Heterogeneous Distribution of Nickel in Hydrous Silicates from New

AmericanMineralogist, V olume 70, pages 549-558, 1985

Heterogeneousdistribution of nickel in hydroussilicates from New Caledoniaore deposits

Ar,,lnr M,q,l.rcenunxo Gnoncps Clus Laborstoire de Miniralogie et Cristallographie,LA CNRS 09 Uniuersitisde Paris 6 et7 4 placeJussieu 75230 Paris Cedex05 France and Lqboratoirepour I'U tilisation ilu RayonnementElectromagnitique ( LU RE),CN R5,91405Orsay France

Abotract Four nickel-bearingclay mineralsfrom New-Caledoniabelonging to the lizardite-nepouite and the kerolite-pimelite serieshave beeninvestigated in order to study the mechanismsof Ni-Mg substitution. Local order around Ni was determined by optical absorption spectroscopy and X-ray absorption spectroscopyat the Ni-K edge.Optical spectrahave been reinterpretedthrough the Kubelka and Munk formalism which lead us to reject the optical evidencesfor the trigonal distortion of the octahedralNi site. New data were also obtained concerningMg-Ni ordering in theseminerals. Analysis of the ExtendedX-ray Absorption Fine Structure(nxers) indicates that the intracrystallinedistribution of nickel is not random: Ni atoms are segregatedinto discretedomains, the minimal size of which have been calculated and are interpreted differently dependingon whether the mineral belongs to the 7A (solid state transformed)or to the 10A (solution precipitated)structure type. This departure from ideal behavior of the Mg-Ni substitution is compared to the chemicaland structural variationsinvolving modulatedstructures. These heterogeneities seem to be quite common in low temperatureformation conditions. Introduction other phyllosilicatesare still scarce(Brindley et al., 1979). Furthermore the local order beyond the first coordination Nickel concentrationsresulting from the weatheringof shell is not known in any of thesephases and limits knowl- ultrabasic rocks under tropical conditions have been the edgeof the substitutionprocesses in theseMg-Ni minerals. subject of numerousstudies in order to understandbetter In this paper we presentfirst resultsof a systematicstudy the physico-chemicalprocesses which lead to these ore of nickel-bearingphyllosilicates (Manceau, 1984) by means bodies. Their complex mineralogy is characterizedby a of various spectroscopictechniques. Diffuse reflectanceand mixture of various hydrous silicates,often referred to as K-edge absorption spectroscopiesare used to obtain pre- "garnierites".The two main minerals(Brindley and Hang, cise crystal chemicalparameters concerning the first coor- 1913)are lizardite (serpentine)and kerolite (10A talc), the dination shell whereasExtended X-ray Absorption Fine nickeliferousend-members of which are nepouite (Maksi- Structure (nx.rns)gives data on local order at a scale of movic, 1973;Brindley and Wan, 1975)and pimelite (Mak- severalangstroms. The resultsobtained on carefullyselect- simovic, 1966; Brindley et al., 1979),respectively. Associ- ed new-caledoniansamples are used in discussingintra- ated phasesinclude smectitesand more rarely chloritesand crystalline distribution in the two main series, namely sepiolites.Intimate mixing of 10A and 7A phasesis cleaily lizardite-nepouiteand kerolite-pimelite. exhibited on X-ray diffraction patterns and has been con- Location and characterization of the studied samples firmed recently by high-resolution electron microscopy (Uyeda et al., 1973;Pelletier, 1984).It is thereforeusually The nickel ore depositsof New Caledoniahave been extensively 1984). diffrcult to obtain monomineralic phasesby mechanical investigated(Trescases, 1975; Troly et al., 1979; Pelletier, horizons may be separatedin the alteration zone: (l) the separation. Three ultrabasicparent rock, mainly of harzburgiticcomposition; (2) the Severalstudies have already been published concerning silicated zone resulting from hydrothermal alteration of this 1: 1 Ni-hydrous silicates optical absorption spectra of parent rock and consistingof primary lizardite, which was subse- (Nussik, 1969; Lakshman and Reddy, 1973; Faye, 1974) quently transformedby supergeneprocesses; (3) the lateritic zone, but only recently was the crystal chemistryof Ni in these mostly consistingof Fe-oxyhydroxides.Nickel-bearing clay min' phasesprecisely studied by thesetechniques (Cervelle and erals originate either from transformationof primary lizarditesor Maquet, 1982).These authors concluded that Ni2+ ions from solution precipitation (neoformation) in cracks. For this are in 6-fold coordination and occupy sites of C." sym- study we have selected a Mg-Fe-Ni lizardite, a pimelite and a metry in lizatdite. Spectroscopicdata concerningnickel in Mg-Ni kerolite which weresampled at Poro and Nepoui Mines in 0m3-{04x/85/0506-0549$02.00 549 MANCEAU AND CALAS: DISTRIBUTION OF NICKEL IN HYDROTJS SILICA.TE9

