Argentotennantite-(Fe), Ag₆(Cu₄Fe₂)As₄S₁₃, a new member of the tetrahedrite group from the San Genaro mine, Peru: occurrence and crystal structure

Argentotennantite-(Fe), Ag₆(Cu₄Fe₂)As₄S₁₃, has been approved as a new mineral species (IMA 2023-126) by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (CNMNC-IMA) using a sample of quartz gangue with argentotetrahedrite-(Zn), pyrargyrite, baryte, and siderite from the San Genaro mine, Castrovirreyna Province, Huancavelica, Peru. Argentotennantite-(Fe) occurs as anhedral grains up to 100 µm in size. More commonly, it forms micrometer-sized rims around crystals of argentotetrahedrite-(Zn) or replaces the latter around cavities and fissures, along with Ag-rich tennantite-(Fe). Argentotennantite-(Fe) is opaque and dark gray in color, with a metallic luster and black streak. In reflected light, it is isotropic and pale gray, with a brownish shade. The reflectance values for wavelengths recommended by the Commission on Ore Mineralogy of the IMA, measured in air, are as follows [λ (nm, R (%)]: 470, 29.9; 546, 30.1; 589, 29.8; and 650, 28.9. The chemical formula of the grain used for the crystal structure study, recalculated on the basis of ΣMe = 16 atoms per formula unit, is (Ag_3.36Cu_2.70)_Σ6.06(Cu_4.05Fe_1.88Zn_0.07)_Σ6.00(As_2.29Sb_1.66)_Σ3.95S_12.82. Argentotennantite-(Fe) is cubic, $I\stackrel{\mathrm{‾}}{\mathrm{4}}$3m, with a=10.4365(5) Å, V=1136.75(17) ų, and Z=2. Its crystal structure has been refined by single-crystal X-ray diffraction data to a final R₁=0.0384 on the basis of 246 unique reflections with F>4σ_F and 20 refined parameters. Argentotennantite-(Fe) is isotypic with other members of the tetrahedrite group. Structural relationships between argentotennantite-(Fe) and selected members of the tetrahedrite-group minerals are discussed, and previous findings relating to this species are briefly reviewed.

1 Introduction

Tetrahedrite-group minerals (TGMs) are the most common sulfosalts in many hydrothermal ore deposits. These chalcogenides form a complex isotypic series, with the general structural formula ^M(2)A${}_{\mathrm{6}}^{M\left(\mathrm{1}\right)}$(B₄C₂)^X(3)D${}_{\mathrm{4}}^{S\left(\mathrm{1}\right)}$Y${}_{\mathrm{12}}^{S\left(\mathrm{2}\right)}$Z, where A = Cu⁺, Ag⁺, □ (vacancy), or they can be represented by an (Ag₆)⁴⁺ cluster; B = Cu⁺ and Ag⁺; C = Zn²⁺, Fe²⁺, Hg²⁺, Cd²⁺, Ni²⁺, Mn²⁺, Cu²⁺, Cu⁺, In³⁺, and Fe³⁺; D = Sb³⁺, As³⁺, Bi³⁺, and Te⁴⁺; Y = S²⁻ and Se²⁻; and Z = S²⁻, Se²⁻, and □ (Biagioni et al., 2020). Thus, tetrahedrite-group minerals are characterized by different homovalent and heterovalent substitutions and represent an interesting link between mineralogy and ore geochemistry. This wide chemical variability, often observed across a deposit or individual veins, has been used by some authors to constrain ore-forming processes (e.g., Kovalenker and Bortnikov, 1985; Staude et al., 2010; Kemkin and Kemkina, 2013; Wei et al., 2021; Šoster et al., 2024; Pažout et al., 2026; Biagioni et al., 2026).

Even if TGMs are one of the main Ag carriers in ore deposits, due to their widespread occurrence, they also represent an environmental risk under weathering conditions, being the source of potentially toxic elements (e.g., Sb, As, Cu, Cd, Hg; Keith and Telliard, 1979). Recently, some authors studied the associations of supergene minerals derived from TGM alteration, giving some insights into the mobility of these toxic elements in different geological environments (e.g., Borčinová Radková et al., 2017; Keim et al., 2018; Majzlan et al., 2018; D'Orazio et al., 2024). Moreover, TGMs are also currently being actively studied for their high-tech, especially thermoelectric, properties (e.g., Chetty et al., 2015; Weller and Morelli, 2022; Rout et al., 2023; Sun et al., 2024).

