nickel-bearing clay minerals: optical spectroscopic study of nickel

Clay Minerals (1985) 20, 367-387

I. N I C K E L - B E A R I N G C L A Y M I N E R A L S : O P T I C A L S P E C T R O S C O P I C S T U D Y OF

N I C K E L C R Y S T A L C H E M I S T R Y

A. M A N C E A U , G. C A L A S AND A. D E C A R R E A U *

Laboratoire de MinJralogie-Cristallographie, UA CNRS 09, Universitds Paris 6 et 7, 4 Place Jussieu. 75230Paris Cedex 05, and *Laboratoire de Gdochimie des Roches S~dimentaires, UA CNRS 0723,

Bat. 504, Universit~ de Paris-Sud, 91405 Orsay Cedex, France

(Received 25 February 1985)

A B S T R A C T: Nickel crystal chemistry was systematically studied in various phyllosilicates, mainly the natural phases selected from the 'garnierites' of the supergene ore deposits of New Caledonia. Minerals which do not usually occur in New Caledonian parageneses were synthe sised, as they could represent intermediate phases of genetic importance. In the kerolite- pimelite series, a linear relationship occurred between the ratio I(13,20~/I(02,11 ) of the hk bands and Ni-content. Diffuse reflectance spectra were used to derive the crystal chemical parameters of Ni. These confirmed its divalent character and its occupation of octahedral sites; the resulting structural distortion was slight and could not be detected in some minerals. There was no optical evidence for Ni atoms in 4-fold coordination. The two main parameters which showed significant variations among the studied phases were site distortion and crystal field stabilization energy (CFSE). Site distortion was at a maximum in trioctahedral smectites and sepiolite. CFSE depended on the mineralogy, crystallinity and chemical composition (Al-content) of the phase. Finally, clay minerals are classified according to the increasing stability of Ni in the octahedral sheet, which has been tentatively related to the geochemical distribution of this element. Secondary minerals are usually enriched vs. primary ones and among them are nepouite and kerolite which exhibit a high CFSE in contrast to sepiolite.

Although the green-coloured 'garnierites' have been studied for many years (Faust, 1966), their chemical and structural characteristics are not entirely clear. An accurate study of these hydrous Ni-Mg silicates requires investigations at various microscopic scales, from the study of the relations between the various layers to the determination of the precise nature of the sites occupied by Ni atoms.

In this paper we present some results quantifying Ni crystal chemistry, which have been derived from the study of diffuse reflectance spectra of various phyllosilicates. Special emphasis is laid on parameters such as coordination number, site distortion, nickel-oxygen bond covalency and crystal field stabilization energy (CFSE). These results should promote a better understanding of Ni geochemistry during the formation of supergene ore deposits. A subsequent paper will be devoted to the intracrystalline distribution of Ni studied by X-ray absorption spectroscopy in the same phases.

E X P E R I M E N T A L

Pure phases were selected on the basis of X-ray diffraction (XRD) and wet chemical analyses. The ignition losses of minerals previously heated at 110~ (H20 +) were

�9 1985 The Mineralogical Society

368 A Manceau et al.

determined for the kerolite-pimelite series. A Philips EM300 Transmission Electron Microscope (TEM) was used with a X-ray dispersive energy spectrometer (XDS). Diffuse reflectance spectra were obtained with a Cary 17D spectrophotometer and an integrating sphere attachment. They were recorded by taking BaSO 4 as a reference compound.

Reflectance spectra, R = Rsample/Rreference, were collected using a PDP 11/04 computer interfaced with the spectrophotometer by averaging ten measurements at 1.5 nm intervals between 1700 and 325 nm.

C H A R A C T E R I Z A T I O N OF T H E S A M P L E S

The samples were mainly collected in New Caledonia whose nickel ore deposits have been the subject of several studies (Trescases, 1975; Troly et al., 1979; Pelletier, 1983).

Kerolite-pimelite series

Chemical analyses. The chemical analyses and structural formulae are reported in Table 1. They are in fair agreement with those reported in the literature for the kerolite/pimelite

Ni Ni+Mg

1 -

0.75--

0.50-

0.25"

P 2

P1

__L~_ K,~_2

K3 K4 L 7

K 6

K5

L8 - 4 . ~

G M K1 i I I I 2 3 4 5

1(13.20)/I (02.11 .~

FIG. 1. Plot of Ni content vs. 13,20/02,11 intensity ratio in the kerolite-pimelite series (GM: sample from Goles mountain, Yugoslavia (Brindley et al., 1977)) and in lizardites.

