# State of Ag in Pyrrhotite: Insights from X-ray Absorption Spectroscopy

Olga N. Filimonova, Didier Wermeille, Kristina O. Kvashnina

► To cite this version:

Olga N. Filimonova, Didier Wermeille, Kristina O. Kvashnina. State of Ag in Pyrrhotite: Insights from X-ray Absorption Spectroscopy. ACS Earth and Space Chemistry, 2023, 7 (9), pp.1648–1660. 10.1021/acsearthspacechem.3c00002 . hal-05080004

HAL Id: hal-05080004

https://hal.science/hal-05080004v1

Submitted on 22 May 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

# State of Ag in Pyrrhotite: Insights from X-ray Absorption Spectroscopy

Olga N. Filimonova,* Didier Wermeille, and Kristina O. Kvashnina

<figure><img src="image_3.png" /></figure>

Cite This: ACS Earth Space Chem. 2023, 7, 1648–1660

<figure><img src="image_4.png" /></figure>

Read Online

## ACCESS

<figure><img src="image_5.png" /></figure>

Metrics & More

<figure><img src="image_6.png" /></figure>

Article Recommendations

<figure><img src="image_7.png" /></figure>

Supporting Information

**ABSTRACT:** Ag-bearing pyrrhotites $Fe_{1-x}S$ were synthesized by using the salt flux technique. The concentration of Ag in pyrrhotite reached 0.08 wt % at 540 °C and 1.3 wt % at 750–760 °C. The synthesized samples are divided into two groups according to the values of sulfur fugacity in the experimental system and the content of Fe ($C_{Fe}$) in pyrrhotite. In group (i) at low sulfur fugacity (log $f(S_2)$ ≈ −6.5, $C_{Fe}$ > 48.8 at. %), Ag is disseminated in pyrrhotites in an “invisible” form but mostly concentrates on the grain boundaries of crystals as metallic rims. In group (ii) at high sulfur fugacity (log $f(S_2)$ ≈ −1.0, $C_{Fe}$ < 48.8 at. %), Ag occurs as an invisible form disseminated in the pyrrhotite matrix together with the minor form of Ag-bearing submicron inclusions. Analysis of Ag K-edge XAS spectra recorded at ambient temperature revealed that the samples of group (i) contain a small fraction of the invisible Ag⁺ form, while the majority of Ag presents in pyrrhotite crystals as Ag°. The samples of group (ii) at a temperature of 750 °C contain the invisible form of Ag–$Ag_2S^+$-like clusters. The decrease in temperature leads to the partial decomposition of the invisible form and the appearance of the second (minor) form, Ag-bearing submicron inclusions. The predicted concentrations of Ag in pyrrhotite coincide with those from natural samples of various origins.

**KEYWORDS:** synthetic minerals, silver, pyrrhotite, concentration, X-ray absorption spectroscopy, “invisible” silver, submicron inclusions, miscibility

<figure><img src="image_8.png" /></figure>

## 1. INTRODUCTION

Pyrrhotite $Fe_{1-x}S$ ($p_o$) is a typical iron sulfide occurring in deposits of various origins, including sediment-hosted orogenic gold, hydrothermal, metamorphic, and magmatic deposits.¹⁻⁹ In these ores, Ag can exist in compounds with Cu, Au, S, Te, Se, As, etc. However, due to the high affinity of PGEs (platinum group elements), Au, and Ag to iron sulfides, pyrrhotite can carry a significant amount of these trace elements in the so-called “invisible” form. The invisible form can be represented by nanosized and submicron inclusions/clusters, or solid solution, which cannot be detected by scanning electron microscopy.¹⁰ Natural pyrrhotites can contain from a few ppm (e.g., 1–3 ppm⁷) up to a few tens of ppm of Ag (e.g., 54 ppm⁶). However, a sub-ppm level of concentration is insufficient to study the capacity of pyrrhotite to incorporate Ag and the state of Ag in the pyrrhotite matrix by spectroscopic methods. To determine the content and the state of Ag in pyrrhotite, multiple experimental studies were carried out. Taylor¹¹,¹² determined phase relations in the Ag–Fe–S system at 150–1200 °C by quench experiments with the use of evacuated, sealed silica tubes as reaction vessels. Interpreting the results of the experiment performed at 600 °C, Taylor¹¹ noticed that traces of crystallized liquid were observed in polished sections, suggesting that the amount of

Ag in pyrrhotite solid solution was considerably less than the measured 0.1 at. %. Taylor¹² proposed that the observed $Ag_2S$ was produced by exsolution upon cooling and that the Ag–pyrrhotite solid solution cannot be quenched. Simon et al.¹³ performed experiments to elucidate the effects of magnetite, ulvöspinel–magnetite solid solution, and pyrrhotite crystallization on the budgets of Au, Cu, and Ag at magmatic conditions. The experiment at 800 °C/vapor pressure demonstrated that the contents of Ag in pyrrhotite and magnetite reach 0.3 and 0.02 wt %, respectively. Since the system was not saturated with respect to Ag, no additional Ag-bearing phases were identified in association with pyrrhotite and magnetite. Au and Ag contents in pyrite $FeS_2$, pyrrhotite $Fe_{1-x}S$, and troilite FeS were determined experimentally by Pal'yanova et al.¹⁴ The detected concentration of invisible Ag in pyrrhotite and pyrite synthesized at 500 °C was <0.03 wt % and that in troilite was 0.08 wt %. Microcrystals of Au–Ag

Received: January 6, 2023

Revised: August 7, 2023

Accepted: August 8, 2023

Published: August 21, 2023

<figure><img src="image_9.png" /></figure>
<table><caption>Table 1. Parameters of Synthesis, Starting Materials, Compositions, and Empirical Formulae of Synthetic Pyrrhotite</caption><thead><tr><th>Synthesis temperature, °C (at the hot/cold end of the ampoule)</th><th>Sample no.</th><th>Starting materials<sup>c</sup></th><th>Synthesis products<sup>d</sup></th><th>C<sub>Ag</sub> min-max, average ± 2SD, wt %<sup>e</sup></th></tr></thead><tbody><tr><td rowspan="2">600/540<sup>a</sup></td><td>1432</td><td>Fe<sub>0.87</sub>S + Ag<sub>2</sub>S (here and below Ag<sub>2</sub>S = acanthite)<sup>c</sup></td><td>po + py + Ag + Cs-Ag-rich phase</td><td>1e-04<sup>e</sup>-0.07, 0.02 ± 0.03</td></tr><tr><td>1433</td><td>Fe<sub>0.87</sub>S + Ag<sub>cr</sub></td><td>po + py + Ag<sub>2</sub>S</td><td>1E-04<sup>e</sup>-0.08, 0.01 ± 0.03</td></tr><tr><td rowspan="7">820/750<sup>b</sup></td><td>4010</td><td>Fe<sub>0.86</sub>S + Ag<sub>2</sub>S</td><td>po + Ag-rich phase</td><td>0.23-0.27, 0.25 ± 0.01</td></tr><tr><td>4011</td><td>Fe<sub>0.86</sub>S + Ag<sub>2</sub>S</td><td>po + Ag-rich phase</td><td>0.23-0.28, 0.25 ± 0.01</td></tr><tr><td>4012</td><td>Fe<sub>0.9</sub>S + Ag<sub>2</sub>S</td><td>po + Ag-rich phase</td><td>0.23-0.34, 0.28 ± 0.03</td></tr><tr><td>4013</td><td>Fe<sub>0.9</sub>S + Ag<sub>2</sub>S</td><td>po + Ag-rich phase</td><td>0.20-0.27, 0.24 ± 0.03</td></tr><tr><td>4014</td><td>Fe<sub>0.95</sub>S + Ag<sub>2</sub>S + Ag<sub>cr</sub> plate (2/3 of ampoule)</td><td>po + Ag-rich phase + Ag<sub>cr</sub></td><td>0.23-0.37, 0.28 ± 0.04</td></tr><tr><td>4015</td><td>Fe<sub>0.95</sub>S + Ag<sub>2</sub>S + Ag<sub>cr</sub> plate (2/3 of ampoule)</td><td>po + Ag<sub>cr</sub></td><td>0.14-0.37, 0.27 ± 0.07</td></tr><tr><td>4016</td><td>Fe<sub>1.1</sub>S + Ag<sub>cr</sub> plate (2/3 of ampoule)</td><td>po + Ag<sub>cr</sub></td><td>0.05-0.09, 0.07 ± 0.02</td></tr><tr><td rowspan="7">810/760<sup>b</sup></td><td>4017</td><td>Fe<sub>1.1</sub>S + Ag<sub>cr</sub> plate (2/3 of ampoule)</td><td>po + Ag<sub>cr</sub></td><td>0.04-0.05, 0.04 ± 0.01</td></tr><tr><td>3828</td><td>Fe<sub>0.86</sub>S + Ag<sub>2</sub>S</td><td>po + Ag-rich phase</td><td>1e-04<sup>e</sup>-0.48, 0.16 ± 0.21</td></tr><tr><td>3829</td><td>Fe<sub>0.86</sub>S + Ag<sub>2</sub>S (hot end) + Ag<sub>2</sub>S (cold end)</td><td>po + Ag-rich phase</td><td>1e-04<sup>e</sup>-1.29, 0.60 ± 0.54</td></tr><tr><td>3830</td><td>Fe<sub>0.95</sub>S + Ag<sub>2</sub>S</td><td>po + Ag-rich phase</td><td>0.14-0.36, 0.25 ± 0.11</td></tr><tr><td>3831</td><td>Fe<sub>0.95</sub>S + Ag<sub>cr</sub> plate (2/3 of ampoule)</td><td>po + Ag<sub>cr</sub></td><td>1e-04<sup>e</sup>-0.33, 0.20 ± 0.15</td></tr><tr><td>3832</td><td>Fe<sub>1.1</sub>S + Ag<sub>cr</sub> plate (2/3 of ampoule)</td><td>po + Ag<sub>cr</sub></td><td>0.03-0.06, 0.04 ± 0.01</td></tr><tr><td>3833</td><td>Fe<sub>1.1</sub>S + Ag<sub>2</sub>S + Ag<sub>cr</sub> plate (2/3 of ampoule) + Ag<sub>cr</sub> (cold end)</td><td>po + Ag<sub>cr</sub></td><td>0.07-0.12, 0.10 ± 0.01</td></tr><tr><td>3834</td><td>Fe<sub>0.9</sub>S + Ag<sub>2</sub>S</td><td>po + Ag-rich phase</td><td>0.20-0.47, 0.31 ± 0.12</td></tr><tr><td>3835</td><td>Fe<sub>0.9</sub>S + Ag<sub>2</sub>S + AgCl</td><td>po + Ag-rich phase</td><td>0.17-0.67, 0.27 ± 0.13</td></tr></tbody></table>

