Markwelchite, TlPbSbS₃, a new Tl–Pb sulfosalt from the hydrothermal deposit of Jas Roux, Hautes-Alpes, France

Abstract

Markwelchite, ideally TlPbSbS₃, is a new mineral from the hydrothermal deposit of Jas Roux, Hautes-Alpes, France. It occurs as a black anhedral crystal associated closely with protochabournéite. Microhardness measurements (VHN₁₅) gave a mean value of 197 kg/mm² corresponding to a Mohs hardness of ~3–4. In plane-polarised incident light, markwelchite is grey in colour. Under crossed polars, it is distinctly anisotropic with greyish white to bluish rotation tints, with bright red internal reflections. Reflectance percentages (R_min and R_max) are: 28.5, 31.5 (471.1 nm); 28.3, 30.7 (548.3 nm); 27.9, 30.3 (586.6 nm); and 27.6, 29.8 (652.3 nm). The mean of 5 electron microprobe spot analyses gave Tl 34.67(45), Pb 31.86(25), Sb 15.06(15), As 2.37(5), S 15.35(20), total 99.31 wt.%, corresponding, on the basis of a total of 6 atoms per formula unit and structural results, to Tl_1.063Pb_0.964(Sb_0.775As_0.198)_Σ0.973S_3.000. Single-crystal X-ray diffraction studies revealed that markwelchite is isotypic with richardsollyite, TlPbAsS₃. It is monoclinic, space group P2₁/c, with the following unit-cell parameters: a = 8.9144(3), b = 8.4513(3), c = 8.6511(3) Å, β = 108.723(4)°, V = 617.27(4) ų and Z = 4. The five strongest observed powder-diffraction lines [d in Å (I_rel)(hkl)] are: 3.88 (100)($\bar{2}$11); 3.78 (90)(210); 3.29 (90)(102); 2.73 (85)($\bar{1}$13); and 2.93 (75)(022). The crystal structure can be described as formed by (100) [Me2(SbS₃)]^– layers sandwiching the Me1⁺ cations. The Me1 site has a seven-fold coordination, whereas the Me2 site has an 6+2 coordination corresponding to a distorted, bicapped trigonal prismatic coordination, and the Sb site displays a trigonal pyramidal coordination with three S atoms and Sb at the apex.

The name markwelchite honours Dr Mark D. Welch of the Natural History Museum, London, UK.

The new mineral has been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2024–001). A discussion on the relationships between markwelchite and synthetic TlPbSbS₃ is also provided.

Introduction

During a mineralogical study of newly collected samples from the Tl-rich hydrothermal mineralization of Jas Roux, La Chapelle-en-Valgaudemar, Gap, Hautes-Alpes, Provence-Alpes-Côte d'Azur, France (44°44′45″N, 6°19′18″E), a new Tl−Pb mineral was discovered. Details of the geological background of the Jas Roux deposit are given in Johan and Mantienne (2000). The Tl−Hg−Sb−As mineralisation occurs within baryte layers mainly hosted in metadolostone and marbles. Johan and Mantienne (2000) gave a full account of the mineralogy of this deposit. Later, Favreau et al. (2011) reported an update, citing a total of 57 different mineral species. Jas Roux is known for the occurrence of ten Tl-mineral species, among which eight have their type locality there: chabournéite (Johan et al., 1981), dewitite (Topa et al., 2020), écrinsite (Topa et al., 2017), ginelfite (Biagioni et al., 2023), jasrouxite (Topa et al., 2013), pierrotite (Guillemin et al., 1970), routhierite (Johan et al., 1974) and vallouiseite (Topa et al., 2023a). Other species first discovered at Jas Roux are laffittite (Johan et al., 1974) and montpelvouxite (Topa et al., 2023b).

This study reports the description of the new mineral markwelchite, together with data on its crystal structure. The new mineral has been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2024–001, Bindi et al., 2024). Holotype material is deposited in the Mineralogical Collections of the Natural History Museum of the University of Florence, Italy, under catalogue number 3739/I. Its mineral abbreviation is Mrw.

