Tennantite-(Cu), Cu12As4S13, from Layo, Arequipa Department, Peru: a new addition to the tetrahedrite-group minerals

Abstract Tennantite-(Cu), Cu12As4S13, was approved as a new mineral species from the Layo epithermal deposit, Castilla Province, Arequipa Department, Peru, where it occurs as black metallic anhedral grains, up to 0.1 mm across, replacing enargite and associated with chalcopyrite and vinciennite. In reflected light, tennantite-(Cu) is isotropic, grey with a bluish shade. Reflectance data for the four COM wavelengths in air are [λ (nm): R (%)]: 470: 29.1; 546: 28.4; 589: 27.4; and 650: 25.0. Electron microprobe analysis for holotype material gave (in wt.% – average of 10 spot analyses): Cu 49.32(27), Fe 2.20(12), Zn 0.09(2), Sn 0.03(5), As 19.45(43), Sb 1.94(10), Te 0.02(5), S 27.75(43), total 100.80(20). On the basis of (As + Sb + Te) = 4 atoms per formula unit (apfu), the empirical formula of tennantite-(Cu) is (Cu11.27Fe0.57Zn0.02)Σ11.86(As3.77Sb0.23)Σ4.00S12.57. Tennantite-(Cu) is cubic, I$\overline 4$3m, with unit-cell parameters a = 10.1710(10) Å, V = 1052.2(2) Å3 and Z = 2. Its crystal structure was refined by single-crystal X-ray diffraction data to a final R1 = 0.0178 on the basis of 263 unique reflections with Fo > 4σ(Fo) and 24 refined parameters. Tennantite-(Cu) is isotypic with other tetrahedrite-group minerals. Previous findings of tennantite-(Cu) are reported and some nomenclature issues, related to the Fe and Cu oxidation states, are discussed. At the Layo epithermal deposit, tennantite-(Cu) is the result of the replacement of enargite under decreasing $f_{{\rm S}_ 2}$ conditions.


Introduction
Tetrahedrite-group minerals are characterised by the general structural formula M(2) A 6 M(1) (B 4 C 2 ) X(3) D 4 S(1) Y S(2) 12 Z, where the capital letters indicate several chemical constituents. Among the different species, the most commons belong to the tetrahedrite and tennantite series and are characterised by A and B = Cu + , D = Sb 3+ or As 3+ , and Y and Z = S 2-. Different C constituents, usually represented by divalent transition elements, identify the species belonging to these series (Biagioni et al., 2020).
Iron and Zn are the most common divalent elements occurring in tennantite-series minerals; moreover, since the publication of the nomenclature of tetrahedrite-group minerals (Biagioni et al., 2020), three other species belonging to the tennantite series have been approved by the International Mineralogical Association -Commission on New Minerals, Nomenclature and Classification (IMA-CNMNC), i.e. tennantite-(Hg) , tennantite-(Ni) (Wang et al., 2021), and tennantite-(Cd) (Biagioni et al., 2022). Other potential end-member compositions are known in the literature. Among them, compositions corresponding to ideal Cu 12 As 4 S 13 have been reported, for instance, from Canada, France and Peru (Johan and Le Bel, 1980;Thouvenin, 1983;Cesbron et al., 1985;Marcoux et al., 1994).
The re-examination of a specimen from the Peruvian epithermal deposit of Layo (Marcoux et al., 1994) allowed the description 79, Calci (PI), under catalogue number 19925, and in the collections of the Mineralogical Museum of Ecole des Mines de Paris (MINES ParisTech) under catalogue number ENSMP 83990.
In this paper the description of this new species belonging to the tetrahedrite group is reported and some crystal-chemical and nomenclature issues are discussed.

Occurrence and physical properties
Tennantite-(Cu), described initially as 'Cu-excess tennantite' by Marcoux et al. (1994), was found in the Layo epithermal deposit (15°11'16"S, 72°14'30"W), Castilla Province, Arequipa Department, Peru (Fig. 1). The Layo vein system is formed by anastomosing veins hosted in NNE-SSW-striking fractures in Miocene-Pliocene volcanic rocks belonging to the Tazaca Group. This rock sequence is formed by two distinct successions: the first one is a 300 m thick sequence of ignimbritic lava flows and pyroclastic tuffs, dacitic to rhyolitic in compositions (Pisaca Formation), whereas the second one, known as the Jullujia Formation, is composed by discordant andesitic lava flows and domes, as well as by pyroclastic rocks. In the Layo area, the volcanic rocks are affected by a pervasive propylitic alteration. Around ore vein systems, propylitised rocks occur as relicts within hydrothermally altered silicic-argillic zones. Marcoux et al. (1994) gave further details about the geological background of the Layo epithermal deposit.