<sup>a</sup>Eutectic salt mixture composition, CsCl/NaCl/KCl; duration of synthesis, 4–5 weeks. <sup>b</sup>Eutectic salt mixture composition, NaCl/KCl; duration of synthesis, 4–5 weeks. <sup>c</sup>Starting materials are loaded at the hot end of the ampoule if not indicated otherwise. <sup>d</sup>According to SEM analysis. <sup>e</sup>Average concentration determined using EPMA method (diameter of the analysis, 20 µm), standard deviation (SD) of the analysis is given in at. % (±2 s), except concentration determined using LA-ICP-MS method (marked with an asterisk), SD in at. % (±2 s) determined via LA-ICP-MS spectra fitting in Iolite program.<sup>24</sup>

alloys of 1–5 µm size were observed on the surfaces of iron sulfides. The X-ray photoelectron spectroscopy (XPS) study of the surfaces of sulfides characterizing a few nanometers thick layer showed preferential occurrence of Ag–Au sulfide phases on the surfaces of ground specimens. Lipko et al.<sup>15</sup> performed an experiment at 400 °C/1 kbar using hydrothermal thermogradiation synthesis. Pyrrhotites synthesized in that work<sup>15</sup> contain 36 ppm of Ag disseminated in the crystals and 940 ppm of Ag on the surfaces of the crystals. Even though invisible silver in pyrrhotites has been detected in a number of studies, understanding of the mode of Ag incorporation is still ambiguous, since no in situ high-temperature measurements were performed.

In this paper, we characterize the capacity of pyrrhotite to incorporate Ag and the state (charge and local atomic environment) of Ag in pyrrhotite. For these purposes, we synthesized Ag-bearing pyrrhotite crystals via the salt flux technique at temperatures of 540 and 750–760 °C. We investigated Ag-doped crystals for several reasons. First, synthetic pyrrhotites contain uniquely high concentrations of Ag of 0.08–1.3 wt %; thus, state-of-the-art high-quality XANES and EXAFS spectra were recorded. Second, synthetic pyrrhotites are free of multiple admixtures, whose presence usually complicates analysis of natural samples by spectroscopic methods. Third, synthetic samples are analogues of pyrrhotites formed in the temperature range typical of hydrothermal, late magmatic, and magmatic deposits. The obtained crystals were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS)

methods. X-ray absorption spectroscopy (XAS), consisting of an X-ray absorption near edge structure and extended X-ray absorption fine structure (XANES and EXAFS, respectively), was applied to study the state of Ag in the iron sulfide matrix. The measurements were carried out at ambient temperature and in situ at elevated temperatures of 150–750 °C using the silica glass capillary technique. The high-temperature XAS experiment provided information about the form of Ag occurrence at the synthesis temperature. The results of the XAS experiment, combined with the data on the concentration of Ag in pyrrhotite, can be used for physicochemical modeling of Ag migration and precipitation in natural systems, building a model capable of describing the concentration of Ag in pyrrhotite, development of ore refinement, leaching, and extraction technologies.

## 2. MATERIALS AND METHODS

### 2.1. Synthesis in the Ag–Fe–S System.
Synthesis in the Ag–Fe–S system was performed using a salt flux technique at a steady-state temperature gradient;<sup>16</sup> the scheme of the experiment is given in Figure 1c in Chareev.<sup>17</sup> The parameters of synthesis together with the charge and salt flux compositions are given in Table 1. We used presynthesized pyrrhotite Fe<sub>1-x</sub>S (0.14 < x < 0) powder, metallic Fe, and crystalline S as initial materials. The charges were loaded into silica glass ampules (8 mm i.d., 11 mm o.d., ~110 mm length) together with Ag<sub>2</sub>S<sub>cr</sub> powder, Ag wire, or Ag plate of 0.5 mm diameter (~2 × 5 × 0.2 mm), the length of which never exceeded half of the ampoule. The maximum possible ampoule volume was filled with the eutectic salt mixtures of various compositions.<sup>17</sup> The ampoules were evacuated, sealed with an oxygen torch gas
<figure><img src="image_10.png" /><figcaption>**Figure 1.** Scanning electron microscopy backscattered electron (BSE) images of the experiment run products of Ag-bearing pyrrhotite. Synthesis at 760 °C: (A) pyrrhotite no. 3832 with no visible Ag inclusions, (B) pyrrhotite no. 3833 containing Ag<sub>cr</sub> next to the grain boundaries, (C, D) pyrrhotite nos. 3834 and 3835, respectively, containing Ag-bearing submicron inclusions of <1 µm size (E) in the pyrrhotite matrix (F).</figcaption></figure>

burner, placed into horizontal tube furnaces, and heated to the synthesis temperatures. The experimental temperatures were 600 °C hot end/540 °C cold end (sample nos. 1432 and 1433), 820/750 °C (sample nos. 4010–4017), and 810/760 °C (sample nos. 3828–3835). The compositions of the eutectic salt mixtures were CsCl/NaCl/KCl+I<sub>cr</sub> (synthesis at 540 °C) and KCl/NaCl (synthesis at 750 and 760 °C). At the synthesis temperature, the initial reagents recrystallized at the cold end of the ampoules. We varied the duration of the experiments depending on the temperature between 4 weeks at 540 °C and 5 weeks at 750–760 °C. After the experiments, the ampoules were extracted from the furnaces and quenched in cold water within 5–10 s. The synthesis run products were extracted from the cold ends of silica glass ampoules and then rinsed 2–3 times in distilled water and acetone using an ultrasonic bath. We dried the run products at 70 °C in a muffle furnace for a few minutes in order to avoid oxidizing in air.

**2.2. Analytical Methods.** Several grains of each sample were mounted in epoxy resin, polished, and then examined using reflected light optical microscopy (Olympus BX-51), SEM/EDS, EPMA, and LA-ICP-MS analyses. SEM/EDS analysis was performed using a JSM-5610LV microscope coupled with an X-Max 80 energy dispersive spectrometer (EDS). Run products were analyzed for S, Fe, and Ag using an electron probe microanalyzer JEOL JXA-8200 WD/ED equipped with five wavelength dispersive X-ray spectrometers. Ag concentration was determined by means of LA-ICP-MS method using Thermo XSERIES 2 quadrupole ICP-MS coupled with New Wave 213 laser (¹⁰⁷Ag and ¹⁰⁹Ag isotopes). External standards used were the in-house pyrrhotite Fe<sub>0.9</sub>S and sulfide reference material MASS-1.¹⁸ The in-house reference material containing ~20 ppm Ag was obtained

using the Wohlgemuth-Ueberwasser et al.<sup>19</sup> experimental technique, and Ag content was measured using reference material prepared by J. H. G. Laflamme and A. Peregoedova at the University of Québec at Chicoutimi as an external standard. Internal standards used were <sup>33</sup>S and <sup>57</sup>Fe isotopes. The EPMA and LA-ICP-MS operating conditions are given in Tables S1 and S2, respectively.

**2.3. X-ray Absorption Spectroscopy.** Only pyrrhotite crystals free of the Ag-rich phase were chosen using a binocular microscope. X-ray absorption spectroscopy (XAS) experiments were performed at the Rossendorf Beamline BM20<sup>20</sup> and XMaS BM28 at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France.

We used the TFY-XAS mode (total fluorescence yield mode) to record Ag K-edge spectra. Rejection of higher harmonics was performed by three Rh-coated mirrors working under total reflection, and the photon energy was scanned by means of the Si(111) monochromator. The 13-element high-purity throughput germanium (BM20) and SDD (BM28) detectors were used. The energy was calibrated using an Ag foil (maximum of the first derivative, 25514 eV). The operating conditions of the storage ring were 6.0 GeV and 80–100 mA. Energy scans from 25300 to 26250 eV were recorded and then averaged for each sample (the number of scans varied from 3 to 7). For the ambient temperature measurements at BM20, several crystals of each pyrrhotite crystal or reference sample (except Ag foil) were ground and then the powders were spread on adhesive Kapton tape.

