# HERFD-XAS evidence for an octahedrally coordinated CoSn-polysulfide precursor as a probe for the mechanism of pyrite formation

Qianyu Deng, Pierre Le Pape, Julie Aufort, Marc Blanchard, Benoit Baptiste, Ludovic Delbes, Camille Baya, Farid Juillot, George Ona-Nguema, Nicolas Menguy, et al.

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Qianyu Deng, Pierre Le Pape, Julie Aufort, Marc Blanchard, Benoit Baptiste, et al. HERFD-XAS evidence for an octahedrally coordinated CoSn-polysulfide precursor as a probe for the mechanism of pyrite formation. Geochimica et Cosmochimica Acta, 2025, 401, pp.104-121. 10.1016/j.gca.2025.06.003. hal-05222719

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https://hal.science/hal-05222719v1

Submitted on 25 Aug 2025

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# HERFD-XAS evidence for an octahedrally coordinated CoS<sub>n</sub>-polysulfide precursor as a probe for the mechanism of pyrite formation

Qianyu Deng<sup>a</sup>, Pierre Le Pape<sup>a</sup>, Julie Aufort<sup>a</sup>, Marc Blanchard<sup>b</sup>, Benoit Baptiste<sup>a</sup>, Ludovic Delbes<sup>a</sup>, Camille Baya<sup>a</sup>, Farid Juillot<sup>a</sup>, George Ona-Nguema<sup>a</sup>, Nicolas Menguy<sup>a</sup>, Jean-Michel Guigner<sup>a</sup>, Olivier Proux<sup>c</sup>, Camille Duquenoy<sup>b</sup>, Romain Guilbaud<sup>b</sup>, Franck Poitrasson<sup>b</sup>, Guillaume Morin<sup>a,∗</sup>

## ARTICLE INFO

Associate editor: Wei Li

Keywords:

Pyrite

Cobalt

Precursor

Paleo-environments

HERFD-XAS

## ABSTRACT

Trace element contents in authigenic pyrite (FeS<sub>2</sub>) are often considered as a reliable geochemical archive of past marine conditions. For instance, cobalt (Co) abundance in marine sedimentary pyrite may track back the extent of past ocean anoxia and is considered as a reverse proxy for the rise of atmospheric oxygen. However, the molecular-scale route of Co incorporation in pyrite at low temperature is not well documented. Thus, any insight on this aspect are expected to help better assess the actual role of pyrite in Co cycling in modern and past subsurface environments. In this study, a series of pyrites were synthesized in solution via the polysulfide pathway under anoxic conditions and at ambient temperature, with various initial aqueous Co concentrations (Co:Fe = 0.13–5). Rietveld refinement analysis of the powder X-ray diffraction (XRD) patterns shows that pyrite is the principal component (69(5) wt%) of the final solid products, with small fractions of marcasite (17(4) wt%) and FeS (10(2) wt%). High Energy Resolution Fluorescence Detected (HERFD) Co K-edge X-ray Absorption Near Edge Structure (XANES) and Extended X-ray absorption fine structure (EXAFS) analysis indicate that a minor fraction (20–36 %) of Co substitutes for Fe in the pyrite structure as compared with a theoretical spectrum of a Co-substituted pyrite supercell calculated using Density Functional Theory (DFT). Besides, in the final products, the major part (64–80 %) of Co persists in the form of an amorphous CoS<sub>n</sub>-polysulfide precursor phase that represents the whole Co speciation before pyrite nucleation. In this precursor observed at the monosulfide FeS pre-pyrite stage, Co early adopts an octahedral coordination as attested by Co pre-edge data, whereas Co is in tetrahedral coordination in our Co-doped mackinawite FeS reference compound. The local structure of the CoS<sub>n</sub>-polysulfide precursor is further elucidated by EXAFS shell-by-shell analysis that points to monomeric units (< 1 nm), where Co is octahedrally coordinated to first neighboring S atoms with at least three of these S neighbors belonging to a polysulfide chain of undetermined length. Aggregation of such monomeric units into an amorphous CoS<sub>n</sub>-polysulfide phase is supported by X-ray scattering-pair distribution function analysis (XRD-PDF) of an analogous amorphous CoS<sub>n</sub> compound. These results could have important implications on our understanding of pyrite nucleation mechanisms via the polysulfide pathway since the observed octahedral CoS<sub>n</sub>-polysulfide precursor differs from the generally proposed models of tetrahedral FeS precursors. In addition, the persistence of these peculiar species and the observed delay of Co incorporation in pyrite highlights the importance of trace elements in pyrite formation kinetics at low temperature. Lastly, our results illustrate the high affinity of Co for polysulfides and raise questions on the possible presence and evolution of this non-pyrite CoS<sub>n</sub> phase in sedimentary archives. In this regard, this study may provide new mechanistic insights that could help explaining the moderate affinity for authigenic pyrite generally reported for Co, in particular in a sedimentary context where other possible bearing phases such as clay minerals and Mn oxides are involved.

<footnote><sup>a</sup> Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), UMR 7590, CNRS, Sorbonne Université, MNHN, IRD, 4 place Jussieu, 75005 Paris, France</footnote>

<footnote><sup>b</sup> Geosciences Environnement Toulouse (GET), UMR 5563, CNRS, Université de Toulouse, IRD, UPS, CNES, Observatoire Midi-Pyrénées (OMP), 14 avenue Édouard Belin, 31400 Toulouse, France</footnote>

<footnote><sup>c</sup> Université Grenoble Alpes, CNRS, IRD, INRAE, Météo France, OSUG, UAR 832 CNRS, 38000 Grenoble, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: guillaume.morin@upmc.fr (G. Morin).</footnote>
# 1. Introduction

Ubiquitous in sulfate reducing environments, pyrite (FeS₂) is a commonly found mineral in marine sediments and, to a lesser extent, in riverine and lacustrine sediments as well as in groundwaters reservoirs (Rickard and Morse, 2005; Rickard and Luther, 2007). Being thermodynamically stable in (sub)surface environments (Rickard, 2012) and acting as an important host for trace elements, pyrite has been viewed as potential phase able to sequester toxic elements in contaminated groundwater, provided that reducing conditions can be maintained over time (Saunders et al. 2018; Lee et al., 2019). It is also used as a geochemical archive of recent and past chemical compositions of marine and fresh-water reservoirs (Large et al., 2017; Gregory et al., 2014; 2022b; Mukherjee and Large, 2020; Tribovillard et al., 2006), and numerous studies have reported high affinities of several trace metals and metalloids for sedimentary pyrite (Huerta-Diaz et al., 1998; Large et al., 2014; Noël et al., 2015; Le Pape et al., 2017; Morin et al., 2017; Baya et al., 2021). In particular, trace element contents and distribution in natural pyrite samples have been analyzed by LA-ICP-MS (Large et al., 2014; Gregory et al., 2015) to further derive information on ancient oceanic chemistry and its evolution. Although potentially powerful, these approaches rely on an estimate of the relation between the trace element contents in pyrite and their concentration in ocean or pore waters (Pisarzowska et al., 2014; Gregory et al., 2014). In this regard, a recent study by Baya et al. (2022) proposes that further elucidating the mechanisms of trace elements incorporation in pyrite, especially at ambient temperature, could help refine the current paleo-reconstruction models of past oceanic composition (Gregory et al. 2022a).

In fact, Co is particularly important for tracing paleoenvironmental sedimentary conditions since it presents a certain affinity for both pyrite and oxidized minerals. Because of the affinity of Co for non-pyrite phases such as clay minerals and iron and manganese oxides (Swanner et al. 2014), the Co contents of marine pyrites is regarded as a powerful reverse proxy for the oxygenation of ancient Earth atmosphere, especially when the Co content in pyrite is compared to that of chalcophile elements such as Se, Mo and Ni (Se:Co, Mo:Co and Ni:Co) (Large et al., 2017). From a biogeochemical point of view, Co is a bio-essential nutrient, present in both prokaryote and eukaryote proteins (Brennan et al., 1996; Kobayashi and Shimizu, 1999; Okamoto and Eltis, 2011). This trace metal thus plays a major role in present and past biogeochemical processes (Robbins et al., 2016) and particular interest has been dedicated to evaluating the role of marine sedimentary and hydrothermal processes in controlling the oceanic Co cycle during Earth's history (Swanner et al. 2014).

From an environmental point of view, even though Co is of moderate toxicity to humans and ecosystems (Gál et al., 2008; Leyssens et al., 2017; see also HSDB and REACH databases), harmful concentrations can be found in sediments directly impacted by mining effluents (Atibu et al., 2013), as well as in acid mine drainage resulting from the weathering of Co-rich pyrite mine tailings (Ziwa et al., 2021). Better evaluating the properties of Co-rich pyrite as a potential source or sink for Co in the environment necessarily implies deepening our knowledge of the modes of insertion of this trace metal in diagenetic pyrite that could form under sulfidic subsurface conditions. Beyond these environmental issues, crystal chemistry of Co in pyrite is also of interest in industrial applications such as fabrication and development of lithium batteries and photovoltaic cell (Grohol and Veeh, 2023; Goffé, 2014).

Despite the isostructural properties of CoS₂ and FeS₂ (Kinner et al., 2016; Faber et al., 2014; Rhodes et al., 2018; Zhang et al., 2016), the affinity of Co for pyrite seems to be controversial, based on natural pyrites analyses. Some studies reported moderate to enhanced Co incorporation in sedimentary pyrite (Scholz and Neumann, 2007; Chen and Campbell, 2021; Qiu et al., 2021). Other researchers proposed a weak Co affinity with pyrite or even an inhibition of pyrite formation in the presence of Co (Huerta-Diaz and Morse, 1992; Swanner et al., 2019). Cobalt distribution is often nonuniform in pyrite in low temperature

deposits in comparison with hydrothermal and metamorphic pyrites (Craig et al., 1998). For instance, in the sedimentary cobalt and copper deposit from Kamoto (Congo), a Co-rich rim was observed around idiomorphic pyrite grains with a cobalt content reaching 4 wt%, and pyrite framboids were reported to be heterogeneous with respect to Co (Bartholomé and Katekesfia, 1971).

The status of Co in Co-doped pyrite has been so far documented on the basis of Co K-edge XANES data analysis only (Wang et al., 2020), and thus deserves further research to unravel the Co molecular environment in such materials. Recent laboratory studies that have investigated Co incorporation in synthetic pyrite showed homogeneous Co distribution at the particle scale in pyrite nanoparticles synthesized at both room temperature (Baya et al., 2022) and 100 °C (Lin et al., 2022). Additionally, Baya et al. (2022) reported a lack of Co release to solution during laboratory-controlled synthesis of Co-doped pyrite, which supports the affinity of this trace metal for the solid phase in a sulfide-rich medium (Morse and Arakaki, 1993). Mansor et al. (2020) revealed that Co precipitates as Co-rich FeS in the presence of excess sulfide or Fe(II). The latter study also proposed other sulfide phases such as non-stoichiometric and amorphous CoS as potential Co-hosts. In addition, the possible formation of other phases such as cattierite (CoS₂) or linnaeite (Co₃S₄) in sulfidic systems is also reported in the same study. All these considerations highlight the need for gaining further insights into the actual modes of Co incorporation into pyrite at low temperature, in order to better assess the role of pyrite in controlling Co cycling in modern and past subsurface environments.

According to these considerations, the mobility and availability of Co in subsurface sulfidic environments may be intimately related to its mode of association with pyrite and associated sulfide mineral phases, which may ultimately control the effects of this potentially toxic or nutrient trace element towards local ecosystems (Huerta-Diaz and Morse, 1992; Large et al., 2014). This mode of association is also a key parameter that could largely influence the ability of pyrite to record past depositional conditions, which would thus constrain the use of pyrite as an archive of ancient ocean chemistry (Gregory et al., 2022a; Large et al., 2017). For these reasons, it is important to examine trace element speciation in pyrite and associated sulfide minerals, which remains insufficiently documented from a mechanistic point of view, in particular at the molecular level. A previous study by Baya et al. (2022) provided a methodological framework for determining on a case-by-case basis the kinetics and solid/liquid partition of a set of trace elements, including Co, introduced as dissolved species during pyrite formation via the polysulfide pathway at ambient temperature. Besides, micro-scale examination of sedimentary pyrites revealed chemical zonation that can be related to growth kinetic effects (Chen and Campbell, 2021). However, no study currently details molecular-level Co speciation in such low temperature synthetic or natural pyrites though it may help understanding the underlying mechanisms controlling Co incorporation in sedimentary sulfides.

