Tetrahedrite-(Hg), a new ‘old’ member of the tetrahedrite group

Abstract Tetrahedrite-(Hg), Cu6(Cu4Hg2)Sb4S13, has been approved as a new mineral species using samples from Buca della Vena mine (hereafter BdV), Italy, Jedová hora (Jh), Czech Republic and Rožňava (R), Slovakia. It occurs as anhedral grains or as tetrahedral crystals, black in colour, with metallic lustre. At BdV it is associated with cinnabar and chalcostibite in dolomite veins. At Jh, tetrahedrite-(Hg) is associated with baryte and chalcopyrite in quartz–siderite–dolomite veins; at R it is associated with quartz in siderite–quartz veins. Tetrahedrite-(Hg) is isotropic, greyish-white in colour, with creamy tints. Minimum and maximum reflectance data for Commission on Ore Mineralogy wavelengths in air (BdV sample), R in %) are 32.5 at 420 nm; 32.9 at 546 nm; 33.2 at 589 nm; and 30.9 at 650 nm. Chemical formulae of the samples studied, recalculated on the basis of 4 (As + Sb + Bi) atoms per formula unit, are: (Cu9.44Ag0.07)Σ9.51(Hg1.64Zn0.36Fe0.06)Σ2.06Sb4(S12.69Se0.01)Σ12.70 (BdV), Cu9.69(Hg1.75Fe0.25Zn0.06)Σ2.06(Sb3.94As0.06)S12.87 (Jh) and (Cu9.76Ag0.04) Σ9.80(Hg1.83Fe0.15Zn0.10)Σ2.08(Sb3.17As0.58Bi0.25)S13.01 (R). Tetrahedrite-(Hg) is cubic, I$\overline 4 $3m, with a = 10.5057(8) Å, V = 1159.5(3) Å3 and Z = 2 (BdV). Unit-cell parameters for the other two samples are a = 10.4939(1) Å and V = 1155.61(5) Å3 (Jh) and a = 10.4725(1) Å and V = 1148.55(6) Å3 (R). The crystal structure of tetrahedrite-(Hg) has been refined by single-crystal X-ray diffraction data to a final R1 = 0.019 on the basis of 335 reflections with Fo > 4σ(Fo) and 20 refined parameters. Tetrahedrite-(Hg) is isotypic with other members of the tetrahedrite group. Mercury is hosted at the tetrahedrally coordinated M(1) site, along with minor Zn and Fe. The occurrence of Hg at this position agrees both with the relatively large M(1)–S(1) bond distance (2.393 Å) and the refined site scattering. Previous occurrences of Hg-rich tetrahedrite and tetrahedrite-(Hg) are reviewed, and its relations with other Hg sulfosalts are discussed.


Introduction
Tetrahedrite-group minerals are the most common sulfosalts in different kinds of hydrothermal ore deposits. They form a complex isotypic series, with the formula M(2) A 6 M(1) (B 4 C 2 ) X(3) D 4 S(1) Y 12 S(2) Z, characterised by homo-and heterovalent substitutions, representing an interesting link between mineralogy and ore geochemistry (Biagioni et al., 2020).
Within the tetrahedrite group, five different series have been identified: tetrahedrite, tennantite, freibergite, hakite and giraudite series. Other unassigned members (e.g. rozhdestvenskayaite and goldfieldite) belong to this group. Biagioni et al. (2020), on the basis of literature data, defined thirty-two potential endmember compositions within the tetrahedrite group; among them eleven can be considered as valid species, whereas the remaining phases have to be approved officially by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA-CNMNC).
The most common mineral species within the tetrahedrite group belong to the tetrahedrite and tennantite series, charac-IMA-CNMNC, in order to give an official certificate of birth to tetrahedrite-(Hg). The mineral and its name have been approved (IMA2019-003, Biagioni et al., 2019). Holotype material is deposited in the mineralogical collections of the Museo di Storia Naturale, Università di Pisa, Via Roma 79, Calci (Pisa), Italy, catalogue number 19895. Both cotype samples are deposited in the mineralogical collection of the Department of Mineralogy and Petrology, National Museum, Prague, Czech Republic, catalogue numbers: P1N 9961 (Jedová hora) and P1N 33538 (Rožňava).