For the heating experiment at BM28, the capillary technique was used.²¹ A small amount of powdered Ag-bearing pyrrhotite crystals was placed into silica glass capillaries (Hilgenberg GmbH, 0.7 mm o.d., 0.68 mm i.d., ~1 cm length), sealed
<figure><img src="image_11.png" /><figcaption>Figure 2. Selected time-resolved laser ablation ICP-MS spectra of Ag-bearing synthetic pyrrhotites studied by XAS (spot size Ø 60 µm). Ag is homogeneously disseminated in the pyrrhotite matrix. Spectra show no evidence of the phases Agcr or Ag₂Scr in the selected crystals. Uncertainties correspond to ±2 s.</figcaption></figure>

under vacuum, and heated up to the experimental temperature of 750 °C, and the XAS spectra were recorded in situ. We collected XANES and EXAFS spectra at high temperatures for each sample within 2–3 h, with the exception of the spectrum of sample no. 3834 at 600 °C, which was collected within 6 h. After the HT temperature experiment, one of the capillaries was slowly annealed within 1 h, and the spectrum was recorded at RT.

## 2.4. Analyses of X-ray Absorption Spectra.

### 2.4.1. XANES Spectra Analysis.
Preliminary data processing included energy calibration, determination of the position of the absorption edge (edge jump, ej) and the first intense feature (white line, WL), background subtraction, and normalization of the spectra and was performed using the Athena program of the IFEFFIT software package.²² The position of the absorption edge is sensitive to the local atomic environment and the valence state of the absorbing atom. Then, the experimental spectra of Ag-bearing pyrrhotites were compared to those of reference substances with a determined local atomic environment and formal oxidation states of Ag: 0 (Agcr) and +1 (Ag₂Scr, (Ag,Au)₂Scr).

### 2.4.2. EXAFS Spectra Analysis.
The EXAFS spectra fitting analysis reveals the parameters of the local atomic environment of the absorbing atom, including numbers of neighboring atoms (Nij), interatomic distances (Rij), and Debye-Waller factors (σij²). These parameters were fitted in order to obtain the best description of the recorded spectra using the EXAFS equation:

$$ \chi_i(k) = S_{0i}^2 \sum_{j=1}^{n} \frac{N_{ij}F_{ij}(k)}{R_{ij}^2 k} e^{-2R_{ij}/\lambda(k)} e^{-2\sigma_{ij}^2 k^2} \sin(2kR_{ij} + \varphi_{ij}(k)) \quad (1) $$

The other parameters (including photoelectron mean free path length λ(k), amplitude Fij(k), and phase shift φij(k)) used for the simulation of the theoretical spectra were calculated ab initio by means of the FEFF6 program.²³ Data processing was performed using the ARTEMIS program of the IFEFFIT software package.²²

## 3. RESULTS

Synthesis in the Ag–Fe–S system was performed at 540 and 750–760 °C in the Ag-saturated system. Each of the 18 synthesis run products consisted of pyrrhotite (+ pyrite in sample no. 1433) crystals of 1–5 mm grain size (pyrrhotite: PDF no. 24-0220, powder diffraction file from the International Centre for Diffraction Data, https://www.icdd.com/). The Ag distributions and chemical compositions of several crystals of each sample were studied by SEM/EDS, LA-ICP-MS, and EPMA methods and statistical analysis (Tables 1, S1, and S2).

### 3.1. Distribution of Ag from SEM and LA-ICP-MS.
SEM images given in Figure 1 show that the distribution of Ag in pyrrhotites (po) is not always uniform. For example, sample no. 3832 shown in Figure 1A contains no visible submicron inclusions, while some grains of sample no. 3833 contain Agcr. In sample no. 3833, metallic Ag mostly forms the rims in the vicinity of pyrrhotite grain boundaries. SEM images of sample
<figure><img src="image_12.png" /><figcaption>**Figure 3.** (A) Maximum content determined via EPMA plotted on the $C_{\text{Fe}}$ vs $C_{\text{Ag}}$ (at. %) diagram. Samples are nominally divided into two groups in accordance with the values of $\log f(S_2)$ ($C_{\text{Fe}}$). Vertical thick black lines denote the Ag/Ag$_2$S (see panel (B)) and Ag + Cs-Ag phase/Ag$_2$S equilibrium lines, respectively. There is an increase of Ag content up to 0.03 at. % (below 47.8 at. % Fe) at 540 °C and 0.1 at. % (below 48.8 at. % Fe) at 750-760 °C. At higher values of $C_{\text{Fe}}$, Ag content remains similar. Content limits are shown in blue and red colors, respectively. (B) Sample compositions $C_{\text{Fe}}$ (at. %) vs $t$ (°C) (adapted from refs 11 and 25). The values of $\log f(S_2)$ are calculated according to the equation from Toulmin and Barton.²⁶</figcaption></figure>

nos. 3834 and 3835 (Figure 1C and D) demonstrate that Ag can be disseminated as Ag-bearing isometric submicron inclusions which are dispersed (Figure 1E) in the pyrrhotite matrix (Figure 1F). In all of the cross sections, isometric submicron inclusions form lamellae-shaped parallel structures. The parallel lines do not cross each other at the angles typical of the pyrrhotite crystal structure.

Time-resolved LA-ICP-MS spectra recorded for samples studied by X-ray absorption spectroscopy demonstrate the smooth shape of the signal ablated across distinct crystal zones, which confirms the homogeneous character of distribution of Ag in the synthetic pyrrhotite matrices on the level of 60 µm (Figure 2).

**3.2. Concentration of Ag from EPMA.** The preliminary EPMA analysis was performed with an unfocused beam and a diameter of analysis of $D = 20$ µm. For example, the concentrations were measured within the area containing Ag-bearing inclusions and within the area with no visible inclusions of sample no. 3834 synthesized at a high sulfur fugacity. The measured contents were 0.47 and 0.35 wt %,

respectively. Thus, we suppose that the majority of Ag is concentrated in the matrix as an invisible form rather than in the Ag-bearing submicrometer inclusions detected by SEM.

The content of Ag in all pyrrhotites, determined by EPMA ($D = 2$ µm) within all zones of the grains, depends on temperature and sulfur fugacity or, in other words, on the composition of pyrrhotite (iron content, $C_{\text{Fe}}$). The content of Ag in pyrrhotite depending on the $C_{\text{Fe}}$ is shown in Figure 3A. At 750-760 °C two groups of samples are distinguished in the diagram of $C_{\text{Fe}}$ vs $t$ °C (Figure 3B): (i) $Ag_{\text{cr}} + Fe_{\text{cr}} + FeS$ (troilite) stability field or $Ag_{\text{cr}} + po$ stability field and (ii) $Ag_2S_{\text{cr}} + po$ stability fields. The concentration of Ag in pyrrhotite increases from group (i) to group (ii). It is worth noticing that the Ag concentration increases steadily with increasing Fe content in group (i), while in group (ii) the average Ag concentration of 0.1 at. % is independent of the pyrrhotite composition. The maximum concentration of Ag in pyrrhotite in this group is 1.3 wt % (sample no. 3829, $t_{\text{synthesis}} = 760$ °C). The same effect is observed for samples synthesized at 540 °C. However, the concentration of Ag in pyrrhotite in group (ii)
<figure><img src="image_13.png" /><figcaption>**Figure 4.** Ag content distribution in pyrites synthesized at 750–760 °C. Values are given in Table S3. According to Figure 2A, all of the samples (A) are formally divided into two groups: C~Fe~ > 48.8 at. % (B) and C~Fe~ < 48.8 at. % (C). Two groups distinguished in (A) are also detectable in (B). (C) Only one distinct contribution. EPMA limit of detection <0.02 at. %.</figcaption></figure>

reaches only 0.08 at. %. Due to the presence of the Cs-Ag-rich phase (Fe~2.5~CsAgS~4~), the Ag~cr~/Ag~2~S~cr~ stability line is shifted to lower C~Fe~ values and does not coincide with those in Figure 3B. According to the analyses of diagrams in Figures 3A and B, we conclude that the state of invisible Ag is expected to be close to Ag° at low sulfur fugacity in the metallic Ag stability field and Ag~2~S at high sulfur fugacity in the Ag~2~S stability field.

In samples synthesized at a relatively high temperature, the concentration of Fe in the crystal grains of Ag~2~S~cr~ (sample no. 3829) which covered the Ag wire during the experiment is derived to be less than 7.5 wt % (Table S4). However, in the other Ag-rich phases of this group, the concentration of Fe reaches almost 25.5 wt %. In the samples synthesized at a relatively low temperature, pyrrhotites coexist with Ag~1.9~Fe~0.1~S (C~Fe~ = 0.3 wt %, Table S4).

