american mineralogist, volume 70,pages -

American Mineralogist, Volume 70, pages 549-558,1985

Heterogeneous distribution of nickel in hydrous silicates from New Caledonia ore deposits

ALAIN MANCEAU AND GEORGES CALAS

Laboratoire de Mineralogie et Cristallographie, LA CNRS 09Universites de Paris 6 et7

4 place Jussieu 75230 Paris Cedex 05 Franceand

Laboratoire pour l'Utilisation du Rayonnement Electromagnetique(LURE),CNRS,91405 Orsay France

Abstract

Four nickel-bearing clay minerals from New-Caledonia belonging to the lizardite-nepouiteand the kerolite-pimelite series have been investigated in order to study the mechanisms ofNi-Mg substitution. Local order around Ni was determined by optical absorption spec-troscopy and X-ray absorption spectroscopy at the Ni-K edge. Optical spectra have beenreinterpreted through the Kubelka and Munk formalism which lead us to reject the opticalevidences for the trigonal distortion of the octahedral Ni site. New data were also obtainedconcerning Mg-Ni ordering in these minerals. Analysis of the Extended X-ray AbsorptionFine Structure (EXAFS)indicates that the intracrystalline distribution of nickel is not random:Ni atoms are segregated into discrete domains, the minimal size of which have been calcu-lated and are interpreted differently depending on whether the mineral belongs to the 7A(solid state transformed) or to the loA (solution precipitated) structure type. This departurefrom ideal behavior of the Mg-Ni substitution is compared to the chemical and structuralvariations involving modulated structures. These heterogeneities seem to be quite common inlow temperature formation conditions.

Introduction

Nickel concentrations resulting from the weathering ofultra basic rocks under tropical conditions have been thesubject of numerous studies in order to understand betterthe physico-chemical processes which lead to these orebodies. Their complex mineralogy is characterized by amixture of various hydrous silicates, often referred to as"garnierites". The two main minerals (Brindley and Hang,1973) are lizardite (serpentine) and kerolite (lOA talc), thenickeliferous end-members of which are nepouite (Maksi-movic, 1973; Brindley and Wan, 1975) and pimelite (Mak-simovic, 1966; Brindley et aI., 1979), respectively. Associ-ated phases include smectites and more rarely chlorites andsepiolites. Intimate mixing of loA and 7A phases is clearlyexhibited on X-ray diffraction patterns and has been con-firmed recently by high-resolution electron microscopy(Uyeda et al., 1973; Pelletier, 1984). It is therefore usuallydifficult to obtain monomineralic phases by mechanicalseparation.

Several studies have already been published concerningoptical absorption spectra of 1: 1 Ni-hydrous silicates(Nussik, 1969; Lakshman and Reddy, 1973; Faye, 1974)but only recently was the crystal chemistry of Ni in thesephases precisely studied by these techniques (Cervelle andMaquet, 1982). These authors concluded that NiH ionsare in 6-fold coordination and occupy sites of C3v sym-metry in lizardite. Spectroscopic data concerning nickel in

0003-004Xj85j0506-0549$02.00

----- --

other phyllosilicates are still scarce (Brindley et aI., 1979).Furthermore the local order beyond the first coordinationshell is not known in any of these phases and limits knowl-edge of the substitution processes in these Mg-Ni minerals.

In this paper we present first results of a systematic studyof nickel-bearing phyllosilicates (Manceau, 1984) by meansof various spectroscopic techniques. Diffuse reflectance andK-edge absorption spectroscopies are used to obtain pre-cise crystal chemical parameters concerning the first coor-dination shell whereas Extended X-ray Absorption FineStructure (EXAFS)gives data on local order at a scale ofseveral angstroms. The results obtained on carefully select-ed new-caledonian samples are used in discussing intra-crystalline distribution in the two main series, namelylizardite-nepouite and kerolite-pimelite.

Location and characterization of the studied samplesThe nickel ore deposits of New Caledonia have been extensively

investigated (Trescases, 1975; Troly et al., 1979; Pelletier, 1984).Three horizons may be separated in the alteration zone: (1) theultrabasic parent rock, mainly of harzburgitic composition; (2) thesilicated zone resulting from hydrothermal alteration of thisparent rock and consisting of primary lizardite, which was subse-quently transformed by supergene processes; (3) the lateritic zone,mostly consisting of Fe-oxyhydroxides. Nickel-bearing clay min-erals originate either from transformation of primary lizardites orfrom solution precipitation (neoformation) in cracks. For thisstudy we have selected a Mg-Fe-Ni lizardite, a pimelite and aMg-Ni kerolite which were sampled at Poro and Nepoui Mines in

549

---

---~ ---------

550 MANCEAU AND CALAS: DISTRIBUTION OF NICKEL IN HYDROUS SILICATES

veins inside the silicated zone underlying the lateritic zone. Thetwo latter samples are characteristic of the most abundant Ni-containing neoformed minerals of the garnierites. A nepouite wasalso collected at Kongouhaou Mine near Thio where it occurstogether with weathered chlorite in veins cutting the silicated aswell as the lateritic zones.