In order to fill this knowledge gap, we investigated the molecular-level Co speciation during the different steps of pyrite synthesis and identified the Co phase-transfer processes during pyrite transformation. A series of pyrites were synthesized with various initial aqueous Co concentrations using a laboratory protocol adapted from previous studies (Morin et al., 2017; 2023; Le Pape et al., 2017; Baya et al., 2021; 2022), via the polysulfide pathway under anoxic conditions and at ambient temperature. Additionally, Co-sulfide model compounds were synthesized and samples of Co-doped pyrites were selected from the study of Baya et al. (2022). The mineralogical composition of solid products was analyzed using powder X-ray diffraction (XRD), while High Energy Resolution Fluorescence Detected Extended X-Ray Absorption Fine Structure (HERFD-EXAFS) spectroscopy was used to investigate the local structure around Co atoms in these iron-rich samples. In addition, DFT modelling of Co-doped sulfides were performed for EXAFS data analysis and XRD-PDF analysis was performed to complement EXAFS results. The results of these investigations provide
evidence on the complexity of the crystal-chemistry of Co in sulfide minerals. Especially, they confirm the expected close properties of Co and Fe but with a distinct behavior during the pyritization process. These results uncover important implications for Co sequestration in authigenic pyrite and its use as a paleo-proxy in sedimentary archives.

## 2. Materials and methods

### 2.1. Syntheses procedures for the Co-doped pyrites and Co model compounds

The set of samples studied is listed in Table 1 and syntheses protocols used in this study are detailed in Table 2. In this study, all syntheses were performed at room temperature (22 ± 3 °C). In order to prevent oxidation of the reduced iron sulfide solids, all syntheses were performed inside a Jacomex™ glovebox (< 5 ppm O₂) and with O₂-free Milli-Q water obtained by N₂ bubbling at 80 °C for 45 min.⁴

#### 2.1.1. Cobalt-doped pyrites

A first Co-doped pyrite sample referred to as Py_05Co_CB1 (Tables 1 and 2) was previously synthesized by Baya et al. (2022) and was analyzed in this study to investigate Co speciation in low-temperature diagenetic-like pyrites. This first synthesis had an excess in Fe(III) or deficit in of HS⁻ with respect to the 1:1 ratio, resulting in an excess of elemental sulfur (orthorhombic – α-S₈) in final solids. Such a deficit in HS⁻ could be attributed to slight degassing of the solution before the synthesis or to excess of hydration of the Na₂S·9H₂O solid used to prepare the stock solution (Baya et al., 2022).

A second Co-doped pyrite referred to as the Py_05Co_CB2 sample was analyzed in this study (Table 1 and 2). It was synthesized with a similar protocol as in Baya et al. (2022) but with stoichiometric proportion of the reactants closer to 1:1, resulting in no detectable elemental sulfur remaining at the end as measured by XRD (Fig. 1).

A multi-trace elements doped pyrite, referred to as Py_TE (Tables 1 and 2), was synthesized according to protocols adapted from previous studies by Noël et al. (2015), Morin et al. (2017), Le Pape et al. (2017) and Baya et al. (2021, 2022). Briefly, an equimolar mixture of ferric chloride and sodium sulfide solutions was aged for several days under vigorous stirring. A brand new pure-grade FeCl₃·6H₂O salt (Sigma-Aldrich #44944-SIGALD; CAS 10025–77-1; Co ≥ 50 ppm) was used to prepare an acidic Fe(III) stock solution at 0.625 M in O₂-free milli-Q water. Similarly, a brand new Na₂S·9H₂O ≥ 99.99 % trace metal basis (Sigma-Aldrich #MKBT8258V) salt was used to prepare the HS⁻ stock solution at 0.629 M. In addition, a Merck ICP multi element standard solution (XXIV #1.09411.0500 Certipur®) containing multiple trace elements (See Table S1 for detailed compositions) was poured into a

<caption>**Table 1**</caption>

List of samples studied indicating synthesis conditions and duration, aqueous Co: Fe ratio at the start of the synthesis and corresponding literature references for synthesis protocols.

<table><thead><tr><th>Sample</th><th></th><th>Co:Fe (mol %)</th><th>Synthesis duration (h)</th><th>pH<sub>final</sub></th><th>Synthesis</th></tr></thead><tbody><tr><td>Py_5Co</td><td>final product</td><td>5</td><td>&gt; 700</td><td>n.m.</td><td>This study</td></tr><tr><td>Py_05Co_CB1</td><td>final product</td><td>0.5</td><td>3100</td><td>4.5</td><td>Baya et al. 2022</td></tr><tr><td>Py_05Co_CB2</td><td>final product</td><td>0.5</td><td>3100</td><td>n.m.</td><td>This study</td></tr><tr><td>Py_TE</td><td>1 ppm TE, product at 141 h precursor at 24 h</td><td>0.13</td><td>141</td><td>3.1</td><td>This study</td></tr><tr><td>PyPrec_05Co</td><td>am CoS<sub>1+x</sub>, 100 °C</td><td>0.5</td><td>24</td><td>n.m.</td><td>This study</td></tr><tr><td>Co-S_100C</td><td>am CoS, 25 °C</td><td>100</td><td>18</td><td>n.m.</td><td>This study</td></tr><tr><td>Co-S_25C</td><td>am CoS, 25 °C</td><td>100</td><td>96</td><td>n.m.</td><td>This study</td></tr><tr><td>Mack_Co</td><td>Fe<sub>0.995</sub>Co<sub>0.005</sub>S</td><td>0.5</td><td>24</td><td>n.m.</td><td>This study</td></tr></tbody></table>

Note: TE and n.m. are abbreviations for “trace elements” and “not measured” respectively.

glass vial (30 mL), sealed with a butyl stopper and brought and opened into the glove box and stirred at a low speed on a magnetic stirring plate overnight to equilibrate it under a N₂ atmosphere. After 141 h, the solid and solution were sampled. Solids were collected by centrifuging tubes of equal-volume synthetic suspension at 6000 rpm during 10 min, washed twice with O₂-free milli-Q water and vacuum dried in a desiccator within the glove-box for at least one day following protocols detailed in Le Pape et al. (2017) and Morin et al. (2017). Supernatants were filtered by 0.2 µm cellulose filters and stored for chemical analysis.

A 5 mol% Co:Fe pyrite, referred to as Py_5Co (Tables 1 and 2), was synthesized using a similar pathway with the following protocol. For stock solutions, 1.01 g of FeCl₃·6H₂O (Sigma-Aldrich #BCBN9139V) were dissolved and stirred in 30 mL degassed milli-Q water. 0.9 g of Na₂S·9H₂O (Sigma-Aldrich #MKBT8258V) were dissolved and stirred in 18.1 mL degassed milli-Q water separately. Then a volume of 1.9 mL of the 0.1 M CoCl₂·6H₂O stock solution used in Baya et al. (2022) was added to the FeCl₃·6H₂O solution before mixing with the Na₂S·9H₂O solutions. Then the resulting solution containing the black Fe-sulfide precipitate was subsequently gently stirred for ~ 1 month according to the protocol described in Baya et al. (2022). This solid end-product was washed twice with degassed HCl (0.01 M) solution during 20 mins and washed twice with O₂-free milli-Q water in the glove box in order to remove residual FeS detected in slightly larger amount than for the other synthetic pyrite products of this study.

For Py_05Co_CB1 and Py_TE synthesis experiments, the dissolved concentration of both Fe and Co was followed overtime using ICP-OES in Baya et al. (2022), and using AAS and ICP-MS for Fe and Co, respectively, in this study (Table S2). Estimated Co:Fe molar ratios in the final products of the pyrite synthesis experiments are reported for all the studied samples in Fig. S1. These results showed that the major fraction of Co was trapped in the solid phase after 24 h of synthesis, which is in agreement with data presented for Py_05Co_CB1 in Baya et al. (2022).

#### 2.1.2. Cobalt-doped precursor

A pyrite precursor referred to as PyPrec_05Co (Tables 1 and 2) corresponding to the same synthesis protocol as Baya et al. (2022) was also synthesized in this study. This type of material sampled within the first 24 h of the synthesis experiment was characterized as FeS<sub>(cluster, nanoMck)</sub> in Baya et al. (2021). The solid was collected after 24 h of ageing of a suspension prepared as reported in Table 2 (see PyPrec_05Co), using FeCl₃·6H₂O and Na₂S·9H₂O salts and the CoCl₂·6H₂O stock solution used in Baya et al. (2022).

#### 2.1.3. Cobalt sulfides model compounds

Three Co model compounds including amorphous CoS (Co-S_25C), amorphous CoS<sub>1+x</sub> (Co-S_100C), and nanocrystalline Co-doped mackinawite (Mack_Co) were synthesized for this study (Table 1 and 2). For this purpose, FeCl₃·6H₂O and Na₂S·9H₂O salts were used to prepare Fe (II) and HS⁻ stock solutions, respectively. The CoCl₂·6H₂O stock solution from Baya et al. (2022) was used for the synthesis of amorphous CoS (Co-S_25C). For the syntheses of Co-doped mackinawite (Mack_Co) and amorphous CoS<sub>1+x</sub> (Co-S_100C), a new stock solution was prepared from the same salt.

The amorphous CoS_25C sample was prepared by equimolar mixing of Co(II) chloride and sodium sulfide solutions and by aging the obtained suspension for 96 h before centrifugation and 2-times washing with O₂-free milliQ water (Table 2).

The amorphous CoS_100C compound was synthesized by mixing the CoCl₂·6H₂O solution with a polysulfide solution (Table 2) and heated to 100 °C for 18 h in a sealed glass vial. The polysulfide solution was prepared by mixing 10 mL of a 0.625 M Na₂S·9H₂O solution with 0.816 g of solid S(0) (Sigma-Aldrich #SZBA2940) to target a theoretical Na₂S₄ or Na₂S₅ molar ratio and then sealed. This solution was then gently stirred for approximately two months, filtered and stored in the glove box in a glass vial with a butyl rubber stopper until use.

The Co-doped mackinawite Mack_Co was synthesized according to
<table><caption>**Table 2**
Proportions of reactants used for Co-doped pyrite samples and Co sulfide model synthesized in this study.</caption><thead><tr><th rowspan="2">Sample</th><th rowspan="2">Co:Fe<br>(mol%)</th><th rowspan="2">Reactants</th><th colspan="3">Stock solutions</th><th colspan="2">Reaction vial</th></tr><tr><th>C (M)</th><th>V (mL)</th><th>m (g)</th><th>n (mmol)</th><th>c (mM)</th></tr></thead><tbody><tr><td rowspan="5">Py_5Co</td><td rowspan="5">5</td><td>FeCl<sub>3</sub>·6H<sub>2</sub>O</td><td rowspan="5">0.1</td><td>30</td><td rowspan="5">1.01</td><td>3.75</td><td rowspan="5">75</td></tr><tr><td>CoCl<sub>2</sub>·6H<sub>2</sub>O<sup>(B)</sup></td><td>1.9</td><td>0.19</td></tr><tr><td>Na<sub>2</sub>S·9H<sub>2</sub>O</td><td>18.1</td><td>3.75</td></tr><tr><td>O<sub>2</sub>-free mQ water</td><td></td><td></td></tr><tr><td>Total</td><td>50</td><td></td></tr><tr><td rowspan="5">Py_05Co_CB2</td><td rowspan="5">0.5</td><td>FeCl<sub>3</sub>·6H<sub>2</sub>O</td><td>0.625</td><td>6</td><td rowspan="5"></td><td>3.75</td><td rowspan="5">75</td></tr><tr><td>CoCl<sub>2</sub>·6H<sub>2</sub>O<sup>(B)</sup></td><td>0.1</td><td>0.19</td><td>0.019</td></tr><tr><td>Na<sub>2</sub>S·9H<sub>2</sub>O</td><td>0.625</td><td>6</td><td>3.75</td></tr><tr><td>O<sub>2</sub>-free mQ water</td><td></td><td>38</td><td></td></tr><tr><td>Total</td><td>50.19</td><td></td><td></td></tr><tr><td rowspan="5">Py_TE</td><td rowspan="5">0.13</td><td>FeCl<sub>3</sub>·6H<sub>2</sub>O pure grade</td><td>0.625</td><td>1</td><td rowspan="5"></td><td>0.625</td><td rowspan="5">12.5</td></tr><tr><td>TE solution</td><td>0.00081</td><td>1</td><td>0.00081</td></tr><tr><td>Na<sub>2</sub>S·9H<sub>2</sub>O ≥ 99.99 %</td><td>0.629</td><td>1</td><td>0.629</td></tr><tr><td>O<sub>2</sub>-free mQ water</td><td></td><td>47</td><td></td></tr><tr><td>Total</td><td></td><td>50</td><td></td></tr><tr><td rowspan="5">PyPrec_05Co</td><td rowspan="5">0.5</td><td>FeCl<sub>3</sub>·6H<sub>2</sub>O</td><td rowspan="5">0.1</td><td></td><td rowspan="5">1.0135</td><td>3.75</td><td rowspan="5">75</td></tr><tr><td>CoCl<sub>2</sub>·6H<sub>2</sub>O<sup>(B)</sup></td><td>0.19</td><td>0.019</td></tr><tr><td>Na<sub>2</sub>S·9H<sub>2</sub>O</td><td></td><td>0.9000</td></tr><tr><td>O<sub>2</sub>-free mQ water</td><td></td><td>49.8</td></tr><tr><td>Total</td><td></td><td>50</td><td></td></tr><tr><td rowspan="5">Co-S_100C</td><td rowspan="5">100</td><td>CoCl<sub>2</sub>·6H<sub>2</sub>O</td><td rowspan="5">0.1</td><td>2.75</td><td rowspan="5">0.151</td><td>0.00025</td><td rowspan="5">20.8</td></tr><tr><td>Na<sub>2</sub>S·9H<sub>2</sub>O</td><td></td><td>0.629</td></tr><tr><td>Polysulfide solution</td><td></td><td></td></tr><tr><td>O<sub>2</sub>-free mQ water</td><td></td><td>0.8</td></tr><tr><td>Total</td><td></td><td>1.2</td><td></td></tr><tr><td rowspan="5">Co-S_25C</td><td rowspan="5">100</td><td>CoCl<sub>2</sub>·6H<sub>2</sub>O<sup>(B)</sup></td><td rowspan="5">0.1</td><td>2.5</td><td rowspan="5"></td><td>0.00025</td><td rowspan="5">20.8</td></tr><tr><td>Na<sub>2</sub>S·9H<sub>2</sub>O</td><td>0.4</td><td>0.00025</td></tr><tr><td>O<sub>2</sub>-free mQ water</td><td></td><td>9.1</td></tr><tr><td>Total</td><td></td><td>12</td><td></td></tr><tr><td></td><td></td><td></td><td></td></tr><tr><td rowspan="5">Mack_Co</td><td rowspan="5">0.5</td><td>FeCl<sub>2</sub>·4H<sub>2</sub>O</td><td>0.606</td><td>4.11</td><td rowspan="5"></td><td>0.249</td><td rowspan="5">49.8</td></tr><tr><td>CoCl<sub>2</sub>·6H<sub>2</sub>O</td><td>0.1</td><td>0.124</td><td>0.0124</td></tr><tr><td>Na<sub>2</sub>S·9H<sub>2</sub>O</td><td></td><td></td><td>0.25</td></tr><tr><td>O<sub>2</sub>-free mQ water</td><td></td><td>41.8</td><td></td></tr><tr><td>Total</td><td></td><td>50</td><td></td></tr></tbody></table>