**3.3. Statistical Analysis from EPMA Data.** Statistical analysis of Ag content distribution in pyrrhotites synthesized at 750–760 °C is shown in Figure 4. This figure is based on the results of single point/line EPMA and LA-ICP-MS analysis given in Table S3. Figure 4A is based on all obtained data: there are two distinctive contributions with maximums at 0.02 and 0.1 at. %. According to the analyses of Figure 3 at C~Fe~ > 48.8 at. %, pyrrhotite coexists with metallic Ag, while at C~Fe~ < 48.8 at. %, pyrrhotite coexists with Ag~2~S~cr~. The set of data was divided according to this criterion and plotted in Figure 4B (C~Fe~ > 48.8 at. %) and Figure 4C (C~Fe~ < 48.8 at. %). This analysis reveals two types of contributions for the samples synthesized at low sulfur fugacity (Figure 4B). We suppose that the first contribution at 0.02 at. % corresponds to metallic Ag, while the minor contribution at 0.1 at. % comes from the invisible Ag. In the samples synthesized at high sulfur fugacity, we found only one distinct contribution (Figure 4C). That allows us to conclude that these samples contain a form (or forms) of Ag that is (are) different from the metallic one (see

Figure 4B). Moreover, the fact that the Gaussian distribution has a very symmetrical shape can be accounted for by the fact that the major invisible and minor Ag-bearing submicron inclusion forms preliminarily identified by SEM/EDS and EPMA were formed by the same process but at different temperatures. For example, Ag-bearing submicron inclusions could have aggregated within the quenching of the pyrrhotite which contained invisible Ag at the temperature of synthesis. This hypothesis will be checked by an in situ XAS spectroscopy experiment.

**3.4. Ag Formal Oxidation State from XANES.** Ag K-edge XANES spectra of Ag-bearing pyrrhotite nos. 3832–3835 are shown in Figure 5 together with the reference materials Ag~ox~, Ag~2~S~cr~ (acanthite), and (Au,Ag~+)~2~S~cr~. As a rule, the spectra of more oxidized atoms are shifted to higher energies in comparison with the metal. However, here we notice that the oxidized Ag~2~S~cr~ is shifted to lower energies from Ag°, and samples (nos. 3834 and 3835) are even more leftward shifted. This appears to be a peculiarity of Ag spectra.²⁷,²⁸

The main experimental features of the spectra are the absorption edge (edge jump, ej) and the first intense feature (white line, WL). The positions of the main features are given in Table 2. The shapes of the WL of spectra of group (i) (nos. 3832 and 3833) are close to those of metallic silver, while the shapes of the WL of spectra of group (ii) (nos. 3834 and 3835) are close to those of Ag~2~S~cr~ and (Au,Ag~+)~2~S~cr~.

**3.4.1. Group (i): Pyrrhotite Synthesized at Low Sulfur Fugacity of log f(S~2~) ≈ -6.5, C~Fe~ > 48.8 at. %** The ej positions of sample nos. 3832 and 3833 vary between 25511 and 25512 eV, while the ej for metallic Ag is 25514 eV. At the same time, the positions of WLs are almost similar to Ag~cr~. The shapes of the spectra of sample nos. 3832 and 3833 are almost identical; however, according to SEM analysis, sample no. 3832 is free of metallic silver. The large difference in the ej
<figure><img src="image_14.png" /><figcaption>**Figure 5.** (A) Normalized Ag K-edge XANES spectra of Ag-bearing pyrrhotites (grey solid lines) and reference materials (black dotted lines). Note the similarity of the main XANES features of the spectra of Ag<sub>cr</sub> and sample nos. 3832 and 3833, and Ag<sub>2</sub>'S and sample nos. 3834 and 3835. (B) Enlarged portion of (A). (C, D) Spectra of sample nos. 3834 and 3835 recorded in situ at temperatures of 25–750 °C. (E, F) Enlarged portions of (C) and (D), respectively.</figcaption></figure>

positions of metallic Ag and the samples suggests that pyrrhotites of group (i) contain a second form of occurrence of Ag that is different from the metallic Ag, with the formal oxidation state close to +1.

### 3.4.2. Group (ii): Pyrrhotite Synthesized at High Sulfur Fugacity of log f(S₂) ≈ -1.0, C<sub>Fe</sub> < 48.8 at. %.
At ambient temperature, the ej positions of sample nos. 3834 and 3835 are 25511.0 and 25510.0 eV, respectively, and these values are close to the spectrum of the reference material, Ag₂S<sub>cr</sub> (acanthite). Positions of the white lines of the standard and the samples are almost identical (25521.0–25522.0 eV). Since no crystalline admixture of Ag₂S was detected in the samples and the statistical analysis revealed a contribution from only one form of occurrence, we suppose that the spectra correspond to a mixture of a major form of an invisible Ag and a minor form of Ag-bearing submicron inclusions.

The heating of both samples to the synthesis temperature (up to 750 °C) demonstrated that the positions of ej and WL

of the samples move toward those of Ag₂S (ej = 25511.0 eV, WL = 25522.0 eV). However, the intensities of the main features decrease gradually with heating. This observation demonstrates that the sizes of Ag-bearing submicron inclusions decrease with temperature²⁹ and Ag redistributes into the invisible form. This observation also corresponds to the statistical analysis, suggesting that at the temperature of synthesis, there is only one mechanism of Ag-enrichment of pyrrhotite. We define this invisible form as Ag₂S-like, based on the similarity of XANES shape and feature positions of the spectra of Ag in pyrrhotite and Ag₂S<sub>cr</sub>.

The positions of the main features of the spectrum of annealed sample no. 3834 are equal to those of the spectrum recorded at ambient temperature before the heating experiment. However, the intensities of the main features of the annealed spectrum are much more pronounced than those of the spectrum recorded before the heating experiment. This effect can be explained by the difference in the quenching time
<table><caption>**Table 2. Positions of the Absorption Edge (ej) and White Line (WL) of Ag K-edge XANES Spectra of Model Substances and Samples<sup>a</sup>**</caption><thead><tr><th>Standard/sample</th><th>Sample ID</th><th>Feature</th><th>Position, eV</th></tr></thead><tbody><tr><td rowspan="2">Ag<sub>cr</sub></td><td rowspan="2">-</td><td>ej</td><td>25514.0</td></tr><tr><td>WL</td><td>25526.3</td></tr><tr><td rowspan="2">Ag<sub>2</sub>S<sub>cr</sub> acanthite</td><td rowspan="2">-</td><td>ej</td><td>25511.0</td></tr><tr><td>WL</td><td>25522.0</td></tr><tr><td rowspan="2">(Ag,Au)<sub>2</sub>S<sub>cr</sub></td><td rowspan="2">-</td><td>ej</td><td>25512.0</td></tr><tr><td>WL</td><td>25524.0</td></tr><tr><td rowspan="4">Group (i): pyrrhotite Fe<sub>1-x</sub>S synthesized at low sulfur fugacity, C̄Fe > 48.8 at. %</td><td rowspan="2">No. 3832, RT</td><td>ej</td><td>25512.0</td></tr><tr><td>WL</td><td>25526.5</td></tr><tr><td rowspan="2">No. 3833, RT</td><td>ej</td><td>25511.0</td></tr><tr><td>WL</td><td>25525.0</td></tr><tr><td rowspan="12">Group (ii): pyrrhotite Fe<sub>1-x</sub>S synthesized at high sulfur fugacity, C̄Fe < 48.8 at. %</td><td>No. 3834, RT</td><td>ej</td><td>25511.0</td></tr><tr><td>WL</td><td>25522.0</td></tr><tr><td>No. 3834, 600 °C</td><td>ej</td><td>25510.5</td></tr><tr><td>WL</td><td>25521.0</td></tr><tr><td>No. 3834, 700 °C</td><td>ej</td><td>25510.0</td></tr><tr><td>WL</td><td>25521.0</td></tr><tr><td>No. 3834, 750 °C</td><td>ej</td><td>25511.0</td></tr><tr><td>WL</td><td>25522.0</td></tr><tr><td>No. 3834, annealed after 750 °C</td><td>ej</td><td>25511.0</td></tr><tr><td>WL</td><td>25522.0</td></tr><tr><td>No. 3835, RT</td><td>ej</td><td>25510.0</td></tr><tr><td>WL</td><td>25522.0</td></tr><tr><td>No. 3835, 150 °C</td><td>ej</td><td>25510.0</td></tr><tr><td>WL</td><td>25522.0</td></tr><tr><td>No. 3835, 300 °C</td><td>ej</td><td>25510.0</td></tr><tr><td>WL</td><td>25522.0</td></tr><tr><td>No. 3835, 400 °C</td><td>ej</td><td>25510.5</td></tr><tr><td>WL</td><td>25522.0</td></tr><tr><td>No. 3835, 500 °C</td><td>ej</td><td>25510.5</td></tr><tr><td></td><td>WL</td><td>25522.0</td></tr></tbody></table>

after the initial synthesis (5–10 s) and heating experiment (1 h). The short time to quench after the synthesis experiment led to the formation of submicrometer inclusions, while the long annealing of the sample after the heating experiment led to the formation of larger inclusions.