First, all samples were hand-picked with care under a binocularmicroscope and homogeneous parts of the garnierites werechosen. Then, only the pure phases were selected by means ofpowder X-ray diffraction, and characterized by Scanning ElectronMicroscopy (SEM) and Transmission Electron Microscopy(TEM) associated with electron microdiffraction. Nepouite occursas well individualized macrocrystallites of 0.2 to 1.0 mm thickness(Fig. 1a). It is made up of regularly superimposed layers whichresult in the transformation of chlorite. The internal texture ofthese crystallites was revealed by TEM to be constituted of platyparticles which display a mosaic structure (Fig. Ib). Because of theexcellent crystallinity the X-ray diffraction patterns exhibit verystrong basal reflections. Contrary to the nepouite, the Mg-Fe-Niserpentine shows tiny particles of about 400A size with regularrims of a lizardite-like mineral (Fig. Ic). Kerolite and pimeliteexhibit a lOA basal reflection and their behavior with ethyleneglycol and heat treatments is characteristic of these minerals(Brindley et aI., 1977). After treatment with ethylene glycol for 15hours, one cannot observe a definite maximum of the 001 peak; insome samples the apparent basal distance expands from 9.36A toabout 16A whereas in others little expansion occurs (Fig. 2). Thisdifference in expansivity among samples depends on several pa-rameters including layer charge and stacking disorder.

Chemical analyses by atomic absorption spectroscopy are re-ported in Table 1. The low totals of oxides must be attributed tothe high H20+ content occurring in these minerals (Brindley andWan, 1975; Brindley et aI., 1979; Gerard and Herbillon, 1983). Incontrast with kerolite and pimelite, lizardite contains varyingamounts of iron depending on its origin (Pelletier, 1984). Signifi-cant amounts of iron (i.e., more than 1.5%) indicate a primary(hydrothermal) origin followed by supergene transformationwhereas iron-poor phases are neoformed (secondary) in garnier-ites. The structural formulae were calculated assuming a totalcation charge of 14 per unit cell for 1: 1 phyllosilicates, and of 22for 2: 1 phyllosilicates. Tetrahedral positions are filled with Siatoms together with AI and trivalent Fe to ensure a number oftwo 4-fold coordinated atoms in TO clay minerals and four atomsin the TOT series. Octahedral sites are filled with (Mg,Ni) atomsand with the remaining AI and FeH. The main features of theseanalyses are that tetrahedral cations exceed two atoms per unitcell in the studied serpentines and are slightly less than four intalc-like samples. These deviations have been repeatedly pointedout by Brindley et aI. (1977, 1979) and Gerard and Herbillon(1983). They are consistent with the assumption of lizardite impu-rities intimately mixed with 2: 1 layers in the kerolite-pimeliteseries and with the presence of silica gels and possibly 2: 1 minorphases in the lizardite-nepouite series (Pelletier, 1984; Manceau,1984).

Spectroscopic cbaracterization of tbe Ni-site

Nickel crystal chemistry was studied by means of twospectroscopic techniques: diffuse reflectance spectroscopyand Ni K-edge structure using synchrotron radiation. Bothtechniques are related in that they give the same kind ofinformation concerning the first coordination shell, i.e., oxi-dation state, coordination number, site distortion andmetal-ligand covalency. The determination of the actual

Fig. 1. (a) Scanning electron microscope photographs of a mac-

rocrystallite of nepouite from Thio. Scale marks 50 pm. (b) Trans-

mission electron micrograph showing a detail of the nepouitelayers. Scale marks 1 pm. (c) Transmission electron micrograph of

a Mg-Fe-Ni lizardite. Scale marks 800A.

L N K P

5102 42.20 32.60 52.71 45.00

A1203 0.15 1.43

Fe203 2.57 l. 57 O.ll 0.25

MgO 35.00 4.47 21.10 3.04

N10 4.50 47.57 14.39 38.94

Total 84.42 87.64 88.31 87.23

51 2.06 1.99 3.79 3.82

Al 0.01

FeOIl) 0.01 0.02

Tetr. 2.06 2.00 3.80 3.84

Al 0.01 0.09

Fe(UI) 0.09 0.07Mg 2.55 0.41 2.26 0.38

N1 0.18 2.34 0.83 2.66

~2.83 2.99 3.09 3.04

. . R2+ + 3/2R3+Oct.L Poro, New Caledonia. N Thio, New Caledonia.K Nepoui, New Caledonia. P Poro ~NewCaledonia.

MANCEAU AND CALAS: DISTRIBUTION OF NICKEL IN HYDROUS SILICATES 551

p K.,.1

Fig. 2. X-Ray powder diffraction patterns of kerolite and pi-melite: behavior with ethylene glycol (EO) and heat treatments.CoKa radiation 10 29/mn.

site symmetry of nickel in lizardite was recently publishedby Cervelle and Maquet (1982) on the basis of optical ab-sorption spectra. These authors concluded that the octa-hedral sites occupied by the Ni atoms exhibit a trigonal

Table 1. Chemical analysis and structural formulae of Mg-Fe-Nilizardite (L), nepouite (N), Mg-Ni kerolite (K) and pimelite (P)

distortion, with significant differences depending on thenickel concentration in the mineral. This site symmetryagrees with the structure proposed by Pavlovic and Krsta-novic (1980) from X-ray diffraction. Our purpose is thus tocompare the Ni-behavior in the two main hydrous silicatefamilies of the garnierites.

Discussion of the optical absorption spectra

Experimental. Optical spectra were obtained using aCary 17D spectrophotometer connected to a PDP 11/04computer. The data were recorded at 1.5 nm intervals byaveraging ten measurements for each step. Spectra of nick-eliferous minerals were recorded in diffuse reflectancemode, with BaS04 as a reference, in order to study thepowders already characterized by the previous techniques.The obtained reflectance measurements are subsequentlytransformed into a remission function

F(R) = (1 - R)2/2R

which is equivalent to the absorption coefficient deducedfrom the Beer-Lambert law (Wendlandt and Hecht, 1966).On a wavenumber basis, the spectra may be fitted intogaussian components in order to discuss site energies andpossible departure from pure octahedral symmetry (Men-dell and Morris, 1982).