The classification and nomenclature of the TGMs, in keeping with the current IMA-accepted rules, were published by Biagioni et al. (2020). Following this revision of the nomenclature of TGMs, a renewed interest in these common sulfosalts, occurring in different kinds of hydrothermal ore deposits, has allowed the identification of several new mineral species and the collection of high-quality crystal chemical data. Since the publication of this nomenclature, 28 new mineral species have been characterized (https://www.mindat.org/min-29338.html, last access: 16 May 2026), emphasizing the chemical variability of this isotypic group.

Silver-rich members with 3 to 6 Ag atoms per formula unit (apfu) (A-constituent, freibergite/arsenofreibergite series) have been known for a long time. Indeed, freibergite was first described from the Hab Acht Mine near Freiberg, Saxony (Germany), by von Weissenbach (1831) and was named by Kenngott (1853). Currently IMA-accepted mineral species belonging to the freibergite series are argentotetrahedrite-(Fe) (Welch et al., 2018), argentotetrahedrite-(Hg) (Wu et al., 2022), argentotetrahedrite-(Zn) (Sejkora et al., 2022), argentotetrahedrite-(Cd) (Mikuš et al., 2023), kenoargentotetrahedrite-(Fe) (the former freibergite – Welch et al., 2018; Biagioni et al., 2020), and kenoargentotetrahedrite-(Zn) (Qu et al., 2024). The arsenofreibergite series comprises fewer and rarer species, being currently represented only by argentotennantite-(Zn) (Spiridonov et al., 1986; Biagioni et al., 2020) and kenoargentotennantite-(Fe) (Biagioni et al., 2026). The occurrence of the new species argentotennantite-(Fe) in a sample from the San Genaro mine (Peru) was mentioned by Velebil et al. (2021) on the basis of electron microprobe data.

The new detailed study of the chemical composition of this sample of argentotennantite-(Fe) and the refinement of its crystal structure allowed the submission of a formal proposal to the IMA-CNMNC, and, subsequently, the mineral and its name (symbol Atnt-Fe) were approved (IMA 2023-126). Holotype material (polished section) of argentotennantite-(Fe) is deposited in the collections of the Department of Mineralogy and Petrology, National Museum in Prague, Cirkusová 1740, 19300 Praha 9, Czech Republic, under the catalogue number P1P 52/2023. The crystal used for the single-crystal X-ray diffraction study is kept in the mineralogical collection of the Museo di Storia Naturale of the Università di Pisa, Via Roma 79, Calci (PI), under catalogue number 20070.

2 Occurrence and physical properties

Argentotennantite-(Fe) was found in a sample from the central part of the San Genaro mine (GPS 13°11^′37^′′ S, 75°08^′57^′′ W), Castrovirreyna Province, Huancavelica, Peru. The San Genaro mine (Fig. 1) is located in the eastern part of the Castrovirreyna mining district, north of the Laguna Orcococha at elevations between 4750 and 5000 m a.s.l. This ore district was discovered in 1591 and had many periods of intermittent operation. There is no recent ore production at the San Genaro area, but the mine is scheduled to be reopened in future.

Figure 1 Central part of the San Genaro mine in 2005. Photo: Jaroslav Hyršl.

The San Genaro mine exploited hydrothermal veins rich in silver sulfosalts (with lengths of up to 2.5 km and thicknesses of about 1 m), which struck more or less E–W within the late Miocene (11–12 Ma) andesite to rhyolite rocks. Hydrothermal mineralization is accompanied by argillic and propylitic alteration. The main ore minerals are galena, sphalerite, enargite, acanthite, polybasite- and tetrahedrite-group minerals, chalcopyrite, and pyrargyrite; the gangue minerals are quartz and pyrite, with minor baryte, calcite, rhodochrosite, siderite, and hematite. The geological background of the San Genaro mine has been described in Lewis (1964), Crowley et al. (1997), Wise (2005), and Hyršl et al. (2010). It is the type locality of two other sulfosalt species: baumstarkite (Effenberger et al., 2002) and sangenaroite (Topa et al., 2019).

Argentotennantite-(Fe) has been identified as part of a specimen (Fig. 2) represented by quartz gangue with abundant dark-red pyrargyrite (up to 3 mm) and dark-gray TGM multi-faced crystals (predominantly argentotetrahedrite-(Zn) with rims of argentotennantite-(Fe) to Ag-rich tennantite-(Fe)) up to 2 mm in size, with minor galena, acanthite, and light-pink siderite and white tabular baryte crystals. Argentotennantite-(Fe) occurs as anhedral grains up to 100 µm in size (Fig. 3), but more commonly it forms micrometer-sized rims around crystals of argentotetrahedrite-(Zn) (Fig. 4a) or replaces the latter around cavities (Fig. 4b) and fissures (Fig. 4c), along with Ag-rich tennantite-(Fe).

