Biagioniite, Tl2SbS2, from the Hemlo gold deposit, Marathon, Ontario, Canada: occurrence and crystal structure

Abstract Biagioniite, ideally Tl2SbS2, is a new mineral from the Hemlo gold deposit, Marathon, Ontario, Canada. It occurs as very rare anhedral crystals up to 65 μm across associated with aurostibite, stibarsen and native gold in a calcite matrix. Biagioniite is opaque with a metallic lustre and shows a black streak. In reflected light biagioniite is moderately bireflectant and not pleochroic. Under crossed polars it is weakly anisotropic with blueish to light-blue rotation tints. Internal reflections are absent. Reflectance percentages for the four standard wavelengths (Rmin and Rmax) are 35.9 and 37.5 (471.1 nm); 34.7 and 36.2 (548.3 nm); 33.8 and 35.3 (586.6 nm); and 31.5 and 33.7 (652.3 nm), respectively. A mean of four electron microprobe analyses gave: Tl 65.12(31), Ag 3.52(9), Sb 20.22(12), S 10.80(8), total 99.66 wt.%, corresponding, on the basis of a total of 5 atoms, to (Tl1.87Ag0.19)Σ2.06Sb0.97S1.97. Biagioniite is monoclinic, space group Pc, with a = 11.0895(9), b = 14.3124(11), c = 7.9352(6) Å, β = 96.230(8)°, V = 1252.02(17) Å3 and Z = 8. The four strongest powder-diffraction lines [d in Å (I/I0) (hkl)] are: 3.56 (100) (310); 3.37 (75) ($\bar{2}$31); 3.79 (60) (012); 3.03 (60) (032). In the crystal structure [R1 = 0.024 for 2655 reflections with I > 2σ(I)], thallium adopts various coordinations extending from quasi-linear to quasi-tetrahedral. Antimony forms Sb–Sb pairs, which lead to the formula [Tl+1]4[Sb2]4+[S2–]4. Biagioniite is isostructural with dervillite, Ag2AsS2. The new mineral has been approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (IMA2019–120) and named for Cristian Biagioni, Associate Professor of Mineralogy at the Department of Earth Sciences of the University of Pisa, Italy.


Introduction
Several elements, having economic value or environmental concern, are hosted in sulfosalts, a group of complex minerals typically occurring in hydrothermal settings. Our ongoing effort in the last two decades (Bindi and Biagioni, 2018, and references therein) has been the study of these minerals from a structural point of view to try to: (1) elucidate the role played by minor components, which could give interesting insights into the physico-chemical conditions of the crystallisation environments, and (2) allow the potential description of unpredictable structures, unexpected crystallographic features, and new mineral species. Such a body of knowledge has been critical for assessing both the technological potentialities of sulfosalts as well as their geological significance.
In the course of this research project (i.e. Biagioni et al., 2016;Bindi et al., 2012aBindi et al., ,b, 2015a, we examined a sample from the Hemlo gold deposit, Marathon, Ontario, Canada (Harris, 1989), belonging to the mineralogical collections of the Museo di Storia Naturale of the University of Florence. The sample consists of tiny criddleite grains associated closely with aurostibite, stibarsen and native gold in a calcite matrix. Among the stibarsen fragments, a small unique grain that turned out to be biagioniite, Tl 2 SbS 2 , was found.
Biagioniite was approved as a new mineral by the International Mineralogical Association -Commission on New Minerals, Nomenclature and Classification (IMA2019-120, Bindi and Moëlo, 2020). The mineral name honours Cristian Biagioni (b. 1981), Associate Professor of Mineralogy at the Department of Earth Sciences of the University of Pisa, Italy. He is the current Italian Member of the IMA-Commission of New Minerals, Nomenclature and Classification and he is co-author of the description of more than 50 new mineral species. In addition, he contributed to the discovery of the Tl-rich nature of pyrite ores from the Apuan Alps (northern Tuscany, Italy), promoting a new scientific investigations of these ore deposits both from a mineralogical and environmental point of view (e.g. Biagioni et al., 2013Biagioni et al., , 2017. The holotype material is deposited in the mineralogical collection of the Museo di Storia Naturale of the University of Florence, Italy, under catalogue number 46582/G.
Here we report the description of the new mineral biagioniite, together with the determination of its crystal structure.
1989; Tomkins et al., 2004). The deposit, discovered in 1982, is of Archean age and occurs at the contact of felsic metavolcanics and pelitic metasediments. It consists of several mineralised zones, of which the main zone extends for a length of 2900 m, for a distance of 2500 m down-dip and ranges in thickness from 3 to 45 m. The ore minerals were formed from hydrothermal fluids that may in part be related to the shear zone.
Native gold is the principal gold mineral in the deposit (Harris, 1989). The gold ore is substantially enriched in Mo, V, As, Sb, Hg, Tl and Ba and contains a diverse assemblage of minerals. Interestingly, there is a close spatial association between the mercury and thallium minerals. In this regard, the realgar-cinnabarrich quartz veins that occur within the central portion of the deposit are common hosts for the thallium minerals (Harris, 1989). Routhierite is the most common thallium mineral, parapierrotite is less abundant, while vaughanite and criddleite are rare.