Ni crystal chemistry in clay minerals 369

z

e~

c~

o

o

E

.'a

[ -

~ ~ ~ _

~ 6 ~ 6 6 6 6 6 ~

ooo o io~176 I i ~ q io~

~II~ I~

r o,I .~-

~4 g~

~ co

~ 1 1 4 6 6 . . ~

0 0

4 I J4 I I . ~ ~

0", ~ 0

,.,4 J 6 , , 4 [ I ~ 6 ~4 ~4

6

= ~

~ > ~

Z.~ ~ N dNm'o o

Z Z ~ ~

~~ ~.~ ~, ,-, , , 6 . . . . . . ~ o ~m

Z~ ~ '

r .-d.-d-..d rA ~

2 ~ m Z Z Z : ~ Z

•

0 0 0 0

Z Z > ~ z

370 A Manteau et al.

series (Brindley et al., 1979; Gerard & Herbillon, 1983). The deviations from ideal composition (tetrahedral atoms <4) have been variously attributed to mixing with

serpentine minerals or as an evidence for tetrahedral Ni. X-ray diffraction results. XRD patterns exhibit two broad basal reflections (001,003)

and hk bands indicate structural disorder. With increasing Ni content the 02,11 band intensity decreases and the 13,20 band intensity increases (Fig. 1), this being an indication of the presence of 6-fold Ni in the lattice. It can be accounted for by the variation of the octahedral-sheet structure factor as a function of the substitution of Mg by a transition element, as observed in synt,hetic Zn-, Co- and Ni-stevensites, and calculated for Zn-stevensites (Decarreau, 1983). The crystal!inity, calculated by the Scherrer equation, leads to mean coherent diameters ranging between 120 and 160/k, similar to those given

by Brind!ey et al. (1979) for the same type of minerals.

Lizardite-nepouite series

Chemical analyses. These are listed in Table 2 with the corresponding structural formu!ae. As already observed by Brindley & Wan (1975), the number of tetrahedral cations exceeds 2 and the octahedral content (R 2+ + 3/2R 3+) is slightly less than 3. Minor

associated phases were observed by electron microdiffraction: chrysotile, antigorite and silica gels (Fig. 2a). XDS investigation (Manceau, 1984) indicated that they were always low in Ni. The high values of SiO 2 revealed by total chemical analyses may be explained by these silica-rich gels, which were also observed by Pel!etier (1983).

TABLE 2. Chemical analyses and structural formulae oflizardites (L) and nepouite (N1). H20 + (not tabulated) ranges between 12-20%; these va!ues are slightly greater than those given by Brindley & Wan (1975).

L1 L2 L3 L4 L5 L6 L7 L8 N1

S i O 2 42.15 40.69 41'64 42.23 44.63 4 1 . 1 1 4 0 . 1 1 41.13 32.62 A I 2 0 3 - - 0.24 0.28 0.11 0.26 0.07 - - - - 0.62 F%O 3 1.47 5.00 4.61 1.93 1-91 4.36 2.39 2.43 0.45 MgO 38.64 3 3 - 1 4 3 2 . 9 5 35.60 33.70 3 1 . 0 5 24.86 20.43 2-77 NiO 2.58 2.91 4-03 4.44 5.67 5.83 13,08 18.07 51.41 Total 84.84 8 1 . 9 8 8 3 . 5 1 8 4 - 3 1 86.17 82.42 8 0 - 4 4 82.06 87.87

Si 2.03 2.05 2.06 2.06 2.13 2.08 2.15 2.21 2.03 A1 . . . . . . . . .

Fe(!II) . . . . . . . . .

Tetr. 2.03 2.05 2.06 2.06 2.13 2.08 2.15 2.21 2.03

AI - - 0.01 0,02 0-01 0.01 . . . . 0.04 Fe(III) 0.05 0,19 0.17 0.07 0.07 0.17 0.10 0.10 0.02 Mg 2-77 2-48 2-43 2-59 2.40 2.34 1-99 1.64 0.26 Ni 0.10 0.12 0.16 0.17 0-22 0.24 0.56 0.78 2.58 Oct.* 2.94 2.90 2.89 2-88 2.74 2.83 2.70 2-52 2.93

* Oct. = R 2+ + 3/2R 3+ L = lizardite N = nepouite LI Poro, Bonini, New Caledonia L2, L3 Nepoui, Kopeto, New Caledonia

L4, L5 Poro, Anniversaire, New Caledonia L6 Mea, New Caledonia L7 Poro, Constantine, New Caledonia L8 Poro, Anniversaire, New Caledonia N1 Mea, Bas Mea, New Caledonia

Ni crystal ehemistry in clay minerals 371

FIG. 2. Electron micrographs showing (a) antigorite (Ag) and silica gels (Si) occurring as impurities in lizardite samples; (b) silica gels (Si) associated with sepiolite (Sp).

X-ray diffraction results. By contrast with the kerolite-pimelite series, lizardites mainly provide patterns with hkl reflections superimposed on the hk bands, which indicates a preferential orientation (Brindley & Wan, 1975). The 13,20/02,11 intensity ratio may only be calculated where hkl reflections are absent, which occurs in samples L2, L7 and L8. This ratio exhibits a linear relationship when plotted against (Ni + Fe)/(Ni + Fe + Mg)

372 A Maneeau et al.

(Fig. 1) but with a slope different from that observed for the kerolite-pimelite series. Nepouite gives a complex pattern with very sharp basal reflections related to an ordered stacking of the layers. It occurs as a well-crystallized phase growing on chlorite crystals in veins cutting the silicified and the lateritic zone. Epitaxic growth phenomena were observed by B. Pelletier (pers. comm.) and may explain its exceptional crystallinity.

Ni-sepiolite

The sepiolite (SP) was sampled in a fracture filling of the Poro mine with associated pimelite (P2). The XRD pattern shows only the characteristic reflections of sepiolite and reveals good crystallinity, in contrast to other Ni-sepiolites previously described (Springer, 1976). Significant deviations from the ideal formula are noted (Table 1) which cannot be interpreted as being due to admixed pimelite, on account of the lower Si/Mg ratio of this phase. Electron micrographs and associated XDS studies show, as in lizardites, the presence of intimately associated amorphous silica without any significant amount of Ni (Fig. 2b).

