Heterogeneous Distribution of Nickel in Hydrous Silicates from New
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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 spec- troscopy 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 calcu- lated 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 re- ported 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 garnier- ites. 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