veins inside the silicated zone underlying the lateritic zone. The two latter samplesare characteristicof the most abundant Ni- containing neoformedminerals of the garnierites.A nepouite was also collected at Kongouhaou Mine near Thio where it occurs together with weatheredchlorite in veins cutting the silicated as well as the lateritic zones. First, all sampleswere hand-pickedwith care under a binocular microscope and homogeneousparts of the garnierites were chosen.Then, only the pure phaseswere selectedby means of powder X-ray diffraction, and characterizedby ScanningElectron Microscopy (SEM) and Transmission Electron Microscopy (TEM) associatedwith electronmicrodiffraction. Nepouite occurs as well individualizedmacrocrystallites of 0.2 to 1.0mm thickness (Fig. 1a). It is made up of regularly superimposedlayers which result in the transformation of chlorite. The internal texture of thesecrystallites was revealedby TEM to be constitutedof platy particleswhich display a mosaicstructure (Fig. lb). Becauseof the excellentcrystallinity the X-ray diffraction patterns exhibit very strong basal reflections.Contrary to the nepouite,the Mg-Fe-Ni serpentineshows tiny particles of about 4O0Asize with regular rims of a lizarditeJike mineral (Fig. tc). Kerolite and pimelite exhibit a l0A basal reflection and their behavior with ethylene glycol and heat treatments is characteristicof these minerals (Brindley et al., 1977).Aft€r treatment with ethyleneglycol for 15 hours,one cannot observea definitemaximum ofthe 001 peak; in somesamples the apparentbasal distanceexpands from 9.36A to about 16A whereasin others little expansionoccurs (Fig. 2). This differencein expansivityamong samplesdepends on severalpa- rametersincluding layercharge and stackingdisorder. Chemical analysesby atomic absorption sp€ctroscopyare reported in Table 1. The low totals of oxidesmust be attributed to the high HrO+ content occurring in theseminerals (Brindley and Wan, 1975;Brindley et al., 1979;Gerard and Herbillon, 1983).In contrast with kerolite and pimelite, lizardite contains varying amounts of iron dcpendingon its origin (Pclletier, 198a).Signifi- cant amounts of iron (i.e., more than 1.5%) indicate a primary (hydrothermal) origin followed by supergene transformation whereasiron-poor phasesare neoformed(secondary) in garnierites. The structural formulae were calculated assuming a total cation chargeof 14 per unit cell for l: I phyllosilicates,and of 22 for 2:l phyllosilicates.Tetrahedral positions are filled with Si atoms together with Al and trivalent Fe to ensurea number of two 4-fold coordinatedatoms in TO clay mineralsand four atoms in the TOT series.Octahedral sites arc filled with (Mg,Ni) atoms and with the remaining Al and Fe3+. The main featuresof these analysesare that tetrahedralcations exc€edtwo atoms per unit cell in the studied serpentinesand are slightly less than four in talc-like samples.These deviations have been repeatedlypointed out by Brindley et al. (1977, 1979)and Gerard and Herbillon (1983).They are consistentveith the assumptionof lizardite impu- rities intimately mixed with 2: 1 layers in the kerolite-pimelite seriesand with the pr€senoeof silica gelsand possibly2:l minor phasesin the lizardite-nepouiteseries (Pelletier, 1984; Manceau, 1984).