Reference for octahedral symmetry: nickel hexahydrate.Optical absorption spectrum of aqueous solution of nickelsulfate is known to be characteristic of a pure octahedralNi site (Burns, 1970; Cotton and Wilkinson, 1968). Wehave recorded the spectrum of a 0.5M solution as a basisfor our decomposition scheme of the spectra of nickelifer-ous clay minerals (Fig. 3). The decomposition exhibits threemain components at 8510 em-I, 13868 em-I and 25460em - 1, together with spin-forbidden bands of low intensitywhich occur in the visible range. The position and assign-ments of the responsible transitions are reported in Table2. It is to be pointed out that the intense spin-allowedtransition at 8510 em - 1, which is directly related to thecrystal field splitting, has a pure gaussian shape: this con-

.~

Fig. 3. Optical absorption spectrum of aqueous nickel sulfate.The absorbance is plotted versus wavenumber (em -1). Dottedline: experimental spectrum. Solid line: gaussian decompositionand sum of the gaussian components.

--~.. ~

Energy width Energy width

(em-I) (em-I)

3T2g(F) 8510 2484 8950 2435

".'"

(

13868 2338 13575 1656

15426 1120 15377 21433TIg(F) 16302 2143 17325 3750

IT2g(D) t 21855 1461 22342 2679JAI(G) J 23900 1558

3TJ (P) 25460 3068 25800 3117


Table 2. Assignment of absorption bands in the visible and near-infrared obtained by decomposition of the optical spectra with

gaussian components

Nature of the Aqueous solution

of Nt sulfate

kerolite

transition from

the 3A2g level

firms the octahedral (Oh) point symmetry of the site. Theother transitions at higher energy are rather more complex:spin-orbit coupling may act on the splitting of the 3T1.band, as in crystalline nickel sulfates (Lakshman andJacob, 1983). We ensured that the optical spectra deducedfrom reflectance measurements through the remission func-tion F(R) give similar results in the case of non-distortedsites (nickel-doped magnesium and ammonium-magnesiumhydrous sulfates, nickel fluosilicate), mainly based on thepure gaussian shape of the absorption band related to thecrystal field.

Optical spectra of Ni-hydrous silicates. AIl the recordedspectra exhibit three absorption bands characteristic of six-fold coordinated divalent nickel (Marfunin, 1979). The cor-responding remission functions may be decomposed ac-cording to the same scheme as for the undisturbed octa-hedral site, using gaussian components with about thesame width (e.g. the kerolite spectrum: Fig. 4 and Table 2).The disagreement with the previous data (CerveIle andMaquet, 1982) arises from the use of the remission functioninstead of the raw reflectance data: these latter-althoughthey give correct values of the absorption maxima-do notallow the use of the formalism currently used for interpret-ing absorbance measurements, such as a fitting procedureinto gaussian components. It is interesting to point out thatthe optical spectra are not sensitive, under the precision ofthe Kubelka-Munk approximation, to the trigonal distor-tion of the Mg site in lizardite revealed by the structurerefinement (Pavlovic and Krstanovic, 1980; Mellini, 1982).

The crystal field splitting parameter Dq is given by theenergy of the 3

A2. -> 3T2. transition, which allows to calcu-late the Crystal-Field Stabilization Energy (CFSE)of nickelin the four studied samples (Table 3). When taking intoaccount the precision of the decomposition procedure theaccuracy of the CFSEvalues is about :to.l kCal/mole. TheCFSE decreases in the foIlowing order:

nepouite > pimelite = kerolite > lizardite.

The identity of the values obtained in the kerolite-pimelite series demonstrates that the Mg-Ni substitutionprocesses do not affect the nickel crystal chemistry; thisinformation is important in view of the EXAFSevidence ofheterogeneous substitution in these minerals. On the otherhand, the marked difference in CFSEvalues between nep-ouite and Mg-Fe-Ni lizardite is explained by the structur-al modification which occurs in this series at high Ni-contents. One of the principal characteristics of the serpen-tine minerals is a "misfit" between the tetrahedral and theoctahedral sheet. The a,b parameters of the latter beingmuch higher than those of the tetrahedral sheet, thisimplies a structural modification for juxtaposing these twosheets. In lizardite there exist two non-equivalent Mg sites,which reduces the lateral ab dimension of the octahedralsheet (Krstanovic, 1968; Pavlovic and Krstanovic, 1980;Mellini, 1982). The marked difference in the CFSEvaluesbetween nepouite and Mg-Fe-Ni lizardite may be ex-plained by a structural modification of the octahedral sheetleading to only one cation site and bringing about a de-crease of the b parameter (Cervelle and Maquet, 1982).This structural modification is possible because the NiBions are smaller than the MgB ions (0.77A and 0.80Arespectively, Whittaker and Muntus, 1970).