Figure 2 Abundant dark-red pyrargyrite and dark-gray TGM crystals, formed by predominant argentotetrahedrite-(Zn) with rims of argentotennantite-(Fe) to Ag-rich tennantite-(Fe). Associated minerals are also minor galena and acanthite on quartz gangue with light-pink siderite crystals and large white tabular baryte crystals in the bottom part of the sample; San Genaro mine, Peru. Sample size: 4.3 cm. Collection of Jaroslav Hyršl, Prague (no. 4.3-17). Photo: Jaroslav Hyršl.

Figure 3 Reflected light (a) and backscattered electron (b) images of the holotype sample. In (b), argentotennantite-(Fe) corresponds to the red circles, whereas blue circles correspond to Ag-rich tennantite-(Fe). Light gray is argentotetrahedrite-(Zn) (Attr-Zn), whereas white is pyrargyrite (Pyg). The grain used from the single-crystal X-ray diffraction study was extracted from the red box.

Figure 4 Backscattered electron images of the rims of argentotennantite-(Fe) to Ag-rich tennantite-(Fe) (dark gray) (a) around crystals of argentotetrahedrite-(Zn), (b) around the surface and cavities of argentotetrahedrite-(Zn), and (c) around fissures in argentotetrahedrite-(Zn). Argentotetrahedrite-(Zn) (light gray) usually contains tiny inclusions of pyrargyrite (white).

It is opaque and dark gray in color, with a black streak. The luster is metallic. The Mohs hardness may be close to 4, in agreement with other members of the tetrahedrite group. Argentotennantite-(Fe) is brittle, with an indistinct cleavage and a conchoidal fracture. Density was not measured, owing to the small amount of available material; on the basis of the empirical formula and the single-crystal unit-cell parameters, the calculated density is 4.922 g cm⁻³. In reflected light, argentotennantite-(Fe) is isotropic. It is pale gray, with a brownish shade. Internal reflections were not observed. Reflectance values, measured in air using a spectrophotometer MSP400 Tidas at Leica microscope, with a 50× objective, are given in Table 1 and shown in Fig. 5, where they are compared with published data for kenoargentotennantite-(Fe) and tennantite-(Fe).

Table 1 Reflectance data for argentotennantite-(Fe)^∗.

^∗ The reference wavelengths required by the Commission on Ore Mineralogy (COM) are given in bold.

Figure 5 Comparison of the reflectance curves of Fe-dominant members of the tennantite and arsenofreibergite series; argentotennantite-(Fe) from the San Genaro mine, Peru (this paper); kenoargentotennantite-(Fe) from the Pollone mine, Valdicastello Carducci, Italy (Biagioni et al., 2026); tennantite-(Fe), Wheal Jewel, Gwennap, Cornwall, UK (Criddle and Stanley, 1993).

3 Experimental

3.1 Chemical analyses

Quantitative chemical analyses were carried out using a Cameca SX100 electron microprobe at the National Museum of Prague (Czech Republic). Experimental conditions were as follows: WDS mode, accelerating voltage of 25 kV, beam current of 20 nA, and beam diameter of 0.7 µm. Standards (element, emission line) were as follows: Ag metal (AgLα), Bi₂Se₃ (BiMβ), CdTe (CdLα), chalcopyrite (CuKα, SKα), pyrite (FeKα), PbS (PbMα), NiAs (AsLβ), Sb₂S₃ (SbLα), Tl(Br,I) (TlLα), and ZnS (ZnKα). The peak counting times were 20 s for all elements, and 10 s for each background. Other elements (i.e., Au, Cl, Co, Ga, Ge, Hg, In, Mn, Ni, Sn, Se, and Te) were found to be below the detection limits (0.02 wt %–0.10 wt %). Matrix correction by PAP procedure (Pouchou and Pichoir, 1985) was applied to the data. As reported in other papers (e.g., Biagioni et al., 2020; Sejkora et al., 2021), there are several different approaches to calculate the chemical formula of the TGMs. Due to possible vacancies at the S(2) site in the Ag-rich members, the best approaches are as follows: (1) normalization on the basis of ΣMe = 16 apfu, assuming that no vacancies occur at M(2), M(1), and X(3), or (2) normalization on the basis of (As + Sb) = 4 apfu, taking into account that previous studies (e.g., Johnson et al., 1986) revealed that negligible variations with respect to the ideal number of X(3) atoms occur. The results of our study indicate that only very minor vacancies, if any, possibly occur at M(2), M(1), and X(3) sites, and, therefore, the first approach was selected for calculation. Chemical data for the grain of argentotennantite-(Fe) used for the single-crystal X-ray diffraction study are given in Table 2, whereas Table S1 in the Supplement reports the full dataset (386 spot analyses in total) of the chemical analyses of the described TGMs.