Spcctrmcopic chsracterizetion of the Ni-site Nickel crystal chemistry was studied by means of two spectroscopic techniques: diffuse reflectanc€ spectroscopy and Ni K-edge structure using synchrotron radiation. Both Pig. l. (a) Scanningelectron microscope photographs ofa mac- techniques are related in that they give the same kind of rocrystalliteof nepouitefrom Thio. Scalemarks 50 pm. (b) Trans- information concerning the first coordination shell, i.e., oxi- mission electron micrograph showing a detail of the nepouite dation state, coordination number, site distortion and layers.Scale marks 1 pm. (c) Transmissionelectron micrograph of metal-ligand covalency. The determination of the actual a Mg-Fe-Ni lizardite.Scale marks 800A. 551 MANCE,AU AND CALAS: DISTRIBLJTIONOF NICKEL IN HYDROT]SilLICATES

distortion, with significant differencesdepending on the nickel concentration in the mineral. This site symmetry agreeswith the structureproposed by Pavlovic and Krsta- novic (1980)from X-ray diffraction. Our purposeis thus to comparethe Ni-behavior in the two main hydrous silicate familiesof the garnierites.

Discussionof the optical absorption spectr&

mode, with BaSOa as a reference,in order to study the powdersalready characterizedby the previous techniques' ihe obtained reflectancemeasurements are subsequently transformedinto a remissionfunction F(R): (1 - RF/2R which is equivalent to the absorption coelficientdeduced from the Beer-Lambertlaw (Wendlandt and Hecht' 1966)' On a wavenumberbasis, the spectra may be frtted into || gaussiancomponents in order to discusssite energiesand a. ze t3 to 3' ao symmetry (Men- fossible departure from pure octahedral Fig. 2. X-Raypowder difrraction patterns of keroliteand pi- dell and Morris, 1982)' glycol(EG) and heattreatments' melite:behavior with ethylene Referencefor octahedralsymmeny: nickel hexahyiltate' CoKc radiationl" 20lmn. Optilal absorption spectrumof aqueoussolution of nickel ,uifut" i, known to be characteristicof a pure octahedral (Burns, 1970; Cotton and Wilkinson, 1968)'We site symmetry of nickel in lizardite was rec€ntlypublished Ni site recordedthe spectrumof a 0.5M solution as a basis by Cervelleand Maquet (1982)on the basisof optical ab- have sorption spectra.These authors concluded that the octahedral sites occupied by the Ni atoms exhibit a trigonal

Table 1. Chemicalanalysis and structural formulae of Mg-Fo-Ni (K) pimelite(P) lizardite(L), nepouite (N), Mg-Ni kerolite and ments of the responsibletransitions are reported in Table 2. It is to be pointed out that the intense spin-allowed transition at 8510 cm-1, which is directly relatedto the gaussianshape: this con- sto2 42.20 32.50 52.7t 45.00 crystal field splitting, has a pure Ar2o3 0,r5 1.43 Fe203 2.57 t,57 0.l r 0.25 Mgo 35.00 4.47 21.l0 3.04 Nlo 4,50 47.57 14.39 38,94 Tota1 84.42 87,6t+ 88,31 87,23

st 2.06 3.79 3.82 AI 0.0I Fe(III) 0.01 0.02 Tetr. 2,06 2.00 3.80 3.84

A1 0.0t 0.09 Fe(III) 0.09 0.07 Mc 2.55 0.41 2.26 0,38 Nl 0.18 2.14 0.83 2.66 * oct. 2,83 2.99 3.09 3,04 spectrumof aqueousnickel sulfate' ----5* Fig. 3. Optical absorption n2l + r/zn3+ Thc absorbanceis plotted versus wavenumber(crn-l). Dott€d L Poro' New Caledonia. N Thlo' New Caledonta' line: experimentalsPectrum. Solid line: gaussiandecomposition K Nepoul, ldew Caledonla. P Poro, l'leu CaLedonla' and sum of the gaussiancomponents. 552 MANCEAI] AND CALAS: DISTRIBUTIIN oF NICKEL IN HYDRIUS SILICATES