Tennantite-(Cu) was collected in the eastern zone of the ore field, in the so-called Vetas 7 and 8. In these veins, a breccia, formed by silicified and alunitised angular rock clasts, is cemented by an ore assemblage composed of complex Cu-As-Fe-Sn sulfides, showing banded and cockade textures related to the rhythmic alternation of pyrite and Cu-As-Sn sulfides, i.e. enargite, tennantite-(Cu), chalcopyrite and vinciennite (Marcoux et al., 1994). The mineralogy is dominated by early pyrite succeeded by a late complex Cu-As-Sn association where tennantite-(Cu) is the prominent sulfide (Fig. 2). Tennantite-(Cu) is associated with enargite, chalcopyrite and vinciennite, that often form ameboid oriented patches up to 80 μm in size, sometimes developed from relicts of enargite. Secondary minerals also formed, such as bornite, covellite and digenite, at the expense of chalcopyrite, and scarce luzonite at the expense of enargite.
Tennantite-(Cu) is black in colour, with a black streak and metallic lustre. Mohs hardness was not measured, owing to the small size of the grain studied and the intimate association of other sulfides, but it should be close to 3½-4, in agreement with other members of the tetrahedrite group. Tennantite-(Cu) is brittle, with a conchoidal fracture and an indistinct cleavage. Due to the small size of the grains studied and their admixture with other sulfides, density was not measured; on the basis of the empirical formula and the single-crystal X-ray diffraction data, the calculated density is 4.656 g⋅cm -3 .
In reflected light, tennantite-(Cu) is isotropic. It is grey, with a bluish shade (Fig. 2). Internal reflections were not observed. Reflectance values measured in air on the holotype sample using a spectrophotometer MSP400 Tidas at Leica microscope, with a 50× objective, are given in Table 1 and shown in Fig. 3, where the reflectance curve for tennantite-(Cu) is compared with published data for other tetrahedrite-group minerals.

Chemical data
Quantitative chemical analyses were carried out using a Cameca SX 100 electron microprobe (National Museum of Prague, Czech Republic) and the following experimental conditions: wavelength dispersive spectroscopy mode, accelerating voltage = 25 kV and beam current = 20 nA, beam diameter = 1 μm. Standards (element, emission line) were: chalcopyrite (CuKα and SKα), pyrite (FeKα), ZnS (ZnKα), NiAs (AsLβ), Sn (SnLα), Sb 2 S 3 (SbLα) and PbTe (TeMα). The contents of other sought elements with Z > 8 (Ag, Au, Bi, Cd, Co, Ga, Ge, Hg, In, Mn, Cl, Ni, Pb, Se and Tl) were below detection limits. Matrix correction by the PAP procedure (Pouchou and Pichoir, 1985) was applied to the data. Electron back-scattered images showed that tennantite-(Cu) is slightly zoned, with a domain richer in Sb, 30-40 μm in size, located on the margin of the grain. Results are given in Table 2.

X-ray crystallography
Single-crystal X-ray diffraction intensity data were collected on an anhedral grain of tennantite-(Cu), 60 μm × 40 μm × 30 μm in size, using a Bruker Apex II diffractometer (50 kV and 30 mA) equipped with a Photon II CCD detector and graphitemonochromatised MoKα radiation (Dipartimento di Scienze della Terra, Università di Pisa, Italy). The detector-to-crystal distance was set at 50 mm. Data were collected using w scan mode in 0.5°slices, with an exposure time of 30 s per frame, and they were corrected for Lorentz, polarisation, absorption and background effects using the software package Apex3 (Bruker AXS Inc., 2016). The refined unit-cell parameters are a = 10.1710(10) Å, V = 1052.2(2) Å 3 ; and space group I43m. The crystal structure of tennantite-(Cu) was refined using Shelxl-2018 (Sheldrick, 2015) starting from the structural model of Johnson and Burnham (1985). The following neutral scattering curves, taken from the International Tables for Crystallography (Wilson, 1992) were used initially: Cu vs □ at M(2), Cu vs Fe at M(1), As vs Sb at X(3), S at S(1) and S(2) sites.
