# Vertical changes of the Co and Mn speciation along a lateritic regolith developed on peridotites (New Caledonia)

Gabrielle Dublet, Farid Juillot, Jessica Brest, Vincent Noel, Emmanuel Fritsch, Olivier Proux, Luca Olivi, Florian Ploquin, Guillaume Morin

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Gabrielle Dublet, Farid Juillot, Jessica Brest, Vincent Noel, Emmanuel Fritsch, et al.. Vertical changes of the Co and Mn speciation along a lateritic regolith developed on peridotites (New Caledonia). Geochimica et Cosmochimica Acta, 2017, 217, pp.1-15. 10.1016/j.gca.2017.07.010. hal-01765615

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Vertical changes of the Co and Mn speciation along a
lateritic regolith developed on peridotites (New Caledonia)

Gabrielle Dublet<sup>a,b,c,*</sup>, Farid Juillot<sup>a,d</sup>, Jessica Brest<sup>a</sup>, Vincent Noël<sup>a,e</sup>,
Emmanuel Fritsch<sup>a,d</sup>, Olivier Proux<sup>f,g</sup>, Luca Olivi<sup>h</sup>, Florian Ploquin<sup>a</sup>,
Guillaume Morin<sup>a</sup>

Received 21 September 2016; accepted in revised form 6 July 2017; available online 21 July 2017

Abstract

Because they can host significant amounts of Co, the Mn-oxides are commonly considered as the major Co-bearing min-
eral species in lateritic environments. However, little is known about the process leading to the formation and/or the weath-
ering of these Co-rich Mn-oxides. This lack of knowledge is mainly due to the fact that Co concentrations are too low in
primary silicates for classical speciation analysis. In this study, we investigated both Co and Mn speciation in a 64 m thick
lateritic regolith developed upon peridotites in New Caledonia, by combining High Energy Resolution Fluorescence Detec-
tion X-ray absorption Near Edge Structure (HERFD-XANES) spectroscopy at the Co K-edge with classical XANES spec-
troscopy at the Mn K-edge, bulk chemistry (ICP) and mineralogy (XRD). The results obtained provide new insights into the
evolution of Co and Mn speciation as a function of the weathering stages. Co and Mn primarily occur as Co(II) and Mn(II,
III) in olivine and serpentine in the bedrock. During the first weathering stage, these forms of Co and Mn are progressively
oxidized toward Co(III) and Mn(III,IV), which occur mainly as Co(III)-bearing Mn(III/IV)-oxides in the transition between
the saprolite and the laterite. In the uppermost lateritic horizons, long-time weathering resulted in a strong leaching of Co and
Mn, and the remaining of these elements occurs as Co(II) and Mn(III) substituting for Fe(III) in goethite. This latter scav-
enging process emphasizes the importance of Fe-oxides for the long-term stabilization of Co and Mn in such deeply weathered
laterites.

Keywords: Laterite; Cobalt; Manganese; Speciation; Weathering; X-ray absorption spectroscopy

<footnote><sup>a</sup> Institut de Minéralogie et de Physique des Matériaux et de Cosmochimie (IMPMC), UMR CNRS 7590, UR IRD 206, Université Pierre
et Marie Curie (UPMC), Campus Jussieu, 75252 Paris Cedex 05, France</footnote>

<footnote><sup>b</sup> CEREGE, UMR 6635 CNRS/Aix-Marseille Université, Europôle de l'Arbois, 13545 Aix-en-Provence, France</footnote>

<footnote><sup>c</sup> Ecole Polytechnique Fédérale de Lausanne (EPFL), Environmental Microbiology Laboratory (EML), EPFL-ENAC-IIE-EML,
Station 6, CH-1015 Lausanne, Switzerland¹</footnote>

<footnote><sup>d</sup> Centre IRD Noumea, 101 Promenade Roger Laroque, BP A5, 98848 Noumea Cedex, New Caledonia</footnote>

<footnote><sup>e</sup> Stanford Synchrotron Radiation Lightsource (SSRL), 2575 Sand Hill Road, Menlo Park, CA 94025, USA</footnote>

<footnote><sup>f</sup>Observatoire des Sciences de l'Univers de Grenoble, BP 53, 38041 Grenoble Cedex 9, France</footnote>

<footnote><sup>g</sup> European Synchrotron Radiation Facility (ESRF), FAME Beamline, French CRG, BP 220, F-38043 Grenoble Cedex, France</footnote>

<footnote><sup>h</sup>Sincrotrone Trieste (ELETTRA), Area Science Park, Strada Statale, 34012 Basovizza, Trieste, Italy</footnote>

<footnote>* Corresponding author at: Ecole Polytechnique Fédérale de Lausanne (EPFL), Environmental Microbiology Laboratory (EML), EPFL-ENAC-IIE-EML, Station 6, CH-1015 Lausanne, Switzerland.</footnote>

<footnote>*E-mail address:* gabrielle.dublet@epfl.ch (G. Dublet).</footnote>

<footnote n="1">¹ Present address.</footnote>
# 1. INTRODUCTION

Peridotites contain high amounts of cobalt (Co) (i.e. 0.01–0.04 wt% Co), compared to the WHO (2006) estimation of 0.0020–0.0025 wt% Co for the Earth crust. The long-term weathering of these rocks under tropical climate, can then lead to exceptional Co contents (several wt% Co) in lateritic covers that can be mined as Co-ores (Wilburn, 2012; Butt and Cluzel, 2013). The knowledge of Co speciation in these lateritic ores is useful to understand how they form and to optimize their processing (Coto et al., 2008; Watling et al., 2011). It is also crucial for predicting and managing the potential impacts of mining activities on the mobility and toxicity of this metallic element (Sasmaz and Yaman, 2006; Collins and Kinsela, 2010). Co is an essential or beneficial element for living organisms but, depending on the Co species, can be toxic when quantities involved become important (Kamnev et al., 2004; WHO, 2006; Collins et al., 2010; Simonsen et al., 2012; Bresson et al., 2015).

According to several studies, Co is generally considered to be associated with Mn-oxides in soil environments (Taylor, 1968; Burns, 1976; Means et al., 1978; Manceau et al., 2000; Beak et al., 2011). Such preferential association of Co with Mn-oxides has been observed in separate mineral phases from lateritic covers in New Caledonia (Llorca and Monchoux, 1991; Manceau et al., 2000), Cuba (Roque-Rosell et al., 2010) and Cameroon (Dzemua et al., 2013). However, selective chemical extractions of laterite samples from New Caledonia also suggested that a minor fraction of Co can be associated with Fe-(oxyhydr)oxides (Quantin et al., 2002). Such association is supported by laboratory experiments showing that Co(II) can efficiently sorb at the surface of goethite, with a behavior similar to that of Ni(II) (Coughlin and Stone, 1995). These considerations raise the question of the actual role of Mn- vs. Fe-(oxihydr)oxides on Co speciation at the whole scale of lateritic regoliths. In particular, the speciation of Co and Mn in peridotites is not well known and, to our knowledge, the effect of the weathering of these ultramafic rocks on the evolution of Co and Mn speciation has never been assessed.

New Caledonia is among the few places in the world where peridotites have been weathered under tropical climate, yielding lateritic covers that are locally highly enriched in Co and Mn (Wilburn, 2012; Butt and Cluzel, 2013). The mineralogical sequence of these lateritic regoliths has been extensively described (Trescases, 1975; Golightly, 1981; Dublet et al., 2012, 2015). It results mainly from the alteration of (phyllo)silicates from the bedrock into secondary (phyllo)silicates and Fe- and Mn-(oxihydr) oxides. The first weathering stage, i.e. saprolitization, is dominated by the formation of secondary phyllosilicates, while the later weathering stage, i.e. lateritization, is dominated by the formation of Fe-(oxihydr)oxides. These weathering processes also lead to the formation of Mn-oxides, which have been shown to be concentrated at the transition between the saprolite and laterite units (Llorca and Monchoux, 1991; Fandeur et al., 2009).

In the present study, we used X-ray absorption spectroscopy (XAS) to determine the vertical evolution of Co

and Mn speciation along a lateritic regolith developed upon peridotites and representative of New Caledonian laterites (Dublet et al., 2012, 2015). The use of High Energy Resolution Fluorescence Detected (HERFD) XAS spectroscopy was required for this study because of the high-fluorescence background from the Fe Kβ emission lines, which precludes collecting Co K-edge XAS data on lateritic samples using classical X-ray fluorescence solid-state detectors. These data, together with classical Mn K-edge XAS data, reveal the vertical changes in Mn and Co speciation associated with the differentiation of lateritic regoliths upon peridotites. These results yield useful information for understanding the genesis of Co/Mn ores and for evaluating the long-term stability of Co- and Mn-bearing mineral species in lateritic environments.