Table 2. Assignmentof absorptionbands in the visible and near- The identity of the values obtained in the kerolite- infrared obtained by decompositionof the optical spectra with pimelite seriesdemonstrates that the Mg-Ni substitution gaussrancomponents processesdo not afect the nickel crystal chemistry; this Nature of the Aqueous solutlon kerollte information is important in view of the ex.cFsevidence of transllion fron of Nl 6ulfate heterogeneoussubstitution in 3a2, theseminerals. On the other the tevel hand, the marked differencein crsr values betweennep- Energy sidth Energy oidrh ouite and Mg-Fe-Ni lizardite is explained (cn') (cn-r) by the structur_ al modification which occurs in this series at high Ni- 3rrr{ r) 8510 2484 2435 contents.One of the principal characteristicsof the serpen_ tine mineralsis a "misfit" betweenthe tetrahedral trr

z I oF z uf

z o o o t! E

2g.ooo 2o.ooo t2.ooo cM-r accuracyof the crse valuesis about *0.1 kCal/mole. The Fig. 4. Optical absorption spectntm of kerolite. The absorb_ CFSE decreasesin the following order: ance is plotted versuswavenumber (crn-r). Dotted line: experi_ mental spectrum- Solid line: gaussiandecomposition and sum of nepouite> pimelite : kerolite > lizardite. the gaussiancomponents gLICATES MANCEAU AND CALAS: DISTRIBUTION OF NICKEL IN HYDROUS 553

Table 3. Crystal chemical parameters of nickel in the investigated minerals

Ni-o distance Ni-Ni disranc" Average $to "T:l:::""irli'll, oq=o a"ru$$ b in the second shell dimension of (".-l; paranete! Kcal/nole X-ray Structural nickeliferous E\AFS I ExAFs EXAFS donains (A) diffraction fornulae b/3 ([)

(H^0). Nis0, 8s00 ,o , solution Reference 2.09 2.95 t2 t2 N io

Reference compound 8950 30.1 o t1 2.08 3.04 3,04 Ni-talc Pimelite 8950 30.7 9.14 2,O65 r.05 3.05 5,3 5.4+0,5 (P)

Nepouite 9120 31.3 9,t7 2.Oa 3.06 J.Ub 4.j 5+0.5 (N) Mg,Ni kerolite 8950 30.7 9.15 2.07 3.05 3.05 r,7 4.7+0.5 lsA-408 (K)

Mg, Ni, Fe I 8A-5oA lizardite 8845 30.4 9.20 2.09 3.O7 3.05 0.4 4.9+0.5 (L)

StO Oo = crystal Field Splitting. S$catt = crystal Iield stabilization Energy.

in Fe coordination numbr, site distortion and m€talJiSandco- energyon oxidation state and coordination number posi- valency.As the theoreticalaspects are not preciselyknown silicatesand a comparabletr€nd in Ni silicates,the yet, it is necessaryto work by comparing the studied sam- tion of this inflexion point suggeststhat Ni in these hy- ples with well known referen@compounds where these drous silicatesis divalent. The splitting of the maximum veriouseffects may be clearlyseparated. The spectra were obtained at the Laboratoire pour I'Utilisation du RayonnementSynchrotron, Orsay, France (runn) using the radiation of the DCI storage ting (1.72 GeV). The experimentalapparatus has already been described (Raoux et al., 1980).High spectral resolution is obtained with a "channel-cut" monochromator using the oUJ 400 reflection of silicon. Intrinsic limitations (coreJevel z experimentalconditions permit resolution of width) and @ 1.2 eY. However, becauseof tr featuresseparated by about o the excellentstability of the beam, it is possibleto detect (t rD relativeenergy shifts as small as 0.2 eV. Absorption K-edges of nickel in the studied clay minerals are shown in Figure 5 together with two reference samples,a 0.5 M aqueous solution of nickel sulfate and LaNiO3 (Ni3+: Crespinet al., 1983).Of the variousedge features,only the pre-edgeregion is well known. It occurs on the low energyside of the edgeat about 8326eV, which is attributed to a transition of ls electronsto 3d orbitals through a partial hybridization with the 2p oxygen ot- bitals. Its low intensity may be partly explained by the presenceof only two holesin the 3d-like levelsin the divalent nickel (3dsion) but the predominantfact is that nickel is in octahedralsites: the tetrahedralsymmetry is known to pre-edge,as was shown in Ni- enhanaesignificantly this 8320 6340 containing glasses(Petiau and Calas, 1982).The first in- ENEnGY(cv) flexion point may be usedas an indication of the oxidation Fig. 5. X-ray abeorptionrK-edge structure of Nr in 6fold coor- state of the metal: it shows a 3.8 eV shift towards the dinaiion: a) aqueous nickel sutfate (0.5 M); b) LaNiO3 (Ni3+) higher energiesin the Ni3* referencevs. the aqueoussolu- from Crespin et al., 1983;c) nepouiteand Mg-Ni-Fe lizardite; d) tions of Ni2+. Basedon the observeddependence of edge pimeliteand Mg-Ni kerolite. 554 MANCEAU AND CALAS: DISTRIBUTI1N oF NICKEL IN HYDR}US ,ILICA.TES