Physical and optical properties
Biagioniite occurs as very rare crystals grown on a calcite matrix (Fig. 1). The mineral exhibits a subhedral to anhedral grain morphology, and shows no inclusions of, or intergrowths with, other minerals. The maximum grain size of biagioniite is ∼65 μm. It is black in colour and shows a black streak. The mineral is opaque in transmitted light and exhibits a metallic lustre. No cleavage is observed, and the fracture is uneven. The calculated density (for Z = 8) for the empirical formula (see below) is 6.192 g/cm 3 . Unfortunately, the density could not be measured here because of the small grain size. The Mohs hardness, estimated with respect to the surrounding calcite, is ∼3.
In plane-polarised incident light, biagioniite is creamy in colour, moderately bireflectant and not pleochroic. Between crossed polars, biagioniite is weakly anisotropic with blueish to light-blue rotation tints. Internal reflections are absent and there is no optical evidence of growth zonation.
Reflectance measurements were performed in air by means of a MPM-200 Zeiss microphotometer equipped with a MSP-20 system processor on a Zeiss Axioplan ore microscope. The filament temperature was ∼3350 K. An interference filter was adjusted, in turn, to select four wavelengths for measurement (471.1, 548.3, 586.6 and 652.3 nm). Readings were taken for specimen and standard (SiC) maintained under the same focus conditions. The diameter of the circular measuring area was 0.04 mm. Reflectance percentages for R min and R max are: 35.9 and 37.5 (471.1 nm); 34.7 and 36.2 (548.3 nm); 33.8 and 35.3 (586.6 nm); and 31.5 and 33.7 (652.3 nm), respectively.

Chemical composition
A preliminary chemical analysis using energy-dispersive spectrometry performed on the crystal fragment used for the structural study did not indicate the presence of elements (Z > 9) other than Tl, Sb, S and minor Ag. Analyses were carried out using a JEOL 8200 microprobe (wavelength dispersive

