Geochimica et Cosmochimica Acta, Vol. 64, No. 21, pp. 3643–3661, 2000 Copyright © 2000 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/00 $20.00 ⫹ .00

Pergamon

PII S0016-7037(00)00427-0

Crystal chemistry of trace elements in natural and synthetic goethite A. MANCEAU,1,* M. L. SCHLEGEL,1 M. MUSSO,1 V. A. SOLE,2 C. GAUTHIER,2 P. E. PETIT,2 and F. TROLARD3 1

Environmental Geochemistry Group, LGIT-IRIGM, University Joseph Fourier and Centre National de la Recherche Scientifique (CNRS), F-38041 Grenoble Cedex 9, France 2 European Synchrotron Radiation Facility, B.P. 220, F-38043 Grenoble Cedex, France 3 INRA-UR Sol et Agronomie, 65 rue de St Brieuc, 35042 Rennes Cedex, France (Received November 22, 1999; accepted in revised form April 12, 2000)

Abstract—The crystal chemistry of Cr (0.73%), Mn (0.88%), Co (0.166%), Ni (0.898%), Cu (0.263%), and Zn (0.026%) in natural goethite (␣FeOOH) from an earthy saprolite formation in West-Africa was investigated by fluorescence-yield extended X-ray absorption fine structure (EXAFS) spectroscopy. Synthetic goethites and phyllomanganates were used as model compounds for structural determinations. The measurement of EXAFS spectra at energies higher than the Fe K-edge (Co, Ni, Cu, and Zn) is notoriously difficult because the fluorescence signal from trace elements is typically two orders of magnitude less intense than the Fe fluorescence from the matrix. This difficulty was circumvented by measuring total intensity (signal ⫹ background) with high precision on the ID26 undulator EXAFS spectrometer of the European Synchrotron Radiation Facility (ESRF) with a fast, highly linear, and low-noise diode detector. Cr, Cu, and Zn have the same local structure in natural and synthetic goethites. With the exception of the Cu polyhedron, which is distorted by the Jahn-Teller effect, Me-O and Me-Fe distances are similar to Fe-O and Fe-Fe distances in goethite. No significant steric effect was observed around Cu and Zn in spite of the ⬃14% increase in ionic radii compared to Fe3⫹. The compliance of the Fe site to these substitutional impurities probably is accomplished by displacement of nearest O and Fe shells (relaxation effect) and modification of interpolyhedral angles (compliance effect) owing to the corner-sharing topology of the goethite structure. X-ray absorption near-edge structure analysis reveals an average valence of ⬃3.7 to 3.8 for Mn, and EXAFS indicates that manganese is present as a phyllomanganate impurity having a hexagonal layer symmetry like asbolane, lithiophorite, and hexagonal birnessite. Cobalt is trivalent and located in the phyllomanganate layer, and also possibly in the interlayer, substituting for Mn. Selective uptake of cobalt by the Mn oxide impurity results from the oxidation of soluble Co2⫹ to insoluble Co3⫹ by Mn3⫹/Mn4⫹, this mineral-surface redox reaction accounting for the well-known geochemical affinity of Co for Mn at the earth’s surface. No more than ⬃20% of the amounts of Mn and Co in natural goethite substitute for Fe in the structure, if present at all. About 75% of total Ni is substituted for Fe in natural goethite and ⬃25% is associated with the phyllomanganate impurity as a Ni(OH)2–MnO2 mixed-layer phase (Ni-asbolane). The Ni site in synthetic goethite is strongly relaxed with a contraction of the goethite structure in the plane of edge-sharing double chains (bc plane), and an expansion in the direction of corner linkages (approximately the ab plane). This anisotropic relaxation of the Ni site locally reduces the distortion of the goethite structure, and could be due to a clustering of Ni atoms. Copyright © 2000 Elsevier Science Ltd voluminous literature on aluminous goethites has appeared relative to the effect of this substitution on the crystallographic and physico-chemical properties of goethite to use it as an indicator of pedogenic processes (Cornell and Schwertmann, 1996 and refs. therein; Stucki et al., 1988). The reality and extent of the Al for Fe isomorphic substitution have been investigated from the linear relationship between unit-cell parameters and the Al content (Vegard’s law) (Fazey et al., 1991; Schulze, 1984; Schwertmann and Carlson, 1994), and also indirectly by chemical extraction from the congruence of the Fe and Al dissolution (Schwertmann, 1984). An impressive number of cations have been incorporated in synthetic goethite besides Al: divalent Ni, Zn, and Cd; trivalent Cr, Ga, V, Mn, Co, and Sc; and tetravalent Pb, Ge, and Si (Inouye et al., 1972; Lim–Numez and Gilkes, 1985; Stiers and Schwertmann, 1985; Vandenberghe et al., 1986; Cornell and Giovanoli, 1987; Cornell and Giovanoli, 1989; Diaz et al., 1989; Ebinger and Schulze, 1989; Schwertmann et al., 1989; Gerth, 1990; Cornell et al., 1992; Schwertmann and Pfab, 1994; Vempati et al., 1995; Martin et al., 1997; Ford et al., 1997; Gasser et al., 1999;

1. INTRODUCTION

Goethite (␣FeOOH) is a common mineral of soils (Stucki et al., 1988), ore deposits (Schellmann, 1983), and continental and marine sediments such as ferromanganese nodules (Burns and Burns, 1977a). Natural goethite is rarely stoichiometric and usually contains a number of substitutional cations isovalent or heterovalent to Fe3⫹ (Burns and Burns, 1977a; Ku¨hnel et al., 1975). Owing to the widespread occurrence and abundance of goethite in the environment, the geochemical uptake of trace elements (TEs) in this mineral is important in terms of mass balance, control of the concentration and migration of metals in natural waters, and availability of nutrients and mobility of toxic elements to living organisms. Among all Fe-substituting elements, Al has been the most studied. Al for Fe substitution was originally demonstrated in soil goethites by chemical analysis and X-ray diffraction (XRD) line shift (Norrish and Taylor, 1961). Since then, a * Author to whom correspondence ([email protected]).

should

be

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Fig. 1. A) Intergrowth of hexagonal anionic close-packed domains of phyllomanganate and ␣FeOOH in a natural Mn-containing goethite. After Manceau et al. (1992). B) Epitaxial growth of ␣Fe2O3 nuclei on ␣AlOOH. After Hazemann et al. (1992). C) Epitaxial growth of ␥CrOOH nuclei on ␣FeOOH. Fe1, Fe2, and Fe3 are the three nearest Fe shells around Fe. After Charlet and Manceau (1992).

Scheinost et al., 2000). However, except for Al, which can reach 33 ⫾ 3 mol% (Fitzpatrick and Schwertmann, 1982; Tardy and Nahon, 1985; Fabris et al., 1986; Schwertmann and Carlson, 1994; Carlson, 1995), foreign cations are generally present in trace concentrations in natural samples. Their incorporation in the goethite structure has been inferred from interelemental correlations by microprobe chemical analyses (Roorda and Queneau, 1973; Schellmann, 1983; Taylor et al., 1964) and dissolution experiments (Schwertmann and Latham, 1986; Singh and Gilkes, 1992; Trolard et al., 1995), and demonstrated in the case of Ge by extended X-ray absorption fine structure (EXAFS) spectroscopy (Bernstein and Waychunas, 1987). Trolard et al. (1995) followed the leaching of Cr, Mn, Cu, and Ni upon dithionite-citrate-bicarbonate (DCB), citratebicarbonate (CB), hydroxylamine hydrochloride (HH), and oxalic-oxalate (Tamm) treatment of a nearly pure goethite sample (F40) from a lateritic profile developed on a Ni-enriched peridotite in East Africa. They proposed that Ni is

quantitatively incorporated into the goethite structure; Cu and Cr are partitioned between goethite and an Fe-Cr spinel inherited from the peridotite; and Mn forms a separate oxide. One major limitation of selective and sequential chemical extractions is their lack of Fe oxide specificity. For instance, the F40 sample contains ⬃5% maghemite and 3% hematite, which are dissolved simultaneously with goethite (though at different rates), thus preventing the determination of partitioning of TEs among the three Fe oxides. Also, TEs do not necessarily substitute for Fe, and the reality of this substitution, or the crystal chemistry of TEs in the case of phase impurity, requires the use of structural methods. Three examples of phase mixture at the local scale have been described: ␣FeOOH:Mn (Manceau et al., 1992), ␣AlOOH:Fe (Hazemann et al., 1992), and ␣FeOOH:Cr (Charlet and Manceau, 1992). In the first case, XRD and EXAFS spectroscopy showed that natural Mn-containing goethite consists of an intergrowth of ␣FeOOH and MnO2 units (Fig. 1A). In the second case, polarized EXAFS