In this paper, the description of tetrahedrite-(Hg) is given, along with a brief history of previous studies on this mineral species.

Occurrence and physical properties
Tetrahedrite-(Hg) was found in the pyrite ± baryte ± iron-oxide ore deposit of Buca della Vena mine (43°59'55''N, 10°18'37''E), Stazzema, Apuan Alps, Lucca Province, Tuscany, Italy. The ore bodies are located at the contact between a metavolcanicmetasiliciclastic sequence belonging to the Palaeozoic basement of the Apuane Unit and the Triassic metadolostone of the Grezzoni Formation (Benvenuti et al., 1986). Their textural and mineralogical features are affected by the greenschist-facies metamorphism of Alpine age that characterises all the deposits of the southern Apuan Alps (e.g. D'Orazio et al., 2017). Ore bodies have a simple primary mineralogy, basically formed by baryte, magnetite, hematite and pyrite. However, the Buca della Vena mine is renowned among mineralogists and mineral collectors for several very rare mineral species that have been described from here since the end of the 1970s. Among them, twelve have this mine as their type locality: allanite-(La), apuanite, bohuslavite, dessauite-(Y), mapiquiroite, marrucciite, oxycalcioroméite, pellouxite, pillaite, rouxelite, scainiite and versiliaite. Tetrahedrite-(Hg) is the thirteenth new mineral species found at this locality.
Tetrahedrite-(Hg) was identified in samples collected in an exploitation void known, among mineral collectors, as the Sala del Castello. Here, a metre-sized metadolostone lens occurs embedded in microcrystalline baryte + iron-oxides and it is cut by several sets of veins and fractures. Tetrahedrite-(Hg) occurs as anhedral grains or equant crystals, up to 0.2 mm in size (Fig. 1a), black in colour and with a black streak. The lustre is metallic. The Mohs hardness may be close to 3½-4, in agreement with other members of the tetrahedrite group and with the sample of Hg-rich tetrahedrite from Moschellandsberg (Germany) that has a Vickers hardness of 236-277 kg/mm 2 (Criddle and Stanley, 1993). Tetrahedrite-(Hg) 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 5.326 g/cm 3 .
In reflected light, tetrahedrite-(Hg) is isotropic. It is greyishwhite, with creamy tints. Internal reflections were not observed. Reflectance values, measured in air using a spectrophotometer MSP400 Tidas at Leica microscope, with a 100× objective, are given in Table 1 and shown in Fig. 2.
During this study, in addition to the sample from the Buca della Vena mine, two other samples were examined. Tetrahedrite-(Hg) was identified from the Jedová hora deposit (49°47 ′ 31 ′′ N, 13°53 ′ 13 ′′ E), Hořovice, Central Bohemian Region, Czech Republic (Fig. 1b), and from the Rožňava ore field (48°40 ′ 37 ′′ N, 20°32 ′ 29 ′′ E), Košice Region, Slovakia (Fig. 1c). The sample from the Jedová hora deposit (vertical fissures with hydrothermal mineralisationquartz, siderite, dolomite, pyrite, baryte, cinnabar and tetrahedrite-(Hg)crosscutting two stratiform bodies of iron oressiderite and hematite) is represented by groups of imperfect crystals of tetrahedrite-(Hg) up to 4 mm in size on baryte, in association with chalcopyrite (sample 8 cm × 4.5 cm × 2.5 cm). The sample from the Rožňava ore field probably came from the upper part of the Mária vein (hydrothermal siderite-quartz vein with chalcopyrite, pyrite and tetrahedrite with variable Hg contents) and it is represented by massive tetrahedrite-(Hg) aggregates up to 10 cm in association with quartz (sample 11 × 6 × 7 cm). It is worth noting that tetrahedrite-group minerals from the Rožňava ore field are typically Hg-poor. However, the sample studied came to the museum in 1941 from the well-known and careful scientist Radim Nováček (1905-1942, who investigated this sample and published his results (Nováček, 1942) (see Fig. S1, available as Supplementary Material). Optical properties of both samples are similar to those reported for the Italian type material and the reflectance values are given in Table 1 and shown in Fig. 2.