# 2. MATERIALS AND METHODS

## 2.1. Samples location and preparation

The samples studied come from a 67 m deep core (S4) drilled by the mining company Vale NC through a whole regolith that is characteristic of the lateritic covers developed upon peridotites in New Caledonia. This core is located at the edge of the plateau of the Kue watershed in the deeply incised peneplain capped by an iron pan in the Goro region (Southern Province; Fig. 1). A previous study by Dublet et al. (2015) showed similarities in chemistry, mineralogy and Ni speciation between the S4 core studied here for Co and Mn speciation and other lateritic regoliths from New Caledonia (Trescases, 1975), including another core (Y5) studied by our group in the Koniambo klippe (Northern Province) (Fandeur et al., 2009; Dublet et al., 2012, 2015).

Each studied sample is representative of a 10 cm to 1 m long section, centered at a specified depth, along the S4 core. One half of each section sample was dried at 105 °C during 12–24 h, ground to powder and analyzed by ICP-AES, before being stored under ambient conditions in sealed containers. These powdered samples were later ana-

<figure><img src="image_2.png" /><figcaption>Fig. 1. Localization of the studied regolith at the scale of New Caledonia (map adapted from Cluzel and Vigier, 2008).</figcaption></figure>
lyzed by powder X-ray Diffraction (XRD) and XAS. The second half of each section, also stored under ambient conditions, was used to prepare hand-polished thin sections after impregnation in epoxy resin for Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy (SEM-EDXS) analysis and Electron Probe Micro-Analyses (EPMA). The precise depth of the samples analyzed by these various techniques is provided in Dublet et al. (2015).

## 2.2. Chemical and mineralogical analyses

Bulk contents of major ($SiO_2$, $MgO$, $Fe_2O_3$, $Al_2O_3$, $TiO_2$, $K_2O$, $CaO$, $Na_2O$, $MnO$, $P_2O_5$) and trace (Ni, Co, Zn, Cu, Cr, V) elements in the powdered samples were determined by ICP-AES at Vale NC (Table A1).

Powder XRD patterns were collected on the natural bulk samples at Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC) of Université Pierre et Marie Curie (UPMC) with a PANALYTICAL® XPert Pro MPD diffractometer equipped with an Xcelerator solid-state detector, and using Co $K\alpha$ radiation to minimize X-ray absorption by Fe. Data were recorded in continuous mode over the 5–100° $2\theta$ – range with 0.017° steps. The following JCPDS reference files of goethite (#00-017-0536), hematite (#00-024-0072), chromite (#00-034-0140), gibbsite (#00-029-0041), talc (#00-019-0770), quartz (#00-005-0490), asbolane (#00-043-1459), lithophorite (#00-041-1378), chlorite (#00-019-0768), enstatite (#00-029-0853), serpentine (#00-018-0779) and olivine (#00-007-0074) were used for mineral identification.

Electron Probe Microanalyses (EPMA) were performed at the Centre d'Analyse des Minéraux de PARIS (CAM-PARIS) of UPMC on thin sections or pressed pellets with a CAMECA SX50 equipped with four Wavelength Dispersive Spectrometers (WDS) and operating at 15 kV and 30 nA.

## 2.3. X-ray absorption spectroscopy (XAS) data collection

Mn K-edge X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) spectra were collected at room temperature in transmission mode on the bending magnet XAFS beamline at the ELETTRA Synchrotron Facility (Trieste, Italy). The energy of the incoming X-ray beam was selected with a Si (111) double-crystal monochromator that was detuned by 20% in order to minimize harmonics. The vertical beam size was slit to 1 mm height to achieve an energy resolution around 0.8 eV. The energy of the Mn K-edge XAS data was calibrated by recording the spectrum of a Mn metal foil in double transmission setup and setting the position of the first maximum of its first derivative to 6539 eV. For these transmission measurements, appropriate mass of finely ground sample were prepared as pressed pellets either as pure material or mixed with cellulose.

Co K-edge XANES and EXAFS spectra were collected at room temperature on the bending magnet FAME beamline at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). The energy of the incoming

X-ray beam was selected with a Si(220) double-crystal monochromator and calibrated by setting the position of the first maximum of the derivative of a Co metal foil at 7709 eV. Two mirrors at 5 mrad (Si mirrors covered by a Rh layer) were used to suppress higher harmonics. Data were collected in fluorescence mode, using a crystal analyzer spectrometers (CAS) based on a Johann-type geometry (Llorens et al., 2012). Two Si(440) crystals with a 0.5 m radius of curvature were used to separate the Co $K\alpha$ fluorescence line from the contribution of the matrix (mainly coming from Fe in the Fe rich samples). The photons diffracted by the analyzer were collected by a Silicon Drift Detector with an energy resolution around 250 eV to discriminate them from the other unwanted photons scattered by the crystals. At the Co K-edge, beamline resolution is close to the intrinsic resolution of the Si(220) crystals of the monochromator, i.e. 0.4 eV (Proux et al., 2006). The total energy bandwidth of the crystal analyzer spectrometer, measured by scanning the monochromator, was estimated to 3.0 eV. Samples were prepared as pressed pellets either as pure material or mixed with cellulose to have less than 1 wt% Co in the pellet.

All Co and Mn K-edges XAS spectra were averaged, baseline substracted and normalized using the software package Athena (Ravel and Newville, 2005). Fast Fourier transforms of the $k^3$-weighted EXAFS data were obtained after weighting by a Kaiser-Bessel window.

## 2.4. LC-LSF analysis of the XANES data

Co and Mn K-edges XANES spectra of the core samples were analyzed with the Linear Combination Least-Square Fitting (LC-LSF) procedure provided by the Athena code (Ravel and Newville, 2005), using model compounds spectra as fitting components. Fit quality was estimated by a reduced $\chi^2$ parameter and uncertainties on the fitted proportions of the components were approximated by their standard deviations, both returned by the non-linear minimizing routine in Athena. Our set of model compounds spectra at the Co and Mn K-edges included synthetic compounds, i.e. Co- or Mn-bearing goethites, co-precipitated Co- or Mn-phylosilicates, Co-sorbed birnessite, co-precipitated Co-siderite, CoOOH and Co(OH)$_2$, manganite, as well as natural model compounds, i.e. Mn-oxides, Co- or Mn-bearing phyllosilicates and Co- or Mn-bearing forsterite. These model compounds were chosen based on the mineralogy found in this laterite profile and in other New Caledonian lateritic materials. Details on the mineralogy and chemical purity of these natural model compounds can be found in Dublet et al. (2012, 2014) and in Fig. A1. The synthesis procedures for the model compounds and the crystal chemistry information obtained from XRD and EXAFS analyses are provided hereafter.

## 2.5. Synthetic model compounds

### 2.5.1. Co-precipitated Co-goethite (Co-Gt)

In an anoxic glove box (<40 ppm $O_2$), 4.5 g of Fe ($NO_3)_3 \cdot 9H_2O$ and 42 mg of $Co(NO_3)_3 \cdot 6H_2O$ were separately dissolved, each in 25 mL of deionized water, and then
mixed in a 1 L polypropylene bottle. A precipitate was obtained by adding 50 mL of 1.35 M NaOH, to a pH value of 13. The volume of the suspension was increased to 1 L with 33 mM NaOH, and the suspension was aged 48 h upon mild stirring in a water bath at 65 °C. The final suspension was then centrifuged 10 min at 10,000 rpm, to separate the solid from the supernatant. The solid was washed for 2 h at 45 °C with 3 M H₂SO₄, then washed 3 times with deionized water and dried under vacuum in a desiccator in the glove box. Assuming that all Co and Fe co-precipitated, the ratio Co/(Co + Fe) of this Co-Gt is 0.01.

### 2.5.2. Goethite co-precipitated with oxidized Co (Co-Gt ox)

In ambient atmosphere, 4.5 g of Fe(NO₃)₃·9H₂O and 42 mg of Co(NO₃)₃·6H₂O were separately dissolved, each in 25 mL of deionized water, and then mixed in a 1 L polypropylene bottle. A precipitate was then obtained by adding at the same time 50 mL of 1.35 M NaOH and 10 mL of 30% H₂O₂ upon strong stirring. The suspension was aged 48 h upon mild stirring in a water bath at 65 °C. The final suspension was then centrifuged 10 min at 10,000 rpm to separate the solid from the supernatant. The solid was washed for 2 h at 45 °C with 3 M H₂SO₄, washed 3 times with deionized water and dried at 45 °C for 24 h. Assuming that all Co and Fe co-precipitated, the ratio Co/(Co + Fe) of this Co-Gt ox is 0.01.

