sheet is smectite. displacedby a/3whereit inverts to form the 2:1 Two solid-statemechanisms seem to be respon- layer. A+a/3shiftis require dforhydroge n siblefor the formatio nofkaolinit e:(1) the bondingbetween OH ofthe newly- formedbrucite- transformationof 1 smectitelayer into 1 kaolinite likeinterlayer and O atomsof thenewly-formed 2:1 layer,by stripping a tetrahedralsheet and the layer.The sense of the shift is determined by the adjacentinterlayer region (Fig. 2); and (2) the stronginteraction between the octahedral cations in intercalationof 1 kaolinitelayer into smectite. thebrucite- likeinterlayer and the Si inthe 2:1 Determinationof illite-smectite structures. The layer,because direct superimposition is strongly determinationof the structure of illite- smectite(I- S) unfavourable.Distortion at the inversion point isimportant because these are common minerals probablylength ensSi Obondsin the next whosestructure changes through diagenesis, weath- tetrahedron,facilitating the relocation of Si onthe eringand hydrothermal activity. The interstratifica- otherside of the basal O plane.The reversal of the tionis usually estimated from peak migration octahedralslant in the 2:1 layer occurs because the curvesin XRD patterns(Drits & Sakharov,1976; Order-disorder in clay mineral structures 3 FIG.1. HRTEMstructural image down [100], illustrating serpentine 1:1 layers (top centre) passing smoothly into dozyite. Asterisks mark continuous layers in which the stacking sequences differ. Note that changes in stacking occur at the point of tetrahedral inversion, where two 1:1 layers pass into achlorite unit (from Banfield &Bailey, 1996). S´rodon´,1980,1981, 1984; Watanabe, 1981, 1988; orderingfor R =1,2 or3, but not for segregated I Reynolds,1981, 1988; Tomita et al.,1988; Moore orS layersor with intermed iatedegrees of &Reynolds,1989;Drits et al.,1994).This ordering.Moreover these curves have mainly been techniquecan, however, be used for two- component usedfor glycolated I- Sassumingthat all smectite I-Swith random (Reichweite R =0),or maximum interlayerscontain two glycol layers and that their 4 A. Planc° on FIG.2. Lattice fringe images of various interstratified kaolinite-smectites of the 10 % kaolinite sample.S=Stype smectite layer (1 nm thick).K=kaolinite layer (0.72 nm thick).In part c, the lateral transition S ? K is indicated by arrows (from Amouric &Olives, 1998). swellingproperties do not depend on the interlayer amountof non-swelling layers is 0.90 but 0.10 of cation.The most efficie nttechniq uefor the theselayers have d(001)= 10.33A Ê .Forall these determinationof the structural parameters is based modelsthe layer types are distributed