The empirical formulae of the samples of tetrahedrite-(Hg), based on (As+Sb+Bi) = 4 atoms per formula unit (apfu), are given in the footnote of Table 2, along with the formulae written according to the partitioning of Cu and Ag between A and B constituents.
The specimens studied show a deficit of (Cu + Ag) (up to 0.43 apfu in the sample from Buca della Vena mine), and two samples also a slight S deficit (up to 0.30 apfu). The deficit of (Cu + Ag) is discussed below.

X-ray crystallography
Powder X-ray diffraction pattern of tetrahedrite-(Hg) from the Buca della Vena mine (Table 3) was collected using a 114.6 mm Gandolfi camera and Ni-filtered CuKα radiation (Department of Earth Sciences, University of Pisa, Italy). Unit-cell parameters were refined on the basis of 18 unequivocally indexed reflections using the software UnitCell (Holland and Redfern, 1997): a = 10.5072(6) Å and V = 1160.0(2) Å 3 . Powder X-ray diffraction data of both cotype samples were recorded using a Bruker D8 Advance diffractometer equipped with solid-state LynxEye detector and secondary monochromator producing CuKα radiation housed at the Department of Mineralogy and Petrology, National Museum, Prague, Czech Republic. The instrument was operating at 40 kV and 40 mA. In order to minimise the background, the powder samples were placed on the surface of a flat silicon wafer. The powder patterns were collected in Bragg-Brentano geometry in the range 3-75°2θ, step 0.01°and counting time of 20 s per step (total duration of experiment was ca. 3 days). The positions and intensities of diffractions were found and refined using the Pearson VII profile-shape function of the ZDS program package (Ondruš, 1993) and the unit-cell parameters were refined by the least-squares program of Burnham (1962). Unit-cell parameters, refined from powder X-ray diffraction data given in Table 3, are a = 10.4939(1) Å and V = 1155.61(5) Å 3 for the sample from Jedová hora, Czech Republic and a = 10.4725(1) Å and V = 1148.55(6) Å 3 for the sample from Rožňava, Slovakia. For the refinement of the crystal structure, X-ray diffraction intensity data were collected on tetrahedrite-(Hg) from Buca della Vena mine using a Bruker Smart Breeze diffractometer ) 50 kV and 30 mA) equipped with a Photon II CCD detector and graphite-monochromatised MoKα radiation. The detector-tocrystal distance was set at 50 mm. Data were collected using ω scan mode in 0.5°slices, with an exposure time of 10 s per frame, and they were corrected for Lorentz and polarisation factors as well as for absorption using the software package Apex3 (Bruker AXS Inc., 2016). The refined unit-cell parameter is a = 10.5057(8) Å and V = 1159.5(3) Å 3 ; and space group I 43m. The crystal structure of tetrahedrite-(Hg) was refined using Shelxl-2018 (Sheldrick, 2015) starting from the structural model of tetrahedrite given by Johnson and Burnham (1985). The occurrence of racemic twins was modelled, with a ratio between the two components of 94(3):6(3). The following neutral scattering curves, taken from the International Tables for Crystallography (Wilson, 1992) were used: Cu at M(2); Cu vs. Hg at M(1); Sb at X(3); and S at S(1) and S(2) sites. The anisotropic structural model converged to R 1 = 0.0190 for 335 reflections with F o > 4σ(F o ) and 20 refined parameters. Details of data collection and refinement are given in Table 4. Fractional atomic coordinates and equivalent isotropic displacement parameters are reported in Table 5. Table 6 reports selected bond distances. Finally, Table 7 gives the bond-valence  calculations obtained using the bond-valence 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).