Note: <sup>(B)</sup>Same solution as in Baya et al. (2022).

the protocol reported by Noël et al. (2014), adapted in this study by adding appropriate amount of CoCl₂·6H₂O in the FeCl₂·4H₂O solution to reach 0.5 mol% Co:Fe as reported in Table 2.

## 2.2. Mineralogical analyses

The mineralogical composition of synthesis products and model compounds was characterized by powder XRD analysis using Co Kα radiation. Data were collected in Bragg-Brentano geometry using a PANalytical X'Pert Pro diffractometer equipped with an X'Celerator detector.

Considering their amorphous nature, additional details on the molecular-scale structure of Co-S_100C and Co-S_25C were obtained by powder XRD-PDF analysis. The details of this method are provided in the Supplementary Material.

Rietveld analysis of the XRD powder patterns of the Co-doped pyrites and mackinawite samples was performed using the XND code (Berar and Baldinozzi, 1998). The space groups, atomic positions and isotropic Debye-Waller factors of pyrite, marcasite FeS₂, elemental sulfur (orthorhombic – α–S₈) and mackinawite FeS structures were taken from Buerger (1937), Rettig and Trotter (1987), Wyckoff (1963) and Lennie et al. (1995), respectively. Unit-cell parameters, mean coherent domain (MCD) sizes and proportion of the identified mineral were determined using Rietveld refinement of the XRD powder patterns. A maximum of four mineral phases were included in the refinements, namely pyrite

(cubic – FeS₂), marcasite (orthorhombic – FeS₂), mackinawite (tetragonal – FeS) and elemental sulfur (orthorhombic – α–S₈). The scale factors were refined for all phases. The unit-cell parameters were refined for pyrite and marcasite only. Mean coherent domain (MCD) sizes were calculated from the width of the Lorentzian component of the line-shape, using the Scherrer formula. The line-shape parameters were refined for all phases except mackinawite. Atomic parameters were fixed for all phases to the previously reported crystallographic data.

Transmission Electron Microscopy (TEM) and Scanning Transmission Electron Microscopy (STEM) was performed using a JEOL 2100F field emission gun microscope operating at 200 kV, on Py_TE, Py_05Co_CB2 and Py_5Co final solids. To examine the nano-scale distribution of Co in the samples, qualitative chemical mapping and elemental compositions were acquired by energy dispersive X-ray spectroscopy (EDXS) with a Si(Li) JEOL detector. The corresponding data are included as Supplementary Material.

## 2.3. High energy resolution Fluorescence Detected (HERFD) X-ray absorption spectroscopy at Co K-edge

Synchrotron-based X-ray absorption spectroscopy at the Co K-edge was used to determine Co speciation in the synthetic pyrites and their precursors. High-resolution X-ray Absorption Near Edge Structure spectroscopy (XANES) was used to fingerprint the local Co coordination geometry (including redox state), and structural analysis of the
<figure><img src="image_4.png" /><figcaption>Fig. 1. Rietveld refinement (red) of experimental powder XRD patterns (black) of a) Pyrite precursor with 0.5 mol% Co and mackinawite with 0.5 mol% Co (black). b) Pyrite with 0.5 mol% Co from the first synthesis of Baya et al. (2022) c) Co-doped pyrites synthesized in this study. Calculated components refer to the above pattern in the figure. Notes: K refers to broad peaks arising from the Kapton® window of the anoxic sample chamber. The ? mark refers to an undetermined peak possibly arising from an edge of the zero-background silicon sample holder.</figcaption></figure>

Extended X-ray Absorption Fine Structure (EXAFS) spectra was performed on the basis of ab initio calculations. The data were collected in the high-energy resolution fluorescence detected (HERFD) mode at cryogenic temperature (~10 K) in a liquid helium cryostat on BM16 FAME-UHD beamline at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). The X-ray beam energy was monitored using 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. Data were collected in fluorescence mode using a crystal analyzer spectrometer (CAS) in order to improve the energy resolution of the fluorescence signal (HERFD), and so that the Co Kα emission line (6930.32 eV) could be properly separated from the Fe Kβ (7057.98 eV) one. For these analyses, selected pyrite and pyrite precursor samples (Py_05Co_CB1, Py_05Co_CB2, Py_TE and PyPrec_05Co) were prepared as pressed pellets (5 mm in diameter) made with finely grounded mixtures of 20–30 mg of pure solid plus about 5 mg of cellulose. By contrast, the pressed pellets of Co-S (Co-S_100C and Co-S_25C) were made with approximately 1 mg of finely grounded sample and approximately 20 mg of cellulose. All samples were prepared in the glove box and the pellets were then cautiously placed into separate labelled Eppendorf tubes, stored in a glass bottle with oxygen filter bags, and sealed with a butyl rubber stopper. The bottles were then placed in a sealed plastic bag with an oxygen filter for transportation to the synchrotron facility. Next to the beamline, the sample pellets were mounted on a PEEK® polymer sample holder (3–4 samples) in a N₂-filled glove bag and transported in a sealed glass vial to the beamline. The holders were then mounted on a cryostat sample rod under N₂ gas flow. The sample holder was immediately placed in a N₂-filled nitrile glove for transport to the experimental hutch where it was rapidly inserted into the cryostat. The number of scans depended on the concentration of Co in the solid samples, as estimated by the counts obtained in the Co Kα emission line with the Ketek® Si-drift detector in total fluorescence mode. Between 5 and 18 scans were necessary to obtain a reliable signal-to-noise ratio, depending on the sample. For each sample, scans were merged, normalized and EXAFS extracted using the ATHENA software (Ravel

and Newville, 2005). XANES pre-edge peaks were extracted by subtracting the baseline using the Larch software with a combination of linear and Voigt functions on a 7704.0 to 7715.0 eV energy range. The baseline skip range was set from 7707.0 to 7713.5(20) in order to obtain the best baseline fit. For Fe K-edge spectra, the energy range is set from 7103.0(14) to 7119.0(10) eV and the baseline skip range was set from 7108.5(5) to 7116.0(10) eV.

## 2.4. Density Functional Theory (DFT) modelling of Co-doped pyrite

The theoretical method employed here to model Co-doped pyrite is the same as that presented in Le Pape et al. (2017) for arsenic-bearing pyrite. A structural model with periodic boundary conditions was constructed by substituting a single Co atom for a Fe atom in a 2 × 2 × 2 neutral pyrite supercell (structural formula Fe₃₁CoS₆). Structural relaxations were performed in the framework of density functional theory (DFT) using the PWscf code of the Quantum ESPRESSO package (Giannozzi et al., 2009). The ionic cores of Fe, S and Co were described by Optimized Norm-Conserving Vanderbilt pseudopotentials from the SG15 ONCV library (Schlipf and Gygi, 2015). The incorporation of a Co atom into diamagnetic FeS₂ pyrite implies the presence of unpaired electrons, requiring spin-polarized calculations, which were carried out using the generalized gradient approximation (GGA) to the exchange-correlation functional with the Perdew-Burke-Ernzerhof (PBE) parametrization (Perdew et al., 1996). The electronic wave functions and charge density were expanded in plane waves with cutoffs of 100 and 1200 Ry, respectively. The Brillouin zone sampling was restricted to a single k-point (Γ-point). These computational parameters were chosen to achieve a convergence of the total energy better than 1 mRy/atom.

## 2.5. EXAFS and XANES data analysis

Shell-by-shell fitting of unfiltered Co K-edge k³-weighted EXAFS spectra was performed using a Fortran 90 custom-built least-squares fitting software based on a Levenberg-Marquardt minimizing algorithm,
with a code used in previous studies (Morin et al., 2016; Lefebvre et al., 2022). In this code, the $\chi(k)$ function is calculated with the plane-wave formalism (Teo, 1986), applying phase-shift and back-scattering amplitude and mean-free-path functions calculated with the Feff8.1 code (Ankudinov et al., 1998). These Co-centered single and multiple backscattering functions were calculated from the crystal structures by Bayliss (1977) and Lennie et al. (1995), which were used for fitting the Co K-edge spectra of the pyrite and mackinawite samples, respectively. The pyrite-derived backscattering functions were also used for fitting samples Co-S_100C and PyPrec_05Co data, whereas the mackinawite-derived backscattering functions were used for Co-S_25C sample data. Additional functions for a Co-Co path at 3.3 Å were calculated from the structure of linnaeite (Lundqvist and Westgren, 1938), Co₃S₄, for fitting the iron-free Co-S_100C sample data.

Besides, the experimental $k^3$-weighted Co K-edge EXAFS spectra of the samples were interpreted by comparison with EXAFS spectra simulated from either a DFT relaxed supercell or crystal structure data. For Co-substituted pyrite (DFT Co:pyrite), a relaxed supercell was calculated by DFT by replacing the center Fe atom by a Co atom (See section 2.4). Straightforward simulations were also conducted for comparison, by replacing a single Fe atom by a Co atom in the pyrite (Bayliss, 1977) and marcasite (Buerger, 1937) structures. The Atom and Feff8.1 codes were thus used to calculate theoretical Co-centered phase-shift and backscattering amplitude functions over a radius of 10 Å, including multiple scattering paths up to NLEG = 4 with criteria = 0.5. All these path functions were used to reconstruct the theoretical EXAFS spectrum. These calculated spectra were then least-square fitted to the experimental spectra of our Co-doped pyrite samples using a custom-built software based on a Levenberg-Marquardt minimization algorithm, refining the threshold energy shift $\Delta E_0$ parameter. For this calculation, the reduction factor $S_0^2$ was fixed to 1 and an overall Debye-Waller parameter $\sigma$ of 0.055 Å was applied. This latter value was calibrated by least-square fitting the Feff8 theoretical spectrum of pyrite (Bayliss, 1977) to the experimental spectrum of a natural crystalline pyrite (IMPMC mineral collection) at the Fe K-edge (Fig. S2). This experimental spectrum was recorded in transmission detection mode at liquid He temperature at the BM23 ESRF beamline. The mean-free path parameter was also optimized on this latter spectrum by changing the Vi potential value to 3 eV in the Feff8 calculation using a trial-and-error approach.

Co K-edge EXAFS spectra of Co-doped synthetic pyrites were fitted using a Linear combination least squares (LC-LS) method with a custom-built software based on the Levenberg-Marquardt minimization algorithm (Morin et al., 2017). The fitted components considered for the linear combinations were the simulated EXAFS spectra of Co-substituted pyrite and marcasite and the experimental Co K-edge spectra of the synthetic samples from this study.

A reduced chi-squares of the form, $\chi_r^2 = \frac{N_{ind}}{(N_{ind}-N_p)N_{points}} \frac{1}{e^2} \sum (k^3\chi_{exp}(k) - k^3\chi_{cal}(k))^2$, was used to estimate the fit quality for both LC-LS and shell-by shell fitting, where $N_{ind} = \frac{2k\Delta R}{\pi}$, is the number of independent parameters, $N_p$ is the number of fitting components, $N_{points}$ is the number of data points, and $e$ is the measurement uncertainty. Each fitting parameter $p$ has an uncertainty estimated to 99.7 % confidence as $3\sqrt{VAR(p)\chi_r^2}$, where $VAR(p)$ is the variance of the parameter $p$ returned by the Levenberg-Marquardt routine for the lowest value of $\chi_r^2$ (Morin et al., 2017).