The name markwelchite honours Dr Mark D. Welch (b. 1960) of the Natural History Museum, London, United Kingdom. Dr Welch is a well-known mineralogical crystallographer who has researched a wide range of minerals, many of which have complex structures and unusual crystal chemistry. In particular, he is well-known for amphiboles, high-pressure studies and recently tetrahedrites. He was Principal Editor for the Mineralogical Magazine from 2006–2011.

Material studied

The geological outcrop, where the sample containing markwelchite was discovered, is located on the north flank of the Torrent de Chabournéou, a small alpine valley at ~2100 m of altitude in the Les Écrins National Park, Hautes-Alpes, France. Jas Roux is a hydrothermal stratiform deposit hosted in dolomitic metalimestones of Triassic age and appears as several sedimentary lenses embedded in the Pigeonnier anatexites. These lenses protrude and form small cliffs parallel to the valley (Boulliard et al., 2010). Samples from this study consist of boulders weathered out of the cliffs which contained small veinlets of primary sulfosalts accompanied by a variety of secondary minerals.

The sampling site is at 44°48′43.2″N, 6°19′20.1″E (Fig. 1). It is at the top of a boulder heap below the middle part of the Jas Roux cliffs and is located at an altitude of ~2156 m. The yellow areas outline the Jas Roux cliffs (Fig. 1). Sampling was done by one of us (HGA) in July 2014 with PN authorisation no. 280/2014.

Figure 1. (a) The location where the sample containing markwelchite was collected near Jas Roux, La Chapelle-en-Valgaudemar, Gap, Hautes-Alpes, Provence-Alpes-Côte d'Azur, France (44°48′43.2″N, 6°19′20.1″E). (b) Photograph indicating the sampling site below the cliff, looking to the East. (c) Photograph showing the sampling site by looking W towards the westernmost cliff (taken in 2014).

Markwelchite is associated with stibnite, twinnite, guettardite, pierrotite, realgar and protochabournéite (Fig. 2), the latter was verified by single-crystal X-ray diffraction.

Figure 2. Back-scattered electron image of the markwelchite-containing polished section. Small grains of markwelchite (Mrw) occur in the brighter regions, closely associated with protochabournéite. Holotype specimen 3739/I.

The formation of markwelchite is linked to the activity of Tl−Pb−Sb-rich fluids during the hydrothermal evolution of the Jas Roux deposit.

Physical and optical properties

Markwelchite occurs as black anhedral crystals up to 40 μm in size 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 density could not be measured owing to the small grain size. The calculated density (for Z = 4) for the empirical formula and unit-cell parameters obtained by single-crystal X-ray diffraction (see below) is 6.696 g/cm³. Five micro-indentation measurements carried out with a VHN load of 15 g give a mean value of 197 kg/mm² (range: 188–218), corresponding to a Mohs hardness of ~3–4.

In plane-polarised incident light, markwelchite is grey in colour. Under crossed polars, it is distinctly anisotropic, with greyish white to bluish rotation tints and bright red internal reflections. Reflectance was measured in air using a Zeiss MPM-200 microphotometer equipped with a MSP-20 system processor on a Zeiss Axioplan ore microscope. The filament temperature was ~3350 K. Readings were taken for specimen and standard (SiC) maintained under the same focus conditions. Reflectance percentages (R _min and R _max) for the four Commission on Ore Mineralogy (COM) wavelengths are: 28.5, 31.5 (471.1 nm), 28.3, 30.7 (548.3 nm), 27.9, 30.3 (586.6 nm) and 27.6, 29.8 (652.3 nm).

Chemical composition

A preliminary chemical analysis (energy dispersive spectroscopy) of the crystal used for the structural study did not indicate the presence of elements (Z > 9) other than Tl, Pb, Sb, As and S. Quantitative analyses (5 spots) were performed using a JEOL 8200 microprobe using wavelength dispersive spectroscopy (WDS, 20 kV, 40 nA, focussed beam (1 μm), counting times 20 s for peak and 10 s for background). For the WDS analyses, the following lines were used: PbMα, TlMα, SbLα, AsLα and SKα. Iron, Cu, Zn, Ag and Se were sought but found to be below the detection limit (0.02 wt.%). The crystal fragment was homogeneous within analytical error. Chemical data and analytical details are given in Table 1.

Table 1. Results of electron microprobe analyses (in elemental wt.%) of markwelchite.