Crystal structure description
The crystal structure of tetrahedrite-(Hg) agrees with the general features of the members of the tetrahedrite isotypic group. It can be described as a collapsed sodalite-like framework of cornersharing M(1)-centred tetrahedra, with cages hosting S(2)-centred M(2)-octahedra and four X(3)S(1) 3 trigonal pyramids (e.g. Johnson et al., 1988). In the sample studied, the triangularly coordinated M(2) site has an average bond distance of 2.262 Å. Refined site scattering, as well as the calculated bond-valence sum (BVS) of 1.02 valence units (vu), agree with the full occupancy at this position by Cu. Electron microprobe analysis suggest the occurrence of a minor vacancy (0.43 apfu). This may be due to some analytical problems, such as the migration of Cu under the electron beam. However, this phenomenon was reported only for Cu-excess tetrahedrites, both natural and synthetic (e.g. Lind and Makovicky, 1982). If a minor vacancy is real, this may be coupled with the partial oxidation of Cu + to Cu 2+ at the M(1) site, giving an excess of divalent metals, as observed in pošepnýite (Škácha et al., 2020). It is not unlikely that such a minor amount of vacancy could be overlooked, because the overestimated site scattering (29 electrons vs. 27.1 electrons, as calculated from chemical data) would be partially masked by the relatively large U eq value due to the positional disorder affecting the M(2) site, that is actually split into two flat pyramidal sub-sites, as reported previously by other authors (e.g. Andreasen et al., 2008;Welch et al., 2018). The splitting of this position did not improve significantly the crystal structure refinement of tetrahedrite-(Hg) and consequently an unsplit model was preferred.
The tetrahedrally coordinated M(1) site has an average bond distance of 2.391 Å. Assuming the site population (Cu 0.66 Hg 0.28 Zn 0.05 Fe 0.01 ), based on chemical data, the theoretical bond distance at M(1) is 2.393 Å, as obtained using the atomic radii proposed by Johnson et al. (1988). The refined site scattering (44.3 electrons per site) is in accord with the site scattering calculated from the site population (43.0 electrons). Bond-valence sums show an overbonding at M(1). This is a usual feature agreeing with previous studies on sulfosalts having tetrahedrally coordinated Hg (e.g. arsiccioite, AgHg 2 TlAs 2 S 6 -Biagioni et al., 2014c) and it is possibly due to the inaccuracy of the bond parameters for the Hg-S pair or to a small shift of the S positions when the M(1) site is occupied by Hg.
The X(3) site has an average bond distance of 2.447 Å, fully consistent with a pure Sb occupancy, as indicated by the BVS of 3.03 vu.
The S(1) site is four-fold coordinated, being bonded to two M(1), one M(2) and one X(3). Its BVS is 2.16 vu. S(2) is    octahedrally coordinated by atoms hosted at M(2) sites, with BVS of 2.16 vu. No hints of vacancies at S(2) were observed during the structure refinement.
Taking into account the crystal radii proposed by Johnson et al. (1988) and the site population given above, the M(1)-S(1) bond distance in the sample from Buca della Vena should be 2.393 Å, in perfect agreement with the observed value, 2.393 Å. Moreover, Foit and Hughes (2004) proposed a linear relationship between Hg content and the M(1)-S(1) bond distance. According to their relationship, the M(1)-S(1) distance in tetrahedrite-(Hg) from Buca della Vena mine should be 2.385 Å, slightly shorter than the observed value, probably as a consequence of the occurrence of minor Zn and Fe replacing Cu. The observed value is comparable with the M(1)-S(1) distance reported by Karanović et al. (2003) in tetrahedrite-(Hg) from Dragodol, Serbia, 2.382 Å, and in the only Hg-dominant sample studied by Foit and Hughes (2004), 2.371 Å. These values are definitely longer than those reported in tetrahedrite-(Fe) and tetrahedrite-(Zn) (e.g. 2.342 Å -Wuensch, 1964).