# 3. Results

## 3.1. Mineralogy of the synthetic pyrites and model compounds

The mineralogical composition of the final products of our Co-doped pyrite synthesis experiments was determined using Rietveld refinement

of their powder XRD patterns (Fig. 1b and c). Pyrite is the principal component, with weight proportions varying from 56(9) to 79(3) wt% and an average of 69(5) wt%. In the Co-doped Py_05Co_CB1 pyrite sample from Baya et al. (2022), elemental sulfur $\alpha$-S₈(0) is remaining in the final solid (Fig. 1b), which is attributed to a non-stoichiometric Fe (III)/HS⁻ starting ratio. The second Co-doped Py_05Co_CB2 pyrite exhibits no residual elemental sulfur but possible residual FeS (Fig. 1c), which can be attributed to the initial Fe(III)/HS⁻ stoichiometry of the synthesis closer to 1:1. As observed using STEM, the size of the final pyrite crystals was in the ~ 500–800 nm range for the Py_TE, Py_05Co_CB2 (Fig. S3) and Py_05Co_CB1 samples (Baya et al. 2022). The size was smaller, in the ~ 100–300 nm range, for the Py_5Co sample, for which the initial Co:Fe was the highest. The pyrite crystals actually consist of aggregates of nano-crystallites with platy shape as shown for sample Py_5Co in Fig. S3d, and as typically observed for pyrites synthesized following our protocol (Baya et al. 2021, 2022). Consistently, Rietveld refinement of the powder XRD patterns indicated Mean Coherent Domain sizes in the 5–25 nm range for pyrite in our samples (Table 3).

The unit-cell parameter varies from $a = 5.43(4)$ Å to 5.44(2) Å, with an average of 5.44(2) Å, slightly larger than those reported by Wyckoff (1963) ($a = 5.4067$ Å) and Bayliss (1977) ($a = 5.417(1)$ Å) for pyrite (Table 3). Indeed, the unit cell dimension of Py_05Co_CB1 and Py_05Co_CB2 ($a = 5.43(4)$ and 5.438(9) Å) are close to that of the trace elements-doped pyrite sample containing Co plus other trace elements at initial concentration of 1 ppm in the synthesis solution ($a = 5.44(2)$ Å). Similar unit-cell parameter is obtained for the Py_5Co sample doped at 5 mol% CoS₂ ($a = 5.443(9)$ Å). According to Vegard's law, the unit cell expansion expected from Co²⁺ for Fe²⁺ substitution, with respective effective ionic radii of 0.65 and 0.61Å in low-spin octahedral coordination (Shannon, 1976) would lead to $a = 5.448$ Å for 5 mol% substitution, which matches the unit-cell dimension observed for our 5 mol% but also that of the much more diluted ones. Hence, the cell expansion observed in all our nano-pyrites compared to reported crystallographic data for bulk pyrite is attributed to structural defects as well as to the nanometric size of our synthetic pyrite crystallites (MCD ~ 6–23 nm), since unit-cell strain and expansion is expected for such sizes (Diehm et al., 2012). Moreover, the DFT calculations performed here for a Fe₃₁CoS₆₄ composition corresponding to 3.1 mol% Co:Fe (Table S3) result in a unit cell parameter lengthening of only +0.007 % instead of +0.358 % assuming Vegard's law. This DFT result, together with the regular Co-S distance returned by the DFT calculation (Table S3), suggests that the accommodation of Co²⁺ in pyrite leads to a minor structural relaxation, weaker than expected with Vegard's law, likely because of the covalent character of the metal-sulfide bond.

The proportion of marcasite in final synthesis products varies from 11(7) to 25(4) wt% with an average of 17(4) wt% (Table 3). The presence of marcasite is consistent with the acidic pH, as reported by Schoonen and Barnes (1991) and references therein, since the final pH of syntheses ranges between 3.1 and 4.5 (Table 1). According to this reported empirical influence of pH on the nature of the iron disulfide forming from FeS precursors (Schoonen and Barnes 1991), the higher proportion of marcasite in Py_TE compared to that in Py_CB1 might be attributed to the lower pH during this experiment (Table 1), due to the addition of the acidic (HNO₃ 1 wt% solution) multi-trace elements solution at the start of the synthesis.

In all the final pyrite products, a small amount of remaining untransformed FeS<sub>NanoMck</sub> precursor (Baya et al., 2021) is observed (Fig. 1, Table 3). Indeed, as determined by Rietveld refinement, nano-mackinawite with MCD of 2–4 nm accounts for 78 w% of the precursor in Baya et al. (2021), whereas elemental sulfur, $\alpha$-S₈(0), accounts for the remaining 22 wt%. These proportions are close to those determined by Rietveld refinement analysis of the PyPrec_05Co sample in the present study, i.e., 65(1) wt% of nano-mackinawite and 35(1) wt% of $\alpha$-S₈(0) (Fig. 1a, Table 3).

The remaining fraction of this FeS<sub>NanoMck</sub> phase is estimated to range
<caption>**Table 3**</caption>

Crystallographic parameters and proportions of mineralogical phases derived from XRD-Rietveld analysis for the samples listed in bold font in the first column. Uncertainties in the parameters are given in brackets referring to the last digit. Other parameters were fixed. Starting crystal structure data were taken from Wyckoff (1963), Buerger (1937), Lennie et al. (1995) and Rettig and Trotter (1987) for pyrite, marcasite, mackinawite and α-S<sub>8</sub>(0), respectively. Unit-cell parameters of mackinawite were fixed to those reported by Baya et al. (2021). The – sign indicates the same value as the previous one in the same row. For samples PyPrec_05Co and Mack_Co, the MCD<sub>iso</sub> values refer to the ab plane extension.

<table><thead><tr><th>Mineral phases</th><th>a(Å)</th><th>b(Å)</th><th>c(Å)</th><th>MCD<sub>iso</sub>(nm)</th><th>MCD<sub>[111]</sub>(nm)</th><th>MCD<sub>[001]</sub>(nm)</th><th>wt%</th></tr></thead><tbody><tr><td><strong>Py_5Co</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Pyrite</td><td>5.443(9)</td><td>—</td><td>—</td><td></td><td>6(1)</td><td>20(1)</td><td>79(3)</td></tr><tr><td>Marcasite</td><td>4.37(9)</td><td>5.5(1)</td><td>3.38(7)</td><td>7(3)</td><td></td><td></td><td>15(3)</td></tr><tr><td>Mackinawite</td><td>3.7</td><td>—</td><td>5.2</td><td>2</td><td></td><td></td><td>6(1)</td></tr><tr><td><strong>Py_05Co_CB1</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Pyrite</td><td>5.43(4)</td><td>—</td><td>—</td><td></td><td>6(11)</td><td>14(48)</td><td>56(9)</td></tr><tr><td>Marcasite</td><td>4.37</td><td>5.42</td><td>3.37</td><td>4</td><td></td><td></td><td>11(7)</td></tr><tr><td>Mackinawite</td><td>3.7</td><td>—</td><td>5.2</td><td>3</td><td></td><td></td><td>19(5)</td></tr><tr><td>α-S<sub>8</sub>(0)</td><td>10.4646</td><td>12.866</td><td>24.486</td><td></td><td></td><td></td><td>14(5)</td></tr><tr><td><strong>Py_05Co_CB2</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Pyrite</td><td>5.438(9)</td><td>—</td><td>—</td><td></td><td>6(3)</td><td>19(1)</td><td>78(3)</td></tr><tr><td>Marcasite</td><td>4.37(8)</td><td>5.4(1)</td><td>3.38(6)</td><td>7(1)</td><td></td><td></td><td>17(3)</td></tr><tr><td>Mackinawite</td><td>3.7</td><td>—</td><td>5.2</td><td>2</td><td></td><td></td><td>5(1)</td></tr><tr><td><strong>Py_TE</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Pyrite</td><td>5.44(2)</td><td>—</td><td>—</td><td></td><td>7(1)</td><td>23(2)</td><td>63(3)</td></tr><tr><td>Marcasite</td><td>4.4(1)</td><td>5.4(1)</td><td>3.36(8)</td><td>6</td><td></td><td></td><td>25(4)</td></tr><tr><td>Mackinawite</td><td>3.7</td><td>—</td><td>5.2</td><td>2</td><td></td><td></td><td>12(2)</td></tr><tr><td><strong>PyPrec_05Co</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mackinawite</td><td>3.67</td><td>—</td><td>5.25</td><td>3</td><td></td><td></td><td>65(1)</td></tr><tr><td>α-S<sub>8</sub>(0)</td><td>10.4646</td><td>12.866</td><td>24.486</td><td></td><td></td><td></td><td>35(1)</td></tr><tr><td><strong>Mack_Co</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mackinawite</td><td>3.67(1)</td><td>—</td><td>5.14(2)</td><td>7(1)</td><td></td><td>5(1)</td><td>100</td></tr></tbody></table>

between 5(1) and 19(5) wt% in the final pyrite products (Table 3). This nano-mackinawite phase is interpreted as “freshly precipitated” mackinawite with expanded unit-cell compared to the crystalline structure caused by the decrease in crystallite size and possibly by intercalation of water molecules (Baya et al., 2021). We thus used these expanded unit cell parameters to fit this phase (Table 3). In addition, a marked preferred orientation effect is observed for this phase when deposited on the sample-holder due to platy crystallites that favor basal-plane Bragg reflections. This effect was accounted for by including a strong preferential orientation parameter in the Rietveld refinement procedure for this phase. For this latter reason and because the broad basal reflection of nano-mackinawite at 20° 2θ is difficult to distinguish from the second bump signal of the sample-chamber Kapton window, the actual amount of nano-mackinawite is difficult to accurately quantify using the sole XRD-Rietveld analysis. Moreover, the detected nano-mackinawite particles are so small (2–3 nm; Table 3) that they reach the size boundary between crystalline and amorphous material, which produces very broad Bragg reflections. Accordingly, based on x-ray scattering pair distribution function (XRD-PDF) and EXAFS analyses, the FeS precursor phase was actually described by Baya et al. (2021) as a continuum between nano-mackinawite (FeS<sub>NanoMck</sub>, 47 wt%) and amorphous FeS that could be interpreted as consisting of aggregated clusters (FeS<sub>cluster</sub>, 53 wt %).

Our Co-doped mackinawite model compound Mack_Co is found to consist only of nanocrystalline mackinawite (Fig. 1a; Table 3) with MCD of 5–7 nm and c unit cell parameters slightly larger than that reported for bulk mackinawite in Lennie et al. (1995). The refined unit-cell parameters, i.e., a = 3.67(1) Å and c = 5.14(1) Å in average, are close to those reported in Baya et al. (2021) (a = 3.70(2) Å and c = 5.24(7) Å) and are larger than those of crystalline mackinawite reported by Lennie et al. (1995) (a = 3.6735(4) Å and c = 5.0328(7) Å).

The diffraction pattern of the Co-S<sub>25C</sub> sample, which consists of amorphous-CoS (Fig. S4), exhibits very broad bands of low intensities with angular position that did not match those of CoS crystalline phases

and was thus not analyzed by Rietveld refinement. For the Co-S<sub>100C</sub> sample, an attempt of Rietveld refinement was performed by hypothesizing a nanosized cattierite (CoS<sub>2</sub>) local structure, based on a rough match to the main Bragg peaks of this phase and on the resemblance of the Co K-edge EXAFS spectrum of the precursor with that of Co-doped pyrite, as discussed in the next sections. The refined unit cell parameter a = 5.2(1) Å was found to be much smaller than that of crystalline cattierite a = 5.5385 Å (Nowack et al., 1991) (Fig. S4), which indicates that the actual local structure of the Co sulfide phase in Co-S<sub>100C</sub> is not simply matching with nano-cattierite. In addition, the very low MCD<sub>iso</sub> of 1–2 nm found for this sample reaches the limit of the Rietveld refinement method that is only strictly valid for periodic crystal structures. Further investigation of the local structure around Co in this sample is then performed in the next section using XANES, EXAFS and XRD-PDF.

## 3.2. Pre-edge analysis of Co first-neighbor coordination

Fig. 2 displays the Co K-edge HERFD-XANES spectra including detailed pre-edge region for all samples investigated in this study. The Co-doped pyrite samples exhibit rather similar XANES spectra, together with the PyPrec_05 and Co-S<sub>100C</sub> samples, whereas the Co-S<sub>25C</sub> and Mack_Co model compounds are different. More precisely, the pre-edge of Co-doped mackinawite (Mack_Co) and amorphous CoS (Co-S<sub>25C</sub>) shows a single peak at 7708.6 eV, whereas the pre-edges of the pyrite precursor (PyPrec_05), the amorphous CoS<sub>1+x</sub> model compound (Co-S<sub>100C</sub>) and the Co-doped pyrite samples all consist in a doublet at 7709.6 eV and 7711.7 eV.