Table 2 Chemical data for argentotennantite-(Fe) (holotype sample – grain used for single crystal study).

Mean and range of five spot analyses; e.s.d. denotes estimated standard deviation.

3.2 X-ray crystallography

A single-crystal X-ray diffraction study of argentotennantite-(Fe) was performed on a small grain ($\mathrm{0.035}\times \mathrm{0.030}\times \mathrm{0.025}$ mm) extracted from the rim of an aggregate of pyrargyrite and argentotetrahedrite-(Zn) (Fig. 3). Data collection was performed using a Bruker D8 Venture four-circle diffractometer equipped with an air-cooled Photon III detector and microfocus MoKα radiation (Centro per l'Integrazione della Strumentazione scientifica dell'Università di Pisa, University of Pisa, Italy). The detector-to-crystal distance was set to 38 mm. Data were collected using φ scan modes, in 0.5° slices, with an exposure time of 20 s per frame. A total of 216 frames were collected, and they were integrated with the Bruker SAINT software package using a narrow-frame algorithm. Data were corrected for Lorentz polarization, absorption, and background. Unit-cell parameters were refined on the basis of the XYZ centroids of 498 reflections above 20 σI with $\mathrm{5.52}<\mathrm{2}\mathit{\theta }<\mathrm{56.30}$° as a=10.4365(5) Å, and V=1136.75(17) ų, with space group $I\stackrel{\mathrm{‾}}{\mathrm{4}}$3m.

The crystal structure of argentotennantite-(Fe) was refined using Shelxl-2018 (Sheldrick, 2015) starting from the atomic coordinates of Johnson and Burnham (1985). The occurrence of the racemic twin was modeled, and the structure had to be inverted. The following neutral scattering curves, taken from the International Tables for Crystallography (Wilson, 1992), were used: Cu vs. Ag at the M(2) site, Cu vs. Fe at M(1), As vs. Sb at X(3), and S at the S(1) and S(2) sites. The scattering factors of Cu (Z = 29) and Fe (Z = 26) are similar, and, consequently, the site occupancy at M(1) was fixed on the basis of electron microprobe data, idealizing the occupancy at (Cu_0.67Fe_0.33). After several cycles of isotropic refinement, the R₁ converged to 0.0842, confirming the correctness of the structural model. The anisotropic model for both cations and anions converged to R₁=0.0384 for 246 reflections with F>4σ_F and 20 refined parameters. The details of data collection and refinement are given in Table 3. Fractional atom coordinates and equivalent isotropic displacement parameters are reported in Table 4. Table 5 reports selected bond distances, whereas weighted bond-valence sums, calculated according to Brese and O'Keeffe (1991), are shown in Table 6. The crystallographic information files (CIFs) are deposited in the Supplement of this paper.

Powder X-ray diffraction data could not be collected due to the paucity of available material. Consequently, powder X-ray diffraction data, given in Table 7, were calculated through the software PowderCell 2.3 (Kraus and Nolze, 1996) using the structural model of argentotennantite-(Fe) discussed below.

Table 3 Crystal and experimental data for argentotennantite-(Fe).

^a $w=\mathrm{1}/\left[{\mathit{\sigma }}^{\mathrm{2}}\right(F_o^{\mathrm{2}})+$(0.0001P)²+31.5623P]. ^b Flack (1983).

Table 4 Site, site occupancy (s.o.), fractional atom coordinates, and equivalent isotropic displacement parameters (Ų) for argentotennantite-(Fe).

Table 5 Selected bond distances (in Å) for argentotennantite-(Fe).

Table 6 Weighted bond-valence sums (in valence units) in argentotennantite-(Fe).

Table 7 Calculated X-ray powder diffraction data for argentotennantite-(Fe). Intensity and d_hkl were calculated using the software PowderCell 2.4 (Kraus and Nolze, 1996) on the basis of the structural model given in Table 4. Only reflections with I_calc>1 are listed. The five strongest reflections are given in bold.