Structure of the Ni K-edge

The use of X-ray absorption spectra for the study ofchemically or structurally disordered systems has recentlygrown with the generalized use of synchrotron radiation.The radiation is utilized as a powerful white-beam X-raysource with various kinds of applications in mineralogy(CaIas et aI., 1984). Two distinct parts of the spectra con-tain different kinds of information: the detailed structure ofthe K (or L) edge of the studied element and the ExtendedX-Ray Absorption Fine Structure (EXAFS)which will betreated in the foIlowing section. The K-edge structure andenergy position carry information about oxidation state,

.4

zo;:o~

.2

~ii.i

'"!iwII:

28.000 20.000 12.000

Fig. 4. Optical absorption spectrum of kerolite. The absorb-ance is plotted versus wavenumber (em-I). Dotted line: experi-mental spectrum - Solid line: gaussian decomposition and sum ofthe gaussian components


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

b

Number of heavyelements (Ni, Fe)

in the second shellAveragedimension

ofnickeliferous

domains

Ni-O distance Ni-Ni distanceCFSE§§

Kcal/mole parameter

A EXAFS(A)

X-ray Structuraldiffraction EXAFS

formulae EXAFSb/3 (A)

NiS04(H20)6 8500solution

Referencecompound

NiO

29.2

ReferencecompoundNi-talc

Pimelite(P)

8950 30.7 9.13

30.7 9.148950

Nepouite(N)

Mg,Ni kerolite8950(K)

9120 31.3 9. I 7

30.7 9.15

Mg, Ni, Felizardite

(L)8845 30.4 9.20 2.09 3.07 181.-601.

§10 Dq

§§CFSECrystal Field Splitting.

Crystal Field Stabilization Energy.

coordination number, site distortion and metal-ligand co-valency. As the theoretical aspects are not precisely knownyet, it is necessary to work by comparing the studied sam-ples with well known reference compounds where thesevarious effects may be clearly separated.

The spectra were obtained at the Laboratoire pourl'Utilisation du Rayonnement Synchrotron, Orsay, France(LURE)using the radiation of the DCI storage ring (1.72GeV). The experimental apparatus has already been de-scribed (Raoux et aI., 1980). High spectral resolution isobtained with a "channel-cut" monochromator using the400 reflection of silicon. Intrinsic limitations (core-levelwidth) and experimental conditions permit resolution offeatures separated by about 1.2 eV. However, because ofthe excellent stability of the beam, it is possible to detectrelative energy shifts as small as 0.2 eV.

Absorption K-edges of nickel in the studied clay min-erals are shown in Figure 5 together with two referencesamples, a 0.5 M aqueous solution of nickel sulfate andLaNi03 (Ni3+: Crespin et aI., 1983). Of the various edgefeatures, only the pre-edge region is well known. It occurson the low energy side of the edge at about 8326 eV, whichis attributed to a transition of Is electrons to 3d orbitalsthrough a partial hybridization with the 2p oxygen or-bitals. Its low intensity may be partly explained by thepresence of only two holes in the 3d-like levels in the diva-lent nickel (3d8 ion) but the predominant fact is that nickelis in octahedral sites: the tetrahedral symmetry is known toenhance significantly this pre-edge, as was shown in Ni-containing glasses (Petiau and Calas, 1982). The first in-flexion point may be used as an indication of the oxidationstate of the metal: it shows a 3.8 eV shift towards thehigher energies in the Ni3+ reference vs. the aqueous solu-tions of Ni2+. Based on the observed dependence of edge

2.09 2.95 12 12

2.08 3.043.04

2.065 3.05 3.05 5.3 5.4+0.5

2.08 3.06 4.7 6+0.53.06

2.07 151.-401.3.05 3.06 1.7 4.7+0.5

3.06 0.4 4.9+0.5

energy on oxidation state and coordination number in Fesilicates and a comparable trend in Ni silicates, the posi-tion of this inflexion point suggests that Ni in these hy-drous silicates is divalent. The splitting of the maximum

w()z.cIS)IXooIS).c

..

"'

\\~

a

8320 8340 8380ENERGY(ev)

Fig. 5. X-ray absorption K-edge structure of Ni in 6-fold coor-

dination: a) aqueous nickel sulfate (0.5 M); b) LaNi03 (Nj3+)

from Crespin et aI., 1983; c) nepouite and Mg-Ni-Fe lizardite; d)pimelite and Mg-Ni kerolite.

--

--- - --~----

554

--

MANCEAU AND CALAS: DISTRIBUTION OF NICKEL IN HYDROUS SILICATES

observed in these latter phases may be interpreted as re-sulting from a weak site distortion, by comparison withFe-bearing minerals (Calas and Petiau, 1983; Waychunaset aI., 1983). It is to be pointed out that in nickel hexahy-drate, where the Ni coordination is a regular octahedron,the K-edge does not exhibit such a splitting. The K-absorption edge structure of the Ni-bearing minerals arevery similar. Therefore, the Ni site geometries are thoughtnot to vary much, in agreement with the conclusionsreached from the optical spectra. Moreover, it may be con-cluded that neither optical nor K-edge spectra provide evi-dence for significant amounts of NiH. Trivalent nickelwould have given specific additional features on opticalspectra. Furthermore, the oxidation state, which can bedetermined from the "chemical shift," is classically deter-mined by the energy position of the absorption slope as theenergy of the crest is also sensitive to the site symmetry.The shape of the Ni K-edge of these Ni phyllosilicates isthus fully consistent with divalent nickel. In addition, arecent study of the Ni "pre-edge" peak of these samplesunder high resolution conditions (Manceau and Calas, tobe published) showed no shift of the maximum relative tohexa-aqua Ni2+ complexes in solution.