Crystal chemistry of trace elements in goethite

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Table 1. Chemical bulk composition of the natural Fe oxide F40 (Trolard et al., 1995). SiO2a

Al2O3a

Fe2O3a

MnOa

MgOa

CaOa

Na2Oa

K2Oa

TiO2a

P2O5a

Crb

Cob

Cub

Nib

Znb

3.66

3.13

73.85

1.14

0.37

0.10

0.06

0.08

0.18

0.27

7300

1660

2630

8980

260

a b

%. ppm.

showed that Fe atoms form epitaxial ␣Fe2O3 clusters on ␣AlOOH (Fig. 1B). A similar epitaxial growth was then produced in the laboratory by sorbing Cr on goethite (Fig. 1C). In this study, the crystal chemistry of Cr, Mn, Co, Ni, Cu, and Zn in F40 (MeF40) and in synthetic goethites has been investigated by EXAFS spectroscopy. F40 was selected for three reasons: (1) previous investigation of the substitution of TEs for Fe by dissolution techniques (Trolard et al., 1995) offers a good opportunity to compare results from macroscopic (i.e., solution chemistry) and microscopic (i.e., structural chemistry) approaches; (2) this sample contains a large number of first row transition elements; and (3) F40 results from the weathering of peridotite and is therefore representative of metalliferous goethites commonly found in lateritic mantles covering ultramafic massifs in tropical and subtropical areas (Ku¨hnel et al., 1978; Trescases, 1986; Troly et al., 1979). EXAFS spectra for dilute systems generally are obtained by measuring the integrated intensity of the K␣/L␣ radiation for the studied element (Stern and Heald, 1979). The measurement of TEs EXAFS spectra at energies higher than the Fe K-edge (Co, Ni, Cu, and Zn) is notoriously difficult because the fluorescence of TEs is eclipsed by the most intense Fe fluorescence-yield from the matrix. Above the Fe K-edge, the ratio of the background Fe to the TE fluorescence signal is typically higher than 100:1. Therefore, obtaining treatable EXAFS spectra with acceptable statistics requires long counting times and an extremely intense X-ray source. For this reason, EXAFS spectra were measured on the undulator XAFS spectrometer ID26 at ESRF where the flux of photons on the sample is typically 1013 ph/s (Gauthier et al., 1999; Signorato et al., 1999).

content in the supernatant. Any remaining amorphous Fe oxides were eliminated by washing five times with an 0.2 mol/L oxalic acid solution. Amorphous materials in Cr-substituted goethite were removed with 2 mol/L H2SO4 (Schwertmann et al., 1989). All goethites were then washed three times with 0.1 mol/L KNO3 and three times with bidistilled water, filtered with a Sartorius 0.1 ␮m cellulose nitrate paper, and air-dried. Goethite powders were ground in a mortar, sieved through a 50 ␮m sieve, and pressed into a pellet for EXAFS measurements. The crystallinity and purity of the goethite samples was verified by XRD using a Siemens D5000 powder diffractometer (Iselin, NJ, USA) equipped with a Kevex Si(Li) solid-state detector, and Cu K␣ radiation (40 kV, 40 mA). 2.2. EXAFS

2.1. Natural and Synthetic Goethite

EXAFS spectra were collected on ID26 at the ESRF in Grenoble, France, with a storage ring energy of 6 GeV, a ring current of ⬃200 mA, and a 2/3 ring-filling mode. The white beam was monochromatized with two pairs of Si (220) crystals cooled to ⬃140° C, and higher harmonics were rejected with two mirrors (Si and fused silica) (Signorato et al., 1999). Measurements were performed in fluorescence detection mode and at ambient temperature. The X-ray fluorescence spectrum of F40 measured above the Zn K-edge (Fig. 2) shows that K␣ fluorescence counts issued from Co, Ni, Cu, and Zn (‘good’ counts) are about two orders of magnitude lower than the background signal from the Fe K␣ fluorescence and the elastic scattering. In addition, the K␣ fluorescence of a Z element and the K␤ fluorescence of the Z-1 element overlap. This problem is critical for Co owing to the overwhelming intensity of the Fe K␣ over Co K␣ radiation. In EXAFS spectroscopy a minimum of 106 good counts per point are needed for a treatable spectrum, requiring a minimum of 108 total counts (i.e., good plus background counts). Because the maximum total counting rate of state-of-the-art multi-element energy-resolving detectors is typically 1 MHz for a resolution of ⬃180 eV, the measurement of a 250 points EXAFS spectrum with the desired statistics requires ⬃7 h at the Ni K-edge, ⬃4 h at the Cu K-edge, and ⬃21 h at the Zn K-edge, whereas the Co-EXAFS spectrum cannot be measured with a solid-state detector. The low fluorescence signal satistics resulting from the limitation in counting rate can be circumvented by measuring the total intensity (signal ⫹ background) on a high flux source with a fast, highly linear,

The chemical composition of F40 is given in Table 1. Most TEs are about two orders of magnitude less concentrated than Fe (51.6 wt%), and Zn is more than three orders (Zn/Fe ⫽ 5 ⫻ 10⫺4). Hematite (⬃5%) and maghemite (⬃3%) were detected by XRD, but this sample probably also contains some quartz, gibbsite, and Fe-Cr spinel impurities (Trolard et al., 1995). Me-containing goethite references (MeGt with Me ⫽ Cr3⫹, Mn3⫹, Co2⫹, Ni2⫹, Cu2⫹, Zn2⫹) were synthesized using ultra-pure water from a Milli-Q system and chemicals of ACS reagent grade. The Me/(Me ⫹ Fe) mole fraction equaled 0.03 for all metals except Co (0.045). Although Mn(NO3)2 was used to synthesize Mngoethite, Mn is known to be incorporated in its trivalent form in synthetic goethite (Stiers and Schwertmann, 1985; Vandenberghe et al., 1986; Cornell and Giovanoli, 1987; Diaz et al., 1989; Ebinger and Schulze, 1989; Gerth, 1990; Vempati et al., 1995; Ford et al., 1997; Gasser et al., 1999; Scheinost et al., 2000). Me-ferrihydrites were first precipitated from CO2-free 1.5 mol/L KOH and 0.25 mol/L Fe(NO3)3 solutions containing the appropriate doping element. Goethites were then obtained by aging 1.3 g of each ferrihydrite sample in 125-mL polypropylene bottles containing 0.5 mol/L KOH at 70°C for 93 days (Schwertmann and Cornell, 1991). Synthetic goethites were rinsed three times with 0.5 mol/L KOH to decrease the dissolved Fe and Me

Fig. 2. X-ray fluorescence spectra of F40 at the excitation energy of 9800 eV.