Serbia. Electron microprobe data (EDS mode) gave the formula Cu 9.28 Hg 1.64 Zn 0.76 Fe 0.02 As 0.17 Sb 4.19 S 13 [Ev(%) = +4.6]. This formula can be recalculated to Cu 8.57 Hg 1.52 Zn 0.70 Fe 0.02 Sb 3.87 As 0.13 S 12.00, on the basis of (Sb + As) = 4 apfu. The poor quality of chemical data contrasts with the high quality of structural data [R 1 = 0.0148 for 165 reflections with F o > 4σ(F o )], confirming the occurrence of Hg at the tetrahedral M(1) site. Further structural data on Hg-rich tetrahedrite [actually a solid solution between tetrahedrite-(Zn) and tetrahedrite-(Hg)] were given by Foit and Hughes (2004), who studied the structural variations in Hg-rich tetrahedrite from the Spring Creek Claims. The sample richest in Hg, corresponding to tetrahedrite-(Hg), has the composition Cu 6 (Cu 4.08 Hg 1.15 Zn 0.67 Fe 0.03 Co 0.01 ) Σ5.94 (Sb 2.64 As 1.36 ) Σ4.00 S 13 . Synthetic Hg-rich tetrahedrites (up to 2 Hg apfu) were studied by Johnson (1991), who reported powder X-ray diffraction data and a unit-cell parameter a = 10.5071(1) Å, and by Makovicky (2003, 2004), who examined Hg-rich tetrahedrite and the solid solution between Zn-rich and Hg-rich tetrahedrite.
'Tennantite-(Hg)' has the unit-cell parameter a = 10.34 Å; the a parameter, shorter than that of tetrahedrite-(Hg), is in keeping with the As-to-Sb substitution and agrees with the value calculated applying the relation of Johnson et al. (1987), i.e. a = 10.35 Å.
Mercury is an important constituent also in the routhierite-group minerals (hereafter routhierites). In these phases, the occurrence of two distinct kinds of tetrahedral sites, showing different occupancies, lowers the symmetry to I 42m. Among the currently five known mineral species, two contain essential Hg: routhierite, CuHg 2 TlAs 2 S 6 (Bindi, 2008;Biagioni et al., 2014b) and arsiccioite, AgHg 2 TlAs 2 S 6 (Biagioni et al., 2014a). Doubling the formula of routhierite, one obtains Cu 2 Hg 4 Tl 2 As 4 S 12 (Z = 2), that can be written as |Tl 2 As 4 | [Cu 2 Hg 4 S 12 ], discriminating the composition of the tetrahedral framework and the content of the distorted Laves polyhedron (Fig. 3c). Passing from tetrahedrite-(Hg) to routhierites and then to galkhaites, the content of Hg in the tetrahedral framework increases, from 2 apfu to 5 apfu. The excess of positive charges is balanced by decreasing the charges hosted in the cavities, replacing six Cu + atoms with one Tl 2 dimer in routhierites and one large monovalent ion (Cs or Tl) in galkhaites. It seems likely that the tetrahedrite-like tetrahedral framework may be able to adapt to the variable ore geochemistry, allowing the incorporation of variable amounts of divalent elements (Hg, Zn and Fe) in tetrahedrite-group minerals as well as favouring the crystallisation of peculiar mineral phases like galkhaites and routhierites in environments characterised by the occurrence of large monovalent cations like Tl + and Cs + .

Conclusions
This study permitted the characterisation of tetrahedrite-(Hg) as a specific mineral species belonging to the tetrahedrite group, in accordance with the nomenclature rules defined by Biagioni et al. (2020). It confirms the structural plasticity of the tetrahedrite structure, favouring the incorporation of a wide range of monovalent and divalent cations typical of hydrothermal ore deposits.