### 2.5.3. CoOOH and Co(OH)₂

In anoxic glove box, 100 mL NaOH of 0.1 M were added dropwise to 160 mL of 0.05 M Co(NO₃)₂·6H₂O and the suspension was aged in a water bath at 45 °C during 24 h. The suspension was then centrifuged in order to separate the solid from the supernatant and the solid collected was washed 3 times before being split into 2 sub-samples, (A) and (B). The Co(OH)₂ compound was obtained by directly drying the sub-sample (A) in a desiccator (Fig. A1). The sub-sample (B) was suspended in 20 mL of deionized water and stirred in a water bath at 45 °C. Then, 10 mL of 8 M NaOH and 4 mL of 30% H₂O₂ were added dropwise and simultaneously in the suspension, which was further aged in the water bath for 18 h. The suspension was then centrifuged at 10,000 rpm for 10 min and a brown precipitate was collected, washed with deionized water and dried at 65 °C for 2 days. The XRD powder pattern of this precipitate indicated that it was CoOOH (Fig. A1).

### 2.5.4. Co(II)-bearing serpentine (Co-Srp)

In a polyethylene flask, 29 mL of 0.2 M Mg(NO₃)₂·6H₂O, 1 mL of 0.2 M Co(NO₃)₂·6H₂O, 20 mL of 0.2 M Na₂SiO₃·5H₂O and 20 mL of 0.2 M NaOH were mixed and stirred during 1 h at room temperature. The suspension was then centrifuged at 10,000 rpm for 10 min and the solid collected was washed 3 times with deionized water and dried during 24 h at 40 °C. The dried solid was then ground in an agate mortar, and about 250 mg of the powdered solid were suspended in 30 mL of deionized water and further autoclaved at 210 °C under saturated water pressure in a 50 mL Teflon® coated sealed container for 15 days. During that time, the container was shaken manually 2 times per

day to ensure homogeneity during the transformation of the solid. After 15 days, a poorly crystalline serpentine with a Mg₂.₉Co₀.₁Si₂O₅(OH)₄ theoretical formula (i.e. with a ratio Co/(Co + Mg) of 0.03) was obtained (Fig. A1).

### 2.5.5. Co-sorbed birnessite (Co-Bir)

Birnessite was first synthesized as described in Dublet et al. (2012). Then, in a N₂-filled anoxic glove box, 150 mg of this birnessite were suspended in 100 mL of N₂-sparged deionized water and sonicated during 15 min. 800 mg of KNO₃ were dissolved in 900 mL N₂-sparged deionized water and added to the birnessite suspension. The pH of this mixture was adjusted to 5 with 0.1 M HNO₃, and it was regularly re-adjusted to 5 for more than 12 h before reaching stability. After that time, 9.3 mg of Co(NO₃)₂·6H₂O were dissolved in 100 mL of N₂-sparged deionized water and added to the birnessite suspension. After 24 h stirring at room temperature, the suspension was centrifuged 10 min at 8500 rpm and the solid collected was washed 2 times with deionized water and vacuum-dried. The mineral purity of the Co-sorbed birnessite obtained was checked by XRD (Fig. A1). Assuming that all of Co is sorbed on birnessite, and using a birnessite formula of (K₀.₅)(Mn⁴⁺, Mn³⁺)₂O₄·1.5 H₂O, we can overestimate a ratio Co/(Co + Mn) of 0.02.

### 2.5.6. Co-precipitated Mn-goethite (Mn-Gt)

The recipe for Mn-Gt was the same as that for Co-Gt, but using 4.98 g Fe(NO₃)₃·9H₂O, 200 mg Mn(NO₃)₂·4H₂O and 39 mg Ni(NO₃)₂·6H₂O. The theoretical ratio Mn/(Mn + Ni + Fe) is 0.06.

### 2.5.7. Co-precipitated Mn-serpentine (Mn-Srp)

The co-precipitated Mn-serpentine was prepared in the same way as the co-precipitated Co-serpentine, but with 25 mL of 0.2 M Mg(NO₃)₂·6H₂O and 5 mL of 0.2 M Mn(NO₃)₂·6H₂O. The theoretical formula of this Mn-coprecipitated serpentine is expected to be Mg₂.₅Mn₀.₅Si₂O₅(OH)₄ (so the ratio Mn/(Mn + Mg) is 0.17).

### 2.5.8. co-precipitated Mn-kerolite (Mn-Ker)

In a polyethylene flask, 24.5 mL of 0.2 M Mg(NO₃)₂·6H₂O, 5.5 mL of 0.2 M MnCl₂·4H₂O, 40 mL of 0.2 M Na₂SiO₃·5H₂O and 40 mL of 0.2 M NaOH were mixed and stirred for 1 h under ambient conditions. The suspension was then centrifuged 10 min at 7500 rpm and the solid collected was washed 3 times with deionized water before being vacuum dried in a desiccator. The theoretical ratio Mn/(Mn + Mg) is 0.17.

### 2.5.9. Mn(OH)₂ and MnOOH (manganite)

In a polyethylene bottle placed in a water bath at 45 °C and in a N₂ filled anoxic glove box, 100 mL of 0.1 M NaOH were added dropwise to 160 mL of a 0.05 M MnCl₂·4H₂O solution. After 24 h, the suspension was centrifuged and the solid collected was washed 3 times, before splitting in 2 sub-samples, (A) and (B). The Mn(OH)₂ compound was obtained by vacuum drying the sub-sample (A) overnight. The sub-sample (B) was suspended in 40 mL of deionized water and stirred in a water bath at 45 °C for 18 h. During
that stirring, 50 mL of 8 M NaOH and 10 mL of 30% H₂O₂ were simultaneously added dropwise in the suspension. The suspension was then centrifuged, the solid collected was washed 3 times with deionized water and dried at 65 °C for 2 days. MnOOH (manganite) was identified by XRD as the only mineral phase in the final product (Fig. A1).

### 3. RESULTS

#### 3.1. Co and Mn contents in bulk samples

In the studied regolith, bulk Co contents (ranging from 0.01 to 0.41 wt%; Fig. 2a, Table A1) are globally five times lower than bulk Mn contents (ranging from 0.08 to 2.81 wt %; Fig. 2a, Table A1). Co and Mn contents remain at their minimum in the bedrock and in the saprolite units, whereas they are several orders of magnitude higher in the laterite (Fig. 2a). The change in the Co and Mn contents between the saprolite and the laterite is sharp, as are the mineralogical and geochemical changes between these two distinct units (Dublet et al., 2012, 2015; Fig. 2a and b). In the laterite units, the Co and Mn contents are variable and they globally decrease toward the top of the regolith. Discrepancies between Co and Mn contents begin to be significant in the uppermost horizons, i.e. above 22 m (Fig. 2a). Altogether, Co and Mn contents are correlated along the

<figure><img src="image_3.png" /><figcaption>Fig. 3. Co content as a function of Mn content in log scales in the studied regolith. Samples collected above and below 22 m are respectively displayed in red and black. The coefficient of the linear regression (R²) between Co and Mn contents is 0.94 for the whole data set, but it falls to 0.23 for the data set corresponding to the samples above 22 m.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 2. Crystal chemistry of Co and Mn along the studied regolith. (a) Co and Mn contents as a function of depth (note that the scale is x5 for Co). Horizontal lines localise the samples selected for Co and Mn K-edges XAS analyses. (b) XRD patterns showing the occurrence of serpentine (srp), olivine (ol), goethite (gt) as major phases and quartz (qtz), gibbsite (gib), hematite (hem), chromite (chr), asbolane/lithiophorite type phyllomanganate (a/l) and possibly birnessite (bir) as secondary phases. Dashed vertical lines show the expected position for asbolane, lithiophorite and birnessite according to XRD patterns recorded on model compounds (Fig. A3). (c) and (d) Results of the LC-LSF procedure applied on Mn and Co K-edges XANES spectra, respectively. * The Mn-oxide species include a Co-sorbed birnessite, a natural asbolane/lithiophorite mixture and a natural lithiophorite, as detailed in Tables 2 and 3. The arrows “S” and “L” show the extent of the saprolite and laterite units, respectively.</figcaption></figure>
studied regolith, except in the uppermost lateritic samples (above 22 m) where this correlation is weaker (Fig. 3).

## 3.2. Mineralogy of the S4 regolith

The mineralogy of the studied regolith is described in Dublet et al. (2015). Briefly, the bedrock mainly consists of olivine (forsterite), pyroxene (enstatite) and serpentine that are progressively replaced by secondary phyllosilicates (mostly serpentine-like and talc-like minerals; Fritsch et al., 2015) and goethite in the overlying horizons. Phyllosilicates dominate in the saprolite unit, whereas the laterite unit mainly consists of goethite, with a sharp transition between these two units (Fig. 2b).