Distribution of nickel in the octahedral sheet

The possibility of a clustered (heterogeneous) arrange-ment of a certain type of cation in octahedral sheets ofphyllosilicates versus a random (homogeneous) arrange-ment cannot be evaluated by the previously used spec-troscopic techniques, which are only sensitive to the firstcoordination shell. In contrast Extended X-ray AbsorptionFine Structure (EXAFS)which extends to several hundred eVabove an absorption edge can give information about first,second, and in certain cases, more distant shells. This tech-nique is particularly suitable for structural studies of amor-phous and poorly organized compounds and has been ap-plied to various mineralogical systems (see the review ofCalas et al., 1984). The information obtained involves in-teratomic distances between the absorbing atom and thefirst coordination shells and the number and chemicalnature of neighbors in the shell; generally the nearest andnext-nearest neighbors may be studied and in simple sys-tems some information may also be derived concerning thethird coordination shell.

Acquisition and analysis of EXAFSspectra

The experimental technique is the same as for study ofedge structure, although the spectral resolution in EXAFSisnot as important as for study of edge structure. For com-parison purpose all the spectra were recorded in the sameexperimental session during dedicated runs at LURE.Figure 6 shows the X-ray absorption spectrum of nepouitenear the Ni K-edge. One separates the edge region whichwas studied in the previous section from the EXAFSwhichbegins from about 60 eV to several hundred eV above theabsorption edge. The theory of K-EXAFSis now firmly es-tablished (Lee et aI., 1981) and a quantitative analysis ofthe experimental spectra is thus made possible.

K- E DG ESTRUCTURE

OJucro

..cL.-aV1

..c<i

EXAFS OSCillATIONS

.. f~....

:;

-' E nerg y (eVJ

8 5 0 0 8 90 0Fig. 6. Whole X-ray absorption spectrum at the Ni K-edge of

nepouite showing the edge region and the EXAFSoscillations.

The EXAFSis basically an interference phenomenon be-tween the photoelectrons ejected at the absorption edgeand those which are backscattered by the various sur-rounding atomic shells. It may be described by a sum ofdamped sinusoids which depend on the photoelectronwavevector k according to the relationship

where the index j refers to the scattering by the jth atomicshell. Rj is the distance between the absorber atom and thejth shell, t/lj is a phase factor due to both the central andthe backscattering atoms, and Aj(k) is the amplitude factor

A (k) = ~ f (k 7t)e -2G2k2e-2RJ!).jR~

j , j

J

where Nj is the coordination number on the jth shell,fj(k, 7t) the back scattering amplitude function correspond-ing to the atomic species on this shell, Gj is the standarddeviation of the Rj distances and ). the mean free pathlength of the photoelectron.

To ensure the reliability of the comparison between thestudied samples we have used the same parameters in theanalysis procedure for all of them. First the X-ray absorp-tion background A(O) is removed from the experimentalspectrum by substracting a Victoreen function in a regionbeginning about 300 eV below the absorption edge. Themean absorption A(I) of the sample above the edge is ob-tained by fitting a polynomial function. The EXAFSoscil-lations X(E)(Fig. 7) are obtained from

X(E) = (A(E) - A(I»/(A(I) - A(O»,

where A(E) is the experimental absorbance. The energyscale (E) is subsequently converted into a wavevector scale(k), by choosing the reference energy Eo at the inflexion

en .1IJ..0(XW

CWN::i0(

::i:c: -.10Z


........... \.

,,:

.

--.2

I600 EV

Fig. 7. Normalized EXAFSof the nepouite. Normalized In(lo/l) is plotted versus photon energy (eV).

point of the Ni K-edge (Eo = 8340 eV). A Fourier trans-form is performed on the function k.x(k). As the shape ofthe windows used for computing this Fourier transformmay strongly influence it, we used constant transformationconditions, a flat window in the range 3.7A -1-11.ZA-1ended on both sides by a cosine function of o.sA- 1 width.

The magnitude of this Fourier transform concerning thefour minerals studied in the previous section is reported inFigure 8, together with the Ni-talc (willemseite) as a refer-ence compound whose Mg analog has a well known struc-ture (Rayner and Brown, 1973). It is possible to get thecontribution of a definite shell to the EXAFSby backtrans-forming the corresponding peak on the Fourier transforminto the k-space. Figure 9a shows the damped sinusoidobtained by backtransforming the second peak of the wil-lemseite. The interatomic distances are calculated by usingtheoretical phase shifts t/1(k)(Teo and Lee, 1980). The valid-ity of these phase shifts was confirmed by studying crys-talline references, namely willemseite and bunsenite (NiO).On account of the agreement of the EXAFSdata with theactual Ni-0 and Ni-Ni distances in these compounds, theaccuracy of the distance measurements is estimated to beo.ozA; a relative precision of about o.olA may be attainedby comparing spectra studied under identical treatmentconditions.

The first peak of the Fourier transform (Fig. 8) is as-signed to the oxygen coordination shell: the correspondingNi-0 distances are reported in Table 3. They confirm theoctahedral site of nickel in all the studied compounds. Thesecond peak corresponds to the next nearest neighbors, Ni,Fe or Mg. It is assigned to the Ni-Ni distance in NiO andNi-talc, but we must discuss in more detail the chemicalnature of the atoms constituting this shell for the mineralswhich have a more complex composition.

FIRST"

SECOND SHELLNi-O Ni-Ni,

Fig. 8. Comparison of the partial distribution function (Fouriertransform modulus curve) of the EXAFSspectra at the Ni K-edge offive Ni-bearing clay minerals.