### 3.5. Ag Local Atomic Environment from EXAFS.

Experimental Ag K-edge EXAFS spectra and their Fourier transforms (FTs) are shown together with the simulated spectra in Figure 6. Fit results are given in Table 3. The most common phases in the Ag–Fe–S system are metallic silver (Ag°, Fm3m<sup>30</sup>), acanthite and argentite (P2<sub>1</sub>/n and Im3m, Ag<sub>2</sub>S<sup>31</sup>), argentopyrite (AgFe<sub>2</sub>S<sub>3</sub>, P2<sub>1</sub>/n<sup>32</sup>), lenaite (AgFeS<sub>2</sub>, I42d<sup>33</sup>), and sternbergite (AgFe<sub>2</sub>S<sub>3</sub>, C<sub>mb</sub><sup>34</sup>). In the pure Ag<sub>cr</sub>, the absorbing atom is surrounded by 12 Ag atoms at 2.89 Å in the first coordination shell.<sup>30</sup> In accordance with the fitting of the Ag<sub>2</sub>S EXAFS spectrum, Ag is surrounded by ~1 S atom at 2.49 ± 0.01 Å and ~2 Ag atoms at 3.00 ± 0.02 Å (Table 3). In the argentopyrite structure, Ag atoms are surrounded by 4 S atoms at 2.24–2.78 Å and 2 Fe atoms at 3.40 and 3.68 Å, and no Ag atoms are located in the Ag vicinity. In the lenaite structure, the Ag atom is surrounded by 4 S atoms at 2.47 Å, 4 Ag atoms at 3.84 Å, and 8 Fe atoms at the same distance. In

the sternbergite structure, there are 2 Ag atoms in the vicinity of Ag at 1.87–1.94 Å, and 4 S atoms at 2.48–3.12 Å; the nearest Ag atoms are located at 3.55 Å. In the pure pyrrhotite structure, Fe is surrounded by 6 S atoms at 2.44 Å, and 2 and 6 Fe atoms at 2.84 and 3.43 Å, respectively.<sup>35</sup> The preliminary fits demonstrated that the presence of Ag in argentopyrite, lenaite, sternbergite, and pyrrhotite (solid solution) could be ruled out because in the local atomic environment of Ag in their structures there are more than 2 atoms of S.

3.5.1. Group (i): Pyrrhotite Synthesized at Low Sulfur Fugacity of log f(S<sub>2</sub>) ≈ −6.5, C<sub>Fe</sub> > 48.8 at. %. The best fits of experimental spectra of sample nos. 3832 and 3833 were achieved when Ag atoms were surrounded by 0.1–0.2 S atoms at 2.50–2.55 Å and ~6 Ag atoms at ~2.86 Å. Considering that the interatomic distance Ag–Ag is close to that in Ag<sub>cr</sub>, whereas the coordination number of Ag atoms is 2 times lower compared to Ag<sub>cr</sub>, we suggest that in these samples at ambient temperature 50–80% of silver exists in the Ag metal form. The minor fraction of silver can be accounted for by the presence of Ag in the invisible form, which results in the contribution from S atoms in the first coordination shell of Ag.

3.5.2. Group (ii): Pyrrhotite Synthesized at High Sulfur Fugacity of log f(S<sub>2</sub>) ≈ −1.0, C<sub>Fe</sub> < 48.8 at. %. The best fit of experimental spectra nos. 3834 and 3835 recorded at ambient temperature was obtained when the Ag atom was surrounded by 1.3–2 S atoms at ~2.50 Å, 2.8 Ag atoms at ~2.93 Å, and 2 Fe atoms at 3.35 Å. The contribution of Fe arises from either the presence of Fe in submicron inclusions or the contribution of the Fe-rich pyrrhotite matrix. The invisible form is most likely presented by Ag<sub>2</sub>S-like clusters where Ag is surrounded by 2 S atoms. We use the term “cluster” to emphasize the disordered nature of the distant coordination shells.

Heating of sample no. 3834 up to 700 °C leads to a decrease of the amount of S atoms in the first coordination shell of Ag from 1.3 to 0.5, and the interatomic distance R<sub>Ag–S</sub> decreases from 2.45 ± 0.03 to 2.51 ± 0.01 Å. As a rule, the decrease in the coordination number and interatomic distance in the nanoparticles means that the surface of the particles increases; in other words, the size of the particles decreases. However, here we avoid the use of the term “nanoparticles” because the size of nanoparticles usually increases with heating,<sup>36</sup> while we observe the opposite effect. Our observations, together with the decrease of the number of Ag atoms in the second coordination sphere and an increase of Debye–Waller parameters for Ag and Fe in the distant coordination spheres, indicate the decrease of the size of Ag<sub>2</sub>S-like submicrometer inclusions with heating. At high temperature, Ag redistributes into the invisible form of Ag<sub>2</sub>S-like clusters in the pyrrhotite structure.

## 4. DISCUSSION AND IMPLICATIONS

### 4.1. Content of Ag in Synthetic and Natural Pyrrhotite.
In this study, we synthesized Ag-bearing pyrrhotites at temperatures of 540 and 750–760 °C. The content of Ag in pyrrhotite obtained at 540 °C was 0.08 wt % (sample no. 1433, EPMA analysis) and at 750–760 °C was 1.3 wt % (sample no. 3829, EPMA analysis). The compilation of these data with the concentrations described in the literature<sup>11–15</sup> is given in Figure 7. The content limit can be described by the following equation:

$$ \lg(C_{Ag}, \text{wt \%}) = -5.0 \times (1000/T) + 5.1 \quad (2) $$

<footnote><sup>a</sup>The uncertainty of the energy values is ±0.5 eV.</footnote>
<figure><img src="image_15.png" /><figcaption>Figure 6. Results of Ag K-edge EXAFS spectra fits performed using the ARTEMIS program. The figures show Fourier transforms (FTs) of the k²-weighted background-subtracted EXAFS spectra (not corrected for phase shift). Inserts: k²-weighted EXAFS spectra. Black lines, experimental spectra; dotted red lines, simulated spectra.</figcaption></figure>

The concentrations measured in Simon et al.¹³ were not considered because the system they investigated was not saturated with respect to Ag. According to the diagram, the calculated Ag content in pyrrhotite in the Ag-saturated system at 150 °C can be estimated as 2 ppb, at 300 °C as 2 ppm, at 450 °C as 150 ppm, at 600 °C as 2360 ppm, and at 750 °C as 1.6 wt %. These calculations are consistent with the data for natural samples of pyrrhotite. For example, Mansur et al.⁸ compiled data on Ag contents in pyrrhotites from late-magmatic Ni-Cu deposits based on the data from several deposits, e.g., Duluth and Norilsk-Talnakh (approximate temperature of formation is 400–540 °C). The concentrations in these pyrrhotites reached 50 ppm. This content falls within the interval of the calculated values between 47 ppm (at 400 °C) and 890 ppm (at 540 °C). Walker et al.⁶ detected 54 ppm of Ag in pyrrhotite from Albany-Fraser Orogen in Western Australia, which hosts significant magmatic sulfide mineralization of the late-magmatic or postcrystallization origin. The temperature of formation can be estimated as 400–500 °C. In this case, the content of 54 ppm lies within the interval of 47 ppm (at 400 °C)–430 ppm (at 500 °C) calculated from the experimental data. Steadman and Large⁵ and Rottier et al.⁴ studied pyrrhotites from sediment-hosted orogenic gold (Homestake deposit, South Dakota, U.S.A.) and base metal with imposed hydrothermal stage (Cerro de Pasco Cordilleran, Peru) deposits, respectively. The obtained concentrations were 10 and 0.74 ppm, respectively. The calculated concentrations for temperatures characteristic of these deposits (250–350 °C) are 0.4–12 ppm. The measured contents fall within the interval of the calculated concentrations.

We also compiled the data from Taylor¹¹,¹² and this study and calculated the content of Fe in the low-temperature Ag₂S

phase (Figure 7); the data can be described by the following equation:

$$ \lg(C_{\text{Fe}}, \text{wt \%}) = -5.2 \times (1000/T) + 6.5 \quad (3) $$

The calculated Fe content in Ag₂S in the Fe-saturated system at 150 °C can be estimated as 20 ppb, at 300 °C as 25 ppm, at 450 °C as 2050 ppm, at 600 °C as 3.6 wt %, and at 750 °C as 26.8 wt %. We note that the content of Fe in the Ag-rich phase is roughly 1.25 log units (wt %) higher than the content of Ag in pyrrhotite.

In spite of the LA-ICP-MS patterns showing a homogeneous distribution of Ag in the pyrrhotite matrix, SEM/EDS, EPMA, and statistical analyses revealed contributions from more than one form of Ag. On the one hand, at low sulfur fugacity (C<sub>Fe</sub> > 48.8 at. %, sample nos. 3831–3833 and 4014–4017) silver mostly forms metallic rims around pyrrhotite grains, and a minor admixture of Ag enriches pyrrhotite as an invisible form that cannot be identified by optical and scanning electron microscopy. On the other hand, at high sulfur fugacity (C<sub>Fe</sub> < 48.8 at. %, sample nos. 3828–3830, 3834–3835, and 4010–4013), Ag occurs in pyrrhotite mostly as an invisible form with a minor admixture of Ag in Ag-bearing submicron inclusions (Figure 4).