Bragg peaks at 2θ values around 11° and 22° (Co Kα) on the XRD patterns of Mn-rich samples (e.g. 29.5 m and 32.5 m) suggest the occurrence of the Mn-oxides asbolane and/or lithiophorite (Fig. 2b; Dublet et al., 2015), in agreement with the presence of black mineral phases occurring as veinlets or concretions at similar depths along the studied core (Fig. A2). The large width (Fig. A3) of the XRD peaks suggests that these Mn-oxides are poorly ordered and/or consist of a mixture of variable structural compositions, as it has been already observed elsewhere in New Caledonia (Llorca and Monchoux, 1991). Actually, these broad and weak XRD peaks cannot be easily distinguished from the rather broad and stronger XRD peaks of talc(-like) and chromite in the bulk samples (Fig. 2b and A1; Dublet et al., 2015). However, separate black solids hand-picked in the laterite unit of the same regolith were identified by XRD as asbolane and/or lithiophorite (Fig. A1). In addition, X Ray Fluorescence (XRF), SEM-EDX and EPMA analyses indicate that these hand-picked samples have chemical compositions intermediate between asbolane and lithiophorite (Dublet et al., 2012; Fig. 4). These observations are in agreement with the reported occurrence of asbolane/lithiophorite mixtures in New Caledonia (Llorca and Monchoux, 1991). Furthermore, EPMA analyses

<figure><img src="image_5.png" /><figcaption>Fig. 4. Relative contents of Mn, Co and Al (atom wt%) measured by EPMA in Mn-oxides from the studied regolith. This ternary diagram shows the contribution of lithiophorite-type Mn-oxides (Al₂O₃ end-member) to the Co contents. Reference compositions were calculated on the bases of [1] chemical compositions of natural asbolane and lithiophorite from Manceau et al. (1987) and [2] the theoretical mineral formula from Post and Veblen (1990).</figcaption></figure>

(Table 1, Fig. 4) of Mn-oxides in the core samples (Fig. A2) indicate compositions between those of lithiophorite ((Alₓ,Niᵧ)Mnₓ(IV)Mnᵧ(III)O₂(OH)₂, Post and Appleman, 1994) and asbolane (Mn(O, OH)₂(Ni, Co)ₓ(O, OH)₂·nH₂O, Drits, 1987), with a wide range of Al₂O₃ content, the lowest value (1.8 wt% Al₂O₃) corresponding to asbolane (or birnessite) and the highest one (17.5 wt% Al₂O₃) corresponding to lithiophorite.

The possible occurrence of poorly-crystalline birnessite in the studied regolith cannot be dismissed on the basis of XRD analyses because the position of the main XRD peak of this phyllomanganate (2θ = 14.0 ± 0.2° Co Kα) is close to that of serpentine(-like) (2θ = 14.4 ± 0.2° Co Kα). However, the low Na + K/Mn ratio indicated by EPMA suggests that the Mn-oxides observed in the studied regolith consist in asbolane/lithiophorite rather than in birnessite (Table 1). Overall, mineralogical analyses thus indicate the occurrence of asbolane and lithiophorite as minor constituents of the bulk samples in the Mn-rich horizons of the laterite unit.

## 3.3. XAS analysis of Mn speciation

### 3.3.1. Mn speciation in the model compounds used for XANES data analysis

The main edge of the XANES spectra of model compounds is shifted toward higher energy for model compounds with higher oxidation state (e.g. MnO₂) compared to model compounds with lower oxidation states (e.g. Mn(OH)₂) (Fig. 5). The absorption maximum on the XANES spectrum of the MnO₂ model compound occurs as a sharp peak at about 6562 eV, which is consistent with the XANES fingerprint of Mn(IV) in an Mn-oxide (Bargar et al., 2000). In addition, the XANES of the Asb/lit and Bir model compounds are very similar to that of MnO₂. This suggests that these model compounds contain mainly Mn(IV), consistently with previous studies of Mn speciation in phylloman ganates (Manceau et al., 1992a). In addition, the first shell of oxygens indicated by shell-by-shell fitting of the EXAFS spectra of these model compounds is found at a short distance of 1.89 ± 0.02 Å (Table A2). On the opposite, the XANES fingerprint of the Mn(OH)₂ model compound shows a sharp absorption maximum at ~6553 eV, which is consistent with other Mn(II) compounds such as aqueous Mn(II) (Bargar et al., 2000). The first shell of oxygens indicated by shell-by-shell fitting of the EXAFS spectrum of this model compound is found at a longer distance, with 6 O at 2.15 ± 0.01 Å (Table A2).

The XANES fingerprint of the Mn-goethite reference shows an inflexion energy and an absorption maximum (~6558 eV) at intermediate positions between those for MnO₂ and Mn(OH)₂, with a flatter shape of the maximum (Fig. 5). In addition, the first shell of oxygens obtained by shell-by-shell fitting of the EXAFS spectrum of this model compound is found at an intermediate distance of 1.94 ± 0.04 Å (Table A2). We conclude that the oxidation state of Mn is Mn(III) in our Mn-goethite model compound, which is consistent with previous Mn-goethites reported (Manceau et al., 2000). Furthermore, our shell-by-shell analysis of the second shell of atoms around Mn yields 3
<table><caption>Table 1
Average chemical composition (wt% oxides) of the main mineral species analyzed by EPMA at various depths along the studied regolith.</caption><thead><tr><th>Phase</th><th>Depth (m)<sup>*</sup></th><th>SiO<sub>2</sub></th><th>MgO</th><th>Fe<sub>2</sub>O<sub>3</sub> or FeO<sup>**</sup></th><th>Al<sub>2</sub>O<sub>3</sub></th><th>Cr<sub>2</sub>O<sub>3</sub></th><th>NiO</th><th>MnO</th><th>CoO<sup>***</sup></th><th>Na<sub>2</sub>O</th><th>K<sub>2</sub>O</th><th>Total</th></tr></thead><tbody><tr><td rowspan="3">Goethite</td><td>0–7</td><td>(n = 73)</td><td>1.0(9)</td><td>0.3(3)</td><td>77(2)</td><td>4.1(4)</td><td>2.6(3)</td><td>0.6(5)</td><td>0.2(1)</td><td>&lt;0.1</td><td>&lt;0.02</td><td>&lt;0.01</td><td>86(2)</td></tr><tr><td>7–32</td><td>(n = 304)</td><td>2(2)</td><td>0.3(4)</td><td>71(10)</td><td>5(2)</td><td>1.3(6)</td><td>1.2(8)</td><td>0.4(5)</td><td>&lt;0.1</td><td>0.02(6)</td><td>0.01(5)</td><td>75(9)</td></tr><tr><td>32–33</td><td>(n = 93)</td><td>2.9(15)</td><td>0.3(2)</td><td>65(10)</td><td>4(2)</td><td>0.9(12)</td><td>1.6(12)</td><td>0.9(6)</td><td>&lt;0.1</td><td>&lt;0.02</td><td>&lt;0.01</td><td>75(10)</td></tr><tr><td rowspan="2">Mn-oxide</td><td>7–32</td><td>(n = 5)</td><td>1.2(4)</td><td>1.4(2)</td><td>7(4)</td><td>3(1)</td><td>0.07(5)</td><td>15(3)</td><td>36(2)</td><td>2(2)</td><td>0.2(1)</td><td>0.11(6)</td><td>65(5)</td></tr><tr><td>32–33</td><td>(n = 33)</td><td>0.6(3)</td><td>0.6(7)</td><td>6(2)</td><td>8(4)</td><td>0.05(5)</td><td>11(3)</td><td>34(5)</td><td>6(2)</td><td>0.03(4)</td><td>0.09(6)</td><td>65(6)</td></tr><tr><td rowspan="3">Chromite</td><td>7–32</td><td>(n = 12)</td><td>0.2(2)</td><td>7(3)</td><td>23(8)</td><td>19(5)</td><td>38(6)</td><td>0.2(3)</td><td>0.4(2)</td><td>0.01(2)</td><td>0.02(2)</td><td>0.01(1)</td><td>87(8)</td></tr><tr><td>32–33</td><td>(n = 6)</td><td>0.3(3)</td><td>7(3)</td><td>23(3)</td><td>21(2)</td><td>47(3)</td><td>&lt;0.05</td><td>0.3(2)</td><td>&lt;0.1</td><td>0.02(2)</td><td>0.01(2)</td><td>92(8)</td></tr><tr><td>33–66</td><td>(n = 3)</td><td>1.2(8)</td><td>10(3)</td><td>21(2)</td><td>15.9(7)</td><td>47(1)</td><td>0.12(8)</td><td>0.37(3)</td><td>&lt;0.1</td><td>&lt;0.02</td><td>0.03(3)</td><td>95(1)</td></tr><tr><td>Talc</td><td>33–66</td><td>(n = 21)</td><td>54(5)</td><td>27(5)</td><td>4(3)</td><td>0.8(8)</td><td>0.3(4)</td><td>0.5(10)</td><td>0.05(5)</td><td>&lt;0.1</td><td>0.2(2)</td><td>&lt;0.01</td><td>85(9)</td></tr><tr><td rowspan="2">Serpentine</td><td>33–66</td><td>(n = 252)</td><td>40(3)</td><td>36(3)</td><td>6(2)</td><td>0.4(6)</td><td>0.1(3)</td><td>1(1)</td><td>0.08(6)</td><td>&lt;0.1</td><td>&lt;0.02</td><td>0.01(1)</td><td>85(3)</td></tr><tr><td>66</td><td>(n = 4)</td><td>39(1)</td><td>39(2)</td><td>5.3(7)</td><td>0.4(4)</td><td>0.2(3)</td><td>0.3(2)</td><td>0.07(4)</td><td>&lt;0.1</td><td>&lt;0.02</td><td>&lt;0.01</td><td>84(2)</td></tr><tr><td rowspan="2">Olivine</td><td>33–66</td><td>(n = 6)</td><td>42(2)</td><td>49(2)</td><td>7.6(6)</td><td>0.02(3)</td><td>0.02(3)</td><td>0.50(9)</td><td>0.10(5)</td><td>&lt;0.1</td><td>&lt;0.02</td><td>&lt;0.01</td><td>99(2)</td></tr><tr><td>66</td><td>(n = 11)</td><td>41(2)</td><td>49(2)</td><td>8.1(4)</td><td>0.01(2)</td><td>&lt;0.04</td><td>0.4(1)</td><td>0.12(3)</td><td>&lt;0.1</td><td>&lt;0.02</td><td>&lt;0.01</td><td>99(4)</td></tr><tr><td rowspan="2">Pyroxene</td><td>33–66</td><td>(n = 17)</td><td>58(1)</td><td>33(1)</td><td>4.8(2)</td><td>1.38(9)</td><td>0.65(2)</td><td>0.1(1)</td><td>0.13(4)</td><td>&lt;0.1</td><td>0.02(2)</td><td>0.01(2)</td><td>99(1)</td></tr><tr><td>67</td><td>(n = 12)</td><td>58(1)</td><td>34.1(8)</td><td>5.3(3)</td><td>1.19(8)</td><td>0.53(7)</td><td>0.1(1)</td><td>0.14(4)</td><td>&lt;0.1</td><td>&lt;0.02</td><td>0.01(1)</td><td>100(2)</td></tr></tbody></table>