K-EDGE STRUCTURE

(U (J C not to vary much, in agreement with the conclusions ro reachedfrom the optical spectra.Moreover, it may be con_ € cluded C- that neither optical nor K-edge spectraprovide evi_ o dence for significant amounts of Ni3+. Trivalent nickel t4 would have given specific additional features on optical € spectra. Furthermore, the oxidation state, which cin be Energy (eV) I s00 Be00 .'pre-edge" Fig. 6. Whole recent study of the Ni peak of these samples X-ray absorption spectrumat the Ni K-edge of nepouiteshowing the edgeregion and the exAls oscillations. under high resolution conditions (Manceau and Calas, to be published)showed no shift of the maximum relative + to hexa-aquaNi2 complexesin solution. The exers is basically an interference phenomenon between the photoelectrons ejected at the absorption Distribution of nickel in the octahedral sheet edge and those which are backscattered by the various sur- The possibility of a clustered(heterogeneous) arrange_ rounding atomic shells. It may be described by a sum of ment of a certain type of cation in octahedral sheetsof darnped sinusoids which depend on the pbotoelectron phyllosilicates versus a random (homogeneous,arrange_ wavevector k according to the relationship ment cannot be evaluated by the previously used spec_ troscopic techniques,which are only sensitiveto the first I /(k) :; I Aj(k) sin (2kR,(k)+ /dk)) coordination shell.In contrast ExtendedX-ray Absorption K= Fine Structure(nnrs) which extendsto severalhundred eV abovean absorptionedge can give information about first, wherethe index j refersto the scatteringby the jth atomic second,and in certain cases,more distant shells.This tech_ shell.R, is the distancebetween the absorberatom and the nique is particularly suitablefor structural studiesofamor_ jth shell, ti is a phasefactor due to both the central and phous and poorly organizedcompounds and has beenap_ the backscatteringatoms, and A(k) is the amplitudefactor plied to various mineralogical systems(see the review of N. Calas et al., 1984).The : -+ information obtained involves in- A,ft)r' fj(k, n)s-2o?k2e-2Rtt teratomic distancesbetween the absorbing atom and the Ri, first coordination shells and the number and chemical where N, is the coordination number on the jth shell, nature of neighbors in the shell; generallythe nearestand flk, a) the backscatteringamplitude function correspond- next-nearestneighbors may be studied and in simple sys_ ing to the atomic specieson this shell, o, is the standard temssome information may also be derivedconcerning the deviation of the R, distances and,), the mean free path third coordination shell. length of the photoelectron. To ensurethe reliability of the comparison Acquisition and analysis of EXAFSspectra betweenthe studied sampleswe have used the sameparameters in the The experimental techniqueis the same as for study of analysisprocedure for all of them. First the X-ray absorp- edgestructure, although the spectralresolution in exers is tion background A(O) is removed from the experimental not as important as for study of edgestructure. For com_ spectrumby substractinga Victoreen function in a region parison purpose all the spectrawere recordedin the same beginning about 300 eV below the absorption edge.The experimental session during dedicated runs at LURE. mean absorption A(1) of the sampleabove the edgeis ob- Figure 6 showsthe X-ray absorption spectrumof nepouite tained by fitting a polynomial function. The sxAFsoscil- near the Ni K-edge.One separatesthe edgeregion which lations X(E)(Fig. 7) are obtainedfrom was studied in the previous sectionfrom the nx.cFswhich (A(E)- A(1))/(A(1)- beginsfrom about 60 eV to severalhundred eV above the r(E): A(o), absorption edge. The theory of K-ex,tFSis now firmly es_ where A(E) is the experimentalabsorbance. The energy tablished (Lee et al., 1981) and a quantitative analysisof scale(E) is subsequentlyconverted into a wavevectorscale the experimentalspectra is thus madepossible. (k), by choosing the referenceenergy Eo at the inflexion MANCEAU AND CALAS: DISTRIBUTION OF NICKEL IN HYDROUSilLICATES )))