### 4.2. State of Ag in Pyrrhotite.
4.2.1. Group (i): Pyrrhotite Synthesized at Low Sulfur Fugacity of log f(S₂) ≈ -6.5, C<sub>Fe</sub> > 48.8 at. %.
Two pyrrhotites from this group were studied by XAS: pyrrhotite with no visible metallic Ag and pyrrhotite containing Ag-bearing rims around the grains (nos. 3832 and 3833, respectively). The XANES spectra of the samples are similar to those of the metallic Ag but not identical. Thus, we suppose that along with metallic Ag rims around grains, pyrrhotites contain silver disseminated in the pyrrhotite structure in an invisible form. The fitting of the
<table><caption>Table 3. Parameters of Ag Local Atomic Structure in Ag-Bearing Pyrrhotites Determined by Ag K-edge *k*²-Weighted EXAFS Spectra Fitting<sup>*a*</sup></caption><thead><tr><th rowspan="2">Atom</th><th colspan="4">Experimental data <sup>b</sup></th><th colspan="2">Fit quality:</th></tr><tr><th><i>N</i></th><th><i>R</i>, Å</th><th>σ<sup>2</sup>, Å<sup>2</sup></th><th><i>E</i><sup>0</sup>, eV</th><th>Red. χ<sup>2</sup></th><th>R-factor</th></tr></thead><tbody><tr><td colspan="7"><strong>Standard</strong></td></tr><tr><td colspan="7">Ag<sub>2</sub>S<sub>(cr)</sub> acanthite (<i>R</i>-range: 1.3 – 3.3, <i>k</i>- range: 3 – 11)</td></tr><tr><td>S</td><td>0.6 ± 0.1</td><td>2.49 ± 0.01</td><td>0.004 ± 0.001</td><td rowspan="2">0.8 ± 0.7</td><td rowspan="2">2.2</td><td rowspan="2">0.003</td></tr><tr><td>Ag</td><td>1.7 ± 0.5</td><td>3.00 ± 0.02</td><td>0.029 ± 0.005</td></tr><tr><td colspan="7"><strong>Group (i): pyrrhotite synthesized at low sulfur fugacity, C<sub>Fe</sub> > 48.8 at.%</strong></td></tr><tr><td colspan="7">No. 3832 (<i>R</i>-range: 1.3 – 3, <i>k</i>- range: 3 – 12)</td></tr><tr><td>S</td><td>0.1 ± 0.1</td><td>2.50 ± 0.04</td><td>0.002 ± 0.009</td><td rowspan="2">0.4 ± 0.4</td><td rowspan="2">101.1</td><td rowspan="2">0.001</td></tr><tr><td>Ag</td><td>6.3 ± 0.3</td><td>2.86 ± 0.01</td><td>0.008 ± 0.001</td></tr><tr><td colspan="7">No. 3833 (<i>R</i>-range: 1.3 – 3, <i>k</i>- range: 3 – 12)</td></tr><tr><td>S</td><td>0.2 ± 0.4</td><td>2.55 ± 0.07</td><td>0.002 ± 0.018</td><td rowspan="2">-0.6 ± 0.9</td><td rowspan="2">6.6</td><td rowspan="2">0.009</td></tr><tr><td>Ag</td><td>6.2 ± 0.9</td><td>2.86 ± 0.01</td><td>0.008 ± 0.001</td></tr><tr><td colspan="7"><strong>Group (ii): pyrrhotite synthesized at high sulfur fugacity, C<sub>Fe</sub> < 48.8 at.%</strong></td></tr><tr><td colspan="7">No. 3834, RT (<i>R</i>-range: 1.3 – 3.6, <i>k</i>- range: 3 – 11)</td></tr><tr><td>S</td><td>1.3 ± 0.1</td><td>2.50 ± 0.01</td><td>0.007 ± 0.001</td><td rowspan="2">-1.8 ± 0.6</td><td rowspan="2">19.6</td><td rowspan="2">0.003</td></tr><tr><td>Ag</td><td>2.8 ± 0.4</td><td>2.91 ± 0.02</td><td>0.021 ± 0.003</td></tr><tr><td>Fe</td><td>2 <sup>c</sup></td><td>3.34 ± 0.04</td><td>0.023 ± 0.004</td><td></td><td></td><td></td></tr><tr><td colspan="7">No. 3834, <i>t</i> = 600 °C (<i>R</i>-range: 1.3 – 3.7, <i>k</i>- range: 3 – 9)</td></tr><tr><td>S</td><td>0.9 ± 0.9</td><td>2.51 ± 0.09</td><td>0.009 ± 0.014</td><td rowspan="2">2.0 ± 7.3</td><td rowspan="2">4.4</td><td rowspan="2">0.002</td></tr><tr><td>Ag</td><td>1.7 ± 1.9</td><td>2.86 ± 0.07</td><td>0.015 ± 0.019</td></tr><tr><td>Fe</td><td>2 <sup>c</sup></td><td>3.30 ± 0.06</td><td>0.014 ± 0.008</td><td></td><td></td><td></td></tr><tr><td colspan="7">No. 3834, <i>t</i> = 700 °C (<i>R</i>-range: 1.3 – 3.5, <i>k</i>- range: 3 – 8)</td></tr><tr><td>S</td><td>0.5 <sup>c</sup></td><td>2.45 ± 0.03</td><td>0.009 ± 0.005</td><td rowspan="2">2.7 <sup>c</sup></td><td rowspan="2">3.6</td><td rowspan="2">0.017</td></tr><tr><td>Ag</td><td>1.1 <sup>c</sup></td><td>2.95 ± 0.07</td><td>0.025 ± 0.014</td></tr><tr><td colspan="7">No. 3834, annealed (<i>R</i>-range: 1.3 – 3.7, <i>k</i>- range: 3 – 9)</td></tr><tr><td>S</td><td>2.1 ± 0.9</td><td>2.50 ± 0.03</td><td>0.011 ± 0.007</td><td rowspan="2">0.1 ± 2.5</td><td rowspan="2">2.3</td><td rowspan="2">0.019</td></tr><tr><td>Ag</td><td>2.7 ± 1.7</td><td>2.94 ± 0.06</td><td>0.016 ± 0.013</td></tr><tr><td>Fe</td><td>2 <sup>c</sup></td><td>3.34 <sup>c</sup></td><td>0.023 ± 0.036</td><td></td><td></td><td></td></tr><tr><td colspan="7">No. 3835, RT (<i>R</i>-range: 1.2 – 3.5, <i>k</i>- range: 3 – 12)</td></tr><tr><td>S</td><td>1.3 ± 0.3</td><td>2.51 ± 0.01</td><td>0.004 ± 0.002</td><td rowspan="2">-1.1 ± 1.6</td><td rowspan="2">451.5</td><td rowspan="2">0.029</td></tr><tr><td>Ag</td><td>2.8 ± 1.0</td><td>2.96 ± 0.04</td><td>0.017 ± 0.005</td></tr><tr><td>Fe</td><td>2 <sup>c</sup></td><td>3.35 ± 0.05</td><td>0.014 ± 0.005</td><td></td><td></td><td></td></tr></tbody></table>

<sup>*a*</sup>Uncertainties were calculated by the ARTEMIS program.<sup>22 b</sup>Amplitude reduction factor S<sub>0</sub><sup>2</sup> = 0.9.<sup>c</sup>Parameter was restrained after the preliminary fit.

EXAFS spectra of pyrrhotites from this group demonstrates that Ag atoms are surrounded by 0.1–0.2 S atoms at 2.50–2.55 Å and ~6 Ag atoms at ~2.86 Å. The contribution of S atoms appears from the presence of 30–50% of a minor form of Ag in the invisible form.

### 4.2.2. Group (ii): Pyrrhotite Synthesized at High Sulfur Fugacity of log f(S<sub>2</sub>) ≈ −1.0, C<sub>Fe</sub> < 48.8 at.%

The shapes of experimental XANES spectra of two samples (nos. 3834 and 3835) were similar to Ag<sub>2</sub><sup>+</sup>S<sub>cr</sub> (acanthite); thus, the oxidation state of Ag in pyrrhotite was close to +1. The heating of pyrrhotites up to 750 °C demonstrated the decrease of the main features of the XANES spectra, indicating the decrease of the size of Ag-bearing submicron inclusions and redistribution of Ag into the invisible form. The overall positions of the main

features of the XANES spectra collected at 25–750 °C are similar; hence, the invisible form is stable up to the synthesis temperature.

The best fit of experimental EXAFS spectra of this group recorded at ambient temperature was obtained when the Ag atom was surrounded by 1.3–2 S atoms at ~2.50 Å, 2.8 Ag atoms at ~2.93 Å, and 2 Fe atoms at 3.35 Å. Thus, we found out that the invisible form of Ag in pyrrhotite is represented by Ag<sub>2</sub>S-like clusters. The heating of the pyrrhotite from this group (sample no. 3834) up to 750 °C led to the decrease of the size of submicron inclusions.

The presence of other Ag–Fe–S bearing phases (e.g., argentopyrite,<sup>25</sup> lenaite,<sup>26</sup> and sternbergite<sup>27</sup>) was ruled out because (i) of the similarity of the spectra of the samples and
<figure><img src="image_16.png" /><figcaption>**Figure 7.** Sketch diagram illustrating the logarithm of maximum concentration of Ag in pyrrhotite and Fe in Ag-rich phase vs the inverse synthesis temperature. Data are compiled from the literature<sup>11,13−15</sup> and this study.</figcaption></figure>

standards: sample nos. 3832, 3833, and Ag°, and sample nos. 3834, 3835, and Ag<sup>2+</sup>S<sub>cr</sub>; and (ii) the local atomic environment of Ag in their structures is different from those in our samples.