* S.D. – standard deviation

The empirical chemical formula, based on 6 atoms per formula unit and structure (see below) is Tl_1.063Pb_0.964(Sb_0.775As_0.198)_Σ0.973S_3.000. The simplified formula is TlPb(Sb,As)S₃. The ideal chemical formula is TlPbSbS₃, which requires Tl 32.47, Pb 32.91, Sb 19.34, S 15.28, total 100.00 wt.%.

X-ray diffraction studies

Single-crystal X-ray studies were conducted using a Bruker D8 Venture diffractometer equipped with a Photon III detector using graphite-monochromatised MoKα radiation (λ = 0.71073 Å). Markwelchite is monoclinic, with the following unit-cell parameters: a = 8.9144(3), b = 8.4513(3), c = 8.6511(3) Å, β = 108.723(4)°, V = 617.27(4) ų and Z = 4.

The powder X-ray diffraction study (PXRD) was carried out using the same instrument but with CuKα radiation (λ = 1.5418 Å). The diffraction rings from markwelchite were converted into a conventional PXRD pattern. The crystal-to-detector distance was 7 cm. Observed and calculated powder X-ray diffraction data for markwelchite are given in Table 2. The unit-cell parameters from the PXRD data are: a = 8.9057(9), b = 8.4376(8), c = 8.6355(10) Å, β = 108.646(8)° and V = 614.83(7) ų.

Table 2. Observed and calculated powder X-ray diffraction data (on the basis of the structural model given in Table 4) for markwelchite.*

* Only reflections with I _calc > 4 are listed. The five strongest reflections are given in bold.

Crystal structure refinement

Single-crystal X-ray diffraction intensity data were integrated and corrected for standard Lorentz-polarisation factors and absorption with APEX3 (Bruker, 2016). A total of 1803 unique reflections were collected. The statistical tests on the distribution of |E| values (|E ²–1| = 0.912) indicated the presence of an inversion centre. Systematic absences agreed with the space group P2₁/c. Given the similarity of the unit-cell values, space group, and general stoichiometry, the structure was refined using the atomic coordinates given for richardsollyite, TlPbAsS₃ (Meisser et al., 2017), through Shelxl-2014 (Sheldrick, 2015). Table 3 reports details of the selected crystal, data collection, and refinement.

Table 3. Crystal and experimental data for markwelchite.

In the crystal structure of markwelchite, similar to richardsollyite, there are three cation and three anion sites. Scattering curves for neutral atoms were taken from the International Tables for Crystallography (Wilson, 1992). Initially, owing to the similar scattering factors of Pb and Tl, only the scattering curve of the first was used. Consequently, the site occupation factor (s.o.f.) of the three independent cation sites were refined using the following curves: Pb vs □ (structural vacancy) for the Me1 and Me2 sites, and Sb vs □ for the Sb site. All the cation positions but Sb were found fully occupied and their s.o.f. were fixed to 1. The Sb site was subsequently refined using Sb vs As curves and gave a Sb_0.805(11)As_0.195 site occupancy, which is in agreement with that obtained by electron microprobe (i.e., Sb_0.78As_0.20).

Discussion

Crystal structure

Markwelchite is isotypic with richardsollyite. The crystal structure of markwelchite can be described as formed by (100) [Me2(SbS₃)]^– layers sandwiching the Me1 cations (Fig. 3). The Me2 site has a seven-fold coordination, with an eighth ligand at 3.808(2) Å. The Me1 site has an 6+2 coordination (two ligands > 3.6 Å), corresponding to a distorted, bicapped trigonal prismatic coordination. Finally, the Sb site displays a trigonal pyramidal coordination with three S atoms and Sb at the apex.

Figure 3. Crystal structure of markwelchite as seen down [001]. Circles: grey = Me1; orange = Sb; yellow = S. Me2-centred polyhedra are shown in dark grey. Drawn using CrystalMaker®.