2. MATERIAL AND METHODS

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and low-noise PIN diode detector (Gauthier et al., 1999). In this configuration the critical parameter is the measured fluorescence edgestep (⌬I), the minimum of which depends on the total number of photons and on the stability of the spectrometer baseline (i.e., sample homogeneity, stability of optical components and of the X-ray source). For an element Z higher than Fe, ⌬I is approximately equal to If(Z)/ If(Fe), where If(Z) and If(Fe) are the fluorescence intensity of Z and Fe above the threshold energy of Z. If was calculated from (Sole´ et al., 1993): If ⫽ I0

JK ⫺ 1 ␶ Z共E兲 ␻K C JK ␮ 共E兲 ⫹ ␮ 共E f兲 Z

冉

1 ⫺ exp

冋

兹 2共 ␮ 共E兲 ⫹ ␮ 共E f兲兲 ␳ d

册冊

⍀ 4␲

where ␻K is the K fluorescence yield of Z, ␶Z(E) is the total photoelectric cross-section of the sample at the excitation energy E, ␮(E), and ␮(Ef) are the total photon mass attenuation coefficients of the sample at E and at the fluorescence energy Ef, JK is the value of the K-jump, CZ is the mass fraction of Z, ␳ is the density, and d the thickness of the sample, and ⍀/4␲ is the relative solid angle of the detector. Omitting solid-angle and efficiency terms, this expression gives for F40: If/I0(Cr) ⫽ 5 ⫻ 10⫺3, If/I0(Mn) ⫽ 9 ⫻ 10⫺3, If/I0(Fe) ⫽ 25 ⫻ 10⫺3, If/I0(Co) ⫽ 7.5 ⫻ 10⫺4, If/I0(Ni) ⫽ 2.5 ⫻ 10⫺3, If/I0(Cu) ⫽ 8 ⫻ 10⫺4, and If/I0(Zn) ⫽ 0.9 ⫻ 10⫺4. The theoretical fluorescence edge-steps for elements above Fe are thus equal to ⌬I(Co) ⫽ 3%, ⌬I(Ni) ⫽ 10%, ⌬I(Cu) ⫽ 3%, ⌬I(Zn) ⫽ 0.4%. Calculations also showed that ⌬I could be enhanced by a factor of two by reducing the parasitic K␣ Fe fluorescence signal with an optimal combination of absorption filters. On ID26, reasonably good fluorescence-yield EXAFS spectra can be recorded with ⌬I as low as ⬃1 to 2% as shown in this study. Consequently, Cr, Mn, Co, Ni, and Cu EXAFS spectra were measured with a diode detector, whereas the Zn EXAFS spectrum had to be measured with a 12-element Si energy-resolving solid-state detector (Gauthier et al., 1996). Data were analyzed following established EXAFS analysis procedures. The preedge background absorption was determined from a fit to the data roughly 150 to 100 eV below the absorption edge energy and then extrapolated over the entire energy range of the spectrum. The K-edge absorption was then isolated by subtracting the extrapolated preedge background from the postedge absorption signal. The smoothly varying atomic absorption was determined by fitting the postedge data with a cubic-spline function. The absorption spectra were then stepnormalized by dividing by the amplitude of the absorption edge. The energy of the absorption edge (E0) was taken as the half-height of the edge jump (⌬I/2). Interatomic distances (R) and coordination numbers (CN) were determined by using theoretical phase shift and amplitude functions calculated by the FEFF7.02 code (Rehr et al., 1991). 2.2. Mn K-XANES XANES spectra for synthetic goethite (MnGt) and F40 were recorded in fluorescence detection mode, and those for Mn2O3, monoclinic Na-exchanged birnessite (NaBi), and rancieite (Mn4⫹-rich natural birnessite) references were recorded in transmission mode on ID26. Energy calibration was performed in situ by measuring simultaneously with each sample a Mn metal standard foil, and by assigning the maximum of the derivative at 6539 eV. The reproducibility in energy was better than 0.1 eV. XANES spectra were step-normalized to a unit edge jump. 3. RESULTS

3.1. Cr-, Cu-, and Zn-EXAFS An overlay plot of EXAFS spectra for F40 and synthetic goethites is shown in Figure 3. Spectra for the natural and synthetic samples are essentially identical at a given K-edge, which indicates that Cr, Cu, and Zn are predominantly substituted for Fe in natural goethite. In laterites Cu and Zn are often taken up by Mn oxides (Ostwald, 1984; Ostwald, 1985; Fig. 4).

Fig. 3. Comparison of k3-weighted EXAFS spectra at the Cr (A), Cu (B), and Zn (C) K-edge for natural (F40, solid lines) and synthetic (dotted lines) goethite.

The possibility of partitioning Cu and Zn between goethite and a Mn oxide impurity was evaluated by comparing EXAFS spectra for F40 to natural (Cu,Zn)-lithiophorite, chalcophanite, and Cu-sorbed birnessite (Fig. 5). Lithiophorite is built up of interlayered MnO2 and Al(OH)3 sheets (Post and Appleman, 1994; Wadsley, 1952), and Ni, Cu, and Zn are incorporated in the gibbsitic layer (Manceau et al., 1987; 1990; Fig. 4A). The presence of ‘light’ Al atoms in the second coordination shell of the TE in lithiophorite, instead of ‘heavy’ Fe atoms in goethite

Crystal chemistry of trace elements in goethite

Fig. 4. Idealized crystallochemical structures of phyllomanganates. A) lithiophorite; B) Ni-asbolane; C) hexagonal H-birnessite (HBi); D) monoclinic Na-exchanged birnessite (NaBi); E) chalcophanite (Zn2⫹ Mn4⫹ 3 O7.3H2O); F) Co-sorbed birnessite (CoBi); G,H) Co-sorbed birnessite S3 and S1 projected in the ab and bc planes. Interlayer octahedra are in black, and arabic numbers correspond to the oxidation state of Mn atoms. After Wadsley (1952, 1955), Chukhrov et al. (1980a, 1980b, 1982), Manceau et al. (1987, 1990, 1992, 1997), Post and Appleman (1988, 1994), Silvester et al. (1997), and Drits et al. (1997).

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Fig. 5. A) Comparison of (Ni, Cu, Zn)-EXAFS spectra for natural lithiophorite (after Manceau et al., 1987; 1990). The three trace elements are located in the Al(OH)3 gibbsitic sheet as attested by the characteristic splitting (arrows) of the maximum of the first oscillation at 3 to 5 Å⫺1. B–E) Comparison of (Cu, Zn)-EXAFS spectra for F40 with various references: B) Cu-sorbed hexagonal birnessite (CuBi, Manceau et al., in preparation), C) chalcophanite (ZnCha, after Silvester et al., 1997), D,E) Zn-kerolite (ZnKer) and Zn-doped kerolite (MgKer) (after Manceau et al., 2000). On hexagonal birnessite Cu sorbs above/below vacancy sites replacing interlayer Mn.

and Mn atoms in birnessite, gives rise to a characteristic split of the first oscillation maximum with a minimum at 3.8 Å⫺1 (see arrows in Fig. 5A). In contrast, the wave amplitude of CuF40

and ZnF40 is maximum at k ⫽ 3.8 Å⫺1 (Fig. 3B and C). Zn in chalcophanite and Cu in birnessite are both located above/ below layer vacant sites (Fig. 4), and their octahedra share three