A similar difference in the XANES spectrum between pyrite and mackinawite can be observed at the Fe K-edge, as shown in Baya et al. (2021) (Fig. S5), indicating that the slightly split and low intensity pre-edge corresponds to sixfold octahedral coordination of the cation to sulfide first neighbors in the pyrite structure, whereas the sharp pre-edge corresponds to fourfold tetrahedral coordination of the cation to sulfide
<figure><img src="image_5.png" /><figcaption>Fig. 2. Normalized Co K-edge HERDF-XANES spectra of the studied samples (left) and corresponding background-subtracted pre-edge signal (right).</figcaption></figure>

in the mackinawite structure. This qualitative interpretation is fully consistent with extensive literature on the pre-edge analysis of transition metal coordination, especially for iron in oxide compounds (e.g., Waychunas et al., 1983; Petit et al., 2001; Galoisy et al., 2001; Wilke et al., 2001) and in sulfide compounds (Jeong et al., 2008) (Fig. S5), including Co sulfide compounds (Al Samarai et al., 2016). Moreover, the increase in pre-edge intensity from non-centrosymmetric to centrosymmetric coordination environment is clearly supported by theoretical calculation (Cabaret et al., 2010), which predicts a sharper pre-edge for tetrahedral high spin Fe(II) than for octahedral low-spin Fe(II) bound to sulfide ligands (DeBeer George et al., 2008). The local coordination of Co in our samples may then be qualitatively interpreted from our unprecedented HERFD-XANES Co pre-edge data, based on the above literature reports on Fe pre-edge analysis and a previous Co K-edge XANES study of synthetic high-temperature Co-doped pyrite (Wang et al., 2020). Hence, the pyrite precursor (PyPrec_05Co) spectrum at the Co K-edge is similar to that of pyrite (Fig. 2), with the first peak of the doublet at 7709.3 eV and the second peak at the same energy position of that of pyrite, suggesting the major occurrence of octahedral Co and the possible occurrence of minor amounts of tetrahedral Co. The Co-S compound synthesized at 100 °C in the presence of polysulfide has a similar pre-edge. By contrast, the Co-S compound synthesized at 25 °C only with sulfides exhibits a single pre-edge peak similar to that of mackinawite at 7708.8 eV, indicative of tetrahedral units. Although less intense, this latter peak suggests a significant occurrence of tetrahedral Co, as in Co-doped

mackinawite (Fig. 2).

Consequently, our data show that Co occurs mostly in octahedral coordination in the synthetic Co-doped pyrites as well as in the FeS precursor of pyrite (PyPrec_05Co), while it is mostly incorporated in tetrahedral coordination in the amorphous Co-S_25C model compound and in Co-doped mackinawite (Mack-Co). Interestingly, the octahedral coordination found for Co in the FeS precursor of pyrite differs from the mainly tetrahedral one reported for Fe in the same compound (Baya et al., 2021). This suggests that a significant fraction of Co does not substitutes for Fe in the precursor compound, which called for further analysis of Co speciation in this sample, as detailed thereafter.

### 3.3. EXAFS analysis of the local structure around Co

Shell-by-shell fitting of experimental Co K-edge EXAFS spectra was performed to further investigate the local structure and coordination of Co in the studied samples (Fig. 3, Table 4). It is noteworthy that the difference in the Fe-S bond length between tetrahedral high-spin Fe(II) in mackinawite and octahedral low-spin Fe(II) in pyrite is on the order of 0.008 Å according to crystallographic data (Table 4). Since such a difference is not detectable by EXAFS (Teo 1986), our structural interpretation of the shell-by-shell analysis hereafter at the Co K-edge is rather based on the short-range structural motif than on the Co-S distance.

In Co-doped mackinawite (i.e., sample Mack-Co) the environment observed for Co is consistent with Co(II) substitution for Fe(II) in the
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Shell-by-shell fit of EXAFS data recorded at 10 K for studied samples. Experimental and calculated curves are plotted as black and red lines, respectively. (left) Unfiltered EXAFS $k^3\chi(k)$ functions. (right) Fourier Transform of unfiltered $k^3\chi(k)$ functions.</figcaption></figure>

mackinawite structure since fitted distances and number of neighbors (CN) matched the values expected in the mackinawite structure at least up to the 7th shell at 5.6 Å (Lennie et al., 1995; Ikogou et al., 2017; Baya et al., 2021) (Table 4, Fig. 3), given uncertainties and relaxation effects. To this regard, the CN value of 3.9(5) for the first Co-S path (Table 4) is consistent with tetrahedral coordination, given the typical estimated accuracy of ±10–30% on CN numbers in EXAFS (Teo 1986). However, the first shell Co-S distance of 2.224(9) Å found for this sample appears slightly shorter than the Fe-S distance of 2.2558(9) Å reported for the mackinawite bulk structure by Lennie et al. (1995) (Table 4). This difference suggests that Co for Fe replacement in mackinawite may cause a local relaxation of the structure, leading to a slight shortening of the first-neighbors distance. This is consistent with the shorter effective atomic radius of Co (0.58 Å) compared to that of Fe (0.63 Å) in high-spin tetrahedral coordination (Shannon, 1976). Co for Fe substitution in mackinawite is consistent with the previously presented pre-edge fingerprint approach that suggested tetrahedral coordination for Co in this sample (Fig. 2).

In the amorphous Co-S compound synthesized at 25 °C (i.e., sample Co-S_25C), Co is surrounded by a single shell consisting of 4.2(7) S atoms

at 2.226(1) Å. Given the uncertainties on CN numbers in EXAFS, this number of first neighbors is in agreement with a dominantly tetrahedral coordination of Co, as in the mackinawite structure, even if augmented by an additional octahedral component, as suggested by the slightly lower pre-edge intensity for this compound compared to mackinawite (Fig. 2).

For the pyrite precursor (i.e., sample PyPrec_05Co), shell-by-shell fitting of EXAFS data indicates 5.8(5) S atoms in the first shell around Co, with a Co-S distance of 2.262(6) Å that is consistent with the Fe-S distance in the pyrite structure. Similar, though slightly shorter, first-neighbor Co-S distance (2.246(8) Å) and coordination number 5.3(7) are observed in the amorphous Co-S compound synthesized at 100 °C in the presence of polysulfides (Table 4). These results confirm the pre-edge interpretation (Fig. 2) indicating octahedral coordination to S atoms for Co both in the precursor of pyrite and in the Co-S_100C model compound. At longer distances, in the PyPrec_05Co sample, the contributions to the EXAFS arise only from S neighboring atoms at distances close to 3.5 Å and 3.6 Å that are in agreement with those found around octahedral Fe in the pyrite structure. However, the lack of contributions from Fe and/or Co neighbors expected at 3.8 Å (Table 4) suggests that
<caption>**Table 4**</caption>

Shell-by-shell EXAFS fit results for studied samples compared to crystal structure data, from which theoretical phase and amplitude functions were calculated using Feff8 (Ankudinov et al., 1998). R(Å): interatomic distances between the central Co atom and its neighbors; N: number of neighbors; σ (Å): Debye Waller factor, ΔE₀: shift between threshold energy and the experimentally determined threshold energy, in eV. The S₂ parameter was fixed to 1. The sign – indicates the same value as in the line above. Uncertainties on the last digit are given under brackets for 3 sigma, i.e. 99.7% confidence (see text).

<table><thead><tr><th rowspan="2">Samples</th><th colspan="4">EXAFS</th><th colspan="3">Crystal structure data</th></tr><tr><th>χ²/√n</th><th>ΔE₀ (eV)</th><th>σ (Å)</th><th>R (Å)</th><th>N</th><th>Structure</th><th>R (Å)</th><th>N</th></tr></thead><tbody><tr><td>Mack_Co</td><td>2.96</td><td>2.9(12)</td><td>0.070(9)</td><td>2.224(9)</td><td>3.9(5) S</td><td>mackinawite<sup>a</sup></td><td>2.2558(9)</td><td>4 S</td></tr><tr><td></td><td></td><td>–</td><td>0.082(4)</td><td>2.58(1)</td><td>4 Fe</td><td>–</td><td>2.5976(3)</td><td>4 Fe</td></tr><tr><td></td><td></td><td>–</td><td>0.10(1)</td><td>3.71(2)</td><td>4 Fe</td><td>–</td><td>3.6735</td><td>4 Fe</td></tr><tr><td></td><td></td><td>–</td><td>–</td><td>4.24(2)</td><td>8 S</td><td>–</td><td>4.3108</td><td>8 S</td></tr><tr><td></td><td></td><td>–</td><td>–</td><td>5.0(1)</td><td>2 Fe</td><td>–</td><td>5.0328</td><td>2 Fe</td></tr><tr><td></td><td></td><td>–</td><td>–</td><td>5.28(2)</td><td>8 CoFeFe</td><td>–</td><td>5.1951</td><td>8 FeFeFe</td></tr><tr><td></td><td></td><td>–</td><td>–</td><td>5.63(3)</td><td>8 Fe</td><td>–</td><td>5.6636</td><td>8 Fe</td></tr><tr><td>Co-S_25C</td><td>4.29</td><td>1.3(18)</td><td>0.09(1)</td><td>2.226(1)</td><td>4.2(7) S</td><td>mackinawite<sup>a</sup></td><td>2.2558(9)</td><td>4 S</td></tr><tr><td>Co-S_100C</td><td>5.17</td><td>3.8(13)</td><td>0.073(8)</td><td>2.246(8)</td><td>5.3(7) S</td><td>pyrite<sup>b</sup></td><td>2.2640</td><td>6 S</td></tr><tr><td></td><td></td><td>–</td><td>0.05</td><td>3.47(4)</td><td>1.7(9) S</td><td>–</td><td>3.4450</td><td>6 S</td></tr><tr><td></td><td></td><td>–</td><td>–</td><td>3.6129</td><td>1.5(13) S</td><td>–</td><td>3.6129</td><td>2 S</td></tr><tr><td></td><td></td><td>–</td><td>–</td><td>4.65(7)</td><td>4.3(37) CoSS</td><td>–</td><td>4.5280</td><td>6 FeSS</td></tr><tr><td>PyPrec_05Co</td><td>2.84</td><td>5.6(9)</td><td>0.074(5)</td><td>2.262(6)</td><td>5.8(5) S</td><td>pyrite<sup>b</sup></td><td>2.2640</td><td>6 S</td></tr><tr><td></td><td></td><td>–</td><td>0.05</td><td>3.49(2)</td><td>2.4(6) S</td><td>–</td><td>3.4450</td><td>6 S</td></tr><tr><td></td><td></td><td>–</td><td>–</td><td>3.6129</td><td>1 S</td><td>–</td><td>3.6129</td><td>2 S</td></tr><tr><td></td><td></td><td>–</td><td>–</td><td>4.72(4)</td><td>6 CoSS</td><td>–</td><td>4.5280</td><td>6 FeSS</td></tr><tr><td>Py_5Co</td><td>4.11</td><td>3.6(11)</td><td>0.064(7)</td><td>2.256(6)</td><td>5.1(5) S</td><td>pyrite<sup>b</sup></td><td>2.2640</td><td>6 S</td></tr><tr><td></td><td></td><td>–</td><td>0.06(2)</td><td>3.47(2)</td><td>3.2(11) S</td><td>–</td><td>3.4450</td><td>6 S</td></tr><tr><td></td><td></td><td>–</td><td>–</td><td>3.70(7)</td><td>2 S</td><td>–</td><td>3.6129</td><td>2 S</td></tr><tr><td></td><td></td><td>–</td><td>–</td><td>3.83(2)</td><td>3.1(15) Fe</td><td>–</td><td>3.8301</td><td>12 Fe</td></tr><tr><td></td><td></td><td>–</td><td>–</td><td>4.449</td><td>6 S</td><td>–</td><td>4.4492</td><td>6 S</td></tr><tr><td></td><td></td><td>–</td><td>–</td><td>4.52(3)</td><td>6 CoSSoS</td><td>–</td><td>4.5280</td><td>6 FeSFeS</td></tr><tr><td>Py_05Co_CB1</td><td>5.08</td><td>4.3(11)</td><td>0.051(8)</td><td>2.264(6)</td><td>4.9(6) S</td><td>pyrite<sup>b</sup></td><td>2.2640</td><td>6 S</td></tr><tr><td></td><td></td><td>–</td><td>0.04(2)</td><td>3.47(2)</td><td>3.0(9) S</td><td>–</td><td>3.4450</td><td>6 S</td></tr><tr><td></td><td></td><td>–</td><td>–</td><td>3.69(7)</td><td>2 S</td><td>–</td><td>3.6129</td><td>2 S</td></tr><tr><td></td><td></td><td>–</td><td>–</td><td>3.82(1)</td><td>2.8(13) Fe</td><td>–</td><td>3.8301</td><td>12 Fe</td></tr><tr><td></td><td></td><td>–</td><td>–</td><td>4.449</td><td>6 S</td><td>–</td><td>4.4492</td><td>6 S</td></tr><tr><td></td><td></td><td>–</td><td>–</td><td>4.52(2)</td><td>6 CoSSoS</td><td>–</td><td>4.5280</td><td>6 FeSFeS</td></tr><tr><td>Py_05Co_CB2</td><td>5.46</td><td>4.4(12)</td><td>0.069(7)</td><td>2.262(7)</td><td>5.9(7) S</td><td>pyrite<sup>b</sup></td><td>2.2640</td><td>6 S</td></tr><tr><td></td><td></td><td>–</td><td>0.08(3)</td><td>3.50(3)</td><td>4.5(18) S</td><td>–</td><td>3.4450</td><td>6 S</td></tr><tr><td></td><td></td><td>–</td><td>–</td><td>3.7(1)</td><td>2 S</td><td>–</td><td>3.6129</td><td>2 S</td></tr><tr><td></td><td></td><td>–</td><td>–</td><td>3.83(2)</td><td>4.2(25) Fe</td><td>–</td><td>3.8301</td><td>12 Fe</td></tr><tr><td></td><td></td><td>–</td><td>–</td><td>4.449</td><td>6 S</td><td>–</td><td>4.4492</td><td>6 S</td></tr><tr><td></td><td></td><td>–</td><td>–</td><td>4.52(4)</td><td>6 CoSSoS</td><td>–</td><td>4.5280</td><td>6 FeSFeS</td></tr><tr><td>Py_TE</td><td>4.35</td><td>3.6(11)</td><td>0.057(7)</td><td>2.262(6)</td><td>5.0(5) S</td><td>pyrite<sup>b</sup></td><td>2.2640</td><td>6 S</td></tr><tr><td></td><td></td><td>–</td><td>0.05(2)</td><td>3.47(2)</td><td>3.1(9) S</td><td>–</td><td>3.4450</td><td>6 S</td></tr><tr><td></td><td></td><td>–</td><td>–</td><td>3.69(7)</td><td>2 S</td><td>–</td><td>3.6129</td><td>2 S</td></tr><tr><td></td><td></td><td>–</td><td>–</td><td>3.83(1)</td><td>3.4(14) Fe</td><td>–</td><td>3.8301</td><td>12 Fe</td></tr><tr><td></td><td></td><td>–</td><td>–</td><td>4.449</td><td>6 S</td><td>–</td><td>4.4492</td><td>6 S</td></tr><tr><td></td><td></td><td>–</td><td>–</td><td>4.52(2)</td><td>6 CoSSoS</td><td>–</td><td>4.5280</td><td>6 FeSFeS</td></tr></tbody></table>