However, some differences with respect to richardsollyite occur in the coordination of atoms hosted at the Me2 and Me1 sites. The average <Me2–S> is 3.191 Å, with a corresponding bond-valence sum (BVS) of 1.49 valence units (vu), calculated using the bond parameters of Brese and O'Keeffe (1991) for the Pb–S bond, also considering the very long Me2–S3 bond [3.8077(18) Å]. In richardsollyite, this site has an average <Pb–S> distance of 3.122 Å, with distances ranging between 2.91 and 3.45 Å; the corresponding BVS is 1.85 vu. (Meisser et al., 2017). Me1 is eight-fold coordinated, with an average <Me1–S> distance of 3.298 Å; in richardsollyite this site is seven-fold coordinated, with average distance of 3.333 Å and a BVS of 0.98 vu (Meisser et al., 2017). In markwelchite, the BVS, calculated using the value of 2.55 Å for Tl–S bonds (e.g. Biagioni and Moëlo, 2017), is 1.35 vu that could indicate a cross-substitution Tl⁺ + Pb²⁺ = Pb²⁺ + Tl⁺ between Me1 and Me2. A possible fit would give Me2 = Pb_0.55Tl_0.45 and Me1 = Tl_0.55Pb_0.45, corresponding to mean formal charges of 1.55 and 1.45 vu, respectively. The disorder of Tl and Pb at the Me1 and Me2 sites is larger than that possibly occurring in richardsollyite, where a virtually ordered distribution of these two elements was reported (Meisser et al., 2017). This is probably a consequence of the Sb-for-As replacement in markwelchite with respect to richardsollyite. Indeed, the Sb site has a site occupancy (Sb_0.80As_0.20), in accord with electron microprobe data; in richardsollyite this site was virtually As-pure (Meisser et al., 2017). The occurrence of Sb may allow an increase in the size of the Me2 polyhedron, favouring the partial replacement of Pb²⁺ by Tl⁺ (Fig. 4).

Figure 4. Comparison between the crystal structures of markwelchite (left) and richardsollyite (right). The S–S distances (in Å) forming the base of the trigonal pyramids having Sb and As at their vertex are shown.

Structural and chemical information allow us to propose the idealised structural formula as ^{Me 1}(Tl_0.55Pb_0.45)_Σ1.00 ^{Me 2}(Pb_0.55Tl_0.45)_Σ1.00 ^Sb(Sb_0.82As_0.18)_Σ1.00S₃.

Possible stacking faults cannot be excluded, and these could be responsible for the presence of the residual maxima in the difference-Fourier map around Me1 and Me2 (highest peak 5.04 e Å^–3 at 0.95 Å from Me2).

Final atomic coordinates and equivalent isotropic displacement parameters are listed in Table 4, whereas selected bond distances and bond valences are provided in Tables 5 and 6, respectively. The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).

Table 4. Atomic coordinates and equivalent isotropic displacement parameters (Ų) for markwelchite.

Table 5. Selected bond distances (Å) for markwelchite.

Table 6. Bond valences (in valence units, vu) according to the parameters given by Brese and O'Keeffe (1991).

Note: the bond parameters of Pb–S and Tl–S bonds were assumed as 2.55 Å. The site occupancy at the Sb site is (Sb_0.80As_0.20).

Relation to other species

Markwelchite is the Sb-analogue of richardsollyite, TlPbAsS₃, and is isostructural with three synthetic compounds, i.e. KEuAsS₃, RbEuAsS₃ and CsEuAsS₃ (Bera and Kanatzidis, 2008).

Synthetic TlPbSbS₃ shows two polymorphs. The high-temperature one is stable over 620 K and has a structure of TlI type (orthorhombic Cmcm) with a disorder of cations occupying the same structural site (crystal structure determined from single-crystal X-ray data by Balić Žunić et al., 1992). In the low-temperature form the disorder of cations is retained although the symmetry is lowered to monoclinic, space group P2₁/c (structure determination from powder X-ray data by Balić Žunić and Bente, 1995). This is mainly due to a side movement in the stacking of tightly bonded two-layer slabs. The structures of the two polymorphs are influenced by the activities of the lone electron pairs of the cations involved. The side movement in the stacking of tightly-bonded slabs in the low temperature polymorph is probably caused by cation repulsions upon decreasing the slab separation. The structural differences with respect to markwelchite are probably due to differences in the formation conditions (temperature, pressure or time).

To elucidate the structural evolution of markwelchite as a function of temperature, in situ low- and high-temperature single-crystal X-ray diffraction studies are planned, provided that more suitable fragments will become available.