Ni-chlorite

The chlorite exhibits a deficient octahedral sheet content (Table 1) and glycolation expands some layers, these observations both indicating a partial vermiculitization of the interlayer sheet (Brindley & Souza, 1975). The distribution of Ni-atoms among the brucitic and mica-like sheets may be estimated from the 'degree of asymmetry' on X-ray patterns (Petruk, 1964; Bailey, 1972). The value of 1.8 obtained implies that 1-9 Ni(Fe) atoms occur in the mica-like sheet and 0.1 heavy atoms in the brucitic sheet.

Ni-smectites

One Ni-rich nontronite was studied (Table 1). Exchangeable cations of the nontronite were determined by CaC12 (2 M) saturation. As half of the layer charge deficiency (--0.18) is located in the tetrahedral sheet, this sample may be described either as an iron-rich montmorillonite or as belonging to the beidellite-nontronite series. The main exchangeable cation is Mg, and only 6% of the total Ni occurs in interlayer positions. As naturally-occurring trioctahedral Ni-smectites are difficult to separate, their synthetic

TABLE 3. Structural formulae and synthesis conditions for saponites (SAP), stevensites (ST) and pimelite (P).

ST1 ST2 ST3 ST4 ST5 ST6 ST7 ST8 ST9 S T I 0 P3 SAP1 SAP2 SAP3

Si 4 4 4 4 4 4 4 4 4 4 4 3-7 3-4 3 .0 At - - - - - - 0 .3 0 .6 1-0 Ni / (Ni + Mg) 0-11 1 1 1 1 1 l I 0 .9 0 . 86 0-80 0 .56 0 .36 0 .17

Synthesis temperature (~ 75 75 75 75 25 25 25 25 75 75 160 75 75 75

Number of days in distilled

water 14 14 14 14 0 7 11 18 11 18 17 14 14 14

Mean diameter D of

crystallites (A) 38 32 29 30 20 .5 22 26 30 45 .5 58 106 42 40 42


analogues were investigated. Na-saturated stevensites and saponites were synthesised from co-precipitated base components (Table 3), according to a procedure previously described (Decarreau, 1980).

Other Ni-containing phyllosilicates

A natural amesite (A1) was collected on the rims of an altered feldspathic vein crossing a lateritic level at Col de Mouirange, Mont Dote, New Caledonia. Amesite (CH485), Ni-talc (willemseite) and monocrystalline Ni-phlogopite (M), all synthesised under hydrothermal conditions, were also investigated for comparison.

O P T I C A L A B S O R P T I O N S P E C T R O S C O P Y

Initial studies of the optical absorption spectra of Ni containing phyllosilicates established the presence of divalent 6-fold coordinated Ni in serpentine and talc (Nussik, 1969), chrysotile (Faye, 1974) and the kerolite-pimelite series (Brindley et al., 1979). The optical spectra have recently been revised on a quantitative basis, which has led to a certain amount of controversy: some investigators have stated that Ni occurs in distorted octahedral sites (Cervelle & Maquet, 1982; Julg & Julg, 1984) whereas others consider that some of the Ni atoms are in 4-fold coordination (Tejedor et al., 1983).

Analysis of the optical spectra

Diffuse reflectance spectra are not equivalent to absorbance measurements and thus it is not possible to fit the experimental curves into gaussian components. It is necessary to convert the reflectance R into a remission function F(R), according to the Kubelka-Munk formalism (Wendlandt & Hecht, 1966):

F ( R ) = (1 R)Z/2R = k/s

where k is directly proportional to the linear extinction coefficient of the Beer-Lambert law--defined as the optical density reduced to unit thickness (Simmons; 1972)--and s is a scattering coefficient mainly connected with the grain size and which varies slowly with the wavelength for finely divided powders. F(R) has been shown to provide a good representation of the true absorbance (Wendlandt & Hecht, 1966). It is thus necessary to convert the 'apparent absorbance' spectra, log (1/R), into a remission function. The optical spectra, transformed on a wavenumber basis, have been decomposed into a minimum number of gaussian components, each one corresponding to a feature, absorption band or shoulder (Mendell & Morris, 1982). As the Kubelka-Munk formalism is only an approximation, it is necessary to compare the reflectance spectra of the studied samples with those of appropriate references. For Fe-bearing samples, charge-transfer effects were assumed to depend linearly on the energy in the near infrared range and were subsequently subtracted from the experimental spectrum.

Two main crystal chemical parameters may be deduced from the analysis of the optical spectra: the strength of the crystal field, which allows calculation of the CFSE, and the Ni -O covalency which is deduced from the reduction of the interelectronic repulsion (the Racah parameter B). The methods of calculation are not reported here (but see e.g. Marfunin, 1979; Burns, 1970).


TABLE 4. Assignments of the optical transitions of the fluosilicate, Ni(OH)2 and kerolite K3.