Note: <sup>a</sup>Lennie et al. (1995); <sup>b</sup>Bayliss (1977).

Co atoms are only surrounded by S atoms, with a short-range structure that may resemble that of pyrite. Given that the pyrite anionic network relies on the specific arrangement of S₂ disulfide groups, the presence of Co within such a network without any other metal neighbors strongly suggests that Co is in the form of pyrite-like octahedral CoSₙ polysulfide monomeric units, which likely aggregate into an amorphous phase in this sample.

Contributions to the EXAFS beyond the first shell in the Co-S_100C sample are fitted with almost the same neighboring atom shell arranged in a rather similar local structure as in the PyPrec_05Co. An attempt to include a weak Co-Co path at 3.3 Å was performed to investigate possible edge-sharing octahedra in this iron-free sample, such as in the thiospinel linnaeite Co₃S₄, but was found not statistically significant. Results of the XRD-PDF analysis of the Co-S_100C sample suggest similar structural characteristics. The Co-S_100C sample exhibit a highly disordered short-ranged (~ 6 Å) structure (Fig. S6), as compared to the theoretical PDF patterns of crystalline cattierite CoS₂

(Nowack et al., 1991), linnaeite Co₃S₄ (Menzer, 1926) and jaipurite CoS (Wyckoff, 1963). However, a significant structural similarity with cattierite CoS₂ can be observed, with consistent distances for the first Co-S and S-S atom pair contributions, as shown in Fig. S6 (right). Nevertheless, the lack of Co-Co pairs compared to the structure of crystalline cattierite is consistent with our EXAFS results and suggests that the local structure of the amorphous phase observed in PyPrec_05Co consists of the aggregation of monomeric pyrite-like CoSₙ octahedral units.

For the Co-doped pyrite end-products (i.e., samples Py_5Co, Py_05Co_CB1, Py_05Co_CB2, Py_TE) (Fig. 3), the set of Co-S and Co-Fe paths determined by the shell-by-shell fitting of the Co K-edge EXAFS data is consistent with the local structure around the cation in pyrite (Table 4), but the observed number of neighbors is systematically lower than expected for bulk pyrite, especially beyond the first coordination S shell. More precisely, the Co-S distance is observed at 2.26 Å that is consistent with the Fe-S distance in the pyrite structure, and is similar to that observed for the PyPrec_05Co and Co-S_100C samples (Table 4).
The CN values obtained for the first shell, i.e., within the 4.9(6)–5.9(7) range are consistent with octahedral coordination, given the typical estimated accuracy of ± 10–30 % in CN values in EXAFS (Teo, 1986). Besides, the pre-edge HERFD-XANES data attest of a mainly octahedral coordination (Fig. 2). In contrast, the lack of Fe neighbors at 3.8 Å cannot be explained solely by such an uncertainty nor by the nanosized character of our anisotropic pyrite crystallites (5–25 nm; Table 3), since the loss of neighbors observed by EXAFS for this crystallite size range is expected to be between 15 and 5 % (Maillot et al., 2011). Note that the high frequency signal at *k* ~ 6.5 Å⁻¹ is due to long distance MS paths (≥ 4.8 Å) in the pyrite structure that yield a weakly detectable feature at *R*+Δ*r* ~ 5.2 Å in the FFT (Fig. 3). Such long-distance MS paths were not included in the shell-by-shell fit but were rather considered in Feff8 calculation presented in Fig. 4, as discussed in the next paragraph. Altogether, the shell-by-shell analysis indicated a local pyrite environment around cobalt.

The significant lack of cation neighbors at short and medium range suggests that Co only partly substitutes for Fe in pyrite, and could then be also present in an additional solid phase. This hypothesis is further supported by the comparison between the experimental Co K-edge EXAFS spectra of pyrite samples and the theoretical spectrum calculated using Feff8 for a DFT-modeled supercell of Co-doped pyrite (see sections 2.4 and 2.5) (Fig. 4). Indeed, although the distances of the second shell paths appear to be well reproduced by the theoretical calculation, their observed amplitudes are significantly lower than the calculated ones (Table 4).

In order to check for the possible occurrence of additional Co species
in the Co-doped pyrite samples, we attempted to fit the EXAFS spectra of

these samples using linear combinations of simulated spectra of Co in
pyrite and other Co components, including a simulated spectrum for Co
in marcasite as well as the experimental spectrum of Co in the
PyPrec_05Co sample (Fig. S7). Several fit solutions corresponding to the
best combinations of two or three of these components are reported in
Table S4. The most reliable results were obtained when mixing a
simulated spectrum of Co in pyrite with the experimental spectrum of Co
in the precursor of Co-doped pyrite, i.e., sample PyPrec_05Co (Figs. 5
and S8). Indeed, adding a simulated spectrum for Co in marcasite as
fitting component with proportions in the 10–23 % range slightly
improved the fit quality (Table S4), which suggests that such a compo-
nent cannot be excluded. However, the addition of this component
decreased the χ²/R value of only 4 to 30 % among samples (Table S4),
which would not be statistically significant against a F-test at 90 %
confidence (observed F ≤ 1.4 against F ≥ 1.6 for α = 0.1 and ν = 30).
Besides, addition of a Co-doped mackinawite component (Mack_Co) did
not improve the fit, in agreement with the important difference in shape
between this spectrum and those of the Co-doped pyrite, marcasite and
precursor (Figs. 3 and S7).

Accordingly, the spectra of the Co-doped pyrite end-products can be
all reconstructed by combining the theoretical spectrum of the Co-
substituted pyrite supercell obtained from DFT and the experimental
spectrum of the Co-doped precursor (Figs. 5 and S8; Table 5). These LC-
LS fitting results show that most of the Co pool added to the solution at
the start of the synthesis remains in the form of the Co-doped precursor
after pyrite precipitation (65–75 %), while a smaller fraction of Co is
incorporated in pyrite structure (20–36 %). Taking into account for
other acceptable fit solutions for the final Co-doped pyrite products data

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Experimental EXAFS spectrum (black lines) compared to ab initio Feff 8.1 calculated spectrum using the Co-doped pyrite supercell simulated by DFT (left panel). Fast Fourier transform of the *k*³*χ*(*k*) functions (right panel).</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 5. Linear combination least squares (LC-LS) fitting of the experimental Co K-edge data of synthetic Co-doped pyrites using two components: the theoretical spectrum of the Co-substituted pyrite DFT-modeled supercell and the experimental spectrum of the Co-doped precursor. EXAFS-LC-LS fitting with selected fitting components simulated using Feff 8.1 from supercell obtained from DFT calculations: Co-doped pyrite (left panel). Fast Fourier transform of the EXAFS signals with the phase shift for both experimental and simulated EXAFS spectra. Proportions of the fitting components are displayed in the pie chart (right panel). Non-normalized results and estimators of fit quality are given in Table 5. Normalized percentage of each component is represented by pie diagrams (Solid and diagonal stripe filled pattern represent theoretical Co-doped pyrite and pyrite precursor, respectively). Detailed fit parameters are reported in Table 5.</figcaption></figure>

(Table S4), including Co substituted in pyrite and to a lesser extent in marcasite, rather similar results are obtained, with 59–67 ± 10 % of Co in the form of the Co-doped precursor and 28–41 ± 10 % of Co in a FeS₂ (py,mar) phase (Table 5). Moreover, the proportion of untransformed, Co-doped precursor in the final solids appears to increase with the amount of Co added in the synthesis experiment (Fig. 5, right panel).

The EXAFS spectrum of the Co-doped precursor appears rather similar to that of Co-S_100C, whereas it significantly differs from those of Co-S_25C and of Mk_05Co (Fig. 3). Moreover, according to the analysis of XANES pre-edge features, Co is likely 6-fold coordinated in the Co-doped precursor and in Co-S_100C, while it is 4-fold coordinated in Mk_05Co (Fig. 2). This resemblance between the local environment of Co in the pyrite precursor and in this amorphous Co-S_100C compound suggests that it may partly segregate into a specific Co-rich phase before and during pyrite formation, as discussed in the next sections. This latter interpretation is also supported by the STEM-EDX analysis of the final Py_5Co sample showing the presence of an interstitial Co-rich phase at the pyrite grain boundaries (Fig. S9), which likely corresponds to such remaining Co-doped precursor. The lower Co doping level in the other samples studied precluded any reliable STEM-EDX analysis of the spatial distribution of Co.

# 4. Discussion

## 4.1. The Co-polysulfide precursor and its role during pyrite formation

The reaction sequence of pyrite formation in our experiments is similar to that of Wei and Osseo-Asare (1997), Noël et al. (2014), Morin et al. (2017), Le Pape et al. (2017) and Baya et al. (2021, 2022). Briefly, mixing an acidic ferric solution and an alkaline Na₂S solution leads to the rapid and complete reduction of Fe³⁺ into Fe²⁺ by HS(aq), followed by the formation of elemental sulfur α-S(0), FeS(s) and remaining Fe²⁺ within the first 30 min of reaction, as summarized in the following equation:

$$ \mathrm{Fe}^{3+}_{(\mathrm{aq})} + \mathrm{HS}^{-} \rightarrow \frac{1}{2} \mathrm{Fe}^{2+}_{(\mathrm{aq})} + \frac{1}{2} \mathrm{FeS}_{(\mathrm{s})} + \frac{1}{2} \mathrm{S}(\mathrm{0})_{(\mathrm{\alpha}-\mathrm{sulfur})} + \mathrm{H}^{+} \quad (1) $$

Pyrite is then formed by the reaction between FeS(s) and S(0) via the polysulfide pathway (Rickard, 1975; Luther, 1991; Theberge and Luther, 1997; Rickard and Morse, 2005; Rickard and Luther, 2007), that is actually considered to involve aqueous polysulfide species H₂Sₙ(aq) and most likely aqueous FeS species (Rickard and Luther, 1997; Rickard et al., 2001, Rickard and Morse, 2005), as follows:

$$ \mathrm{FeS}_{(\mathrm{s},\mathrm{aq})} + \mathrm{H}_{2}\mathrm{S}_{\mathrm{n}}_{(\mathrm{aq})} \rightarrow \mathrm{FeS}_{2}(\mathrm{s}) + \mathrm{H}_{2}\mathrm{S}_{\mathrm{n}-1} \quad (2) $$

Under our synthesis conditions, the polysulfide species H₂Sₙ(aq) and
<caption>**Table 5**</caption>

Results of the LC-LS fitting procedure applied to Co K-edge EXAFS data of the Co-doped pyrite end-products. The upper part of the Table reports the results obtained with two fitting components: the theoretical spectrum for the DFT supercell for Co-doped pyrite and the experimental spectrum of the pyrite precursor PyPrec_05Co sample, as shown in Figs. 5 and S8. The lower part of the table reports the average results of two and three components fitting solutions that combined the PyPrec_05Co spectrum with the theoretical spectra of either Co in the DFT supercell for Co-doped pyrite, Co at the Fe site in the pyrite structure by Bayliss (1977), Co at the Fe site in the marcasite structure by Buerger (1937). Fitting components are displayed in Fig. S7 and detailed results of these fits are reported in Table S4. This latter analysis results into two main Co species referred to as CoS<sub>n</sub> precursor, represented by the PyPrec_05Co spectrum and Co:FeS<sub>2</sub>(py,mar) representing combinations of the Co:py and Co:mar theoretical spectra.