4 Results and discussion

4.1 Chemical data for argentotennantite-(Fe) and associated tetrahedrite-group minerals

Chemical data for the holotype grain of argentotennantite-(Fe) used for the single-crystal X-ray diffraction study are given in Table 2 and correspond to the following formula: Ag_3.36(9)Cu_6.75(9)Fe_1.88(2)Zn_0.07(1)As_2.29(23)Sb_1.66(22)S_12.82(5). Taking into account the results of the crystal structure refinement (see below), the crystal chemical formula can be written as ^M(2)(Ag_3.36Cu_2.70)_Σ6.06 ^M(1)(Cu_4.05Fe_1.88Zn_0.07)_Σ6.00 ^X(3)(As_2.29Sb${}_{\mathrm{1.66}}{)}_{\mathrm{\Sigma }\mathrm{3.95}}^{\mathrm{S}\left(\mathrm{1}\right)+\mathrm{S}\left(\mathrm{2}\right)}$S_12.82. The end-member formula of argentotennantite-(Fe) is Ag₆(Cu₄Fe₂)As₄S₁₃ (Z=2), corresponding to (in wt %) Cu 14.70, Ag 37.42, Fe 6.46, As 17.33, S 24.10, and total 100.00.

The chemical compositions for other argentotennantite-(Fe) grains and for associated Ag-rich tennantite-(Fe) and argentotetrahedrite-(Zn) are given in Table S1. Late-stage rims of TGM crystals (Figs. 3 and 4) are represented by zoned argentotennantite-(Fe) to Ag-rich tennantite-(Fe), with a negative correlation between Ag and As (Fig. 6b) and a less distinct negative correlation of Ag and S contents (Fig. 6c). All compositions belong to Fe- and As-dominant members with S contents >12.5 apfu (Fig. 6a–c). The silver contents of argentotennantite-(Fe) are found to lie in the range of 3.00–3.68 apfu; the mineral is Fe-dominant (1.64–1.93 apfu), with only minor Zn (up to 0.25 apfu) and traces of Cu²⁺, Pb, and Cd of up to 0.08, 0.04, and 0.01 apfu, respectively. Arsenic (1.98–3.25 apfu) is partly substituted by Sb (0.69–1.97 apfu), and S contents were found to be in the range 12.54–13.07 apfu. For Ag-rich tennantite-(Fe), Ag contents are in the range of 2.23–2.99 apfu; it is Fe-dominant (1.30–1.95 apfu), with minor Zn (up to 0.60 apfu) and, locally, traces of Pb and Cd of up to 0.04 and 0.01 apfu, respectively. The calculated Cu²⁺ contents occasionally reach up to 0.27 apfu but are usually only around 0.05 apfu. The dominant As (2.66–3.82 apfu) is accompanied by Sb (0.11–1.22 apfu), and S contents were determined to be in the range 12.61–13.18 apfu.

The relatively uniform chemical composition of early-stage TGM crystals (Figs. 3 and 4) is significantly different (Fig. 6) and corresponds to argentotetrahedrite-(Zn) (Sejkora et al., 2022), with 3.21–3.29 apfu Ag as the A-chemical constituent. The dominant Me²⁺ element is Zn (1.46–1.92 apfu), accompanied by minor Fe (0.02–0.43 apfu) and Cd (0.02–0.05 apfu); calculated Cu²⁺ contents are low (up to 0.09 apfu). The extent of AsSb₋₁ substitution is limited to 0.60 apfu As, and contents of S (12.76–13.16 apfu) are close to the ideal value of 13 apfu. The average formula (28 spot analyses) can be expressed as (Ag_3.25Cu_2.73)_Σ5.98[Cu_4.00(Zn_1.73Fe_0.19Cu_0.04Cd_0.04)_Σ2.00]_Σ6.00
(Sb_3.76As_0.26)_Σ4.02S_13.02.

Figure 6 Chemical relationships (in apfu) between Ag vs. Fe $/$ (Fe + Zn + Pb + Cd) (a), Ag vs. As $/$ (As + Sb) (b), Zn vs. Fe (c), and Ag vs. S (d) in studied tetrahedrite-group minerals from San Genaro. STR – data of fragment used for the crystal structure study.

4.2 Crystal structure description

The crystal structure of argentotennantite-(Fe) agrees with the general features of the members of the tetrahedrite isotypic group, i.e., a collapsed sodalite-like framework formed by corner-sharing M(1)S(1)₄ tetrahedra with cages hosting S(2)-centered M(2)-octahedra, surrounded by four X(3)S(1)₃ trigonal pyramids (e.g., Johnson et al., 1988).