/: Not analyzed.

Note: Results are given in wt%. n stands for the total number of analyses. The results reported correspond to average values calculated for each depth unit.

* The depth units are presented to match the regolith units as defined in Dublet et al. (2015): red laterite (0–7 m), yellow laterite (7–32 m) transition laterite (32–33 m), saprolite (33–67 m) and the bedrock (67 m).

** Fe contents were calibrated with hematite and are given as Fe₂O₃ wt% for goethite and Mn-oxides whereas they were recalculated as FeO wt% for serpentine, olivine, pyroxene, amphibole and chromite.

*** 0.1 is the limit of quantification during the EPMA analyses of this study. CoO was detected in each mineral phase but the concentrations measured are not significant.
<figure><img src="image_6.png" /><figcaption>Fig. 5. XANES spectra of the Mn K-edge (left) and Co K-edge (right) model compounds used during the LC-LSF analysis. MnO₂: synthetic manganese dioxide; Bir: synthetic birnessite; Asb/Lit: natural asbolane/lithiophorite sample from Goro, New Caledonia (Dublet et al., 2012); Mn-Gt: co-precipitated Mn-goethite; Cal Srp: natural serpentine from California (Dublet et al., 2012); Mn-Srp: co-precipitated Mn-serpentine; Mn-Ker: co-precipitated Mn-Kerolite; Olivine: natural olivine forsterite (Dublet et al., 2012); Manganite: synthetic MnOOH; Mn(OH)₂: synthetic manganese hydroxide; Rhodo: natural rhodochrosite. Co-Bir: Co-sorbed birnessite; Lithio: natural lithiophorite; Asb/Lit: natural asbolane/lithiophorite sample from Goro, New Caledonia; Co-Srp: co-precipitated Co-serpentine; Olivine: natural olivine forsterite; Co-Gt: co-precipitated Co-goethite; Co-Gt ox: goethite co-precipitated with oxidized Co.</figcaption></figure>

± 5 Fe at 3.01 ± 0.05 Å and 2 ± 5 Fe at 3.34 ± 0.04 Å (Table A2). Although the errors are rather high, the number of Fe neighbors (theoretically 8 in total) and the distances found (theoretically about 2 Fe at 3.0 Å, 2 Fe at 3.3 Å and 4 Fe at 3.4 Å) are compatible with Mn(III) structurally incorporated in goethite (Manceau et al., 2000).

The XANES fingerprints of the phyllosilicate model compounds are still different, with several absorption maxima at ~6558 eV and ~6553 eV. These positions of absorption maxima suggest a mix of oxidation states (e.g., Mn(II) and Mn(III)). This is consistent with the fact that the shell-by-shell fit of the EXAFS spectrum of the Mn-serpentine model compound resulted in two distances of Mn-O at

1.9 Å and 2.2 Å (Table A2). Finally, the XANES fingerprint of the olivine model compound shows an absorption maximum at ~6553 eV, suggesting that Mn occurs mainly as Mn(II) in this silicate, and a feature at ~6558 eV that could suggest a small contribution of Mn(III). However, the Mn content of olivine did not allow recording EXAFS data for shell-by-shell fitting and for supporting this statement.

### 3.3.2. Mn speciation in the regolith samples

Two absorption maxima of equal intensities can be observed at 6553 eV and 6558 eV on the XANES spectrum of the bedrock sample (67.4 m), which indicates the pres-
<figure><img src="image_7.png" /><figcaption>Fig. 6. Mn (left) and Co (right) K-edges XANES spectra showing the redox evolution of Mn and Co as a function of depth along the studied regolith. LC-LSF fits are shown as dotted red lines. The proportions of the fitting components are reported in Tables 2 and 3 for Mn and Co, respectively.</figcaption></figure>

ence of both Mn(II) and Mn(III) (Fig. 6). These two features are also present on the XANES spectrum of the sample at 60.2 m, but that of Mn(III) (i.e. 6558 eV) is stronger than that of Mn(II) (i.e. 6553 eV). In addition, a third feature at 6562 eV, indicative of Mn(IV), is also visible on the XANES spectrum of this sample. This feature at 6562 eV becomes dominant on the XANES spectrum of the sample at 56.4 m, and it is almost the only feature visible on the XANES spectra of the samples at 32.5 m and 29.5 m. The feature at 6562 eV (i.e. Mn(IV)) is still predominant on the XANES spectra of the samples at 23.5 m and 14.8 m, but a shoulder can be seen on the low-energy side of the white line, which suggests the occurrence of the feature at 6558 eV (i.e. Mn(III)). On the XANES spectra of the uppermost laterite samples (12.5 m and 2.7 m), the feature at 6558 eV (i.e. Mn(III)) increases at the expense of that at 6562 eV (i.e. Mn(IV)). This vertical evolution of the Mn K-edge XANES spectra is similar to the one that was previously reported for samples collected along another lateritic regolith in the Northern Province of New Caledonia (Y5 core drilled at the Koniambo; Fandeur et al., 2009).

In order to identify the Mn host phases, the XANES spectra of the samples were analyzed by a LC-LSF procedure using appropriate components selected from our model compounds data set (Fig. 5; Fig. A4; Table A2). LC-LSF results indicate that Mn is hosted by serpentine (60%) and olivine (40%) in the bedrock (Fig. 2c; Table 2). A significant fraction of Mn (27%) is already sequestered

as Mn-oxides in the sample at 60.2 m, which corresponds to saprolite. In this sample, the remaining fraction of Mn is hosted by serpentine (73%), whereas olivine is no longer a significant Mn host. In the upper part of the saprolite, the fraction of Mn hosted by Mn-oxides increases at the expense of that hosted by serpentine (Mn-oxides account for 27–55% of the total Mn from 60.2 m to 56.4 m). Additionally, in the top part of the saprolite unit, a small but significant fraction of Mn (14% at 56.4 m) is hosted by goethite. This fraction increases upward in the laterite units at the expense of Mn-oxides, first slightly in the bottom part of the laterite (below 12.5 m) and then strongly in the uppermost samples (Fig. 2c; Table 2). Finally, in the uppermost samples analyzed, Mn only occurs as Mn-bearing goethite.

## 3.4. XANES analysis of Co speciation

### 3.4.1. Co speciation in the model compounds used for XANES data analysis

The main edge of the XANES spectra of Co model compounds is significantly shifted toward higher energy for Co (III) model compounds (e.g. CoOOH) compared to Co(II) model compounds (e.g. Co(OH)₂) (Fig. 5). The maximum absorption feature of the XANES spectrum of CoOOH occurs as a sharp peak at about 7730 eV. In addition, the first shell of oxygens indicated by shell-by-shell fitting of the EXAFS spectrum of this model compound is found at
<caption>Table 2</caption>

Results of the LC-LSF analysis of the Mn K-edge XANES spectra collected on selected samples from the studied regolith. The fraction of each component is given in %. Uncertainties obtained with the software package Athena are given in parentheses.