<table><thead><tr><th>Sample</th><th>PyPrec_05Co</th><th>DFT Co-doped pyrite</th><th>sum</th><th>R<sub>f</sub></th><th>χ<sup>2</sup></th></tr></thead><tbody><tr><td>Py_5Co</td><td>75(5)</td><td>20(3)</td><td>95</td><td>5.65<br>10<sup>-2</sup></td><td>1.24</td></tr><tr><td>Py_05Co_CB1</td><td>73(5)</td><td>33(3)</td><td>105</td><td>6.87<br>10<sup>-2</sup></td><td>1.53</td></tr><tr><td>Py_05Co_CB2</td><td>73(5)</td><td>33(4)</td><td>106</td><td>5.54<br>10<sup>-2</sup></td><td>1.24</td></tr><tr><td>Py_TE</td><td>65(4)</td><td>36(4)</td><td>101</td><td>7.55<br>10<sup>-2</sup></td><td>1.07</td></tr></tbody></table><table><thead><tr><th>Sample</th><th>CoS<sub>n</sub> precursor</th><th>Co:FeS<sub>2</sub>(py, mar)</th><th>sum</th></tr></thead><tbody><tr><td>Py_5Co</td><td>67(8)</td><td>28(8)</td><td>95(8)</td></tr><tr><td>Py_05Co_CB1</td><td>67(10)</td><td>38(11)</td><td>105<br>(11)</td></tr><tr><td>Py_05Co_CB2</td><td>69(10)</td><td>37(10)</td><td>106<br>(10)</td></tr><tr><td>Py_TE</td><td>59(11)</td><td>41(12)</td><td>100<br>(12)</td></tr></tbody></table>

aqueous clusters FeS<sub>(aq)</sub> precursors for such pyrite formation are typically considered to derive from the dissolution of S(0) (α-sulfur) and FeS<sub>(s)</sub>, respectively.

Indeed, the production of polysulfide species (H<sub>2</sub>S<sub>n(aq)</sub>) can be initiated via the oxidation of HS⁻ by Fe<sup>3+</sup><sub>(aq)</sub> in parallel to Eq. (1), as proposed in the study of Morin et al. (2017):

$$Fe_{(aq)}^{3+} + n/[2(n-1)] HS_{(aq)} \rightarrow Fe_{(aq)}^{2+} + 1/[2(n-1)] H_2S_{n(aq)} + (n-2)/[2(n-1)] H_{(aq)}^{+} \quad (3)$$

Then, S(0) can dissolve by reacting with polysulfides (Rickard and Morse, 2005; Baya et al., 2021):

$$S(0)_{(\alpha\text{-sulfur})} + H_2S_{n(aq)} \rightarrow H_2S_{n+1(aq)} \quad (4)$$

Note that the FeS<sub>(s)</sub> solid phase in Eq (1) is actually interpreted as a continuum between FeS<sub>(cluster)</sub> and FeS<sub>(nanoMck)</sub> species in the study of Baya et al. (2021). Accordingly, FeS<sub>(aq)</sub> in Eq (2) could be attributed to aqueous Fe<sub>x</sub>S<sub>x(aq)</sub> clusters deriving from the FeS<sub>(cluster, nanoMck)</sub> solid phase across a continuous range of sizes from colloidal to aqueous species (Rickard and Morse, 2005). Indeed, Baya et al. (2021) suggest that the amorphous FeS<sub>(cluster)</sub> phase observed by EXAFS and XRD-PDF could be consistent with an aggregated form of the aqueous Fe<sub>x</sub>S<sub>x(aq)</sub> clusters proposed by Rickard and Morse (2005), which is here also in relation with the salinity of the aqueous medium. Equation (2) may then be rewritten as in Baya et al. (2021), in which Fe<sub>x</sub>S<sub>x(aq)</sub> clusters react with polysulfides to form pyrite, and considering that the FeS species may be a solid (x ≥ 150; Wolthers et al., 2003; Rickard and Morse, 2005) as well an ion pair in solution (x = 1), as suggested by Luther (1991):

$$Fe_xS_x(s, aq) + H_2S_{n(aq)} \rightarrow Fe_{x-1}S_{x-1}(s, aq) + FeS_{2(s)} + H_2S_{n-1} \quad (5)$$

In this study, EXAFS analysis at the Co K-edge indicate that Co is found incorporated in an amorphous phase exhibiting a very short-range structure, smaller than 1 nm, only 24 h after reaction with initial

aqueous Co<sup>2+</sup> (Table 4, Figs. 3 and 6). Moreover, the local structure of this amorphous phase is close to that of CoS<sub>2</sub> or Co-doped FeS<sub>2</sub> but with lacking cation neighbors (Fig. 6). Finally, HERFD-XANES pre-edge data shows a dominant octahedral environment for Co in this phase. This structural motif is also supported by the results of XRD-PDF analysis of the Co-S_100C compound, which was synthesized by including polysulfides in the starting solution (Fig. S6), while tetrahedral Co is only observed in our samples for Co species synthesized in the presence of sulfides.

As shown by our EXAFS results, the absence of cation neighbors in the amorphous Co-rich precursor forming before pyrite nucleation and persisting in the final pyrite solids, combined with its pyrite-like local structure at the very short range, could suggest that this amorphous phase may consist of the aggregation of monomeric CoS<sub>n</sub> units (Fig. 6) bound with each other via polysulfide bridges. From the polysulfide pathway perspective (Equation 2 or 5), these results could suggest that the reaction of CoS tetrahedral species with polysulfide might not be followed by S-S cleavage and polysulfide detachment, as expected for transient FeS-polysulfide units during pyrite formation (Luther, 1991). This reaction would then lead to the stabilization of such monomeric CoS-polysulfide forms:

$$CoS + H_2S_n \rightarrow [CoS(H_2S_n)] \rightleftharpoons CoS_n \quad (6)$$

Our results may also give birth to the hypothesis that Co<sup>2+</sup> has a stronger affinity for HS<sup>2-</sup> species than for H<sub>2</sub>S ones. Thus, instead of the mechanisms proposed in Equation (2), (5) and (6), Co<sup>2+</sup> would preferentially bind to polysulfides via ligand exchange with sulfides, and would form CoS<sub>n</sub> polysulfide units, as soon as HS<sup>2-</sup> species become available in the medium. Nevertheless, further aggregation of the monomeric CoS<sub>n</sub> units would form an amorphous CoS<sub>n(am)</sub> phase (Fig. S9).

Our EXAFS results suggest that Co is octahedrally coordinated to S atoms in the CoS<sub>n</sub> phase and that at least three of these S first neighbors belong to a polysulfide chain (Table 4; Fig. 6). It is noteworthy that Co here adopts octahedral coordination in the presence of polysulfide as in the pyrite samples and Co-S_100C sample. On the contrary, when there is only sulfide present, Co tends to adopt tetrahedral coordination as in the Mack-Co and Co-S_25C sample. In addition, according to the EXAFS results on the final pyrite products, a limited fraction of Co<sup>2+</sup> substitutes for Fe<sup>2+</sup> at the octahedral sites of pyrite, whereas the major fraction of Co<sup>2+</sup> remains in the form of this CoS<sub>n</sub> precursor phase (Fig. 6). Counterintuitively, the putative early formation of the CoS<sub>n</sub> precursor does not improve Co incorporation in pyrite, though cobalt is octahedrally coordinated to S atoms with a local structure close to that of pyrite.

Moreover, the CoS<sub>n(am)</sub> precursor phase appears as metastable under our experimental conditions and delays and/or limits Co incorporation in pyrite during the synthesis. Indeed, our data show that the proportion of Co incorporated into pyrite tends to decrease with increasing initial Co concentration (Table 5; Fig. 5). This is especially supported by comparing the 1 ppm sample and the 5 mol% Co-doped pyrite samples (Py_TE and Py_5Co, respectively). Since the whole solid fraction of Co is present in the form of the CoS<sub>n(am)</sub> phase after 24 h of synthesis, this phase necessarily served as the precursor for incorporation of Co in the FeS<sub>2</sub> final compounds. Hence, less than half (≤ 40%) of the CoS<sub>n</sub> phase likely served as precursor for Co incorporation in pyrite, and possibly to a lesser extent in marcasite (Tables 5 and S4; Fig. 5), either via direct incorporation of octahedrally coordinated monomeric CoS<sub>n</sub> units or via a dissolution pathway.

Altogether, these results indicate that the presence of polysulfides in the medium leads to the formation of CoS<sub>n</sub>-polysulfide phase, in which Co early adopts the octahedral coordination expected in pyrite, but this CoS<sub>n</sub>-polysulfide precursor does not spontaneously evolve towards pyrite nuclei, contrarily to what was observed for nickel (Morin et al., 2017). This finding suggests that Co might be viewed as a potential probe of the polysulfide pathway of pyrite formation, which here
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Atomic models proposed for the local structure around Co in Co-doped pyrite, Co-doped pyrite precursor and Co-doped mackinawite, based on EXAFS shell-by-shell analysis (Table 4). Corresponding Fast Fourier Transforms of experimental and calculated unfiltered $k^3\chi(k)$ functions are displayed for representative samples. Experimental and calculated curves are plotted as black and red lines, respectively. Co, Fe, and S atoms are blue, brown and yellow respectively.</figcaption></figure>

appears to be partly blocked at a stage in which CoS$_n$ monomeric units aggregate into an amorphous CoS$_{n(am)}$ phase. One may infer that the persistence of this CoS$_n$ polysulfide precursor could be related to a delay in the detachment of the polysulfide tail, whereas this mechanism is typically required for pyrite nucleation especially in the model of Luther (1991) (Equations (2) and (5)). Moreover, our results could reveal further details in the pyrite formation pathway, which would rely on the polymerization of monomeric polysulfide units, still accompanied by polysulfide tail detachment. According to such a scheme, the inability of CoS$_n$ monomeric units to easily polymerize into a pyrite framework and/or the low solubility of the CoS$_{n(am)}$ phase could explain its persistence after FeS$_2$ nucleation and growth in our experiments. Nevertheless, a fraction of the CoS$_n$ monomeric units could likely have co-polymerized with Fe polysulfide species and was then incorporated in pyrite, explaining that a 20–40 % proportion of the CoS$_{n(am)}$ phase was converted into pyrite in our experiments (Table 5, Fig. 5).

## 4.2. Role of trace elements in the kinetics of pyrite formation

The results obtained here on the evolution of Co speciation during pyrite synthesis at ambient temperature open new perspectives to discuss previous findings regarding the role of trace elements in the kinetics of pyrite formation. Indeed, it has been reported that adding Ni

(Morin et al., 2017, Swanner et al., 2019, Baya et al., 2021, 2022), Co (Swanner et al., 2019, Baya et al., 2022) or As (Le Pape et al., 2017) in low temperature pyrite synthesis protocols can significantly change the pyrite formation rate. According to Baya et al. (2022), the presence of Co has a moderate impact on pyrite formation kinetics, whereas Swanner et al. (2019) report a delay in such a reaction in the presence of this trace element. A similar delay in pyrite formation is reported for Ni by Swanner et al. (2019), but Morin et al. (2017) and Baya et al. (2021, 2022) show that Ni significantly accelerates pyrite formation, this discrepancy is discussed below. In the pyrite syntheses reported by Morin et al. (2017), Ni early adopts the octahedral coordination with disulfide in Ni-rich pyrite nuclei during the lag phase, before pyrite can be detected by XRD. Despite similar pyrite growth kinetic in both Ni and Ni-free syntheses, this early nucleation that is promoted by the presence of Ni lead to accelerated pyrite formation compared to Ni-free syntheses.