-- -~--

-----

556

--. - .-.----


.

b

200 600 ENERGY (e v )400

c

".. ...'

d

2 0 400 6 ENERGY(ev)

Fig. 9. Fourier filtered contribution of the second shell (dottedline) and the corresponding calculated curve (solid line) based ontheoretical phase shift and backscattered amplitude values of Teoand Lee (1980): a) nepouite: 6 Ni neighbors at 3.06A; b) Ni-talc:Ni atoms are surrounded by 6 Ni neighbors at 3.04A; c) the samesample as d), but calculations are performed by assuming 6 Mgneighbors at 3.07A: the fit is clearly unacceptable; d) Mg-Fe-Nilizardite: 4.4 Ni neighbors at 3.06A.

Evidence for heterogeneous distribution of nickel

The nickel end members of the two series under investi-gation are characterized by a second shell constituted only

by Ni atoms. As b/3 is equal to the metal-metal distance,the Ni-Ni distances which are calculated by EXAFSanalysisare compared to the b/3 values obtained by X-ray diffrac-tion from the 06.33 reflection in Table 3. The good agree-ment between both techniques confirms the validity of ouranalysis.

The study of minerals of low Ni-content is more difficultto perform because some assumptions must be made con-cerning the constitution of the second shell around Ni asthe phase shifts IjJ(k) depend on the chemical nature of theback scattering atom. We performed two calculations, firstby assuming that all the atoms of the second shell aretransition elements (Ni and Fe have similar backscatteringphases and amplitudes and cannot be distinguished byEXAFS),then by assuming only Mg atoms in this shell. Thestructural constraint is given by X-ray diffraction of theseminerals which shows that the b parameter and thus thecation-cation distances are close in the kerolite-pimeliteand in the nepouite-lizardite series (Table 3). This may beexplained by the similar ionic radii of Mg and Ni (0.80Aand O.77A, respectively).

The fact that the interatomic distance is well definedallows us to overcome the indetermination about thenature of the second nearest neighbors. This is true becausethe distance determined by EXAFSvaries depending on thetype of backscattering atom, and inversely it is possible todetermine its type by knowledge of the actual distance.Distance determination is made by considering the contri-bution of one shell to the whole EXAFSspectrum by back-transforming the corresponding peak in the real space intothe k space (Fig. 9b) and calculating the zeros of this func-tion. If we use the EXAFSformula as defined above, a theo-retical curve may be calculated by adjusting three parame-ters: the number and nature of the surrounding atoms (thislatter affects both phase shift and amplitude values) and thecorresponding inter-atomic distances. Only the latter twoare used for distance calculation as they determine thezeros of the sine function. The Debye-Waller parameterwas fixed to 0.o9A, the value determined for the referencecompound. A good agreement is found between the experi-mental and calculated curves (Fig. 9b and 9d) by takingnickel (or iron) as the backscattering atom and the Ni-(Ni,Fe) distances determined from XRD (3.05-3.06A). Onthe contrary, it is not possible to get a correct fit by con-sidering a second shell only composed by Mg atoms (Fig.9c) unless the Ni-Mg distance is set at the unrealistic valueof 3.13A. Since nickel is essentially surrounded by heavyatoms even in clay minerals of low (Fe,Ni) content, we haveevidence for the heterogeneous character of the substitu-tion of Mg by transition elements.

Estimation of the size of the Ni-enriched areas

As the nature of the atoms present on one coordinationshell is known, it is possible to get their number from theamplitude of the corresponding EXAFS.The backscatteringamplitude of nickel was calculated from the theoreticalvalues of Teo and Lee (1980) and found to be valid by


analysis of the willemseite as a reference compound for theclay minerals we investigated. The number of Ni secondneighbors found by EXAFSin Ni end-members nepouite andpimelite is in good agreement with the structural formulae(Table 3). In the phases of low-(Ni,Fe) content it must benoticed that the second peak has a similar intensity as forthe reference compounds, which indicates the presence ofpredominant (Ni,Fe) atoms on the second shell. It must benoticed that the difference between the backscatterer phaseof Mg and Ni is about n; therefore the presence of someMg atoms together with Ni atoms on the second coordi-nation shell would tend to decrease the amplitude of thewave backscattered by the surrounding atoms. EXAFScannot separate the contributions of heavy atoms from thelight ones, as the A(k) function varies almost linearly inboth cases in this energy range. Apparent values are 4.4:1:0.5and 4.2:1:0.5 Ni(Fe) atoms for lizardite and kerolite

respectively. If we take into account the phase oppositionwhich exists between Ni and the Mg atoms which completeto six neighbors this second shell we obtain the respectivevalues of 4.9:1:0.5 and 4.7 :1:0.5.These values must be com-pared to those estimated from the structural formulae, 0.38in lizardite and 1.66 in kerolite. We have thus additionalevidence for the non-random distribution of nickel in theoctahedral layer of these minerals.

The comparison between the number of Ni(Fe) neigh-bors calculated from structural formulae and measured byEXAFSmay be interpreted quantitatively to estimate theaverage size of the areas which are Ni( + Fe)-enriched. If weassume that these latter have a circular shape and aredevoid of Mg atoms, only the atoms at the boundaries ofthese regions have Mg as neighbors. In this hypothesis weget average diameters of between 20A (i.e., 2 cell units) and60A in Mg-Fe-Ni lizardite and between 15A and 40A inMg-Ni kerolite. In a model assuming a circular shape ofthese segregated areas, it must be noticed that the meannumber of Ni atoms surrounding a central atom increasesfaster for the small dimensions than for the larger ones.Consequently these calculations are noJ_~E~rate. for thelarge dimensions. If Mg atoms are present inside these en-riched areas, the dimensions obtained are significantly in-creased. Therefore the average values given above corre-spond to an lower limit; the upper limit cannot be esti-mated but may lead to a whole layer. Furthermore, as wehave only access to spatially averaged values, a distributionof these dimensions cannot be excluded.