<table><thead><tr><th>Sample/reference</th><th>Natural olivine</th><th>Synthetic serpentine</th><th>Natural Asb/Lit</th><th>Synthetic birnessite</th><th>Total Mn oxides<sup>*</sup></th><th>Synthetic goethite</th><th>Reduced Chi<sup>2</sup></th><th>Sum<sup>**</sup></th></tr></thead><tbody><tr><td colspan="9"><em>S4 profile</em></td></tr><tr><td>2.7 m</td><td></td><td></td><td></td><td></td><td></td><td>100(/)</td><td>2 × 10<sup>-3</sup></td><td>101</td></tr><tr><td>12.5 m</td><td></td><td></td><td></td><td>30(7)</td><td>30(7)</td><td>70(2)</td><td>1 × 10<sup>-3</sup></td><td>101</td></tr><tr><td>14.8 m</td><td></td><td></td><td>31(3)</td><td>36(3)</td><td>67(6)</td><td>33(1)</td><td>3 × 10<sup>-4</sup></td><td>100</td></tr><tr><td>23.5 m</td><td></td><td></td><td>44(2)</td><td>25(3)</td><td>69(5)</td><td>31(1)</td><td>2 × 10<sup>-4</sup></td><td>100</td></tr><tr><td>29.5 m</td><td></td><td></td><td>74(1)</td><td>11(2)</td><td>85(4)</td><td>15(1)</td><td>8 × 10<sup>-5</sup></td><td>100</td></tr><tr><td>32.5 m</td><td></td><td></td><td>77(1)</td><td>9(2)</td><td>86(3)</td><td>14(1)</td><td>2 × 10<sup>-3</sup></td><td>100</td></tr><tr><td>56.4 m</td><td></td><td>31(2)</td><td></td><td>55(4)</td><td>55(4)</td><td>14(5)</td><td>2 × 10<sup>-3</sup></td><td>103</td></tr><tr><td>60.2 m</td><td></td><td>73(0)</td><td></td><td>27(2)</td><td>27(2)</td><td></td><td>4 × 10<sup>-3</sup></td><td>103</td></tr><tr><td>67.4 m</td><td>40(2)</td><td>60(2)</td><td></td><td></td><td></td><td></td><td>4 × 10<sup>-3</sup></td><td>102</td></tr></tbody></table>

a short distance of $1.89 \pm 0.01$ Å (Table A3). On the opposite, the XANES fingerprint of $Co(OH)_2$ shows a sharp absorption maximum at ~7725 eV and the first Co-O distance is found at a longer distance of $2.09 \pm 0.01$ Å. These two model compounds are hereafter used as references for the positions of absorption maxima corresponding to Co (III) and Co(II), respectively, in other (Co, Mn)-oxide compounds.

The XANES fingerprints of the phyllomanganate model compounds (i.e. Co-Bir, Lithio and Asb/lit), are very close to that of COOH, but with an additional small shoulder at ~7725 eV that is slightly more pronounced in the case of Co-Bir. In addition, the first shell of oxygens indicated by shell-by-shell fitting of the EXAFS spectra of these model compounds is found at a short distance of $1.90 \pm 0.01$ Å for Co-Bir and Asb/lit and $1.91 \pm 0.01$ Å for Lithio (Table A3). We conclude that Co occurs mainly as Co (III), and only barely as Co(II), in these phyllomanganate model compounds. The EXAFS fit also shows ~6 Co and/or Mn (these elements cannot be discriminated by XAS due to their similar retrodiffusion properties) as second neighbors at $2.85 \pm 0.02$ Å. This result is consistent with Mn-Me distances found in octahedral layers of phyllomanganates (Manceau et al., 1992b).

The XANES fingerprints of the Co-bearing goethite model compounds (Co-Gt and Co-Gt ox) resemble that of $Co(OH)_2$, with an absorption maximum at 7725 eV (Fig. 5). Consistently, the first shell of oxygens indicated by shell-by-shell fitting of EXAFS spectra of these references is found at a long distance of $2.01 \pm 0.02$ Å (Table A3). We conclude that Co is mainly Co(II) in these two goethite model compounds. In addition, the EXAFS fit of the Co-Gt model compound shows a second shell of neighbors made of Fe atoms at $3.01 \pm 0.03$ Å, $3.21 \pm 0.04$ Å and $3.43 \pm 0.02$ Å (Table A3). This structural environment around Co is consistent with a divalent cation incorporated in the structure of goethite (Carvalho-e-silva et al., 2003).

Finally, the XANES fingerprints of the serpentine (i.e. Co-Srp) and olivine model compounds are different from

both $CoOOH$ and $Co(OH)_2$. Indeed, the absorption maximum on the XANES spectra of these two model compounds is split in two, with a first maximum at ~7725 eV and another at ~7728 eV (Fig. 5). The absorption maximum at 7725 eV, more intense in the case of olivine, suggests that the Co oxidation state is mainly Co(II) in olivine and to a smaller extent Co(II) in Co-srp. The absorption maximum at 7728 eV might be due to the presence of Co(III) in these silicate model compounds. In that case, the 2 eV difference compared to the absorption maximum of $CoOOH$ (7730 eV) could be due to the different electronic structure and atomic environment around Co in silicates compared to those in Co, Mn-(hydro)oxides.

### 3.4.2. Co speciation in the regolith samples

The fingerprints of the Co K-edge XANES spectra vary significantly as a function of depth (Fig. 6). In order to identify the Co-hosting mineral phases, the Co K-edge XANES spectra of the regolith samples were analyzed by LC-LSF. The number of fitting component was chosen after a PCA analysis of the XANES data set, and only the model compounds contributing significantly (>10%) to the fit were retained. The LC-LSF results indicate that Co is hosted by olivine (56%) and serpentine (44%) in the bedrock sample (Fig. 2d; Table 3). At the first stage of weathering (i.e. sample at 63.2 m), the fraction of Co hosted by olivine decreases (from 56% at 67.4 m to 23% at 63.2 m), whereas that of Co hosted by serpentine increases (from 44% at 67.4 m to 77% at 63.2 m). This trend is in agreement with XRD data that show a preferential disappearance of olivine relatively to serpentine (Fig. 2d). After further weathering (i.e. upper in the saprolite unit), the proportion of Co associated with olivine and serpentine decreases to respectively reach 0% and 22% in the sample at 56.4 m (Fig. 2d; Table 3). In parallel, the proportion of Co associated with Mn-oxides strongly increases (i.e. from 0% at 63.2 m to 66% at 56.4 m), and a small fraction of Co is hosted by goethite (i.e. 12% at 56.4 m). The Mn-oxides host the major fraction of Co (i.e. from 69% to 89%) in the laterite samples from 32.5 m to 23.5 m, while goethite is only a minor host

<footnote>* i.e. Sum of the contributions of the natural Asb/Lit and synthetic Birnessite model compounds quantified by the LC-LSF fitting procedure.</footnote>

<footnote>** Sum before normalization to 100%.</footnote>
<caption>Table 3</caption>

Results of the LC-LSF analysis of the Co K-edge XANES spectra collected on selected samples from the studied regolith. The fraction of each component is given in %. Uncertainties obtained with the software package *Athena* are given in parentheses.

<table><thead><tr><th>Sample/reference</th><th>Natural olivine</th><th>Synthetic serpentine</th><th>Natural lithiophorite</th><th>Synthetic goethite</th><th>Reduced Chi<sup>2</sup></th><th>Sum**</th></tr></thead><tbody><tr><td colspan="7"><em>S4 profile</em></td></tr><tr><td>2.7 m</td><td></td><td></td><td></td><td>100(/)</td><td>6 × 10<sup>-3</sup></td><td>101</td></tr><tr><td>12.5 m</td><td></td><td></td><td></td><td>100(/)</td><td>2 × 10<sup>-3</sup></td><td>98</td></tr><tr><td>14.8 m</td><td></td><td></td><td>41(1)</td><td>59(1)</td><td>7 × 10<sup>-4</sup></td><td>101</td></tr><tr><td>23.5 m</td><td></td><td></td><td>84(1)</td><td>16(1)</td><td>3 × 10<sup>-4</sup></td><td>100</td></tr><tr><td>29.5 m</td><td></td><td></td><td>69(1)</td><td>31(1)</td><td>2 × 10<sup>-4</sup></td><td>100</td></tr><tr><td>32.5 m</td><td></td><td></td><td>89(0)</td><td>11(0)</td><td>4 × 10<sup>-4</sup></td><td>99</td></tr><tr><td>56.4 m</td><td></td><td>22(3)</td><td>66(1)</td><td>12(3)</td><td>7 × 10<sup>-4</sup></td><td>100</td></tr><tr><td>60.2 m</td><td>14(3)</td><td>55(4)</td><td>31(2)</td><td></td><td>7 × 10<sup>-4</sup></td><td>101</td></tr><tr><td>63.2 m</td><td>23(1)</td><td>77(1)</td><td></td><td></td><td>6 × 10<sup>-4</sup></td><td>102</td></tr><tr><td>67.4 m</td><td>56(1)</td><td>44(1)</td><td></td><td></td><td>4 × 10<sup>-4</sup></td><td>100</td></tr></tbody></table>

for Co (i.e. 11–31%) in these samples. However, in the uppermost laterite samples, the fraction of Co hosted by Mn-oxides decreases drastically from 84% at 23.5 m to 41% at 14.8 m and 0% at 12.5 m, whereas the fraction of Co associated with goethite increases (Fig. 2d; Table 3). At the last weathering stages, Co is only present as Co-bearing goethite (Fig. 2d; Table 3).