Here, we show that Co also early adopts the octahedral coordination in the presence of polysulfides during the lag phase, but in the form of CoS$_n$ polysulfide units that further aggregate in an amorphous phase. By contrast with Ni, this Co-polysulfide precursor appears metastable and persistent, which rather delays or limits Co incorporation in pyrite. The influence of such behavior on pyrite nucleation rate is unclear since it could depend on the ability of CoS$_n$ monomeric units to interact with FeS precursors but also on the initial Co:Fe ratio. Indeed, the binding of
octahedral CoS$_{n}$ monomeric units with FeS precursors could for instance
help the formation of octahedrally coordinated Fe of similar form and
promote pyrite nucleation, whereas the aggregation of CoS$_{n}$ monomeric
units into an amorphous phase may tend to delay pyrite formation and
limit Co incorporation in pyrite. Regarding such complex possible ef-
fects, Baya et al. (2022) reports a moderate acceleration of pyrite for-
mation in the presence of Co. By contrast, Swanner et al. (2019) report
that Co and Ni decelerate pyrite formation and may thus act as inhibitors
of the transformation of FeS to pyrite. However, these authors synthe-
sized Co-doped and Ni-doped pyrite via the polysulfide pathway by
reacting H$_{2}$S or S$^{0}$ with a freshly and independently precipitated Co-
doped and Ni-doped mackinawite, i.e. (Fe,Co)S$_{m}$ and (Fe,Ni)S$_{m}$ respec-
tively. This method differs from that used by Morin et al. (2017), Baya
et al. (2021, 2022) and in this study, where Ni$^{2+}$ and Co$^{2+}$ were added in
the dissolved form in an acidic iron stock solution. To explain the limited
pyrite formation rate observed by Swanner et al. (2019) compared to the
control, one may thus infer that Ni incorporation in initial mackinawite
could have modified its reactivity compared to a Ni-free mackinawite
precursor. Such a Ni effect on mackinawite reactivity could result from
the induced increase in crystallite size reported by Ikogou et al. (2017).
Assuming a similar behavior for Co, one may infer that the aging of Co-
doped mackinawite during its synthesis at 80 °C and during the lag
phase of pyrite formation at 65 °C, before the nucleation step, might
have promoted crystallite growth of mackinawite and subsequently
limits its transformation rate to pyrite as observed by Swanner et al.
(2019). Such an enhanced mackinawite crystal ripening process is not
expected to have occurred in the synthesis protocols used by Morin et al.
(2017) and Baya et al. (2021, 2022) because Co and Ni were added in the
soluble form to the Fe solution before mixing and further precipitation
with Na$_{2}$S. Under such conditions, Ni mainly precipitated as amorphous
NiS, whereas only a small fraction incorporated in mackinawite. The
amorphous NiS precursor appeared then to readily transform into Ni-
rich pyrite nuclei, which accelerated pyrite nucleation. Such a differ-
ence in the role of Ni on the kinetics of pyrite formation as a function of
its interaction with the pyrite precursors, illustrates the complexity of
the processes that may arise in natural systems in which both dissolved
and FeS-bearing trace elements may coexist prior to pyrite formation.

In the case of Co in our experiments, one may infer that, before the
CoS<sub>n</sub> polysulfides units formed after 24 h, an amorphous CoS<sub>(am)</sub> phase
with Co<sup>2+</sup> in tetrahedral coordination, initially formed via coprecipita-
tion of Co<sup>2+</sup> with HS<sup>-</sup>. This hypothesis is strongly supported by the re-
sults obtained for the Co-S_25C sample in our study and with those
reported by Mansor et al. (2020). Then, the initially precipitated CoS<sub>(am)</sub>
likely converted into an amorphous CoS<sub>n(am)</sub> polysulfide precursor
phase in which Co<sup>2+</sup> is octahedrally coordinated. This conversion is
thought to occur as soon as polysulfides are produced in the medium as a
result of the progressive reduction of Fe(III) by H<sub>2</sub>S. The persistence of
the amorphous CoS<sub>n(am)</sub> polysulfide precursor phase is then found in this
study to delay and/or limit Co incorporation in pyrite, without major
influence on the kinetics of pyrite nucleation when incorporated as trace
level in the synthesis (Baya et al. 2022).

4.3. Implications for Co incorporation into natural sedimentary pyrites

Our results show that whatever the initial Co:Fe ratio, only a minor
fraction of Co is actually substituting for Fe in the pyrite structure, while
the major fraction precipitate as a CoS<sub>n(am)</sub> polysulfides phase. This
observation deserves to be discussed in the light of the strong to mod-
erate affinity generally ascribed to Co towards pyrite and Fe-sulfides
(Huerta-Diaz and Morse. 1992; Morse and Luther, 1999; Tribovillard
et al., 2006; Large et al., 2014; Gregory et al., 2015; Chen and Campbell,
2021; Qiu et al., 2021). In this regard, our results may suggest that once
polysulfides are available in the medium, the early formation of the
Co-polysulfide precursor phase could limit Co mobility and favor asso-
ciation Co with sulfide solid phase. Accordingly, in the study of Huer-
ta-Diaz and Morse (1992) the degree of trace metal pyritization (DTMP)

of Co in a large set of natural sediments tends to increase with the degree
of pyritization (DOP). However, it rarely exceeds 40 % and remains
below 70 % of total reactive Co. Such a behavior differs from that of As
or Cu for which the DTMP can reach 100 %. It suggests that the fraction
of reactive Co that is not associated with pyrite, i.e., extracted by strong
acid, is still significant. This non-pyrite fraction of Co is generally
ascribed to Co associated with iron and manganese (oxyhydr)oxides or
adsorbed to clay minerals, from which would originate the moderate
affinity of Co for pyrite. It is unclear from our results, whether the
Co-polysulfide precursor phase can be ascribed to the pyritic or
non-pyritic pool of reactive Co, in the sense of an acid extraction.
Indeed, the moderate 0.2 M HCl acid wash applied to the Py_5Co sample
is found to have preserved the Co-polysulfide precursor but is not ex-
pected to have extracted the whole AVS phases, which are typically
removed with 0.5–12 M HCl (Tisserand et al., 2021). Nevertheless, the
persistence of the Co-polysulfide precursor in our pyrite synthesis ex-
periments at ambient temperature further suggests that it could possibly
also persist, intimately associated with pyrite crystallites in sedimentary
deposits (Bartholomé and Katekesfia, 1971). Hence, further studies of
natural sedimentary pyrites could target the possible existence of this
Co-polysulfide precursor phase in addition to Co-substituted pyrite.
Indeed, it is unknown whether this Co-polysulfide precursor phase could
be further destabilized during diagenesis, or if its finally converts to
pyrite, or if it persists but could be dissolved upon classical chemical
removal of acid volatile sulfides when analyzing sedimentary pyrites.
One could infer that the Co-polysulfide precursor destabilization during
long-term diagenesis could be favored by the high affinity of Co for
oxyhydroxides and clays, which would contribute to explaining the
decrease in Co-DMTP in the sedimentary record. On the contrary, the
persistence of the precursor or its diagenetic conversion to pyrite would
rather imply that the partition between the oxyhydroxide/clay and py-
rite fractions of Co is acquired at an early diagenetic stage and is rather
controlled by the relatively high solubility of Co monosulfide (Morse and
Luther, 1999).

The Co contents in pyrite are proposed as a reverse proxy for at-
mospheric oxygenation since Co availability in the ocean is considered
to be closely related to adsorption and coprecipitation into Mn and Fe
oxyhydroxides (Large et al., 2017). Cobalt is considered more soluble
under reduced or limited O₂ condition, whereas it is thought to be
immobilized by adsorption onto Fe(III) oxyhydroxides and Mn(III)/(IV)
oxides with increasing atmospheric oxygen level (Large et al., 2017;
Stockdale et al., 2010). Beyond these assumptions, the use of Co in pyrite
as a reverse proxy for atmospheric oxygenation is based on the under-
lying hypothesis that its concentration in pyrite would scale linearly to
that in anoxic oceans. The low concentration of aqueous Co at the end of
the Co-doped pyrite experiments by Baya et al. (2022), indicates an
efficient trapping of Co in the sulfide solid phase, even if a major fraction
persists in the form of the Co-polysulfide precursor in addition to Co-
substituted pyrite, as shown in the present study. In addition, our re-
sults further suggest that the Co partition between the Co-polysulfide
precursor and pyrite is not much influenced by the initial Co:Fe ratio
at trace levels (Fig. 5), which could ensure that the Co:Fe in the final
polysulfide solid might still increase with the concentration of available
Co in the pyrite formation medium. Altogether, these considerations
suggest that the complex Co speciation we observed here in low-
temperature pyrite may not greatly affect the use of the Co content in
sedimentary pyrites as a reverse oxygenation proxy.

5. Conclusion

Using Co K-edge high resolution-XANES and EXAFS data, this study
provides unprecedented information on the state of Co in crystalline Fe-
sulfides produced at ambient temperature and some of their precursors.
The results reveal a precursor of Co-doped pyrite that differs from the
FeS models generally proposed via the polysulfide pathway (Baya et al.
2021 and references therein). Indeed, the observed precursor appears to
consist of a CoSₙ polysulfide phase in which Co is octahedrally coordinated to S first neighboring atoms. Such a presence of octahedrally coordinated divalent cation in “FeS-like” pyrite precursors on the polysulfide pathway to pyrite has already been mentioned, especially nano-greigite (Benning et al., 2000) or FeS_(clusters) (Matamoros-Veloza et al., 2018; Baya et al., 2021) species. This study also indicates that nearly the entire Co pool occurs in this octahedral coordination early in the reaction sequence leading to pyrite, likely as a result of the rapid reaction of Co with (i) sulfides and (ii) polysulfides within the first 24 h. The fact that a large fraction of this Co-polysulfide precursor persists after pyrite formation indicates that it delays and/or limits Co incorporation in pyrite. The fate of the Co-polysulfide precursor upon diagenesis thus deserves further investigations in order to evaluate its implication in the interpretation of the Co content in sedimentary pyrite. Our study also shows that the partition of Co between pyrite and the Co-polysulfide precursor appears to depend weakly on the initial Co:Fe ratio and mostly at high loading. Furthermore, the persistence of a large fraction of Co in this Co-polysulfide precursor phase in the final pyrite solids, as evidenced in the present study, does not particularly affect their ability to efficiently sequester Co, as shown by water chemistry data reported by Baya et al. (2022). Altogether, these considerations thus suggest that this complex speciation of Co in low temperature pyrite is not expected to significantly affect the use of the Co content in sedimentary pyrite as a proxy for reconstructing paleo-environmental conditions. Beyond shedding a new light on the crystal-chemistry of Co in sulfide-rich environments, the identification of a CoSₙ polysulfide precursor also has important implications for our understanding of pyrite nucleation mechanisms via the polysulfide pathway. Indeed, one may infer that such a precursor observed here in the case of Co might have been overlooked in the case of Fe because the average local structure of FeS precursors in reported laboratory syntheses is largely dominated by tetrahedral Fe in the major FeS nano-mackinawite component. Hence, the unveiling of this precursor phase with a particular structure could offer new perspectives to further elucidate the molecular-scale pyrite nucleation mechanisms. The hypothetical existence of such an octahedral monomeric unit for Fe would then point toward a pyrite transformation pathway via the polymerization of cation-polysulfides units, which could help unify the vision of pyrite formation routes.

## Data availability

Synchrotron HERFD-XAS and laboratory XRD data are available through Zenodo at https://doi.org/10.5281/zenodo.15498570.

## CRediT authorship contribution statement

**Qianyu Deng:** Writing – review & editing, Writing – original draft, Visualization, Validation, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Pierre Le Pape:** Writing – review & editing, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. **Julie Aufort:** Writing – review & editing, Visualization, Validation, Supervision, Methodology, Investigation, Formal analysis, Conceptualization. **Marc Blanchard:** Writing – review & editing, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization. **Benoit Baptiste:** Writing – review & editing, Validation, Methodology, Formal analysis, Data curation. **Ludovic Delbes:** Methodology, Data curation. **Camille Baya:** Methodology, Data curation, Conceptualization. **Farid Juillot:** Writing – review & editing, Methodology, Investigation, Data curation, Conceptualization. **George Ona-Nguema:** Supervision, Methodology, Investigation, Conceptualization. **Nicolas Menguy:** Visualization, Validation, Supervision, Methodology, Investigation. **Jean-Michel Guigner:** Visualization, Supervision, Methodology, Investigation. **Olivier Proux:** Writing – review & editing, Validation, Supervision,

Software, Resources, Methodology, Data curation. **Camille Duquenoy:** Writing – review & editing, Methodology, Data curation. **Romain Guilbaud:** Writing – review & editing, Validation, Methodology, Data curation. **Franck Poitrasson:** Writing – review & editing, Validation, Methodology, Data curation. **Guillaume Morin:** Writing – review & editing, Validation, Supervision, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

The authors thank Carole Causserand and Karen Alloncle for AAS analytical support and Olivier Mathon for his help in measuring the Fe K-edge EXAFS spectrum of crystalline pyrite at the BM23 beamline. The FAME-UHD project is financially supported by the French "grand emprunt" EquipEx (MAGNIFIX, ANR-21-ESRE-0011), the French "Programmes et équipements prioritaires de recherche" (DIADEM), the CEA-CNRS CRG consortium and the INSU CNRS institute. We acknowledge the European Synchrotron Radiation Facility (ESRF) for the provision of beamtime through proposals EV-520 and EV-587. The authors acknowledge the ANR - FRANCE (French National Research Agency) for its financial support of the PYRISOFE project (ANR-22-CE49-0012). The atomistic modelling was performed using HPC resources from CALMIP (grant 2024 – P1037) and from GENCI-IDRIS (grant 2024-AD010814170R1).

## Appendix A. Supplementary material

Supplementary Material contains nine figures (S1 to S9) and four tables (S1 to S4), including especially complementary information on the chemical composition of the samples, TEM and STEM-EDX data, XRD and XRD-PDF data and analysis, DFT calculations and LC-LS fitting of EXAFS data. Supplementary material to this article can be found online at https://doi.org/10.1016/j.gca.2025.06.003.

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