Discussion

The data presented in this study permit discussion of thenickel distribution within the two most abundant Ni-containing series of the garnierites.

Ni-Mg cation ordering in the kerolite-pimelite series

Two types of structural configurations may be con-sidered: (1) Ni and Mg can be clustered within the octa-hedral sheet or (2) different types of Ni-(Fe) sheets can beirregularly distributed in the phyllosilicate structure. In thislatter model the 2: 1 phyllosilicates of the garnierites would

557

essentially consist of mixtures of kerolite and pimelite at ascale of several layers. This hypothesis cannot be excludedas it would not be detected by XRD, whereas the opticalspectra of both phases are identical.

Ni-Mg cation ordering in the lizardite-nepouiteseries

The two-cation ordering schemes described previouslycannot apply to this series, as the coexistence of Ni- andMg-layers in the lizardite would give optical spectra similarto that of nepouite. The marked difference between bothspectra leads to rejection of this hypothesis. Data availablepoint to the existence of Ni-rich areas of small dimensionswithout any structural reorganization as it is observed inthe nepouite. This discussion refers to the crystallinity ofthese minerals. From the TEM studies and the presence ofa strong 06.33 reflection, we deduce that there is a goodcrystallinity of the samples in the ab planes, with a200-300A length of coherence. The Ni-containing areas aresmaller than that value.

Conclusion

Few data exist on the actual mechanisms of atomic sub-stitution in minerals. In the case of ions with the samevalence, the condition of equality of charges and of ionicradii is not sufficient to imply the existence of ideal solidsolutions in minerals. The chemical heterogeneity at a scaleof several angstroms that we report in Ni-clay mineralsmay be compared to the chemical variations bringingabout modulated structures shown under high resolutionconditions in some minerals and alloys (Buseck andCowley, 1983). However as EXAFSis not based on any as-sumption concerning the periodic character of the chemicalheterogeneities, we have no information about that param-eter and it is unlikely that they could be imaged by HRTEM.The structural problem is the correlation between bothscales of observation of these heterogeneities, the middlerange order between 5 and lsA being until now impossibleto attain directly either with techniques such as EXAFS(be-cause of the limited mean free path of the photoelectrons)or with imaging techniques. However, the existence ofchemical heterogeneity could be related to the growth con-ditions, with the possible influence of disequilibrium con-ditions. The distinct behavior of loA and 7A mineralspointed out in this study refers also to distinct geologicalformation conditions. The lizardite is derived from thetransformation of primary serpentines by aNi-enrichmentand a subsequent Mg loss. In contrast, the minerals of thekerolite-pimelite series are solution precipitated productsand the existence of distinct Mg- and Ni-octahedrallayerscan be correlated to this distinct formation process. Thecomparison of Ni-crystal chemistry among mineralsformed under various weathering conditions could lead toa better comprehension of heterogeneous substitutionmechanisms, which certainly play an important role in theprocesses of supergene enrichment of nickel. In summary,the most important result of this study is the discovery of aheterogeneous distribution of Ni (i.e., a clustering) in theoctahedral sheet of hydrous Ni layer silicates.

___ ___n -.-_____


Acknowledgments

The authors wish to thank M. Gilbert Troly and MM. Esterle,Bernard Pelletier and Bernard Escande (Societe Metallurgique LeNickel, SLN) who helped us in the knowledge of the New Cale-donian ore deposits and provided facilities during the sampling.We are also grateful to L.U.R.E.for the Synchrotron Radiationfacilities, Mrs. Madeleine Gandais and M. Claude Guillemin forTEM studies and Mrs. Helene Vachey for the chemical analyses.We thank Mrs. Jacqueline Petiau and MM. Gerard Bocquier, G.E. Brown, Bernard Cervelle and Michel Semet for their kind ad-vices. This work is supported by MSTjMRI grant, Valorisationdes Ressources du Sous-Sol, Contract N°82D-0811.

ReferencesBrindley, G. W., Bish, D. L., and Wan, H. M. (1977) The nature of

kerolite, its relation to talc and stevensite. Mineralogical Maga-zine, 41, 443-452.

Brindley, G. W., Bish, D. L., and Wan, H. M. (1979) Compo-sitions, structures and properties of nickel-containing mineralsin the kerolite-pimelite series. American Mineralogist, 64, 615-625.

Brindley, G. W. and Hang, P. T. (1973) The nature of garnierites-I: Structures, chemical compositions and color characteristics.Clays and Clay Minerals, 22, 27-40.

Brindley, G. W. and Wan, H. M. (1975) Compositions, structuresand thermal behaviour of nickel-containing minerals in thelizardite-nepouite series. American Mineralogist, 60, 863--871.

Burns, R. G. (1970) Mineralogical Applications of Crystal FieldTheory. Cambridge University Press, Cambridge, England.

Buseck, Peter and Cowley, J. M. (1983) Modulated and inter-growth structures in minerals and electron microscope methodsfor their study. American Mineralogist, 68, 18-40.

Calas, Georges, Bassett, W. A., Petiau, Jacqueline, Steinberg,Michel, Tchoubar, Denise, and Zarka, Albert (1984) Mineral-ogical applications of synchrotron radiation. Physics andChemistry of Minerals, II, 17-36.

Calas, Georges and Petiau, Jacqueline (1983) Structure of oxideglasses: spectroscopic studies of local order and crys-tallochemistry. Geochemical implications. Bulletin deMineralogie, 33-55.