# 4. DISCUSSION

## 4.1. Evolution of Mn speciation during weathering

### 4.1.1. Saprolitization: from Mn-bearing (phyllo)silicates to Mn-oxides

Mn(II)/Mn(III)-bearing primary silicates progressively disappear during the first weathering stages of peridotite (i.e. saprolitization) (Fig. 2 b and c; Table 2). This trend is considered to reflect the dissolution of the primary minerals from peridotite. In addition, Mn-bearing olivine dissolves more rapidly than Mn-bearing serpentine (Fig. 2c), in agreement with the lower stability of olivine under supergene weathering conditions (Trescases, 1975; Golightly, 1981). Moreover, a fraction of the Mn-bearing serpentine in the saprolite could also correspond to secondary phyllosilicates. Indeed, a previous study by Dublet et al. (2012) demonstrated that secondary phyllosilicates formed in the saprolite samples from New Caledonia can incorporate large amounts of Ni(II), and sixfold Mn(II) and Ni(II) have similar ionic radii (respectively 0.67 Å and 0.69 Å; Shannon, 1976).

In parallel, the first stage of peridotite weathering leads to the oxidation of Mn(II) to Mn(III) and then Mn(IV) (Fig. 6), and the occurrence of Mn as Mn-oxides (Fig. 2c). This evolution suggests that the major fraction of Mn released by the dissolution of silicates precipitated preferentially as Mn-oxides rather than incorporating in newly formed goethite. This assumption is consistent with literature evidences that in many environments, Mn and Fe do not form solid solutions but precipitate as distinct phases (Manceau et al., 1992b, 2000 and references therein).

The amount of Mn-oxides is small in the saprolite, high in the transition between saprolite and laterite, and decreases toward the top of the laterite (Fig. 2). This profile

of Mn concentrations could be due to more favorable conditions (i.e. higher humidity and/or slightly more alkaline pH) for Mn-oxides precipitation and stabilization at the transition level compared to those that prevail at shallower depths (i.e. lower humidity and/or slightly more acidic pH). The range of chemical composition of the Mn-oxides found in the investigated regolith falls between that of lithiophorite and asbolane (Fig. 4, Table 1). These chemical compositions compare well with those already reported for Mn-oxides in lateritic regoliths from New Caledonia (Manceau et al., 1987; Llorca and Monchoux, 1991), Eastern Cuba (Roque-Rosell et al., 2010), Philippines (Fan and Gerson, 2011) and Cameroon (Yongue-Fouateu et al., 2006; Dzemua et al., 2013).

### 4.1.2. Lateritization: residual Mn is incorporated in goethite

Mn K-edge XANES data indicate that in the uppermost horizons of the studied regolith, Mn mainly occurs as Mn (III) substituted for Fe(III) in goethite (Fig. 2c, Table 2). This result is consistent with previous studies that already reported significant Mn contents in goethite sampled in lateritic soils (Singh and Gilkes, 1992; Becquer et al., 2006; Raous et al., 2013). Among these studies, Becquer et al. (2006) reported TEM-EDX analyses indicating 1.1 ± 0.3 to 2.4 ± 0.7 mol% MnO₂ in goethite particles from ferralsols in New Caledonia and Raous et al. (2013) measured 0.8 ± 0.2 wt% Mn in goethite particles from Brazilian limonite. In the laboratory, numerous studies demonstrated that up to 12–15 mol% Mn can incorporate in the structure of goethite (Stiers and Schwertmann, 1985; Gasser et al., 1999; Scheinost et al., 2001; Singh et al., 2002; Wells et al., 2006; Alvarez et al., 2007). As revealed by several XAS studies, the mechanism involved in this incorporation is a Mn(III) for Fe(III) substitution (Scheinost et al., 2001; Singh et al., 2002; Alvarez et al., 2007). The amount of Mn (III) substituted for Fe(III) can be significant because of the similar ionic radii of both elements, either in the high spin (respectively 0.72 Å and 0.69 Å) or low spin (respectively 0.78 Å and 0.78 Å) configuration (Shannon, 1976).

The Mn-bearing goethite occurring in the uppermost weathering horizons could be inherited from earlier weathering stages. Indeed, our Mn K-edge XANES data indicate that a small fraction of Mn already occurs as Mn(III)-

<footnote>** Sum before normalization to 100%.</footnote>
bearing goethite in the poorly-weathered saprolite, and that this fraction increases toward the surface (Fig. 2c; Table 2). The early-stage Mn(III)-bearing goethite could likely have been preserved and relatively enriched upon saprolitization and lateritization because of the preferential dissolution of the other Mn species (i.e. Mn-bearing (phyllo)silicates and Mn-oxides) and leaching of released Mn. This hypothesis is consistent with the low Mn content (0.2 wt% Mn; Table A1) found in the shallowest lateritic samples compared to the whole laterite unit (Fig. 2a; Table A1). However, a fraction of the Mn(III) occurring as Mn-bearing goethite in the uppermost weathering horizons could also originate from the Mn-oxides identified in the underlying laterite horizons (Fig. 2). Indeed, a fraction of Mn(IV) from the Mn-oxides could have been released upon (bio)dissolution of this mineral species, (bio)reduced to Mn(III) and incorporated into the goethite structure at the favor of successive dissolution-crystallization cycles that are known to have operated upon lateritization (Dublet et al., 2015).

In both cases, a preferential dissolution of the Mn-oxides relative to the Fe-oxides would explain the dominance of Mn(III)-bearing goethite in the surface horizons of the studied regolith. Such a difference in solubility in the shallowest horizons is supported by the results of a laboratory experiment that showed an easier dissolution of birnessite and lithiophorite compared to goethite in acidic media (Tokashiki et al., 2003). Indeed, the pH values are rather high in the New Caledonian saprolite (i.e. pH from 9 to 6), whereas they decrease upward and reach acidic values (down to pH 5) in the uppermost horizons of the laterite (Fandeur et al., 2009). In addition, the preferential dissolution of Mn-oxides relative to Fe-oxides in the surface horizons could also be related to the development of temporary reducing conditions upon saturation during wet periods. Such a mechanism could be enhanced by the larger susceptibility of Mn-oxides to bio-reduction, as demonstrated by Quantin et al. (2002) in laterite samples from New Caledonia.

## 4.2. Evolution of Co speciation during weathering

### 4.2.1. Co speciation in the peridotite bedrock

Our Mn and Co K-edge XANES data indicate that Mn and Co are both hosted by the primary minerals olivine and serpentine in the bedrock (Fig. 6; Table 3). Previous EPMA results showed the occurrence of Mn in (phyllo)silicates (Trescases, 1975; Wells et al., 2009), and in olivine and pyroxene from bedrock samples (Trescases, 1975; Latham, 1987) in New Caledonia. However, these studies did not refer to Co, likely because of the low concentrations of this element that are typically below the detection limit of EPMA in the studied samples (Dzemua and Gleeson, 2012). To our knowledge, our XAS data are then the first to directly show that Co is hosted by (phyllo)silicates in peridotites.

### 4.2.2. Saprolitization: Co is trapped in Mn-oxides

Our Co and Mn K-edge XANES data indicate that the first weathering stage is characterized by (i) an oxidation of Co(II) to Co(III) and Mn(II) to Mn(III)/Mn(IV), and (ii)

the formation of Co(III)-bearing Mn(III/IV)-oxides (Fig. 2). This evolution suggests that the oxidation of Co (II) to Co(III) occurs concomitantly with the formation of Mn(III/IV)-oxides. This hypothesis is consistent with previous studies which indicated that Co(II) can be oxidized to Co(III) upon sorption onto the surface of Mn(III/IV)-oxides (Burns, 1976; Manceau et al., 1997; Kay et al., 2001; Murray et al., 2007). Our Co and Mn K-edges XANES data show for the first time the importance of the Co(II) to Co(III) oxidation process related to sorption by Mn-oxides at the scale of a lateritic regolith.