Crystal chemistry of trace elements in goethite

corners with layer Mn octahedra instead of edges with Al octahedra in lithiophorite and of edges and corners with Fe octahedra in goethite. Consequently, chalcophanite and Cusorbed birnessite, like (Cu,Zn)-lithiophorite, have markedly different EXAFS spectra from (Cu,Zn)-goethite (Figs. 3 and 5B and C), which means that (Cu,Zn)-bearing Mn oxide species are minor, if present at all. Zn is also known to have a high geochemical affinity for silica in natural systems owing to the low solubility of Zn-containing trioctahedral clay minerals (Decarreau, 1985). As F40 contains ⬃65 times more Si (1.7 wt.%) than Zn (260 ppm), the possibility of some Zn-clay species in F40 was also considered in the spectral analysis. The ZnEXAFS spectra for the Zn-rich and Zn-poor end-members of the trioctahedral series Si4Zn3O10(OH)2-Si4Mg2.97Zn0.03O10(OH)2 are presented in Fig. 5D and E. These spectra are completely different from all others considered previously because in clays Zn octahedra are edge-linked to (Mg, Al, Fe) octahedra and corner-linked to (Si,Al) tetrahedra. Interestingly, the maximum of the first oscillation for the Zn-doped clay reference is split as in lithiophorite (Fig. 5E) due to the presence of ‘light’ Mg atoms as nearest cations, thus confirming the sensitivity of this spectral feature to the nature of neighboring atoms in edgesharing octahedral layers. Finally, the close similarity of CuF40 and CuGt, and of ZnF40 and ZnGt, and the high spectral sensitivity of EXAFS to different coordination environments as illustrated with birnessite, chalcophanite, lithiophorite, and clay minerals, provide conspicuous evidence for the incorporation of Cu and Zn in the goethite structure and allow us to dismiss the presence of other TE species in significant amounts. Note that chromium cannot be taken up by manganates because insoluble chromic ions are readily oxidized to soluble chromate ions by Mn4⫹ and Mn3⫹ (Eary and Rai, 1987; Manceau and Charlet, 1992; Fendorf and Zasoski, 1992; Silvester et al., 1995). The signal-to-noise ratio is lower in CuF40 and ZnF40 than in CrF40 owing to the low amount of Cu (2630 ppm) and Zn (260 ppm) and to the intense Fe fluorescence-yield signal at these two K edges. Therefore, the spectral analysis of CuF40 and ZnF40 was limited to the 3 to 10 Å⫺1 wavevector (k) interval. Fourier transforms at the Me K-edges for F40 (MeFTs) are compared to Fe-FT of goethite on the left side, and to Me-FT of synthetic goethites on the right side, of Fig. 6. Only the magnitude of the FTs (also called radial structure functions, RSFs) are shown on the left, and both the magnitude and the imaginary functions are shown on the right. At the Fe K-edge of goethite (FeGt), the first RSF peak (labeled A in Fig. 6A) corresponds to the contribution from the nearest three oxygen atoms at 1.95 Å and three hydroxyls at 2.09 Å (Szytula et al., 1968). Peaks B and C correspond to the contributions from the four nearest Fe atoms across edges (two in the [001] direction, Fe1, and two in the [035] and [035៮ ] directions, Fe2) and the four corner-sharing octahedra in adjacent chains (Fe3, Fig. 1) (Manceau and Combes, 1988; Manceau and Drits, 1993). Peaks A have almost the same position in (Cr,Cu,Zn)-RSFs and the Fe-RSF, which indicates that Me-(O,OH) bond lengths are similar. More interestingly, peaks B and C in Fe-RSF are also observed in all these Me-RSFs, which offers strong evidence that TEs have principally the same structural environment as Fe. This inference is further supported by the comparison of FTs for F40 and synthetic goethites (plots on the right in Fig.

3649

6), which shows that both the magnitude and imaginary parts of the FTs are similar in the two sets of samples. Results from the quantitative data analysis are summarized in Tables 2 and 3. Two oxygen subshells (O1 and O2) were required to fit the Fourier-filtered Me-(O,OH) contributions to EXAFS (␹O). The effects of the split first coordination shell are clearly visible on inverse FTs of peaks A, which show a marked beat-pattern at k ⬃8 Å⫺1 (Fig. 7E and G). The O1 subshell for Cr and Zn consists of ⬃1 to 2 oxygen atoms at 1.86 to 1.98 Å, and the O2 subshell of 4.6 to 5.4 oxygen atoms at 2.01 to 2.12 Å. Significantly different results were obtained for CuF40 and CuGt owing to the Jahn–Teller distortion of the copper octahedron (Burns, 1970; Douglas et al., 1994). The two apical oxygen atoms at ⬃2.2 to 2.4 Å are hardly detected by EXAFS and oxygen atoms generally sum up to ⬃4 instead of ⬃6 (Charnock et al., 1995; Cheah et al., 1998; Farquhar et al., 1996; 1997; Mckeown, 1994; Mosser et al., 1990; Parkman et al., 1999; Weesner and Bleam, 1997). The splitting of the first coordination shell for these three TEs is comparable to that found in the bulk structure of ␣FeOOH where, for electrostatic reasons, d(Fe-OH) (O2 subshell) is longer than d(Fe-O) (O1 subshell). The three first Fe-Fe distances in goethite [d(Fe-Fe1) ⫽ 3.01 Å, d(Fe-Fe2) ⫽ 3.28 Å, d(Fe-Fe3) ⫽ 3.46 Å] are resolved in EXAFS and give rise to two wave nodes, one at k ⬃5 Å⫺1 and one at k ⬃11 Å⫺1, on inverse FTs of peaks B and C (␹Fe, Fig. 7B). This double-beat node pattern, characteristic of goethite structure, is well marked in CrF40 and CuF40 (Fig. 7D and F). To reduce the degree of freedom during the fitting procedure, the Debye–Waller term (␴) was kept identical for the three Fe shells. The maximum number of ajusted parameters allowed in the spectral fit is given by the number of independent points, Nidp ⫽ 2␦k␦R/␲ ⫹ 2. Here, ␦k (Å⫺1) is the interval in k-space on which the fit is performed, and ␦R is the R ⫹ ⌬R interval of the inverse FT (Stern, 1993). The total number of variable parameters was equal to 8 (3 R values, 3 CN values, 1 ␴ value, and the threshold energy variation, ⌬E) for all samples as compared to Nidp ⫽ 10 for CrGt, CrF40, CuGt, and 8 for CuF40, ZnGt, and ZnF40. Therefore, it was permissible to use a three-shell approach to fit the data. This quantitative analysis showed that Me-Fe distances are, within precision, close to Fe-Fe distances in ␣FeOOH (Table 3). Consequently, Cr, Cu, and Zn are substituted for Fe and their incorporation in trace amounts in the goethite framework engenders little structural distortion. 3.2. Mn-XANES This technique examines the variation of the absorption of X-rays with wavelength in the vicinity of an absorption edge, and the energy position of the edge is correlated with the valence state of the atom in the sample (Wong et al., 1984). At the Mn K-edge, the shift in energy of the absorption threshold equals ⬃4 eV from Mn2⫹ to Mn3⫹, and ⬃3 eV from Mn3⫹ to Mn4⫹ (Fig. 8A and C). XANES spectra and their first derivatives for MnGt and MnF40 are presented in Fig. 8B and D. The energy position of MnGt coincides with that of the Mn3⫹ reference (Mn2O3), confirming that Mn is incorporated in its trivalent state in synthetic goethite. The F40 edge spectrum is right-shifted from the Mn3⫹ reference by ⬃2 eV, but left-

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Fig. 6. Left column: comparison of RSFs (modulus of the Fourier transforms, FTs) at the Cr, Cu, and Zn K-edges for F40 (solid lines) with goethite at the Fe K-edge (dashed lines). Right column: comparison of FTs (modulus and imaginary part) at the Cr, Cu, and Zn K-edges for F40 (solid lines) and synthetic (dotted lines) goethite. First peaks in RSFs (A) correspond to the oxygen-shell contribution, second peaks (B) to Me-Me edge-sharing contributions, and third peaks (C) to Me-Me corner-sharing contributions. RSFs are not corrected for phase shift, accordingly peaks are shifted toward shorter distances by ⌬R ⬃0.3 to 0.4 Å with respect to crystallographic values. Real distances obtained from least-squares fits are reported in Tables 2 and 3.

shifted from the Mn4⫹ reference by ⬃1 eV. The threshold energy of F40 was found to be similar to that of Na-exchange monoclinic birnessite (MBi) of ideal structural formula Na0.333 4⫹ 3⫹ Mn2⫹ 0.055 (Mn0.722 Mn0.222) O2 (crystallites of Type II, Drits et al., 1997; Silvester et al., 1997). Therefore, XANES shows that Mn has a mixed valent state in F40, and that its average valence is ⬃3.7 to 3.8 as in MBi. 3.3. Mn-EXAFS MnF40 and MnGt have different EXAFS spectra (Fig. 9A). The lower wave frequency and the higher wave amplitude of