X-ray crystallography and crystal-structure determination
The same crystal fragment (40 × 50 × 65 μm) used to obtain the chemical data was selected for X-ray single-crystal diffraction.
Data were collected using a Bruker D8 Venture diffractometer equipped with an Photon II CCD detector, with graphitemonochromatised MoKα radiation (λ = 0.71073 Å). Biagioniite was found to be monoclinic, with a = 11.0895 (9)   led to the choice of the space groups Pc and P2/c. Although the statistical tests on the distribution of |E| values (|E 2 -1| = 0.812) indicated the absence of an inversion centre, suggesting the choice of the space group Pc, the structure was preliminarily solved in the P2/c space group. A residual R 1 = 0.18 value was achieved quickly. However, the preliminary structural model obtained indicated a large atomic disorder. The structure model was subsequently optimised, and an ordered model was sought, but no improvement in R could be achieved. At this point, a thorough analysis of the structure (essentially based upon the observation of the very large atomic displacement parameters for particular atoms) suggested that some symmetry element of the P2/c space group should be removed. The reflection and atomic position data sets were then adapted to the Pc space group (showing the same reflections conditions) and the structure refined. After several cycles, an ordered solution with full site occupancies was finally determined by carefully removing atoms with low site occupancies and/or non-realistic distances with neighbouring atoms and adding significant positions found in the difference-Fourier syntheses. The structure could be smoothly refined in Pc without any damping factor or restrictions by the program SHELXL (Sheldrick, 2008). The occupancy of all the sites was left free to vary (Tl vs. □; Sb vs. □; S vs. □,   where □ = a vacancy) but all the positions were found to be fully occupied. Neutral scattering curves for Tl, As and S were taken from the International Tables for X-ray Crystallography (Wilson, 1992). At the last stage, with anisotropic atomic displacement parameters for all the atoms and no constraints, the residual value settled at R 1 = 0.0243 for 2655 observed reflections [2σ(I ) level] and 182 parameters and at R 1 = 0.0315 for all 4520 independent reflections. Note that the acentric structural model we obtained does not show high values in the correlation matrix between pairs of atoms which are equivalent in the centrosymmetric space group P2/c. To test whether the acentric model is to be preferred to the centric one we also tested the presence of twinning by inversion in the non-centrosymmetric structure refinement. Indeed, as is well known, a centrosymmetric structure that is refined as noncentrosymmetric will show a twin scale factor, equivalent to the Flack parameter in the case of inversion twinning (Flack et al., 2006;Müller et al., 2006), that refines to 50% within analytical uncertainty. We found the racemic twin-component scale factor refined to 0.09(2), consistent with a highly asymmetrical distribution of the enantiomorphic components and indicating the acentric model as the right choice.
Experimental details and R indices are given in Table 2. Fractional atomic coordinates and atomic displacement parameters are reported in Table 3. Bond distances are given in Table 4. The crystallographic information files have been deposited with the Principal Editor of Mineralogical Magazine and are available as Supplementary material (see below).
The coordination of these pairs is represented in Fig. 4. Coordination of each pair with four S atoms corresponds to a hemi-octahedron cut along a symmetry plane (through two opposite edges of the cube - Fig. 5). Two neighbouring hemioctahedra brought closer allow a single octahedron to be rebuilt.
The coordination of Tl atoms of the Tl 4 S 2 layer is shown in Fig. 6. There is a central zig-zag row of tetrahedral Tl atoms, flanked by two stripes of triangular Tl, and then two stripes of linear Tl. Contrary to the Tl 4 (Sb 2 ) 2 S 6 ribbon-layer (see below), here there is linear Tl on one mirror position, and tetrahedral Tl along the second mirror position. It is worth noting the linear coordination of Tl7. To the best of our knowledge, biagioniite seems to be Fig. 5. Coordination of each Sb pair (down, reduced to a central blue atom) with four S atoms corresponding to a hemi-octahedron cut along a symmetry plane (through two opposite edges of the cube). Two neighbouring hemi-octahedra brought closer allow the rebuilding of the octahedron (above). Fig. 6. Coordination of Tl atoms in the Tl 4 S 2 layer in the structure of biagioniite. Tl2: tetrahedral; Tl6 and Tl8: triangular; Tl7: linear. Fig. 7. Sb 2 pairs have been replaced by a single atom (G, green) at their gravity centre, and an anionic vacancy (□, red) has been added. One Tl 4 (Sb 2 ) 2 S 6 ribbon (now Tl 4 G 2 S 6 □ 2 ) has been selected. Fig. 8. One Tl 4 G 2 S 6 □ 2 ribbon in the crystal structure of biagioniite can be described as a distorted derivative of PbS structure.
the first example of a natural chalcogenide showing Tl in linear coordination. For a review of Tl chalcogenides see Makovicky (2018).
In Fig. 7, the Sb 2 pairs have been replaced by a single atom (G) at their gravity centre, and an anionic vacancy (□) has been added. One Tl 4 (Sb 2 ) 2 S 6 ribbon (now Tl 4 G 2 S 6 □ 2 ) has been selected. Projection of this ribbon (Fig. 8) shows that it is a distorted derivative of the PbS structure. The Pb 6 S 8 ideal ribbon (Fig. 9) is parallel to [310], two-atoms-thick, and three-octahedra large. The junction between two ribbons along [010] (mirror in the structure, with a shift along c, to preclude S-S short bond in the interface) corresponds to (131) of PbS. The two sides of such a junction (the two different mirror positions) have the same topology.
Bond-valence calculations (Table 6) have been computed on the basis of the following bond parameters: R Sb,S = 2.45 (Brese and O'Keeffe, 1991), R Tl,S = 2.55 (Biagioni et al., 2014) and R Sb,Sb = 2.82 (O'Keeffe and Brese, 1992). Tl atoms are overbonded (bond valence from 1.11 up to 1.36 valence units, vu), but we have not considered in the computation the presence of minor Ag disordered at the Tl positions, although it would be very minor. Sb-Sb bond lengths (2.830 and 2.843 Å) agree well with R Sb,Sb corresponding to one vu; nevertheless, Sb atoms are underbonded varying from 2.31 to 2.57 vu. This feature has also been observed in sartorite (Berlepsch et al. 2003) and in minerals such as dadsonite (Makovicky et al., 2006), and was discussed in detail by Mills et al. (2009). The overall bond-valence sum of the eight S sites is 15.95, very close to the theoretical value (16 vu). These results may be due to uncertainty in the positions of some S atoms: a small shift closer to Sb atoms would reduce Sb underbonding together with Tl overbonding, without significant change of the S bond-valence sum.
The presence of dimeric [Sb 2 S 4 ] 4ions with a central Sb-Sb bond in biagioniite means the formula could be written as [Tl +1 ] 4 [Sb 2 ] 4+ [S 2-] 4 . However, it is difficult to analyse such polycationic compounds in strict bond-valence terms, as the electronegativity of such elements lies between that of common cations and common anions. The weak Tl-Sb bonds in biagioniite are good examples of the 'anionic' behaviour, which could be explained through dative donation of the Sb lone pair to the closed-shell d 10 Tl cations. On the contrary, in dervillite, short Ag-As bonds are lacking (Bindi et al., 2013).
Tl 2 SbS 2 has never been described either in Nature or as synthetic compound. Actually, the experimental TlSbS 2 -Tl section includes three ternary compounds, namely Tl 2 SbS 2 , Tl 4 SbS 2 and Tl 5 SbS 2 . The former two compounds melt congruently at 613 and 683 K, whereas the latter one decomposes by peritectic reaction at 663 K (Jafarov et al., 2016). But, in fact, the Tl 2 SbS 2 compound has been found to be Tl 3 SbS 3 + Sba two-phase mixture instead of a single, independent chemical compound. Such a S-deficient formula for biagioniite indicates its formation at Hemlo at low f S2 , in accordance to its association with stibarsen, SbAs, aurostibite, AuSb 2 , and criddleite, TlAg 2 Au 3 Sb 10 S 10 (also S-deficient).