Transition from the 3A 2g level

Fiuosiiicate Ni(OH) 2 Kerolite gaussian curves gaussian curves gaussian curves

Energy Width Energy Width Energy Width (era-') (cm -1) (era l)

3T2g(F ) 8680 2820 8800 2680 8950 2440 lEg(D) t 14 060 2680 13 240 1700 13 580 1660 3TIu(F ) J 15 9!0 1560 15 040 2580 15 380 2140

17 280 1460 17 320 3750 IT~g(D) } ~Ag(D) 22 150 I460 22 300 2680 22 340 2680

3T~g (P) i 25 600 3020 23 560 1700 23 900 1560 25 700 3080 25 800 3120

J

Octahedral references

Some nickel hexahydrate complexes have been used as references for non-distorted octahedral sites: an aqueous solution of nickel sulphate ( M a n c e a u & Calas, 1985a), hydrous fluosilicate (Pryce et al., 1964) and Ni-doped ammonium-magnesium double sulphate (Tutton salt) (Lakshman & Jacob, 1983). Nickel hydroxide served as a reference for a phyllite with slightly distorted octahedra (C3v: Szytula et al., 1971) without any tetrahedral Ni. Assignments of the optical transitions are reported in Table 4 and the crystal field parameters (site energy, distortion intensity) in Table 5.

References for a regular octahedron: nickel fluosilicate as an example. The spectrum of the fluosilicate shows three main features which are characteristic of the presence of 6-fold divalent Ni (Fig. 3):

at 8700 cm -1 (1150 nm), the 3A2g--3Tz~(F) transition (v~ band); at 14 300 cm 1 (700 nm), the 3Azx--3Tlr ) transition (v z band); at 25 000 cm ~ (400 nm), the 3Azx--3T~x(P ) transition (v~ band).

The v~ and v 3 bands have a pure gaussian shape, as in the nickel sulphate and Tutton salt: the absence of any splitting confirms that Ni occupies a regular octahedron and proves the validity of an analysis based on the remission function. The line-shape of the v~ band is particularly sensitive to the site geometry as it only depends on the crystal field splitting. The fit of the v 2 band is more difficult, owing to the spin-orbit coupling which mixes the spin-allowed and spin-forbidden transitions and to the vibronic character of these transitions, only partially resolved at room temperature (Pryce et al., 1964). Both unresolved spin-orbit and vibronic couplings are also responsible for a significant broadening of all the absorption bands (some hundreds of cm -j) as compared with the aqueous nickel sulphate and Tutton salt (Table 5).

The effect of a slight distortion: nickel hydroxide. The non-gaussian shape of the v~ band may be assigned to the trigonal distortion of the Ni-site (Fig. 3). As a fit into two gaussian components is not unique, we chose to quantify the distortion by measuring the deviation from a pure gaussian shape. In order to avoid the influence of the OH stretching band at 7200 cm -1, the total width is measured at 1/e of the gaussian maximum, and the


TABLE 5. Crystal field parameters (site energy and distortion intensity) for Ni 2+ in phyllosilicates and model compounds.

Parameters ofthe gaussian curves

Samples Intensity of D (references and Position Width the distortion parameter CFSE phyllosilicates) (cm l) (cm ~) A. kCal/Mole

Tutton salt 8850 2490 0 - - 30.3 Fluosilieate 8680 2820 0 - - 29.8 Aqueous nickel sulphate 8500 25 I0 0 - - 29.1 Ni(OH)2 8800 2460 350 - - 30.2*

Nepouite 9120 2350 0 250 31.3 Chlorite 9060 2240 0 320 31.1 Keroiite-pimeiite 8950 2320 0 140 30.7 Wiilemseite (Ni-talc) 8950 2210 0 200 30.7 Amesite 8850 2460 0 - - 30.3 Nontronite 8800 2190 0 125 30.2 Phlogopite 8820 2460 0 - - 30-2 Lizardite 8850 2350w 115 100 30.3*

SAP3 8900 2460 200 42 30.5" Saponite ~SAP2 8850 2460 270 40 30.3*

[ SAP 1 8800 2460 270 42 30.2* Sepiolite 8680 2460 190 160 29.8*

( STI0 8700 2460 190 58 29.8* ST9 8650 2460 190 45 29.7*

Stevensite | ST8 8500 2460 270 30 29.1"

\ STT-ST5 8500 2460 350 20-26 29.1"

D: mean diameter D of the crystallites; for series D *: estimated from the apparent maximum of v l w same width as in nepouite

represents average value

energy and half-width of the gauss ian curve were chosen such as to obta in a good fit on the h igh-energy side o f the v~ band F o r N i ( O H ) 2 the gauss ian is at 8800 cm -~ with a

half-width o f 2460 cm -1 (width at 1/e . . . . --- 1810 cm-1). The intensi ty of the dis tor t ion so

measured is 350 cm -1 (Table 5). This p rocedure m a y be used for a relat ive es t imat ion o f

the site d is tor t ion in phyllosi i icates, a l though a more quant i ta t ive app roach might require

the s tudy of single crystals .

A l though the opt ical spec t rum is similar to that o f the sulphate, Tu t ton salt and

fluosilicate, the relat ive intensity o f the two apparent m a x i m a of the v2 band is reversed in

the spec t rum o f N i ( O H ) v This inversion is di rect ly related to a s t ronger crys ta l field

( M a n c e a u & Calas , in preparat ion) , and is a lways observed in the phyllosi l icates.