MnF40 provide evidence of a shortening of interatomic distances and an increase of structural order. This is consistent with the higher average oxidation state of Mn in F40 relative to MnGt. Indeed, the d3 electronic configuration of Mn4⫹, with its completely filled t2g orbitals and empty eg orbitals, yields more coherent interatomic distances, and thus a lower disorder, than the d4 electronic configuration which leads to a Jahn–Teller distortion. The Mn mineral species were identified by comparing MnF40 to a large database of Mn references taken from our previous studies on Mn oxides (Manceau and Combes, 1988;

Crystal chemistry of trace elements in goethite

3651

Table 2. EXAFS parameters for the first coordination shell. 1st Sub-shell

2nd Sub-shell

Sample

R window (Å)

k range (Å⫺1)

R (Å)

CNa

R (Å)

CN

␴(Å)b

⌬E (eV)c

Rpd

CrGt CrF40 MnGt MnF40 FeGt CoGt CoF40 NiGt NiF40 CuGt CuF40 ZnGt ZnF40

1.1–2.1 1.1–2.1 1.0–2.1 1.1–2.0 1.1–2.1 1.1–2.0 1.1–1.9 1.1–2.1 1.1–2.1 1.1–2.1 1.1–2.2 1.1–2.1 1.1–2.1

3.5–11.6 3.5–11.5 3.5–11.4 3.5–10.6 3.0–14.2 3.5–11.7 3.5–9.1 3.5–12.4 3.5–12.5 3.5–12.5 3.5–10 3.5–10.5 3.5–10.2

1.92 1.88 1.89 1.91 1.95 1.95 1.90 — — 1.98 1.94 1.87 1.86

1.5 1.1 1.9 5.4 3.1 4.7 5.1 — — 2.6 2.1 1.1 2.1

2.01 2.01 2.04 — 2.09 — — 2.07 2.07 2.11 2.12 2.05 2.06

4.7 4.9 1.4 — 2.5 — — 5.3 5.3 1.2 2.1 5.4 4.6

0.05 0.05 0.05 0.06 0.08 0.07 0.09 0.06 0.06 0.08 0.05 0.09 0.09

⫺0.7 ⫺0.4 2.8 ⫺0.1 ⫺0.8 ⫺0.4 2.1 ⫺0.5 0.7 ⫺1.0 ⫺2.5 0.9 2.4

0.01 0.006 0.07 0.03 0.012 0.006 0.08 0.016 0.02 0.05 0.02 0.05 0.06

a CN is the coordination number. The precision on CN is estimated to ⫾1 and on R to ⫾ 0.02Å. The amplitude reduction factor S20 in FEFF calculations was set to 0.85. The low number of Cu-O pairs in CuGt and CuF40 (⌺ CN ⫽ 3.8 and 4.2) and Mn-O pairs in MnGt (⌺ CN ⫽ 3.3) is due to the existence of two undetected apical oxygen atoms associated with the Jahn–Teller distortion of Cu2⫹ and Mn3⫹. b ␴ is the Debye–Waller factor. c ⌬E is the shift in threshold energy (E0) relative to FEFF phase shift function. d Rp: Figure of merit for the spectral fitting. Rp ⫽ ⌺ (k3 ␹exp ⫺ k 3 ␹ th ) 2 /⌺ (k 3 ␹ exp)2.

Manceau et al., 1992; Silvester et al., 1997). The best spectral match was obtained with the phyllomanganates (Ni,Co)-asbolane and Co-sorbed hexagonal birnessite (sample S3 in Manceau et al., 1997; Fig. 9G and I). (Ni,Co)-asbolane has a mixed-layer structure like lithiophorite, but the non-manganiferous layer consists of Ni(OH)2 instead of Al(OH)3 in lithiophorite (Fig. 4B). S3 has the average structural formula Mn2⫹ 0.02 3⫹ 3⫹ 4⫹ 3⫹ 3⫹ Co2⫹ 0.06 Mn0.03 Co0.03 (Mn0.74 Mn0.05 Co0.11 䊐0.10) O1.75 (OH)0.25, where cations in parenthesis are located in the octahedral sheet and others are in the interlayer of the phyllomanganate structure (Fig. 4G). The sensitivity of EXAFS to phyllomanganate structural varieties can be assessed by comparing MnF40 to the two structurally close Mn references, Co-free hexagonal birnessite [ideally 2⫹ 4⫹ 3⫹ Mn3⫹ 0.116Mn 0.05(Mn 0.74Mn 0.093䊐 0.167) O 1.7(OH) 0.3, HBi) and

monoclinic birnessite (MBi) (Fig. 4C and D, Drits et al., 1997; Manceau et al., 1997; Silvester et al., 1997). Asbolane, S3 (Co-sorbed birnessite), HBi, and MBi references essentially differ by the ratio of the number of edge- (i.e., layer) to corner(i.e., interlayer) sharing octahedra, and also by the amount of layer Mn3⫹ cations. Asbolane contains only edge-sharing octahedra and is devoid of heterovalent Mn ions as indicated by the hexagonal symmetry of the MnO2 layer and the low value of its b parameter (2.83 to 2.84 Å, Chukhrov et al., 1980a,b; Manceau et al., 1992). The fraction of layer Mn3⫹ is ⬃20% in MBi Type II and ⬃10% in HBi. Owing to the bigger size of Mn3⫹ relative to Mn4⫹, average Mn-Mn distances across edges are correspondingly larger, and this difference is reflected in EXAFS spectra by a shift to lower k values (i.e., higher fre-

Table 3. EXAFS parameters for the two or three nearest cation shells. 1st Shell

2nd Shell

3nd Shell

Sample

R window (Å)

k range (Å⫺1)

R (Å)

CN

R (Å)

CN

R (Å)

CN

␴(Å)

⌬E (eV)

Rp

CrGt CrF40 MnGt MnF40 FeGt CoGt CoF40 NiGt NiF40 CuGt CuF40 ZnGt ZnF40

2.2–3.7 2.2–3.7 2.2–3.7 2.1–3.1 2.3–3.6 2.1–3.7 2.0–3.9 2.3–3.8 2.4–3.8 2.3–3.7 2.3–3.8 2.2–3.6 2.2–3.7

3.5–11.6 3.5–11.5 4.0–11.3 4.0–10.6 3.5–14.2 4.0–11.2 4.0–9.2 4.0–12.4 4.0–12.4 4.0–12.5 4.0–10.1 4.0–10.1 4.0–10.1

3.02 3.01 3.06 2.85 3.05 3.01 2.86 3.00 3.07 3.00 3.05 3.00 3.02

2.4 3.0 2.5 5.4 3.0 2.8 4.0 2.1 2.1 1.8 2.4 1.9 1.0

3.24 3.24 3.29 — 3.28 3.22 — 3.18 3.18 3.23 3.26 3.23 3.23

3.1 2.6 2.6 — 2.6 3.2 — 1.4 0.5 3.3 3.5 2.8 1.4

3.46 3.48 3.50 3.47 3.46 3.46 3.66 3.62 3.65 3.48 3.50 3.44 3.50

3.8 3.5 2.6 0.8 3.7 2.4 3.0 2.5 2.0 3.1 2.9 2.1 1.7

0.08 0.09 0.08 0.07 0.08 0.07 0.10 0.08 0.08 0.09 0.09 0.09 0.09

⫺2.2 ⫺4.1 ⫺6.8 ⫺2.6 ⫺4.5 ⫺4.0 ⫺4.4 ⫺0.7 ⫺0.8 ⫺3.5 ⫺5.5 ⫺5.1 ⫺7.0

0.01 0.01 0.002 0.01 0.003 0.02 0.07 0.04 0.03 0.01 0.02 0.02 0.007

S20 ⫽ 1.0. Based on goethite structure, the precision on CN can be estimated to ⫾1 for most samples. It is probably greater than ⫾1 for CuF40, ZnGt, and ZnF40 owing to the limited fitting k range. The precision on R is estimated to ⫾0.05 Å. Fits were performed by varying individual CN values and taking a single ␴ value for all atomic shells. This procedure is arbitrary and another fitting strategy would be to fix CN values to their crystallographic values (when known), and to adjust ␴ for each shell. For MnGt, this alternative method yielded: CN1 ⫽ 2.0, ␴1 ⫽ 0.07 Å, R1 ⫽ 3.05 Å, CN2 ⫽ 2.0, ␴2 ⫽ 0.07 Å, R2 ⫽ 3.25 Å, CN3 ⫽ 4.0, ␴3 ⫽ 0.12 Å, R3 ⫽ 3.47 Å, ⌬E ⫽ ⫺7.8 eV, Rp ⫽ 0.004.