Several cycles of isotropic refinement converged to R 1 = 0.115, confirming the correctness of the structural model. The modelling of the racemic twin suggested that the structure had to be inverted. An anisotropic refinement for cations only converged to R 1 = 0.0280. The relatively large U eq value of the M(2) site suggested its split nature, in agreement with previous authors (Andreasen et al., 2008;Welch et al., 2018). After the addition of the split position, found in the difference-Fourier map, the R 1 value was lowered to 0.0210. The site occupancy factors (s.o.f.) at the two split positions M(2a) and M(2b) were constrained to be 1, as no Cu excess at the M(2) site was detected. After several cycles of anisotropic refinement for all the atoms, the R 1 converged to 0.0178 for 263 unique reflections with F o > 4σ(F o ) and 24 refined parameters. Details of the data collection and crystal structure refinement are reported in Table 3. Fractional atomic coordinates and equivalent isotropic displacement parameters are reported in Table 4, whereas Table 5 reports selected bond distances and Table 6 the weighted bondvalence sums (BVS) calculated according to the bond parameters of Brese and O'Keeffe (1991). The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).
Powder X-ray diffraction data were not collected, owing to the small size of the available grains and admixture with other phases (in particular, vinciennite). Table 7 reports the calculated X-ray powder diffraction pattern.

Chemical formula
As discussed in previous papers (e.g. Sejkora et al., 2021), there are different approaches to recalculate the chemical formulae of tetrahedrite-group minerals. The two better ones normalise the number of atoms on the basis of ΣMe = 16 atoms per formula unit (apfu) or on the basis of (As + Sb + Te + Bi) = 4 apfu. The former approach assumes that no vacancies occur at the M(2), M(1), and X(3) sites, whereas the latter is based mainly on the results discussed by Johnson et al. (1986) who revealed that negligible variations in the ideal number of X(3) atoms usually occur.
The tetrahedrally coordinated M(1) site is a mixed (Cu,Fe) site, with only minor amounts of Zn. The refined site scattering is 171.48 electrons per formula unit (epfu), to be compared with the value calculated from the site population proposed on the  The M(2) site is split into two sub-positions, M(2a) and M(2b), separated by 0.60 Å, whereas the distance between two neighbouring M(2b) positions is 1.20 Å. These distances are shorter than those observed in Cu-rich unsubstituted tennantite described by Makovicky et al. (2005), having M(2a)-M(2b) and M(2b)-M(2b) distances of 1.08 and 2.00 Å, respectively. The M(2a) position has a triangular planar coordination, whereas M(2b) has a flat trigonal pyramidal one. This feature agrees with previous studies (e.g. Andreasen et al., 2008;Welch et al., 2018). Average bond distances are 2.230 and 2.307 Å for M(2a) and M(2b) positions, respectively. Such values can be compared with those reported in Cu-pure and unsplit M(2) sites occurring in tetrahedrite and tennantite studied by Wuensch (1964) and Wuensch et al. (1966), respectively, i.e. 2.259 and 2.240 Å, respectively, as well as with those reported by Makovicky et al. (2005), i.e. 2.217 Å for Cu2A and 2.467 Å for Cu2B. The Cu2B of Makovicky et al. (2005) is also close to the As site, 2.41 Å, whereas the M(2b)-X(3) distance is 2.835 Å in the sample from Layo. In this latter sample, copper was the only cation at the M(2) site, in agreement with electron microprobe data. The larger size of the M(2b) position is probably due to its average nature. The splitting of the M(2) position is variable in the tetrahedrite-tennantite sub-group. In Fe-bearing compounds synthesised at 450°C (Fe between ∼ 0.3 and 2 apfu), Andreasen et al. (2008) only found a split (24g) site. A single-crystal synchrotron X-ray diffraction study of synthetic Cu 12 Sb 4 S 13 and Cu 12 As 4 S 13 was performed recently by Hathwar et al. (2019) from room temperature down to 70 K. Whereas in synthetic Cu 12 Sb 4 S 13 there is only a single (12e) site (but with a high atomic displacement perpendicular to the triangle) the use of high resolution data allowed the resolution of the M(2) site of synthetic Cu 12 As 4 S 13 into six (24g) sub-positions.
The X(3) site shows an average bond distance of 2.266 Å. Taking into account the electron microprobe data, the site occupancy (As 0.95 Sb 0.05 ) can be proposed. It corresponds to a mean atomic number of 33.90 electrons, to be compared with the refined mean atomic number of 34.03 electrons. Assuming idealised X-S distances of 2.26 and 2.45 Å for As 3+ and Sb 3+ , respectively (calculated according to the bond parameters of Brese and O'Keeffe, 1991), an average X(3)-S(1) distance of 2.270 Å can be expected. The BVS is 3.06 vu.