ct) l! x tu o UJ N J tr oz

Fig. 7. Normalizednx*s of the nepouite.Normalized ln(Ig/I) is plotted versusphoton energy(eV).

point of the Ni K-edge (Eo: 8340 eV)' A Fourier trans- SECOND SHELL form is performed on the function k'xft). As the shapeof the windows used for computing this Fourier transform may strongly influenceit, we usedconstant transformation a flat window in the range 3.7A-1-ll.2l-r 1 ended"onditionr,on both sidesby a cosinefunction of 0.5A- width. The magnitudeof this Fourier transform concerningthe four mineralsstudied in the previoussection is reported in Figure 8, together with the Ni-talc (willemseite)as a refer- has a well known struc- z encecompound whoseMg analog o ture (Rayner and Brown, 1973).It is possible to get the - contribution of a definite shell to the nxlrs by backtrans- o forming the correspondingpeak on the Fourier transform tr into the k-space.Figure 9a shows the damped sinusoid o obtained by backtransformingthe secondpeak of the wil- o lemseite.The interatomic distancesare calculatedby using theoreticalphase shifts r/(k) (Teo and Lee, 1980).The validity of these phaseshifts was confirmed by studying crystalline references,namely willemseiteand bunsenite(NiO). On account of the agreementof the ExAFsdata with the actual Ni-O and Ni-Ni distancesin thesccompounds, the accuracyof the distancemeasurements is estimatedto be 0.02A; a relative precisionof about 0.01A may be attained by comparing spectra studied under identical treatment conditions. The first peak of the Fourier transform (Fig. 8) is as- sigrredto the oxygencoordination shell: the corresponding Ni-O distancesare reported in Table 3. They confirm the octahedralsite of nickel in all the studied compounds'The secondpeak correspondsto the next nearestneighbors, Ni' Fe or Mg. It is assignedto the Ni-Ni distancein NiO and Ni-talc, but we must discussin more detail the chemical Fig. 8. Comparisonof the partial distribution function (Fourier nature of the atoms constituting this shell for the minerals transformmodulus curve)of the ExAFsspectra at the Ni K-edgeof which havea more complexcomposition. five Ni-bearing clay minerals. 556 MANCEAU AND CALAS: DISTRIBU.TrcN oF NIjKEL IN HYDR)T]S ilLICATES