The M(2) site shows a planar triangular coordination, with an average 〈M(2)–S〉 distance of 2.342 Å. The refinement of its site occupancy points to Ag_0.53(4)Cu_0.47(4), corresponding to a mean atomic number (MAN) of 38.54 electrons, to be compared with the value obtained during electron microprobe analyses, i.e., ∼39 electrons (38.99 normalizing the sum of Ag and Cu to one atom at the M(2) site). In other isotypic Ag members of the tetrahedrite group, e.g., argentotetrahedrite-(Zn) (Sejkora et al., 2022) and zvĕstovite-(Zn) (Sejkora et al., 2021), the M(2) site was found to be split. It is very probable that, in argentotennantite-(Fe), such a splitting also occurs, as testified to by the high U_eq value observed at this site, i.e., 0.076(2) Ų, as well as the high residuals around the M(2) site and the M(2)–S distances, physically too short when the site is occupied by Ag. A split model suggests the preferential occupancy of Cu at M(2a) and Ag at M(2b), as supported by observed average bond distances. However, some correlations higher than 0.8 were found during the refinement of the split model, and consequently an unsplit model is presented. The bond-valence sum is 1.36 valence units (v.u.).

The tetrahedrally coordinated M(1) site has an average 〈M(1)–S〉 bond distance of 2.329 Å. Electron microprobe analysis agrees with an ideal (Cu_0.67Fe_0.33) site occupancy, with a minor replacement of these elements by Zn. On the basis of this proposed site occupancy, the bond-valence sum at the M(1) site is 1.60 $v.u.$, larger than the theoretical value of 1.33 v.u.

The X(3) site is a mixed (As, Sb) position, with an average 〈X(3)–S〉 distance of 2.340 Å. Considering the ideal As–S and Sb–S distances of 2.26 and 2.45 Å, respectively, calculated on the basis of the bond parameters of Brese and O'Keeffe (1991), the observed average distance should correspond to the site occupancy (As_0.58Sb_0.42), to be compared with the results of crystal structure refinement (As_0.53Sb_0.47) and electron microprobe analysis (As_0.58Sb_0.42). The bond-valence sum, calculated on the basis of the refined site occupancy, is 3.18 v.u.

The S(1) and S(2) sites are fully occupied by S. The S(2)-centered octahedron has a volume of ∼15 ų, in accord with an S(2)M(2)₆-occupied octahedron (e.g., Welch et al., 2018; Sejkora et al., 2022; Biagioni et al., 2026). The bond-valence sums of S atoms at S(1) and S(2) are 2.25 and 3.48 $v.u.$, respectively. Both S atoms are overbonded, mainly due to the too-short Ag–S distances. This is particularly true for the S atom hosted at the S(2) position, resulting in an exceptionally high overbonding. This feature has been observed in all of the other known Ag members of the tetrahedrite group, e.g., in argentotetrahedrite-(Zn) (bond-valence sums ranging between 3.00 and 3.30 $v.u.$ for the three refined samples), zvĕstovite-(Zn) (3.24 $v.u.$), and zvĕstovite-(Fe) (2.76 $v.u.$) (Sejkora et al., 2021, 2022; Biagioni et al., 2024). This is due to the short M(2)–S(2) bond distance, definitely shorter than the ideal 〈^[III]Ag–S〉 distance, i.e., 2.56 Å. The physically unrealistic distances may be due to some disorder affecting both the S(2) position (e.g., Sejkora et al., 2022) and the M(2) site. The former shows a high U_eq value (0.12 Å). A high value of U_eq could be related to a positional disorder or to an incorrect refinement of the site occupancy at S(2). As no significant vacancies are suggested by the chemical analyses, by the refinement of its site occupancy (pointing to a full occupancy), or by the volume of the S(2)-centered octahedron, the former hypothesis is more probable. This could be in accord with Peterson and Miller (1986), who proposed that, in order to achieve longer Ag–S distances, S(2) should be displaced from the 2a position. Indeed, as in argentotetrahedrite-(Fe), in argentotennantite-(Fe), a residual at the 8c (x, x, x) position, with x=0.0448, was also observed in the difference-Fourier map. Moreover, as discussed above, the U_eq value at the M(2) site is high and also suggests a splitting of this position into at least two sub-sites able to achieve physically sound bond distances (e.g., Biagioni et al., 2024).