The S(1) site is four-fold coordinated and is bonded to two M(1), one M(2) [i.e. M(2a) or one of the two mutually-exclusive M(2b)] and one X(3). Its BVS is 2.04 vu. S(2) is octahedrally coordinated by atoms hosted at M(2) sites, with a BVS of 2.16 vu. Both S sites were found fully occupied.

Comparison between tennantite-(Cu) and previous findings of Cu-rich tennantite
Natural members of the tennantite series are usually characterised by the formula Cu 6 (Cu 4 Me 2 )As 4 S 13 , where Me is commonly Fe and Zn. However, synthetic Cu 12 As 4 S 13 has been synthesised, in some cases showing a Cu excess with respect to the ideal 12 apfu. For instance, Maske and Skinner (1971) studied the system Cu-As-S and found a compositional field Cu 12+x As 4+y S 13 , with 0 < x < 1.72 and 0 < y < 0.08. Unit-cell variation from 10.168 to 10.222 Å was reported for compositions ∼Cu 12 As 4 S 13 and ∼Cu 14 As 4 S 13 , respectively. Lind and Makovicky (1982) highlighted an analytical problem during electron microprobe analysis of synthetic tetrahedrite-group phases; indeed, those compositions having Cu > 12 apfu gave the same analytical results as those having 12 Cu apfu. This effect was noted for both Sband As-members of this sulfosalt group.
Other findings of tennantite-(Cu) were reported by Kouzmanov et al. (2004) from the Radka deposit, Bulgaria. The samples studied by these authors show Cu contents ranging from 10.88 to 11.26 apfu, Fe between 0.79 and 1.14 apfu, and Zn below the detection limit. The observed As/(As+Sb+Bi) atomic ratio is in the range 0.90-0.98. Ideally, samples from this locality correspond to the formula Cu 11 Fe(As 3.8 Sb 0.2 )S 13 . Catchpole et al. (2012) reported chemical data of tetrahedrite-group minerals from the Morococha base metal district, Peru. Chemical compositions vary between tetrahedrite-(Zn) in the Ag-Pb zone to tennantite-(Zn) in the Zn-Pb-Ag and Zn-Cu zones, and to tennantite-(Cu) in the Cu zone. Actually, in this last zone Cu varies between 11.10 and 11.60 apfu, and Fe is the second most abundant C constituent (ranging from 0.44 to 0.70 apfu), with low contents of Zn (from below the detection limit to 0.23 apfu). The As/(As+Sb+Te) atomic ratio ranges between 0.59 and 0.88. Repstock et al. (2015) described Cu-rich tennantite from the Pefka mine, Greece; at this locality, tennantite-(Cu) shows As/ (As+Sb+Te) atomic ratios ranging between 0.73 and 0.94 (also samples with Sb > As were observed) and Fe/(Fe+Zn+Hg) varying between 0.01 and 0.75, i.e. from nearly Fe-free samples, with Zn as the second most abundant C constituent, to Fe-rich phases. Velebil et al. (2021) described Zn-free tennantite samples from Julcani ore district, Peru, with 0.61-0.94 Fe apfu and Sb only up to 0.09 apfu which also correspond to tennantite-(Cu).
Finally, Voudouris et al. (2022) reported an interesting In-and Cu-rich tennantite from the Pefka mine, Greece. This occurrence deserves further discussion below.