by Ni atoms. As b/3 is equal to the metal-metal distance, the Ni-Ni distanceswhich are calculatedby nx.lrs analysis are comparedto the bl3 valuesobtained by X-ray difrac- tion from the 06.33reflection in Table 3. The good agreement betweenboth techniquesconfirms the validity of our analysis. The study of mineralsof low Ni-content is more diflicult to perform becausesome assumptionsmust be made concerning the constitution of the secondshell around Ni as the phaseshifts ry'(k)depend on the chemicalnature of the backscatteringatom. We performedtwo calculations,first by assuming that all the atoms of the second shell are transition elements(Ni and Fe have similar backscattering phases and amplitudes and cannot be distinguished by ExAFs),then by assumingonly Mg atoms in this shell.The structural constraint is given by X-ray diffraction of these minerals which shows that the b parameterand thus the cation--cationdistances are close in the kerolite-pimelite and in the nepouite-lizarditeseries (Table 3). This may be explainedby the similar ionic radii of Mg and Ni (0.80A and O.77A, respectively). The fact that the interatomic distance is well defined allows us to overcome the indetermination about the nature of the secondnearest neighbors. This is true because the distancedetermined by rx.lrs variesdepending on the type of backscatteringatom, and inverselyit is possibleto determine its type by knowledge of the. actual distance. Distancedetermination is made by consideringthe contribution of one shell to the whole ExAFsspectrum by backtransformingthe correspondingpeak in the real spaceinto the k space(Fig. 9b) and calculatingthe zerosof this function. If we use the ExAlsformula as definedabove, a theoretical curve may be calculatedby adjustingthree parameters: the number and nature of the surroundingatoms (this latter affectsboth phaseshift and amplitudevalues) and the correspondinginter-atomic distances.Only the latter two are used for distance calculation as they determine the zeros of the sine function. The Debye-Waller parameter was fixed to 0.09A,the value determinedfor the relerence compound.A good agreementis found betweenthe experimental and calculated curves (Fig. 9b and 9d) by taking nickel (or iron) as the backscatteringatom and the Ni- (Ni,Fe) distancesdetermined from XRD (3.05-3.06A).On the contrary, it is not possibleto get a correct fit by con- sidering a secondshell only composedby Mg atoms (Fig. 9c) unlessthe Ni-Mg distanceis set at the unrealisticvalue Fig. 9. Fourier filtered contribution of the secondshell (dotted of 3.13A. Since nickel is essentiallysurrounded by heavy line) and the correspondingcalculated curve (solid line) basedon atomseven in clay mineralsof low (Fe,Ni)content, we have theoreticalphase shift and backscatteredamplitude valuesof Teo evidencefor the heterogeneous and Lee (1980):a) nepouite:6 Ni neighborsat 3.06A; b) Ni_talc: character of the substitution of Mg Ni atoms are surroundedby 6 Ni neighborsat 3.04A;c) the same by transition elements. sampleas d), but calculationsare performed by assuming6 Mg neighborsat 3.07A: the fit is clearly unacceptable; d) Mg-Fe-Ni Estimation of the size of qreas lizatdite:4.4 Ni neighborsat 3.06A. the Ni-enriched As the nature of the atoms presenton one coordination shell is known, it is possibleto get their number from the Euidence heterogeneous distribution for of nickel amplitude of the correspondingExArs. The backscattering The nickel end members of the two series under investi_ amplitude of nickel was calculated from the theoretical gation are characterized by a second shell constituted onlv values of Teo and Lee (1980)and found to be valid bv MANCEAU AND CALAS: DISTRIBUTION OF NICKEL IN HYDROUSilLICATES 557 analysisof the willemseiteas a referencecompound for the essentiallyconsist of mixtures of kerolite and pimelite at a clay minerals we investigated.The number of Ni second scaleof severallayers. This hypothesiscannot be excluded neighborsfound by ExAFsin Ni end-membersnepouite and as it would not be detectedby XRD, whereasthe optical pimelite is in good agreementwith the structural formulae spectraof both phasesare identical. (Table 3). In the phasesof low-(Ni,Fe) content it must be Ni-Mg cation ordering in the lizardite-nepouite noticed that the secondpeak has a similar intensity as for series the referencecompounds, which indicates the presenceof The two-cation ordering schemesdescribed previously predominant(Ni,Fe) atoms on the secondshell. It must be cannot apply to this series,as the coexistenceof Ni- and noticed that the differencebetween the backscattererphase MgJayers in the lizardite would give optical spectrasimilar of Mg and Ni is about z; thereforethe presenceof some to that of nepouite.The marked differencebetween both Mg atoms together with Ni atoms on the secondcoordi- spectraleads to rejectionof this hypothesis.Data available nation shell would tend to decreasethe amplitude of the point to