The structural formula of argentotennantite-(Fe), based on electron microprobe data and crystal structure refinement, could be written as ^M(2)(Ag_3.18Cu${}_{\mathrm{2.82}}{)}_{\mathrm{\Sigma }\mathrm{6.00}}^{M\left(\mathrm{1}\right)}$(Cu_4.00Fe${}_{\mathrm{2.00}}{)}_{\mathrm{\Sigma }\mathrm{6.00}}^{X\left(\mathrm{3}\right)}$(As_2.12
Sb${}_{\mathrm{1.88}}{)}_{\mathrm{\Sigma }\mathrm{4.00}}^{\mathrm{S}\left(\mathrm{1}\right)+\mathrm{S}\left(\mathrm{2}\right)}$S₁₃ (Z=2).

Johnson et al. (1987) proposed the following relation between the unit-cell parameter a and the chemistry of the studied sample: a (Å) = 10.379 + 0.082(Ag) − 0.01(Ag²) − 0.009(Cu*) + 0.066(Hg) − 0.038(As) + 0.144(Bi). On the basis of the formula derived from the structural analysis, the calculated unit-cell parameter is a=10.46 Å, which aligns with the observed value of 10.44 Å.

4.3 Crystal chemical features of arsenofreibergite-series minerals and the role of Sb in their stability

The name “arsenofreibergite” was introduced by Biagioni et al. (2020) as a series name to indicate the As analogues of the freibergite-series minerals. At that time, only argentotennantite-(Zn) from the Kvartsitoviye Gorki deposit, Kazahstan, was formally approved (Spiridonov et al., 1986), whereas Števko et al. (2018) had reported chemical data corresponding to argentotennantite-(Fe). Both minerals are characterized by the ideally fully occupied S(2) site, but no crystallographic data were available. Only recently were the first crystal chemical data on members of the arsenofreibergite series made available due to the description of kenoargentotennantite-(Fe) (Biagioni et al., 2026) and due to the present work, where argentotennantite-(Fe) is described. These data allow some comparison among these species (Table 8).

Table 8 Comparison of members of the arsenofreibergite series.

Me${}^{\mathrm{2}+}=$ Zn + Fe + Cu${}^{\mathrm{2}+}+$Hg. NA denotes not available. ^a Bond distance for M(2a); ^b bond distance for M(2b); [1] this paper; [2] Spiridonov et al. (1986); [3] Biagioni et al. (2026).

According to Spiridonov et al. (1986), the type material of argentotennantite-(Zn) has the chemical formula (Ag_5.67Cu_0.33)_Σ6.00(Cu_4.15Zn_1.52Fe_0.37Pb_0.01Cd_0.01)_Σ6.06
(As_2.14Sb_1.89)_Σ4.03S_12.90, to be compared with its Fe analogue, whose chemical formula, as reported above, is (Ag_3.36Cu_2.70)_Σ6.06(Cu_4.05Fe_1.88Zn_0.07)_Σ6.00(As_2.29
Sb_1.66)_Σ3.95S_12.82. The former has a higher Ag content, reflected in a larger unit-cell parameter, i.e., a=10.58 Å, expanded with respect to that observed in argentotennantite-(Fe), a=10.44 Å. In both species, a significant amount of Sb occurs, with As $/$ (As + Sb) atomic ratios of 0.531 and 0.579 for the Zn and Fe isotypes, respectively. A non-negligible content of Sb also occurs in the keno-counterpart of argentotennantite-(Fe), i.e., kenoargentotennantite-(Fe), whose empirical formula has been reported by Biagioni et al. (2026) as (Ag_4.41Cu_1.59)_Σ6.00(Cu_4.02Fe_1.22Zn_0.39Hg_0.37)_Σ6.00(As_2.80
Sb_1.19)_Σ3.99S_12.12□_0.88, with an As $/$ (As + Sb) atomic ratio of 0.702. The presence of a significant amount of Sb in kenoargentotennantite-(Fe) has been interpreted considering the fact that, following Barton and Skinner (1978), the stability field of As₂S₃ is at a higher f(S₂) than the Ag₂S–Ag equilibrium, and, consequently, As³⁺ could not coexist with Ag metal; on the contrary, below 300 °C, the equilibrium Sb₂S₃–Sb is below the Ag₂S–Ag equilibrium, suggesting that minor Sb seems necessary to stabilize kenoargentotennantite-(Fe). This hypothesis cannot be applied to argentotennantite-(Fe), where no hints of Ag–Ag bonding were observed. It is indeed possible that the occurrence of Sb is also necessary for steric reasons.