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Fig. 7. Fourier-filtered EXAFS Me-O (left column) and Me-Fe (right column) contributions at the Fe K-edge of goethite (A,B), and Cr K-edge (C,D), Cu K-edge (E,F), and Zn K-edge (G,H) of F40. Solid line: experimental spectrum; dotted lines: optimal spectral simulation.

Crystal chemistry of trace elements in goethite

3653

Fig. 8. Mn K-edge XANES spectra (A,B) and first derivatives (C,D) for Mn2⫹ (MnCO3), Mn3⫹ (Mn2O3), Mn4⫹ (rancieite), and mixed valent Mn (MBi) reference compounds, and for synthetic (MnGt) and natural (MnF40) goethite.

quency) of the wave phase (Fig. 9C) and in TFs by a shift to higher R ⫹ ⌬R values of the modulus and imaginary part of peak B for MBi (Fig. 9D). In Co-sorbed birnessite, aqueous Co2⫹ ions are oxidized to Co3⫹ by Mn3⫹, which is reduced to the more soluble species Mn2⫹. The replacement of Mn3⫹ by Co3⫹ in the octahedral sheet of birnessite has two important structural consequences. First, it decreases the average value and the distribution of Mn-(Mn,Co) distances across edges because Co3⫹ is smaller than Mn3⫹ (0.545 vs. 0.645 Å, Shannon, 1976) and similar in size to Mn4⫹ (0.53 Å). Second, and more noteworthy, the layer distortion also decreases because Co3⫹ does not undergo a Jahn–Teller distortion. Therefore, the Co3⫹ for Mn3⫹ substitution increases the structural order around Mn atoms, and this phenomenon engenders an increase in amplitude of the EXAFS oscillations (Fig. 9E), and an enhancement of peaks A (Mn-O pairs) and B (Mn(Mn,Co) pairs) on RSFs (Fig. 9F). Consequently, although synthetic HBi contains a fair amount of heterovalent Mn atoms, the oxidation and substitution of Co3⫹ for Mn3⫹ in the octahedral sheet decreases this amount and increases the layer symmetry. These crystallochemical characteristics explain why asbolane and Co3⫹-substituted birnessite have similar Mn-EXAFS spectra, and that these two distinct references provided a similar match to MnF40 (Fig. 9G and I).

3.4. Co-EXAFS EXAFS spectra for CoGt and CoF40 differ in phase and amplitude in a similar fashion as MnGt and MnF40 (Fig. 9A and 10A). CoF40 has a lower wave frequency than CoGt, and its two first RSF peaks are accordingly shifted to lower R ⫹ ⌬R values (Fig. 10B). These observations suggest the presence of trivalent cobalt in F40. The CoF40 EXAFS spectrum is distinctly different in amplitude from S3 at the Co K-edge (CoS3, Fig. 10C), and resembles those of the (Ni,Co)-asbolane 3⫹ 3⫹ (CoAsb) and the Co-sorbed birnessite S1 (Mn2⫹ 0.06 Mn0.05 Co0.01 3⫹ 3⫹ (Mn4⫹ 0.74 Mn0.10 Co0.05 䊐0.11) O1.71 (OH)0.29) in Manceau et al. (1997) (Fig. 4H and 10E). S1 differs from S3 by its amount of Co2⫹ and Mn3⫹. S1 contains less cobalt than S3 and, therefore, more layer Mn3⫹ and no divalent cobalt because all sorbed Co2⫹ ions were oxidized to Co3⫹. The oxygen-shell contribution in CoF40 was successfully fitted by assuming a single Co-O distance of 1.90 Å (Table 2), and the derived distance is consistent with all Co atoms in the trivalent state (Burns, 1976; Manceau et al., 1987; Manceau et al., 1997). The optimal fit for the second and third shell contributions (peaks B and C) was obtained with 4.0 Mn at 2.86 Å and 3.0 Mn at 3.66 Å. The short distance is characteristic of edge-sharing Co3⫹-Mn4⫹ pairs in the octahedral sheet. The longer distance corresponds to cornersharing Co-Mn octahedra, but the location of Co atoms in-

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Fig. 9. EXAFS results obtained at the Mn K-edge for natural goethite (F40). Comparison of k3-weighted EXAFS spectra (left column) and FTs (modulus and imaginary part, right column) for F40 with Mn-doped goethite (MnGt) (A,B); monoclinic birnessite (MBi, after Silvester et al., 1997) (C,D); hexagonal birnessite (HBi, after Silvester et al., 1997) (E,F); Co-sorbed birnessite S3 (MnS3, after Manceau et al., 1997) (G,H); and (Ni, Co)-asbolane (MnAsb, after Manceau et al., 1987, 1992) (I,J).

Crystal chemistry of trace elements in goethite

Fig. 10. EXAFS results obtained at the Co K-edge for natural goethite (F40). Comparison of k3-weighted EXAFS spectra (left column) and Fourier transforms (modulus and imaginary part, right column) for F40 with Co-doped goethite (CoGt) (A,B); Co-sorbed birnessite S3 (CoS3, after Manceau et al., 1997) (C,D); Co-sorbed birnessite S1 (CoS1, after Manceau et al., 1997) (E,F); and (Ni, Co)-asbolane (CoAsb, after Manceau et al., 1987, 1992) (G,H).

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Fig. 11. EXAFS results obtained at the Ni K-edge for natural (F40) and synthetic (NiGt) goethite (A), and comparison with reference spectra; B) Ni-asbolane (NiAsb, after Manceau et al., 1987, 1992); C) Ni(OH)2; D) Ni-serpentine (after Manceau, 1990).

volved in this type of linkage is less certain. They can be located either in the octahedral sheet near an interlayer Mn atom, or in the interlayer above/below a layer vacancy site (Fig. 4). Finally, Co atoms in F40 are all associated with the phyllomanganate impurity, and they are either partly, or entirely, substituted for Mn in the octahedral sheet. 3.5. Ni EXAFS EXAFS results for NiGt and NiF40 are presented in Figs. 11 and 12. The RSF for NiGt (Fig. 12A) exhibits three peaks as in goethite, but their shape and position differ significantly from those in FeGt (Fig. 12A), CrGt, CuGt, and ZnGt (Fig. 6). Peaks A and C are shifted to higher distances and peak B has an asymmetrical shape with a shoulder on the right wing. The oxygen contribution (␹O) of NiGt was successfully fitted with a single shell of 5.3 ⫾ 1 O at 2.07 Å. The fit of metal shell contributions yielded 2.1 Fe1 at 3.00 Å and 1.4 Fe2 at 3.18 Å (peak B) and ⬃2.5 Fe3 at 3.62 Å (␴ ⫽ 0.08 Å, precision on CN estimated to ⫾1, Table 3). In spite of the bigger size of Ni2⫹ relative to Fe3⫹ (0.69 Å vs. 0.64 Å), some interatomic distances are almost equal (O2, Fe1), or even shorter (Fe2), than those for Fe in goethite: d(Fe-O1) ⫽ 1.95 Å, d(Fe-O2) ⫽ 2.09 Å, d(Fe-Fe1) ⫽ 3.01 Å, d(Fe-Fe2) ⫽ 3.28 Å, d(Fe-Fe3) ⫽ 3.46 Å.