**4.3. Ag Incorporation Mechanism in Pyrrhotite.** In the present study, we observed that Ag tends to exist as Ag° (rims, inclusions) and an invisible form of Ag<sub>2</sub>S-like clusters in the pyrrhotite with a high content of Fe. At low Fe content, pyrrhotite is capable of accommodating Ag in an invisible form of Ag<sub>2</sub>S-like clusters together with the minor form of Ag-bearing submicron inclusions appearing during quenching. The formation of the quenching product happens due to the slow annealing and/or due to the fact that experimental systems were saturated with respect to Ag. So, both invisible and submicron forms of Ag could be present in the natural pyrrhotites.

Based on our previous observations on the incorporation of Pt and Au in the pyrite and pyrrhotite structures, the capability of metals to form a solid solution with Fe sulfide and/or nanosized inclusions/clusters strongly depends on only one or many factors simultaneously (i.e., sulfur fugacity, crystal structures of the end-members, and ionic radii<sup>37,38</sup>). Our recent work showed that Pt forms a solid solution (Pt,Fe)<sub>1-x</sub>S and PtS-like clusters; the latter were considered to be a quench product.<sup>39</sup> The similarity of the Fe<sup>2+</sup> (0.6 Å) and Pt<sup>4+</sup> (0.63 Å) ionic radii favored the formation of a solid solution. At the same time, Au can form both a solid solution and Au<sub>2</sub>S-like clusters in the pyrite structure.<sup>38,40,41</sup> Even though the difference in the ionic radii between Fe<sup>2+</sup>(VI) and Au<sup>+</sup>(VI) exceeds 50%, Au can be incorporated into the cationic position in the pyrite structure. The formation of Au<sub>2</sub>S-like clusters can be accounted for by the difference in ionic radii or kinetics. Comparison of the ionic radii of Ag<sup>+</sup>(VI) (1.15 Å) and Fe<sup>2+</sup>(VI) shows a difference of only ~50%.<sup>42</sup> However, based on our data, Ag unlikely forms a solid solution in pyrrhotite, and at both ambient and high temperatures, Ag<sup>+</sup>(II) with the ionic radius of 0.67 Å<sup>42</sup> mostly favors the formation of invisible Ag<sub>2</sub>S-like clusters (with the minor admixture of Ag-bearing submicron inclusions as a quenching product). The formation of Ag<sub>2</sub>S-like clusters cannot be accounted for by the cooling effect.

**5. CONCLUSIONS**

Pyrrhotite can incorporate up to 0.08 wt % Ag at 540 °C and up to 1.3 wt % Ag at 750–760 °C. The proposed concentration limit can be described by the equation lg(C<sub>Ag</sub>, wt %) = -5.0 × (1000/T) + 5.1. The concentrations of Ag in pyrrhotite at 150 °C can be estimated as 2 ppb, at 300 °C as 2 ppm, at 450 °C as 150 ppm, at 600 °C as 2360 ppm, and at 750 °C as 1.6 wt %. The predicted levels of concentrations are consistent with the data obtained for natural pyrrhotites.<sup>4–8</sup> LA-ICP-MS, SEM/EDS, EPMA, and statistical analyses combined with an X-ray absorption spectroscopy study revealed two major states of Ag in pyrrhotite. Group (i) contains pyrrhotites synthesized at low sulfur fugacity (C<sub>Fe</sub> > 48.8 at. %). In these samples, most of the Ag is presented by metallic Ag disseminated in the pyrrhotite matrix. The second, minor contribution arises from the formation of an invisible form of Ag<sub>2</sub><sup>+</sup>S-like clusters. Group (ii) is presented by pyrrhotites synthesized at high sulfur fugacity (C<sub>Fe</sub> < 48.8 at. %). These samples mostly contain the invisible form presented by Ag<sub>2</sub><sup>+</sup>S-like clusters with a minor contribution of the quenching product of Ag-bearing submicron inclusions. The heating of the pyrrhotite from this group up to 750 °C leads to a decrease in the size of the submicron inclusions and redistribution of Ag into the invisible form. The formation of a Ag solid solution is unlikely due to the large difference between the ionic radii of Ag<sup>+</sup>(VI) and Fe<sup>2+</sup>(VI) of almost 50%. Hence, the formation of the invisible form presented by Ag<sub>2</sub><sup>+</sup>S-like clusters is more favorable.

**■ ASSOCIATED CONTENT**

**● Supporting Information**

The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acsearthspace-
chem.3c00002.

EPMA and LA-ICP-MS operating conditions, and compositions of Ag-bearing pyrrhotites and Ag-rich run products of the experiment determined by EPMA method (PDF)

**■ AUTHOR INFORMATION**

**Corresponding Author**

Olga N. Filimonova — *The XMaS Beamline*, FR 38043
*Grenoble, France*; *Department of Physics, University of*
*Liverpool, Oliver Lodge Laboratory, Liverpool L69 7ZE,*
*United Kingdom; ● orcid.org/0000-0001-5713-7890;*
*Email: olga.filimonova@esrf.fr*

**Authors**

Didier Wermeille — *The XMaS Beamline*, FR 38043
*Grenoble, France*; *Department of Physics, University of*
*Liverpool, Oliver Lodge Laboratory, Liverpool L69 7ZE,*
*United Kingdom*

Kristina O. Kvashnina — *The Rossendorf Beamline*, FR 38043
*Grenoble, France*; *Helmholtz-Zentrum Dresden-Rossendorf*
*(HZDR), Institute of Resource Ecology, 01314 Dresden,*
*Germany; ● orcid.org/0000-0003-4447-4542*

Complete contact information is available at:
https://pubs.acs.org/10.1021/acsearthspacechem.3c00002

**Notes**

The authors declare no competing financial interest.
## ACKNOWLEDGMENTS

We acknowledge beamlines BM20 and BM28 at the European Synchrotron Radiation Facility (a U.K. EPSRC funded beamline) for beam time (experiment no. IHES79). The help and support of Romain Garlet and Yves Watier from Sample environment group is greatly appreciated. Valuable comments and suggestions from two anonymous reviewers and AE are greatly appreciated.

## REFERENCES

(1) Barnes, S. J.; Cox, R. A.; Zientek, M. L. Platinum-group element, gold, silver and base metal distribution in compositionally zoned sulfide droplets from the Medvezky Creek Mine, Noril'sk. Russia. *Contrib. to Mineral. Petrol.* **2006**, *152* (2), 187–200.

(2) Piña, R.; Gervilla, F.; Barnes, S. J.; Ortega, L.; Lunar, R. Distribution of platinum-group and chalcophile elements in the Aguablanca Ni-Cu sulfide deposit (SW Spain): Evidence from a LA-ICP-MS study. *Chem. Geol.* **2012**, *302*, 61–75.

(3) Duran, C. J.; Barnes, S. J.; Corkery, J. T. Trace element distribution in primary sulfides and Fe-Ti oxides from the sulfide-rich pods of the Lac des Iles Pd deposits, Western Ontario, Canada: constraints on processes controlling the composition of the ore and the use of pentlandite compositions in exploration. *J. Geochem. Explor. 2016*, **166**, 45–63.

(4) Rottier, B.; Kouzmanov, K.; Wälle, M.; Bendézú, R.; Fontboté, L. Sulfide replacement processes revealed by textural and LA-ICP-MS trace element analyses: example from the early mineralization stages at Cerro de Pasco. *Peru. Econ Geol.* **2016**, *111* (6), 1347–1367.

(5) Steadman, J. A.; Large, R. R. Synsedimentary, diagenetic, and metamorphic pyrite, pyrrhotite, and marcasite at the Homestake BIF-hosted gold deposit, South Dakota, USA: Insights on Au-As ore genesis from textural and LA-ICP-MS trace element studies. *Econ Geol.* **2016**, *111* (7), 1731–1752.

(6) Walker, A. T.; Evans, K. A.; Kirkland, C. L. A novel application of image analysis to interpret trace element distributions in magmatic sulphides. *Lithos* **2020**, *362*, 105451.

(7) Mansur, E. T.; Barnes, S. J.; Duran, C. J.; Sluzhenikin, S. F. Distribution of chalcophile and platinum-group elements among pyrrhotite, pentlandite, chalcopyrite and cubanite from the Noril'sk-Talnakh ores: Implications for the formation of platinum-group minerals. *Miner. Depos.* **2020**, *55* (6), 1215–1232.

(8) Mansur, E. T.; Barnes, S. J.; Duran, C. J. An overview of chalcophile element contents of pyrrhotite, pentlandite, chalcopyrite, and pyrite from magmatic Ni-Cu-PGE sulfide deposits. *Miner. Depos.* **2021**, *S6* (1), 179–204.

(9) Chu, X.; Li, B.; Shen, P.; Zha, Z.; Lei, Z.; Wang, X.; Tao, S.; Hu, Q. Trace elements in sulfide minerals from the Huangshaping copper-polymetallic deposit, Hunan, China: Ore genesis and element occurrence. *Ore Geol. Rev.* **2022**, *144*, 104867.