This oxidation of Co by Mn-oxides has been proposed to explain the uptake of this element into the Mn-oxides layers of birnessite by a Co(III)-Mn(III) exchange. Such an incorporation of Co(III) into the Mn-oxide layers is supported by the short Co-O distance at 1.9 Å found for our Co-bearing Mn-oxide model compounds and by the similarity between the Co and Mn K-edges EXAFS spectra of these model compounds (Fig. A4). All these considerations then strongly suggest that Co(III) is incorporated in the Mn-oxide layer when Co(III)-bearing Mn(III/IV)-oxides are found in the studied regolith. This is consistent with a previous study from Manceau et al. (1987), who showed that Co is trivalent and non-randomly distributed in the Mn layers of asbolane and lithiophorite from New Caledonia. This latter point is important because this whole mechanism of Co(II) to Co(III) oxidation by Mn-oxides followed by incorporation of Co(III) in the Mn layers of Mn-oxides could represent a major process in the formation of Co lateritic ores worldwide (Roque-Rosell et al., 2010; Fan and Gerson, 2011; Dzemua et al., 2013).

The trapping of Co by Mn-oxides is consistent with our results of bulk chemistry. Indeed, Co and Mn contents are correlated in the saprolite and in the first stages of lateritization (i.e. from the bedrock to 22 m depth; Figs. 2a and 3), where Mn-oxides are the most abundant. The correlation between both elements is lost in the ultimate stages of lateritization (i.e. above 22 m depth; Figs. 2a and 3), where the amount of Mn-oxides decreases to finally disappear (Figs. 2 and 3). These observations suggest that the behavior of Co is controlled by Mn-oxides as long as the conditions favor this mineral species. In addition, the strong affinity of Co for Mn-oxides is in agreement with a number of field observations and chemical analyses that revealed atomic-scale association of Co with Mn-oxides (Taylor and McKenzie, 1966; Taylor, 1968; Kuhnel et al., 1978; Manceau et al., 2000).

### 4.2.3. Lateritization: Co-goethite as the ultimate Co-species

Our Co K-edge XANES data show that Co is mainly hosted by goethite in the uppermost laterite samples of the studied regolith. Such an association of Co with goethite rather than Mn-oxides in the uppermost laterite is consistent with the low correlation observed between Mn and Co for the samples above 22 m (Fig. 3). As already discussed for Mn, this loss of correlation between Mn and Co in the upper laterite samples is likely related to the dissolution of the Co-bearing Mn-oxides upon the ultimate weathering stage of lateritization that could be due to the unfavorable conditions for Mn-oxides stabilization in the shallow horizons of the studied regolith.
In addition, our Co K-edge XAS data indicate that Co is associated with goethite as Co(II) in the investigated lateritic regolith (Fig. 5, Table A3). To our knowledge, it is the first time that Co(II)-bearing goethite is evidenced in a lateritic regolith. In a precedent study by Manceau et al. (2000) on a single laterite sample, the oxidation state of Co associated with goethite (i.e. 20% of the total fraction of Co measured in the studied sample) could not be unambiguously determined because of the prevalence of Co associated with Mn-oxides. The evidence of Co(II)-hosting goethite in lateritic regoliths provided by the present study is important because numerous past studies assumed that Co incorporates goethite rather as Co(III), through a Co (III) for Fe(III) homovalent substitution (Cornell and Giovanoli, 1989; Cornell and Schwertmann, 1996; Bousserrhine et al., 1999). This assumption was often based on the similar ionic radii of sixfold Co(III) and Fe(III) in high spin configuration (respectively 0.61 Å and 0.65 Å; Shannon, 1976). However, Cornell and Giovanoli (1989) explained that in nearly all Co(III) complexes and compounds the cobalt ion is in the low-spin configuration. In that case, the ionic radius for $^{VI(LS)}$Co(III) is 0.53 Å (Shannon, 1976), which means a 15% radius difference with $^{VI(HS)}$Fe(III) (i.e. 0.65 Å). This radius difference is similar to that between $^{VI(HS)}$Co(II) and $^{VI(HS)}$Fe(III) (respectively 0.75 Å and 0.65 Å, Shannon, 1976). In addition, this 15% radius difference between Co(II) and Fe(III) is still in the 18% range of difference that can be accepted for the substitution of a metal (Me) to Fe(III) in the goethite structure, according to Cornell and Schwertmann (1996). Finally, Cornell and Schwertmann (2003) also explained that divalent metal cations can substitute for trivalent iron in the Fe(III)-oxide structure in concentrations that are usually below 0.1 mol mol$^{-1}$. This is for example the case with Ni(II) that is known to substitute for Fe(III) in goethite (Gerth, 1990; Carvalho-e-Silva et al., 2002, 2003; Singh et al., 2002), and which has been demonstrated to occur as Ni(II)-bearing goethite in lateritic regoliths from New Caledonia (Dublet et al., 2012, 2015).

Although it probably results in a distortion of the cationic site, all these considerations support the Co(II) for Fe (III) substitution in goethite indicated by our Co K-edge XAS data. As already shown for Ni-substituted goethite (Carvalho-e-Silva et al., 2002), the charge imbalance that arises from the Co(II) for Fe(III) substitution in goethite is likely compensated by an uptake of an additional proton for each substitution.

### 4.3. Implications for Mn and Co mobility upon weathering of peridotites under tropical conditions

As mentioned in the introduction, upon tropical weathering of the New Caledonian peridotites, soluble elements (e.g. Si, Mg) were intensely leached and less soluble elements (e.g. Fe, Cr) were relatively concentrated in the residual laterite. These trends are reflected by the strong correlation between SiO$_2$ and MgO concentrations, on the one hand, and Fe$_2$O$_3$ and Cr$_2$O$_3$ concentrations, on the other hand, in the lateritic regolith of southern New Caledonia (Fig. A6). However, Co and Mn are not simply

following one of these two trends (Fig. A6), suggesting that the mobility of these elements changes during the weathering processes and upon changes of their speciation.

Our results indicate that Co(II) is oxidized to Co(III) upon the weathering of the Co(II)-bearing (phyllo)silicates from the peridotite bedrock. This change of oxidation state favors a significant scavenging of Co(III) by Mn-oxides. Indeed, Co(III) and Mn(III) are typically less soluble than Co(II) and Mn(II), even though the solubility depends on the water composition (Morgan, 2000; Borch et al., 2010; Collins and Kinsela, 2010). Nevertheless, as discussed earlier, the stability of the Mn-oxides decreases upward in the studied regolith, and the mobility of Mn and Co likely increases upon dissolution of Co-bearing Mn-oxides.

On the opposite, the Co(II), Mn(III)-bearing goethite remains in the upper horizons of the regolith (Fig. 2, Table 2), which can be interpreted as an indication of a better stability of this mineral species upon the environmental conditions of the upper laterite. This latter point is supported by previous results, which showed that Mn-oxides are more susceptible to (bio)reduction than goethite in laterites from New Caledonia (Quantin et al., 2002). Nevertheless, in such a context of lateritization, goethite aging through successive dissolution-crystallization cycles has been shown to take place and to be responsible for the leaching of Ni(II) from goethite (Dublet et al., 2015). It is then likely that a fraction of Co(II) and Mn(III) has been leached from goethite upon aging of this mineral species during lateritization. Considering the quite acidic conditions that prevail in the uppermost laterite horizons of the investigated regolith (i.e. pH values down to 5; Fandeur et al., 2009) and regarding the quite high mobility of aqueous Co(II) in such conditions (Borch et al., 2010), it is likely that the fraction of Co(II) and Mn(III) released from Co (II), Mn(III)-bearing goethite was leached toward the transition laterite and saprolite and/or downstream. Co(II) leached toward the transition laterite (or the saprolite) could be re-oxidized to Co(III) at the contact with Mn-oxides and accumulate in the form of Co(III)-bearing Mn-oxides. In addition, aqueous Co(II) species could represent a potential hazard because of the high mobility and the potential toxic character of this species. Further studies are then needed to better assess the behavior of Co in the aqueous phase in the lateritic regolith developed upon peridotites in New Caledonia.

## ACKNOWLEDGMENTS

This study was supported by the Centre National de Recherche Technologique Nickel et son Environnement (CNRT) in Noumea (New Caledonia) under the Project *Ni/Co mineralization factors of laterites derived from ultramafic rocks of New-Caledonia*. CNRT is a Public Interest Group funded by the French and New Caledonian authorities and research organizations, as well as by international mining companies, for promoting local research programs in the field of sustainable mining of geological resources. The authors wish to thank the Vale NC company for providing the studied core samples. Michel Fialin and Frederic Couffignal (CAMPARIS center, UPMC) are also acknowledged for assistance during EPMA. Finally, we thank the Mineral Collection of Université Pierre et
Marie Curie (UPMC) and the Mineral Collection of Ecole Nationale Supérieure des Mines de Paris (ENSMP) for providing the lithiophorite and olivine model compounds, respectively.

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