The anisotropic relaxation of the Ni site can be explained by considering a local contraction of the goethite structure in the plane of edge-sharing double chains (bc plane) and an expansion in the direction of corner linkages (approximate ab plane). This structural interpretation is evident from the crystallographic orientation of the various Fe-Fe pairings (Fig. 1C), but less apparent for the first coordination shell. In goethite, the O1 subshell corresponds to Fe-O1-(Fe1,Fe3) bonds and the O2 subshell to Fe-O2-(Fe1,Fe2) bonds. Therefore, the lengthening of the Fe-Fe3 distance from 3.46 Å to 3.62 Å is accompanied by a lengthening of the Fe-O1 bond length and, reciprocally, the shortening of the Fe-Fe2 distance decreases the Fe-O2 bond length. The anisotropic relaxation around Ni atoms reduces the distortion of the goethite structure because the first coordination shell (O) is no longer split and the second shell of Fe atoms across edges in the [001] (Fe1) and [035] and [035៮ ] directions (Fe2) is less split around the impurity than around Fe. The particular behavior of nickel is surprising because its ionic radius (0.69 Å) is intermediate between Fe3⫹ (0.64 Å) and Cu (0.73 Å) or Zn (0.74 Å), and could result from a clustering of Ni atoms in the goethite structure. A three-shell fit of peaks B and C in NiF40 (␹Fe) yielded 2.1 Fe1 at 3.07 Å (or 2 Ni at 3.05 Å), 0.5 Fe2 at 3.18 Å, and 2.0

Crystal chemistry of trace elements in goethite

3657

Fig. 12. FTs at the Ni K-edge for natural (F40) and synthetic (NiGt) goethite, and comparison with references. A) RSFs (modulus of the FTs) for NiGt (Ni K-edge) and goethite (Fe K-edge, FeGt); B) FTs for NiF40 and NiGt; C) FTs for NiF40 and Ni-asbolane (NiAsb); D) FTs for NiF40 and a mixture of 75% NiGt ⫹ 25% Ni-asbolane.

Fe3 at 3.65 Å (Rp ⫽ 0.03, Table 3). The increase in average distance of the Fe1 shell in NiF40 relative to NiGt (3.07 Å vs. 3.00 Å) is real as indicated by the shift in phase of EXAFS spectra (Fig. 11A) and in position of peaks B (Fig. 12B), and suggests the presence of a second Ni species. The average Ni-Ni distance of 3.05 Å undoubtedly corresponds to an edgesharing linkage. It is shorter than the Ni-Ni distance in Ni(OH)2 (3.12 Å, Brindley and Kao, 1984), but similar to values found in Ni-containing clay minerals (3.06 Å, Manceau and Calas, 1986) and (Co,Ni)-asbolane (3.03 Å, Manceau et al., 1987). From the k3-weighted EXAFS functions shown in Fig. 11B it is evident that NiF40 and (Co,Ni)-asbolane have similar EXAFS spectra. In contrast, the frequency and amplitude of the EXAFS oscillations for Ni(OH)2 and Ni-serpentine are significantly distinct from NiF40 (Fig. 11C and D). The FT for NiF40 also resembles to that of (Co,Ni)-asbolane (Fig. 12C), further suggesting the presence of this species in F40. Adding this Ni component to the EXAFS spectrum of NiGt shifted peak B of NiGt toward that of NiF40. NiF40 was matched best with a mixture of 75% NiGt ⫹ 25% (Co,Ni)-asbolane (Fig. 12D). 4. DISCUSSION

In minerals, TEs are either substituted for a constitutive atom of the host structure (Bernstein and Waychunas, 1987; Reeder et al., 1999; Sturchio et al., 1998) or associated with a separate phase, which can be a precipitated form of the TE (Greegor et

al., 1997) or a discrete phase intimately mixed with the major mineral species. These three possibilities are encountered here: Cr, Cu, and Zn substitute for Fe in goethite; Mn is precipitated as asbolane/birnessite; and Co is substituted for Mn in the phyllomanganate impurity. Nickel is an intermediate case because it is partitioned between goethite and asbolane. 4.1. Crystal Chemistry and Geochemical Behavior of Cr, Cu, and Zn Cr, Cu, and Zn have different electronic structure, coordination chemistry and physico-chemical properties and, consequently, contrasting geochemical behavior in soils. Thus, their simultaneous occurrence as substitutional ions in goethite warrants some discussion. Among all transition metals, Cr3⫹ is probably the most mimetic to Fe3⫹. Both cations are trivalent and have a similar ionic radius (0.615 vs. 0.645 Å, Shannon, 1976) and, hence, comparable hydrolyzing properties (Baes and Mesmer, 1976). In addition, ␣FeOOH and ␣CrOOH (bracewellite) are stable and isostructural, allowing the formation of mixed ␣(Fe,Cr)OOH solids. Chromium is present in chromite and pyroxenes in ultramafic rocks, and their weathering releases Fe2⫹ and Cr3⫹ ions that can precipitate as Cr-containing ferric oxides in laterites (Ku¨hnel et al., 1978; Nahon and Colin, 1982; Schellmann, 1978; Schwertmann and Latham, 1986). In soils, chromium is also associated with

3658

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goethite and is considered to be substituted for Fe. This is consistent with the observation that the best fit of metal-Fe dissolution congruence is seen for the Fe-Cr pair (Trolard et al., 1995). The incorporation of Cu and Zn in the structure of natural goethite is more surprising because these elements have a lower geochemical affinity for Fe oxides, and a higher affinity for Mn oxides and phyllosilicates, than chromium. For instance, Manceau et al. (2000) showed that Zn released by the weathering of willemite (Zn2SiO4) and franklinite (ZnFe2O4) in contaminated sandy soils is predominantly taken up by phyllosilicates, and secondarily by birnessite and Fe oxides. This study suggests that goethite plays a significant role in the geochemical cycle and mass balance of copper and zinc in lateritic soils because this mineral is generally the most abundant in thick oxidized horizons (Ku¨hnel et al., 1978; Herbillon and Nahon, 1988; Muller et al., 1995 and references therein). Trolard et al. (1995) suggested that Cu is partly associated with Fe-Cr spinel remnants from the bedrock. The fraction of Cu and Cr in this phase should be low (⬍10%); otherwise, noticeable differences between EXAFS spectra for F40 and synthetic Cu-substituted goethite would be observed. The ionic radii of Cu (0.73 Å) and Zn (0.74 Å) are ⬃14% greater than Fe (0.645 Å) and, consequently, their substitutional incorporation in the ␣FeOOH framework should result in a displacement of higher shells relative to those in goethite. The differences in Me-Fe1, Me-Fe2, and Me-Fe3 distances (Me ⫽ Fe, Cu, Zn) are within the precision of EXAFS and, therefore, no significant steric effect was observed around Cu and Zn. A similar observation was reported by Reeder et al. (1999) for the incorporation of TEs in calcite, and the compliance of the Ca site to substitutional impurities was attributed to the cornersharing topology of the carbonate structure. A similar interpretation can be proposed for goethite as local variations in interatomic distances can be compensated simultaneously by a displacement of nearest atomic shells (i.e., relaxation effect) and a modification of interpolyhedral angles between cornerlinked octahedra in adjacent chains (i.e., compliance effect). 4.2. Crystal Chemistry and Geochemical Behavior of Mn and Co In oxidizing environments, Mn is predominantly present in trivalent and tetravalent oxidation states. Mn4⫹ and Mn3⫹ are, like Fe3⫹, highly hydrolyzable and therefore sparingly soluble. There is plenty of evidence at the field, microscopic, and atomic scales that Mn and Fe do not form solid solutions but precipitate as distinct phases. Oceanic and soil nodules are well-known examples of the geochemical association of Mn and Fe and, also, of the absence of Mn-Fe mixed solids because Fe is generally present as goethite and hematite, and Mn as phyllomanganate and tectomanganate (Childs, 1975; Burns and Burns, 1977a; Jackson and Bistricki, 1995; Zhang and Karathanasis, 1997; Banerjee et al., 1999). Minute poorly crystalline Mn oxide grains are often observed in ferruginous laterites developed on ultramafic rocks. They generally occur as a discontinuous black diffuse layer in the red saprolite (Llorca, 1987). Therefore, the presence of a Mn oxide impurity in F40 is geochemically sound. EXAFS results indicate that Mn is precipitated as a phyllomanganate with a hexagonal local struc-