Optical spectra of the Ni-Mg phyllosilicates

The spect ra are similar to those o f the oc tahedra l references and the cont r ibut ion o f the same componen t s m a y be recognized (Figs 3, 4). On accoun t o f the uncer ta in ty regarding

the actual coord ina t ion o f Ni in phyllosi l icates, the main character is t ics o f the optical

spect ra are discussed below in s o m e detail.


A

v LL

.5~

.3-

.1-

3 0 0 0 0

F luosi l icate

t

20 0 0 0 10 0 0 0

1.1-

9 .7-

30 0 0 0

Ni (OH) 2

20 0 0 0 10 0 0 0

. 5 ~

Kerol i te K3

ri- LL

.1-

3 0 0 0 0 2 0 0 0 0 10 0 0 0 W A V E N U M B E R , cm-1

FIG. 3. Decomposition into gausslan components of the optical absorption spectra of nickel fluosilicate, nickel hydroxide and kerolite K3.

Shape of the absorption bands. For the keroli te-pimeli te series and the nepouite, the v 1 band may be fitted with only one gaussian component whose width is similar to that of the references corresponding to a regular oc tahedron (Table 5 and Fig. 3): the distort ion of the


A n" V IJ.

Z 0 m

i - (..) Z

u..

Z O

if)

tti t r

3 0 0 0 0 2 0 0 0 0 10 0 0 0 W A V E N U M B E R , c m - 1

FIG. 4. Optical absorption spectra of nickeliferous clay minerals: nepouite (NI), stevensite (ST10), saponite (SAP3), amesite (CH485) and sepiolite (SP).

Ni site described by Cervelle & Maquet (1982) in the nepouite is not confirmed when using a remission function. This result also rules out the possibility of any tetrahedral Ni, as it would have given an additional absorption band in this energy range. No distortion


occurs likewise in chlorite, willemseite, nontronite and amesite and a good fit requires a narrower gaussian curve (Table 5). A significant broadening occurs in the spectra of the other phyllosilicates where the v~ band does not have a gaussian shape. Because of this, a procedure similar to that previously described for nickel hydroxide was chosen. For comparison purposes, the same width for the gaussian (2460 cm ~) was kept after the quality of fit on the left side of v~ was confirmed for each sample. For all samples the CFSE value was measured from the energy maximum of vv The precision of measurement is estimated at +0-1 kcal/mole for a regular octahedron. When the site is distorted an overestimation of the actual CFSE value is obtained, this increasing with the intensity of the distortion.

The shape of the v 2 band is similar to that of Ni(OH) 2. Its asymmetry increases with the CFSE in stevensites ST5 to ST10 and is a maximum for the nepouite. The direct relationship between the intensity of the high-wavenumber component and the crystal field splitting (Manceau & Calas, in preparation)--including references with only octahedral Ni--allows the hypothesis regarding the contribution of tetrahedral Ni to the shape of this band, as proposed by Tejedor et al. (1983), to be rejected. The v 3 band always shows a shoulder at 23 000-24 000 cm -~ (425 nm) which is never observed in reference compounds, except for Ni(OH) 2. Unfortunately there is no definite attribution for this feature but it may result from the splitting of the 3T(P) term arising from some site distortion (Manceau & Calas, 1985). Finally, it must be pointed out that the maximum distortion occurs in the trioctahedral smectites and sepiolite.

Energy position. The main band locations indicate a 6-fold coordinated Ni, with sites similar to the chosen references and particularly Ni(OH) 2. The presence of some tetrahedral Ni would provide supplementary features in the visible and near-infrared range, overlapping the octahedral bands, which have never been observed on the fit of the kerolites, e.g. the v~ band of K3 has a pure gaussian line-shape (Fig. 3). Moreover, the transition characteristic for tetrahedral crystal field--expected near 2200 cm-~--was not reported by Brindley et al. (1979).

Relative intensity. The relative intensity of the v 2 and v 3 bands varies among samples of the same mineral species, but their shapes remain unchanged after normalization. These variations do not correspond to significant modifications of the atomic surrounding and cannot act as a proof for 4-fold Ni.

Presence o f other transition elements

Fe-bearing minerals (Fig. 5) exhibit Ni 2+ bands similar to those of Ni-phyllites. Even when various transition elements are present in the same mineral, it is possible to recognize them on the diffuse reflectance spectra, as in the case of the Niquelandia nontronite

(Fig. 5):

octahedral Ni 1+ has absorption bands at 8800 cm -~ and 13 300 cm -~, as in the other Ni-bearing clay minerals; octahedral Fe 3+ contributes an intense charge-transfer band with a superimposed transition at 24 000 cm -~ as in nontronite (Bonnin et al., 1985); octahedral Cr 3+ contributes intense absorptions at 16 300 cm -~ and 25 300 cm i and spin-forbidden bands near 14 800 cm-~: these bands occur at similar energy in volkonskoite (Calas et al., 1984).


VOLKONSKOITE ~ Cr 3+ Fe 3+

Cr3+ N I - F e - C r ~ , t

NONTRONITE ~ % , j / " ~ : ( N T ) - ' " ~ _

GARFIELD \ NONTRONITE ~ . .