This is supported by some other natural occurrences of (Ag, As)-rich TGMs. For instance, Ixer and Stanley (1983) reported the chemistry of a grain of TGMs corresponding, on the basis of ΣMe = 16 apfu, to the composition (with rounding errors) Ag_6.02(Ag_1.91Cu_1.85Zn_1.44Fe_0.48Cd_0.16Pb_0.16)_Σ6.00(As_2.15
Sb_1.87)_Σ4.02S_12.12 [As $/$ (As + Sb) = 0.535], whereas Zakrzewski (1989) reported two chemical analyses corresponding to Fe- and Zn-rich members of the arsenofreibergite series with As $/$ (As + Sb) atomic ratios of 0.500 and 0.502, respectively. In the zvĕstovite series, the As $/$ (As + Sb) ratio is 0.604 in zvĕstovite-(Zn) (Sejkora et al., 2021) and 0.896 in zvĕstovite-(Fe) (Biagioni et al., 2024).

As discussed by Johnson et al. (1988), the chemical composition of TGMs affects the framework rotation and the percent of distortion from ideality. In particular, the two most important parameters for the stability of TGMs are the size of the “spinner blades” (in the sense of Wuensch, 1964 – see Fig. 4 in his paper) and that of the X(3) cations. The replacement of Cu by Ag at the M(2) site increases both the rotation angle of the framework and its distortion. Figures 8 and 9 in Johnson et al. (1988) show that TGMs with M(2) occupied by Ag and X(3) hosting As show high values of both of these parameters, suggesting their instability owing to high structural strain. However, one can see that substituting As with Sb helps in reducing such a strain, thus allowing the existence of (Ag, As)-bearing TGMs. This is in line with Fig. 6b, which shows a negative correlation between the Ag content and the As $/$ (As + Sb) ratio.

For this reason, even if the end-member formula of argentotennantite-(Fe) is defined to be Ag₆(Cu₄Fe₂)As₄S₁₃, its actual formula would be more correctly represented by the formula Ag₆(Cu₄Fe₂)(As,Sb)₄S₁₃.

4.4 Previous findings of argentotennantite-(Fe): a brief review

Tetrahedrite-group minerals with chemical compositions corresponding to that of argentotennantite-(Fe) were previously reported. To the best of our knowledge, the first chemical data corresponding to argentotennantite-(Fe) were reported by Števko et al. (2018) for part of the zoned TGM rims of argentotetrahedrite-(Zn) grains (Sejkora et al., 2022) from the Kremnica Au–Ag epithermal deposit, Slovakia. After recalculation of published chemical analyses (Števko et al., 2018) on the basis of ΣMe = 16 apfu, 11 spot analyses corresponded to argentotennantite-(Fe) with 3.02–3.77 apfu Ag, 1.31–1.98 apfu Fe, and S contents in the range of 12.54–12.99 apfu. Other parts of these rims are formed by kenoargentotennantite-(Fe), with 3.24–4.50 apfu Ag and 11.70–12.49 apfu S (13 spot analyses), or Ag-rich tennantite-(Fe), with 0.67–2.99 apfu Ag (28 spot analyses). Later, Kolitsch et al. (2019) gave, for the Ag- and As-richest portions of zoned grains (10 µm in size) hosted in fluorite from the Grube Clara, Germany, the empirical formula (Ag_3.36Cu_2.64)Cu₄(Fe_1.61Zn_0.39)(As,Sb)₄S₁₃. Ondruš et al. (2003) published, for “argentotennantite” from Jáchymov, Czech Republic (grains up to 25 µm), contents of S in the range of 12.46–12.69 apfu, but after recalculation on the basis of ΣMe = 16 apfu, S contents were only 12.06–12.44 apfu, and so this phase may correspond to kenoargentotennantite-(Fe), with 3.17–4.29 apfu Ag and 1.52–1.84 apfu Fe.

5 Conclusions

Johnson et al. (1986) suggested the antipathetic relationship between Ag and As in TGMs, supported by some evidence regarding natural specimens collected between the 1970s and the 1980s (e.g., Charlat and Lévy, 1974; Wu and Petersen, 1977; Johnson and Burnham, 1985). However, other authors questioned this issue, supporting the existence of (Ag, As)-rich TGMs whose origin may be connected to P–T–X conditions (e.g., Foit and Ulbricht, 2001). As a matter of fact, up to recent times, no structural data on members of the arsenofreibergite series were available as the crystal structure of argentotennantite-(Zn) described by Spiridonov et al. (1986) was not refined.

The recent advancement in the knowledge of the crystal chemistry of TGMs allowed the collection of structural data on As analogues of the freibergite series, with the description of the new species kenoargentotennantite-(Fe) (Biagioni et al., 2026) and argentotennantite-(Fe). These species are further evidence of the plasticity of the crystal structure of TGMs, able to record the subtle variations of the ore geochemistry during the crystallization of these widespread sulfosalt minerals.