ture such as lithiophorite, hexagonal birnessite, or asbolane. Manganese is at least partly speciated as asbolane because this mineral was unambiguously identified by Ni-EXAFS, but the co-existence of other Mn-layer species cannot be excluded. The atomic-scale association of cobalt with the phyllomanganate impurity in F40 agrees with a number of field observations and chemical analyses (Burns and Burns, 1977b; Ku¨hnel et al., 1978; Ostwald, 1984; Taylor, 1968; Taylor and McKenzie, 1966). The affinity of Co for Mn oxides has been explained by the oxidation of soluble divalent cobalt to insoluble trivalent cobalt by Mn3⫹/Mn4⫹ in birnessite (Murray and Dillard, 1979; Manceau et al., 1997). That the phyllomanganate present in F40 contains less Mn3⫹ than synthetic monoclinic and hexagonal birnessite is consistent with a redox reaction involving the coupled oxidation of Co2⫹ to Co3⫹ and reduction of layer Mn3⫹ to soluble Mn2⫹. However, the exact nature of the Co-bearing manganese species remains unknown. The striking resemblance of Co-EXAFS spectra for F40 and synthetic Cosorbed birnessite, where cobalt was shown to migrate to vacancy sites of the phyllomanganate layer after its oxidation (Manceau et al., 1997), suggests a similar immobilization mechanism. Although lithiophorite was not identified in this study, the uptake of cobalt in the MnO2 layer of this species cannot be excluded because this phyllomanganate was found in Ni ores developed on serpentines (Llorca, 1987; Manceau et al., 1987). The crystal chemistry of cobalt in asbolane is still ambiguous. Selected area electron diffraction coupled with energy dispersive spectroscopy analysis suggested the existence of separate CoOOH-like layers regularly mixed with MnO2 layers (Manceau et al., 1992). But, on the other hand, the CoAsb spectrum was shown to resemble Co-sorbed birnessite S1 and not CoOOH, suggesting a Co for Mn substitution (Manceau et al., 1997). 4.3. Crystal Chemistry and Geochemical Behavior of Ni The serpentinization of Ni-containing peridotites in tropical and subtropical regions leads to thick and highly differentiated lateritic covers. In silicated Ni ores, the saprolitic horizon in contact with the serpentinized rock contains neoformed nickeloan kerolite and serpentine veins (the so-called garnierites, Brindley and Wan, 1975; Brindley et al., 1979), which meander between boulders of fresh rock (Ku¨hnel, 1978; Troly et al., 1979; Melfi et al., 1979; Nahon et al., 1982; Trescases, 1986). This coarse and compact silicated horizon is overlain by an earthy saprolite layer formed by a mixture of clays and Fe oxides, namely goethite and hematite. The upper part of these lateritic profiles resembles a typical reddish limonitic horizon devoid of silicates, and often topped by a hard hematitic and goethitic crust. Because F40 was collected in an earthy saprolite (Trolard et al., 1995) and contains 3.66% SiO2, the presence of a residual Ni-clay impurity was likely. However, EXAFS provides no evidence for a Ni clay species, suggesting that F40 is representative of the crystal chemistry of TEs in limonites. In metallogenic deposits from South Africa (Waal, 1971) and Australia (Zeissink, 1969), the Ni concentration is in excess of a few percent in the limonite, and Fe oxides are exploited instead of garnierites in silicated ores. Electron microprobe analyses showed that the concentration of Ni is correlated to Fe at the micron scale in limonite (Ku¨hnel et al.,

Crystal chemistry of trace elements in goethite

1978), but this does not prove that the two elements are associated at the atomic scale because Ni can be present as discrete micrometer-size mineral species. We show here that nickel is partly incorporated in the structure of natural goethite and, therefore, Ni-substituted goethite could be the major form of Ni in oxidized ores. Partitioning of Ni between goethite and the phyllomanganate impurity is consistent with the overall mineralogy of this type of geological setting because Mn oxide precipitates are generally metalliferous (Ostwald, 1984; Ostwald, 1985). For instance, lithiophorite and asbolane containing several percent NiO were found in mineralized zones of oxidized horizons of New Caledonian Ni laterites (Llorca, 1987; Manceau et al., 1987). Acknowledgments—The authors acknowledge scientific advice from V. A. Drits and comments from two anonymous reviewers. Editorial handling by K. V. Ragnardsdottir is also appreciated. We are grateful to the ESRF at Grenoble for the provision of beamtime. Special handling: K. V. Ragnarsdottir REFERENCES Baes C. F. and Mesmer R. E. (1976) The Hydrolysis of Cations. John Wiley and Sons. Banerjee R., Roy S., Dasgupta S., Mukhopadhyay S., and Miura H. (1999) Petrogenesis of ferromanganese nodules from east of the Chagos Archipelago, Central Indian Basin, Indian Ocean. Mar. Geol. 157, 145–158. Bernstein L. R. and Waychunas G. A. (1987) Germanium crystal chemistry in hematite and goethite from Apex Mine, Utah, and some data on germanium in aqueous solution and in stottite. Geochim. Cosmochim. Acta 51, 623– 630. Brindley G. W. and Wan H. M. (1975) Compositions, structures and thermal behaviour of nickel-containing minerals in the lizarditenepouite series. Am. Mineral. 60, 863– 871. Brindley G. W. and Kao C.-C. (1984) Structural and IR relations among brucite-like divalent metal hydroxides. Phys. Chem. Mineral. 10, 187–191. Brindley G. W., Bish D. L., and Wan H. M. (1979) Compositions, structures and properties of nickel-containing minerals in the kerolite-pimelite series. Am. Mineral. 64, 615– 625. Burns R. G. (1970) Mineralogical Application of Crystal Field Theory. Cambridge Univ. Press. Burns R. G. (1976) The uptake of cobalt into ferromanganese nodules, soils, and synthetic manganese (IV) oxides. Geochim. Cosmochim. Acta 40, 95–102. Burns R. G. and Burns V. M. (1977a) Mineralogy of ferromanganese nodules. In Marine Manganese Deposits (ed. G. P. Glasby), pp. 185–248. Elsevier. Burns R. G. and Burns V. M. (1977b) The mineralogy and crystal chemistry of deep-sea manganese nodules - a polymetallic resource of the twenty-first century. Phil. Trans. R. Soc. London A286, 283–301. Carlson L. (1995) Aluminum substitution in goethite in lake ore. Bull. Geol. Soc. Finland 67, 19 –28. Charlet L. and Manceau A. (1992) X-ray absorption spectroscopic study of the sorption of Cr(III) at the oxide/water interface. II Adsorption, coprecipitation and surface precipitation on ferric hydrous oxides. J. Coll. Interf. Sci. 148, 25– 442. Charnock J. M., England K. E. R., Farquhar M. L., and Vaughan D. J. (1995) A REFLEXAFS study of metal adsorption on a mica surface. Physica B209, 457– 458. Cheah S. F., Brown G. E., and Parks G. A. (1998) XAFS spectroscopy study of Cu(II) sorption on amorphous SiO2 and ␥-Al2O3: Effect of substrate and time on sorption complexes. J. Coll. Interf. Sci. 208, 110 –128. Childs C. W. (1975) Composition of iron-manganese concretions from some New Zealand soils. Geoderma 13, 141–152.

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