Fe3+

Ni - Fe PHLOGOPITE ~

(M)~, �9 2+ 3 + " "

CHLORITE - - ~ Ni2+ Ni 2+ (CL) ~ I

t 30000 20 000 10 000

WAVENUMBER, cm- 1

FIG. 5. Optical absorption spectra of Ni-Fe and Ni-Fe-Cr phyllosilicates.

N I C K E L C R Y S T A L C H E M I S T R Y

One of the most important results given by the optical spectra is the estimation of the CFSE: this parameter is controlled structurally by the coordination number and the metal-oxygen distance r (it varies as r 5), as well as chemically by the nature of the second shell, which affects the polarization of the oxygen ligands.


Incorporation of nickel in the mineral lattice

The location of Ni in hydrous silicates has been extensively discussed in the literature. Intimate mixtures of hydroxides or adsorption phenomena were frequently postulated (Ammou-Chokroum, 1969; Trescases, 1975; Besset & Coudray, 1978). Spectroscopic studies have shown, however, that Ni is incorporated in the lizardite structure (Faye, 1974; Cervelle & Maquet, 1982). We have used two lines of argument for this feature in the phyllosilicates here.

XRD and chemical analyses give indirect evidence of Ni substitution. In particular, the structural formulae of the investigated Mg-Ni clay minerals are consistent with the existence of complete solid solutions in the main series. Evidence for such a Ni-Mg substitution is given by the linear relationship between the Ni-content and the X-ray intensity of the 02,11 and 13,20 bands.

Diffuse reflectance spectroscopy gives a more direct indication of the nickel environment. The optical spectra of nickel oxide (Low, 1958) and hydroxide are distinct from those of the New Caledonian hydrous silicates, which allows their presence to be rejected in the present samples. Moreover the occurrence of silico-nickeloan gels will give rise to absorption bands wider than those observed (Maquet et al., 1984).

It must be pointed out that there is no argument in favour of some Ni in tetrahedral sites. The presence of some 4-fold coordinated Ni was deduced from apparent absorbanee spectra by Tejedor et al. (1983) and used to explain the anomalous swelling properties of kerolites and the tetrahedral layer-charge deficit. Spectroscopic data relying either on optical spectra or on X-ray absorption Ni-K edge structure (Manceau & Calas, 1986) do not lend support to the presence of significant amounts of tetrahedral Ni and show that this element is only 6-fold coordinated in regular or slightly distorted sites: this result is predicted by crystal field theory (Burns, 1969) because of the weaker crystal field in the tetrahedral sites.

Ni-location in minerals with more than one available site

Some doubt, however, remains about the precise location of Ni atoms in smectites (because of the presence of interlayer ions) and chlorites (two distinct octahedral sites).

Nontronite. In order to estimate the contribution of interlayer Ni to the nontronite spectrum, we recorded the optical spectrum of a montmorillonite (Camp-Berteaux) exchanged by Ni. Its CFSE is equal to that of Ni aqueous solution (29.2 kCal/Mole) and is lower than that of nontronite (30-2 kCal/Mole). This indicates that the interlayer ions do not contribute significantly to the nontronite spectrum, and therefore that nickel is mainly incorporated in the octahedral sheet.

Chlorite. In their refinement of the Mg-chlorite structure, Brown & Bailey (1963) demonstrated that octahedral sites have a similar size in both micaceous and brucitic sheets, with the metal-ligand distances of 2.078 + 0.006 & and 2-068 _+ 0.0006 A, respectively. In brucite these distances are significantly greater, 2. 102 _+ 0.003 A (Zigan & Rothbauer, 1967). On the basis of the similarity of the ionic radii of Mg 2§ and Ni 2+, we assume that the metal-oxygen distances are preserved in the Ni-substituted magnesium hydroxide and remain different from those in the brucitic sheet of the Ni-chlorite. As a consequence, the optical spectrum of both compounds--Ni(OH) 2 and chlorite--cannot be compared. The spectra of chlorite, muscovite and kerolite are similar: this could be an


indication of the presence of Ni in the micaceous sheet, which is consistent with XRD results. A distinct crystal field intensity may be expected between both sheets if the different effective charges of the oxygens, which are more polarized in the micaceous sheet, are taken into account. Such a different crystal field was observed in chromium-bearing minerals (Calas et al., 1984); Cr-clinochlore has only brucitic Cr and a higher CFSE than volkonskoite (Cr-smectite) and Cr-muscovite.

Variation of nickel crystal chemistry

Both CFSE and site distortion exhibit significant differences among the studied phases (Fig. 6). They demonstrate that the characteristics of the octahedral sheet may be affected by several parameters: Ni- and Al-content, crystallinity, and nature of the phase. On the contrary, the covalency of the Ni-O bond--measured by the Racah parameter B--remains the same among all the studied minerals, with the noticeable exception of the amesite. In phyllosilicates, the B value is reduced by only 14% compared with the free ion, which confirms a high degree of ionicity of the Ni-O bonding.

ii

.. ,= i

r o

; \ �9 I

/ - "SP

N .1.7 t P2

:" " ~ Aqueous \ ." Ni 2 + N 1~ '

�9

/ \ , '

J w ~ r ,

10 0 0 0 8 0 0 0

W A V E N U M B E R , r

FXG. 6. Normalized crystal field band of nepouite (N1), pimelite (P2), sepiolite (SP) and 0.5 M aqueous solution of nickel sulphate.