(10) Cook, N. J.; Chryssoulis, S. Concentrations of invisible gold in the common sulfides. *Can. Mineral.* **1990**, *28*, 1–16.

(11) Taylor, L. A. The system Ag-Fe-S: Phase equilibria and mineral assemblages. *Miner. Depos.* **1970**, *5* (1), 41–58.

(12) Taylor, L. A. The system Ag-Fe-S: Phase relations between 1200 and 700 °C. *Metall. Trans.* **1970**, *1* (9), 2523–2529.

(13) Simon, A. C.; Candela, P. A.; Piccoli, P. M.; Mengason, M.; Englander, L. The effect of crystal-melt partitioning on the budgets of Cu, Au, and Ag. *Am. Mineral.* **2008**, *93* (8–9), 1437–1448.

(14) Pal'yanova, G.; Mikhlin, Y.; Kokh, K.; Karmanov, N.; Seryotkin, Y. Experimental constraints on gold and silver solubility in iron sulfides. *J. Alloys Compd.* **2015**, *649*, 67–75.

(15) Lipko, S.; Tauson, V.; Smagunov, N.; Babkin, D.; Parkhomenko, I. Distribution of Trace Elements (Ag, Pd, Cd, and Mn) between Pyrite and Pyrrhotite and Selectivity of Surficial Nonautonomous Phases in a Hydrothermal System. *Minerals* **2022**, *12* (9), 1165.

(16) Chareev, D. A.; Volkova, O. S.; Geringer, N. V.; Koshelev, A. V.; Nekrasov, A. N.; Osadchii, V. O.; Osadchii, E. G.; Filimonova, O.

N. Synthesis of chalcogenide and pnictide crystals in salt melts using a steady-state temperature gradient. *Crystallogr. Rep.* **2016**, *61* (4), 682–691.

(17) Chareev, D. A. General principles of the synthesis of chalcogenides and pnictides in salt melts using a steady-state temperature gradient. *Crystallogr. Rep.* **2016**, *61* (3), 506–511.

(18) Wilson, S. A.; Ridley, W. L.; Koenig, A. E. Development of sulfide calibration standards for the laser ablation inductively-coupled plasma mass spectrometry technique. *J. Anal At Spectrom.* **2002**, *17* (4), 406–409.

(19) Wohlgemuth-Ueberwasser, C. C.; Ballhaus, C.; Berndt, J.; Stotter née Paliulionyte, V.; Meisel, T. Synthesis of PGE sulfide standards for laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). *Contrib. to Mineral. Petrol* **2007**, *154* (5), 607–617.

(20) Scheinost, A. C.; Claussner, J.; Exner, J.; Feig, M.; Findeisen, S.; Hennig, C.; Kvashnina, K. O.; Naudet, D.; Prieur, D.; Rossberg, A.; Schmidt, M.; et al. ROBL-II at ESRF: a synchrotron toolbox for actinide research. *J. Synchrotron Radiat.* **2021**, *28* (1), 333–349.

(21) Tagirov, B. R.; Trigub, A. L.; Filimonova, O. N.; Kvashnina, K. O.; Nickolsky, M. S.; Lafuerza, S.; Chareev, D. A. Gold transport in hydrothermal chloride-bearing fluids: Insights from in situ X-ray absorption spectroscopy and ab initio molecular dynamics. *ACS Earth and Space Chem.* **2019**, *3* (2), 240–261.

(22) Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT. *J. Synchrotron Radiat.* **2005**, *12* (4), 537–541.

(23) Zabinsky, S. I.; Rehr, J. J.; Ankudinov, A.; Albers, R. C.; Eller, M. J. Multiple-scattering calculations of X-ray-absorption spectra. *Phys. Rev. B* **1995**, *52* (4), 2995.

(24) Paton, C.; Hellstrom, J.; Paul, B.; Woodhead, J.; Hergt, J. Iolite: Freeware for the visualisation and processing of mass spectrometric data. *J. Anal. At. Spectrom.* **2011**, *26* (12), 2508–2518.

(25) Yund, R. A.; Hall, H. T. The miscibility gap between FeS and Fe₁-xS. *Mater. Res. Bull.* **1968**, *3* (9), 779–783.

(26) Toulmin, P.; Barton, P. B. A thermodynamic study of pyrite and pyrrhotite. *Geochim. Cosmochim. Acta* **1964**, *28* (5), 641–671.

(27) Renones, P.; Collado, L.; Iglesias-Juez, A.; Oropeza, F. E.; Fresno, F.; de la Peña O'Shea, V. A. Silver-gold bimetal-loaded TiO₂ photocatalysts for CO₂ reduction. *Ind. Eng. Chem. Res.* **2020**, *59* (20), 9440–9450.

(28) Wang, L.; Zhang, T.; Li, P.; Huang, W.; Tang, J.; Wang, P.; Liu, J.; Yuan, Q.; Bai, R.; Li, B.; Zhang, K.; et al. Use of synchrotron radiation-analytical techniques to reveal chemical origin of silver-nanoparticle cytotoxicity. *ACS Nano* **2015**, *9* (6), 6532–6547.

(29) Timoshenko, J.; Roese, S.; Hövel, H.; Frenkel, A. I. Silver clusters shape determination from in-situ XANES data. *Radiat. Phys. Chem.* **2020**, *175*, 108049.

(30) Suh, I. K.; Ohta, H.; Waseda, Y. High-temperature thermal expansion of six metallic elements measured by dilatation method and X-ray diffraction. *J. Mater. Sci.* **1988**, *23* (2), 757–760.

(31) Sadovnikov, S. I.; Gusev, A. I.; Rempel, A. A. An in situ high-temperature scanning electron microscopy study of acanthite-argentite phase transformation in nanocrystalline silver sulfide powder. *Phys. Chem. Chem. Phys.* **2015**, *17* (32), 20495–20501.

(32) Yang, H.; Pinch, W. W.; Downs, R. T. Crystal structure of argentopyrite, AgFe₂S₃, and its relationship with cubanite. *Am. Mineral.* **2009**, *94* (11–12), 1727–1730.

(33) Bindi, L.; Spry, P. G.; Pratesi, G. Lenaite from the Gies gold-silver telluride deposit, Judith Mountains, Montana, USA: occurrence, composition, and crystal structure. *Can. Mineral.* **2006**, *44* (1), 207–212.

(34) Pertlik, F. Crystal structure of sternbergite, AgFe₂S₃. *Neues Jahrb. für Mineral. Abhandlungen.* **1987**, *10*, 458–464.

(35) Wyckoff, R. W. G. *Crystal Structures*, Vol. 1; Interscience Publishers: New York, 1963; pp 85–237.

(36) Matos, J.; Ono, L. K.; Behafarid, F.; Croy, J. R.; Mostafa, S.; DeLaRiva, A. T.; Datye, A. K.; Frenkel, A. I.; Roldan Cuenya, B. *In situ* coarsening study of inverse micelle-prepared Pt nanoparticles
supported on γ-Al₂O₃: pretreatment and environmental effects. *Phys. Chem. Chem. Phys.* **2012**, *14* (32), 11457–11467.

(37) Filimonova, O. N.; Nickolsky, M. S.; Trigub, A. L.; Chareev, D. A.; Kvashnina, K. O.; Kovalchuk, E. V.; Vikentyev, I. V.; Tagirov, B. R. The state of platinum in pyrite studied by X-ray absorption spectroscopy of synthetic crystals. *Econ Geol* **2019**, *114* (8), 1649–1663.

(38) Filimonova, O. N.; Tagirov, B. R.; Trigub, A. L.; Nickolsky, M. S.; Rovezzi, M.; Belogub, E. V.; Reukov, V. L.; Vikentyev, I. V. The state of Au and As in pyrite studied by X-ray absorption spectroscopy of natural minerals and synthetic phases. *Ore Geol. Rev.* **2020**, *121*, 103475.

(39) Filimonova, O. N.; Trigub, A. L.; Nickolsky, M. S.; Chareev, D. A.; Kvashnina, K. O.; Kovalchuk, E. V.; Vikentyev, I. V.; Reukov, V. L.; Tagirov, B. R. The state of platinum in pyrrhotite: X-ray absorption spectroscopy study and implications for the role of Fe sulfides as platinum carriers. *Mineral. Mag.* **2021**, *85* (6), 846–861.

(40) Trigub, A. L.; Tagirov, B. R.; Kvashnina, K. O.; Chareev, D. A.; Nickolsky, M. S.; Shiryaev, A. A.; Baranova, N. N.; Kovalchuk, E. V.; Mokhov, A. V. X-ray spectroscopy study of the chemical state of “invisible” Au in synthetic minerals in the Fe-As-S system. *Am. Mineral.* **2017**, *102* (S), 1057–1065.

(41) Pokrovski, G. S.; Escoda, C.; Blanchard, M.; Testemale, D.; Hazemann, J. L.; Gouy, S.; Kokh, M. A.; Boiron, M. C.; de Parseval, F.; Aigouy, T.; Menjot, L.; et al. An arsenic-driven pump for invisible gold in hydrothermal systems. *Geochemical Perspectives Letters* **2021**, *17*, 39–44.

(42) Shannon, R. D. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. *Acta Crystallogr.* A **1976**, *32*, 751–767.