the existenceof Ni-rich areasof small dimensions wave backscattered by the surrounding atoms. EXAFS without any structural reorganizationas it is observedin cannot separatethe contributions of heavy atoms from the nepouite.This discussionrefers to the crystallinity of light ones, as the A(k) function varies almost linearly in the minerals.From the TEM studiesand the presenceof both casesin this energy range. Apparent values are 4.4 these reflection,we deducethat there is a good * 0.5and4.2+0.5 Ni(Fe) atomsfor lizarditeand kerolite a strong 06.33 the samples in the ob planes, with a respectively.If we take into account the phaseopposition crystallinity of length ofcoherence.The Ni-containing areasare which existsbetween Ni and the Mg atoms which complete 200-300A than that value. to six neighborsthis secondshell we obtain the respective smaller valuesof 4.9+0.5and 4.7*0.5. Thesevalues must be com- Conclusion pared to thoseestimated from the structural formulae,0.38 Few data exist on the actual mechanismsof atomic sub- in lizardite and 1.66 in kerolite. We have thus additional stitution in minerals. In the case of ions with the same evidencefor the non-random distribution of nickel in the valence,the condition of equality of chargesand of ionic octahedrallayer of theseminerals. radii is not suffrcientto imply the existenceof ideal solid The comparison betweenthe number of Ni(Fe) neigh- solutionsin minerals.The chemicalheterogeneity at a scale bors calculatedfrom structural formulae and measuredby of several angstromsthat we report in Ni-clay minerals ExAFSmay be interpreted quantitatively to estimate the may be compared to the chemical variations bringing averagesize of the areaswhich are Ni( + Fe)-enriched.If we about modulated structuresshown under high resolution assumethat these latter have a circular shape and are conditions in some minerals and alloys (Buseck and devoid of Mg atoms, only the atoms at the boundariesof Cowley, 1983).However as ExAFSis not basedon any as- theseregions have Mg as neighbors.In this hypothesiswe sumption concerningthe periodic characterof the chemical get averagediameters of between204 (i.e.,2 cell units) and heterogeneities,we have no information about that param- 60A in Mg-Fe-Ni lizardite and between 15A and 4OA in eter and it is unlikely that they could be imagedby rnrru. Mg-Ni kerolite. In a model assuminga circular shapeof The structural problem is the correlation between both these segregatedareas, it must be noticed that the mean scalesof observation of these heterogeneities,the middle number of Ni atoms surrounding a central atom increases rangeorder between5 and 15A being until now impossible faster for the small dimensionsthan for the larger ones. to attain directly either with techniquessuch as nxens(be- Consequentlythese calculations are no_t?gllqreF for the causeof the limited mean free path of the photoelectrons) large dimensions.If Mg atoms are pr€sentinside theseen- or with imaging techniques.However, the existenceof riched areas,the dimensionsobtained are significantly in- chemicalheterogeneity could be related to the growth con- creased.Therefore the averagevalues given above corre- ditions, with the possibleinfluence of disequilibrium con- spond to an lower limit; the upper limit cannot be esti- ditions. The distinct behavior of 10A and 7A minerals mated but may lead to a whole layer. Furthermore,as we pointed out in this study refersalso to distinct geological haveonly accessto spatiallyaveraged values, a distribution formation conditions. The lizardite is derived from the of thesedimensions cannot be excluded. transformationof primary serpentinesby a Ni-enrichment and a subsequentMg loss.In contrast, the mineralsof the Discussion kerolite-pimelite seriesare solution precipitated products The data presentedin this study permit discussionof the and the existenceof distinct Mg- and Ni-octahedral layers nickel distribution within the two most abundant Ni- can be correlated to this distinct formation process.The containingseries of the garnierites. comparison of Ni-crystal chemistry among minerals formed under various weatheringconditions could lead to Ni-Mg cation ordering in the kerolite-pimelite series a better comprehension of heterogeneoussubstitution Two types of structural configurations may be con- mechanisms,which certainly play an important role in the sidered:(l) Ni and Mg can be clusteredwithin the octa- processesof supergeneenrichment of nickel. In summary, hedral sheetor (2) different types of Ni{Fe) sheetscan be the most important result of this study is the discoveryof a irregularly distributed in the phyllosilicatestructure. In this heterogeneousdistribution of Ni (i.e.,a clustering)in the latter model the 2: 1 phyllosilicatesof the garnieriteswould octahedralsheet ofhydrous Ni layer silicates. 558 MANCEAU AND CALAS: DISTRIBUTI1N oF NICKEL IN HYDR\IIS SILI1ATES

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