Nickel and aluminium content. Mg-Ni substitution was studied in the following series: lizardite-nepouite, kerolite-pimelite (including synthetic pimelite: P3), Mg-Ni stevensite (with similar crystallinity: ST1 to ST4). With the exception o f nepouite, the line-shape and energy of the v~ band remain unchanged with a given series whatever its Ni content: the CFSE is a characteristic parameter for a Mg-Ni mineral family. The CFSE is higher in nepouite than in lizardite whereas the site seems no more distorted (Table 5). Such a result confirms the model proposed by Cervelle & Maquet (1982) in which high Ni-Mg substitution reduces the lateral misfit between tetrahedral and octahedral sheets, leading to a structural reorganization of the latter and to a unique 6-fold site (Krstanovic, 1968).

Si-A1 substitution in synthetic saponites increases CFSE (Table 5): this may be explained by a decrease of the Ni -O distances as the tetrahedral A1 content increases. This distance reduction is not accompanied by an increase of the effective charge of the apical oxygens, as the Ni-(O,OH) covalency remains the same. In contrast, amesites show a 5% higher B parameter, which indicates a more ionic Ni-(O,OH) bond compared with some Aloet-OH bonds. This result agrees with the lower OH stretching vibration frequencies reported in amesites (Heller-Kallai et al., 1975; Serna et al., 1979).

Crystallinity. The influence of crystallinity was studied in the synthetic stevensites, for which XRD results allowed a detailed analysis of the processes of lattice organization to be carried out. Both CFSE and site symmetry increase with crystallinity up to 60 • (which corresponds to the highest synthesis temperature). CFSE might help to enhance the crystallinity of natural samples; the fact that their crystalline coherence is always greater than 60 A also supports this assumption (Table 5).

Site distortion. No distortion was detected in some phases (Table 5): the v 1 band has a

I s a p o n l t e i n c r e a s i n g AI i

N I - F e - M g ~ t ~

t Ni 1 . . . . . . . [ - - ] ' t e

N I - M g

I"'

I"" r "'t'l .,,o.,2 J

1 310 I 311 I 29.5 30,5 31.5 s t a b i l i z a t i o n e n e r g y

Kcal/mole

FIG. 7. Measured CFSE of Ni :+ ions in the phyllosilicates. The relative precision is estimated to 0-1 kCal/Mole--as Ni occupies a regular or slightly distorted octahedron--and decreases with increasing distortion. For comparison purposes the CFSE of the well-crystallized Ni

stevensite is reported.


pure gaussian shape with a smaller half-width than the references (however, it must be noted that spin-orbit coupling broadens the fluosilicate spectrum). In the other mineral groups the intensity of the distortion was estimated from the width enlargement of the infrared band and reported in Table 5. It is most noticeable in the trioctahedral smectites and in sepiolite. Although this is not understood in the former, the peculiar structure of the latter may explain this result: some six-fold coordinated Ni 2§ ions are attached to the outer edge of the ribbons and are thus coordinated with four apical oxygens and two molecular water molecules.

CFSE. The energy shift of the vz band vs. the nature of the phase is displayed on Fig. 6. CFSE values obtained for all the samples are reported in Table 5 and Fig. 7. They increase in the following order:

stevensite, sepiolite < saponite, lizardite, nontronite < kerolite < chlorite < nepouite.

Relationship to Mg-Ni partition coefficients

The partitioning of first-row transition elements M 2+ (M = Ni, Co, Zn, Fe, Cu) between magnesian smectites (stevensites and saponites) and water has been studied previously (Decarreau, 1983, 1985). The equilibrium between Mg-stevensite and Ni-bearing solution can be written on a basis of 1 ion/gram of Ni(Mg)

(Si4 /3MgOl0/3OH2/3) + Ni 2+ = (Si4/3NiO10/3OH2/3) + M g 2+.

The layer charge of the stevensite is very small and can be neglected. The partition coefficient

(Ni/Mg) solid DtNi_Mg ) - - (Ni/Mg)water

has a high value (log D(Ni_Mg ) = 3.0), indicating a high stabilization of Ni in the octahedral sites of the smectites. When a given experiment was repeated (up to six times) the greatest variation was +0.09. On this basis the partition coefficient is assumed to be identical for stevensite and saponite and independent of the smectite crystallinity.

These results do not seem to agree with the experimentally obtained CFSE, which increases with the crystallinity of clays, and from stevensite to saponite. We therefore attempted to calculate the increase of D(Ni_Mg ) a s a function of the variation of the CFSE of Ni 2+. The partition coefficient is associated with a variation of the free energy given by

A G~ ........ = - -RT log O(Ni_Mg ) z --4.1 kCal/Mole.

The largest difference of CFSE among minerals of similar crystallinity is 0.8 kCal/Mole (ST9 and SAP3: Table 5). Consequently the free energy of formation of the saponite (SAP3) will be lowered, with respect to stevensite, in proportion to the Ni atoms present in the structure (three octahedral sites occupied by Ni and four tetrahedral sites occupied by Si or A1), i.e. 0.3 kCal/Mole. Thus the free energy associated with the partition coefficient D(Ni-Mg) will be lowered by the same value and will be equal, in this saponite, to --4-4 kCal/Mole. The greatest difference of log Dtyi_Mg ) between stevensite and saponite found by this calculation was 0-22, which is of the same order as the uncertainty of experimental measurements (0.18). The differences of the Ni CFSE in stevensite and saponite are too small to be observed by their chemical effects, especially for high Mg-Ni partition


coefficients. Both methods are complementary to each other and may be applied to systems presenting more contrasted values of CFSE, such as kerolite-stevensite. Preliminary data on this system show that the partition coefficient is greater in kerolite (Montdesir & Decarreau, in preparation).