Notwithstanding these previous occurrences of tennantite-(Cu), the first structural characterisation of a pure Cu-tennantite-series mineral was reported by Makovicky et al. (2005) using a sample from Cerro Atajo Cu-Au deposit, in the Province of Catamarca, Argentina. Its chemical formula, based on (As + Sb) = 4 apfu, is Cu 12.5 (As 3.92 Sb 0.08 )S 12.4 . Makovicky et al. (2005) found ∼10% excess of Cu, with s.o.f. at the split M(2a) and M(2b) sites of Cu 0.75(2) and Cu 0.17(1) , resulting in a site population at M(2) corresponding to M(2) Cu 6.54 . As discussed above, such a Cu excess was not found in the sample from Layo.   Flack (1983) Nomenclature issues in Cu-rich tennantite Type material of tennantite-(Cu) has a chemical composition close to Cu 11.4 Fe 0.6 (As 3.75 Sb 0.25 )S 13 = A Cu 6 [ B Cu 4.0 C (Cu 1.4 Fe 0.6 )] D (As 3.75 Sb 0.25 )S 13 . Following Biagioni et al. (2020), this chemistry can be idealised to the end-member formula Cu + 10 Cu 2+ 2 As 4 S 13 , assuming that formally divalent Cu 2+ is the most abundant C constituent. However, in agreement with the results of Mössbauer studies performed by Makovicky et al. (2003) on synthetic Fe-bearing tennantite, the most probable composition of the sample studied from Layo could be A Cu 6 [ B Cu 4.0 C(Cu 2+ 0.8 Cu + 0.6 Fe 3+ 0.6 )] D (As 3.75 Sb 0.25 )S 13 . Indeed, sample 2052 of Makovicky et al. (2003), with chemical formula Cu 11.24 Fe 0.57 As 3.93 S 13 (similar to that of the natural Peruvian specimen), showed Fe 3+ as the dominant iron species. If so, applying the site-total-charge approach (Bosi et al., 2019), the end-member formula Cu 6 (Cu + 5 Fe 3+ )As 4 S 13 = Cu 11 Fe 3+ As 4 S 13 is achieved. This result is in agreement with the hypothesis of Marcoux et al. (1994) about the presence of Fe 3+ in the specimen from Layo, due to the high f S 2 .
On this basis, composition of tennantite-(Cu) from the Layo epithermal deposit falls in the field of 'tennantite-(Fe 3+ )'. Nevertheless, studies of natural and synthetic samples of tetrahedrite-(Cu) and tennantite-(Cu) using various physical methods revealed a very complex crystal chemistry, not completely understood up to now. After initial examinations in the 1970s, the first detailed 57 Fe-Mössbauer studies were performed on Fe-bearing tetrahedrite in the 1990s (Charnock et al., 1989;Makovicky et al., 1990 and references herein) and completed by Nasonova et al. (2016) and Sobolev et al. (2017). Iron-bearing synthetic tennantite was studied by Makovicky et al. (2003). Though the first studies confirm major Fe 2+ towards the Fe pole, and major Fe 3+ towards the Cu pole, examination of tennantite indicates the presence of Fe 2+ down to 0.5 Fe apfu, as well as mixed valence Fe. Mixed valence iron seems to represent a substantial fraction of total iron at room T, owing to charge-transfer phenomena between Cu and Fe. For instance, at     (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.  (Lu and Morelli, 2013), which would correspond to partial replacement of Cu 2+ by Cu + and one ligand hole (i.e. mobile S electron). It thus appears that in Cu-rich tetrahedrite/tennantite one may have coexistence of Fe 3+ , Fe 2+ , Cu 2+ and Cu + (with ligand hole). The distinction between three species envisaged above on the basis of a simple ionic model is not pertinent, and it is more convenient, for nomenclature purposes, to consider only two species, tennantite-(Fe) and tennantite-(Cu). The sample from Layo can thus be classified as tennantite-(Cu), as Cu is more abundant than Fe as the C constituent.
A special case is represented by In-bearing tennantite-series minerals reported by Voudouris et al. (2022) from Pefka, Greece. These samples show up to 0.893 In apfu, with 11.049 Cu apfu, thus corresponding to the end-member Cu 6 (Cu 5 In 3+ ) As 4 S 13 . As no ambiguity in the oxidation state of In can be assumed, this phase should be regarded as different from tennantite-(Cu), and it may represent the new species 'tennantite-(In)'.

Conclusion
The description of tennantite-(Cu) adds further complexity to the tetrahedrite group, confirming on one side the structural plasticity of these chalcogenides, hosting several metals typical of hydrothermal settings, and on the other side their role in recording the crystallisation conditions of ore assemblages.
In addition to improving the knowledge of ore mineralogy, the description of this new phase gives further information about the crystal chemistry of tetrahedrite-group minerals, with possible technological implications, as revealed by several recent studies focusing on the thermoelectric properties of these compounds (e.g. Chetty et al., 2015;Levinsky et al., 2019). Acknowledgements. CB acknowledges financial support from the Ministero dell'Istruzione, dell'Università e della Ricerca through the project PRIN 2017 "TEOREMdeciphering geological processes using Terrestrial and Extraterrestrial ORE Minerals", prot. 2017AK8C32. The study was also financially supported by the Ministry of Culture of the Czech Republic (long-term project DKRVO 2019-2023/1.II.d; National Museum, 00023272) for JS and ZD. The comments of Peter Leverett and two anonymous reviewers improved the original manuscript.