Cervelle, B. D. and Maquet, Michel (1982) Cristallochimie deslizardites substituees Mg-Fe-Ni par spectrometrie visible etinfra-rouge proche. Clay M;.~erals, 17,377-392.

Cotton, F. A. and Wilkinson, G. (1968) Advanced InorganicChemistry. Interscience Publishers, New York.

Crespin, Michel, Levitz, Pierre, and Gatineau, Lucien (1983) Re-duced forms of LaNi03 perovskite. Journal of the ChemicalSociety, Faraday Transactions 2, 79,1181-1194.

Faye, G. H. (1974) Optical absorption spectrum of NiH in garni-erite: a discussion. Canadian Mineralogist, 12,389-393.

Gerard, P. and Herbillon, Adrien (1983) Infrared studies of Ni-bearing clay minerals of the kerolite-pimelite series. Clays andClay Minerals, 31,143-151.

Krstanovic, P. (1968) Crystal structure of single layer lizardite.Zeitschrift fUr Kristallographie, 126, 163-169.

Lakshman, S. U. and Reddy, B. J. (1973) Optical absorption spec-trum of NiH in garnierite. Proceedings of the Indian Academyof Sciences, A, 77, 269-278.

Lakshman, S. U. and Jacob, S. A. (1983) Absorption spectra ofNiH in (NH..hMg(SO..h . 4H20. Solid State Communications,48, 563-568.

Lee, P. A., Citrin, P. H., Eisenberger, P. and Kincaid, B. M. (1981)

Extended X-ray Fine Structure: its strength and limitations as astructural tool. Review of Modern Physics, 53, 769-806.

Maksimovic, Z. (1966) Kerolite-pimelite series from Goles moun-tain, Yugoslavia. Proceedings of the International Clay Confer-ence, Jerusalem, 1,97-105.

Maksimovic, Z. (1973) Lizardite-nepouite isomorphic series. Za-piski Mineralski Obslich, 102, 143-149.

Manceau, Alain (1984) Localisation du nickel dans les phyllosili-cates: consequences sur les conditions de formation des min-erais de nickel de Nouvelle-Caledonie. These de Specialite,Universite de Paris VII.

Marfunin, A. S. (1979) Physics of Minerals and Inorganic Materi-als. Springer Verlag, Heidelberg.

Mellini, M. (1982) The crystal structure of lizardite IT:hydrogenbonds and polytypism. American Mineralogist, 67, 587-598.

Mendell, W. W. and Morris, R. V. (1982) Band quantification inreflectance spectroscopy. Lunar and Planetary Science XIII,513-514.

Nussik, S. A. M. (1969) Optical absorption spectra of Ni-bearingminerals. Izvestia Akademia Nauk SSSR, Geol. Series, 3, 108-112.

Pavlovic, S. and Krstanovic, P. (1980) Order-disorder structure inlizardite. Bulletin de I'Academie Serbe des Sciences et Arts, 20,55-58.

Pelletier, Bernard (1984) Localisation du nickel dans les minerais"garnieritiques" de Nouvelle-Caledonie. In D. Nahon, Ed., In-ternational Congress on Alteration Petrology, CNRS, Paris.Sciences Geologiques 73, 173-183.

Petiau, Jacqueline and Calas, Georges (1982) Local structureabout some transition elements in oxide glasses using X-rayabsorption spectroscopy. Journal de Physique, C9, 47-50.

Raoux, Denis, Petiau, Jacqueline, Bondot, Paul, Calas, Georges,Fontaine, Alain, Lagarde, Pierre, Levitz, Pierre, Loupias,Genevieve, and Sadoc, Anne (1980) L'EXAFS applique auxdeterminations structurales de milieux desordonnes. Revue dePhysique Appliquee, 15, 1079-1094.

Rayner, J. H. and Brown, G. (1973) The crystal structure of talc.Clays and Clay Minerals, 21,103-114.

Teo, B. K. and Lee, P. A. (1980) Ab initio calculation of amplitudeand phase function for Extended X-Ray Absorption Fine Struc-ture (EXAFS)Spectroscopy. Journal of the American ChemicalSociety, 101,2815-2830.

Trescases, Jean Jacques (1975) L'evolution geologique supergenedes riches ultrabasiques en zone tropicale. Formation des gise-ments nickeliferes de Nouvelle-Caledonie. Memoires ORSTOM,Serie Geologie, 78.

Troly, Gilbert, Esterle, M., Pelletier, Bernard, and Reibel, W.(1979) Nickel deposits in New-Caledonia: some factors influ-encing their formation. International Laterite Symposium, New-Orleans, 85-119.

Uyeda, N., Hang, P. T., and Brindley, G. W. (1973) The nature ofgarnierites II. Electron-optical study. Clays Minerals Bulletin,21,41-50.

Waychunas, G. A., Apted, M. J., and Brown, G. E. (1983) X-rayK-edge absorption spectra of Fe minerals and model com-pounds. Near-edge structure. Physics and Chemistry of Min-erals, 10, 1-9.

Wendlandt, W. M. and Hecht, Harry (1966) Reflectance spec-troscopy. Interscience Publishers, 298 p.

Whittaker, E. J. W., and Muntus, R. (1970) Ionic radii for use in

geochemistry. Geochimica et Cosmochimica Acta, 34, 945-956.

Manuscript received, Apri/26, 1984;

acceptedfor publication, December 3,1984.

american mineralogist, volume 70,pages -

Documents