D I S C U S S I O N

It must be recalled that CFSE acts only as a small (about 10%) perturbation of the crystal lattice energy and will mainly affect the processes affecting the local structures around Ni. CFSE variations may be correlated with the Ni distribution among coexisting phases at equilibrium, although with some caution because of the above-mentioned lack of experimental evidence, and are thus to be taken into account for a thermodynamic modelling of Ni geochemistry.

Nickel abundance in the Ni-bearing phases

Distribution of nickel between primary and secondary phases. Taking into account the CFSE determined in olivine (site M l : 27.3 kCal/Mole: Wood, 1974) and pyroxenes (22.8-28.8 kCal/Mole: White et al., 1971), the following order is obtained:

orthopyroxene < Ca-pyroxene, olivine < aquo-complexes < hydrous silicates.

This order is in good agreement with the Ni-site preference among coexisting phases in mafic rocks and the observed supergene enrichment of nickel.

Distribution of nickel among secondary phases. On the basis of the values reported in Fig. 7, one Ni distribution observed in New Caledonia ore deposits may be tentatively explained. Some 'garnierites' found in cracks (Poro, Anniversaire Mine) are made up of pimelite intimately mixed with sepiolite. The observed distribution favours the former, with Ni contents up to 39% as compared to 13% in the sepiolite, and corresponds to a lower CFSE of Ni in this mineral. Only one occurrence of Ni-rich (<20% NiO) sepiolite ('falcondoite') is known (Springer, 1976), which shows that this phase, with a low CFSE, does not play an important role in Ni concentration processes.

Mineralogical consequences

Decarreau & Perruchot (1980) have shown that experimental weathering of olivines leads to Mg-smectites. Heating synthetic smectites at 165~ leads to the formation of Ni-kerolite (P3). This latter mineral occurs as a neoformed phase in New Caledonia and has chemical and structural properties closely related to stevensite and talc (Brindley et al., 1977). With respect to the observations, it could be suggested that the New Caledonian kerolites derive from stevensites, as a final product of the evolution of neoformed phases. In addition to phase-stability arguments, it must be pointed out that differences in CFSE may be correlated with such an evolution:

interlayer Ni < octahedral sheet in stevensite < kerolite.

The increase of CFSE with the size of coherent domains (up to 60 ,~) is consistent with the absence of disordered or poorly crystallized phases containing substituted nickel. Minerals such as 'chrysoprase' are in fact a mixture of silica and nickeliferous phyllosilicate solid inclusions (Manceau et al., in preparation).


C O N C L U S I O N S

The actual location of Ni in hydrous silicates has been studied mainly in minerals from the New Caledonian ore deposits. X R D patterns can be used to show that the Ni 2+ ions are located in the octahedral sheet in randomly stacked phyllosil icates: the 12,20/02,11 ratio is shown to depend on the Ni / (Ni + Mg) ratio. Diffuse reflectance spect roscopy is a selective technique for characterizing Ni crystal chemistry in clay minerals.

Nickel crystal chemistry depends only on the nature of the mineral and not its origin: identical results were found for natural kerolites from various locations and synthetic varieties. C F S E and degree of site distort ion varied largely among the studied phases. In contrast , C F S E was independent of the Ni-content, but increased with crystall inity up to about 60 A in the 2 : 1 phyllosilicates.

The results obtained in this study may be used to explain the distr ibution of nickel in the so-called 'garnieri tes ' : Ni is always preferentially located in kerolite and lizardite. The derivation of kerolites from stevensite was suspected from phase stability studies and follows also the concentration of Ni in the phases with the highest CFSE.

Crysta l chemistry of other transit ion elements can similarly be studied by means of spectroscopic techniques, Fo r example, such a study was undertaken on Cr-bearing minerals to follow the various phases resulting from the weathering of chromite ore deposi ts (Calas et al., 1984). The range of the C F S E variations found for Cr-containing phyllosilicates was - 7 kCal /Mole , in contrast with ~2 kCa l /Mole for Ni-bearing phyllosilicates. These investigations confirm that the crystal chemistry of transit ion metal i o n s - - s i t e distortion, metal-oxygen covalency, C F S E - - i s a predominant parameter which controls the distribution of these elements among mineral associations, as well as within the structure of these minerals.

A C K N O W L E D G M E N T S

We wish to thank all the persons who provided us with samples of controlled origin and formation conditions, especially Drs Baronnet, Besson and Velde. Sampling of the material from New Caledonia was facilitated by Drs Escande, Esterle, Pelletier and Troly of the Soci&6 Le Nickel (SLN). Chemical analyses were made by Mrs Vachey. This work was supported by MST/MIR grant, Valorisation des Ressources du Sous-Sol, Contract N~

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nickel-bearing clay minerals: optical spectroscopic study of nickel