First occurrence of the M2a2b2c polytype of argentopolybasite, [Ag₆Sb₂S₇][Ag₁₀S₄]: Structural adjustments in the Cu-free member of the pearceite–polybasite group

Abstract

The chemistry and the crystal structure of the recently described mineral argentopolybasite are critically discussed based on the study of two new occurrences of the mineral: Gowganda, Timiskaming District, Ontario, Canada and IXL Mine, Silver Mountain mining district, Alpine County, California.

The crystal structure of argentopolybasite can be described as the sequence, along the c axis, of two alternating layers: a [Ag₆Sb₂S₇]^2– A layer and a [Ag₁₀S₄]²⁺ B layer. In the B layer there are linearly-coordinated metal positions (B sites), which are usually occupied by copper in all members of the pearceite–polybasite group, resulting in a B-layer composition [Ag₉CuS₄]²⁺. In argentopolybasite, however, Ag fills all the metal sites in both A and B layers. By means of a multi-regression analysis on 67 samples of the pearceite–polybasite group, which were studied by electron microprobe and single-crystal X-ray diffraction, the effect of Ag, Sb and Se on the B sites of the B layer was modelled. Although the nomenclature rules for these minerals are based on chemical data only, we think this approach is useful to evaluate the goodness of the refinement of the structure (Ag/Cu disorder) and thus fundamental to discriminate different members of the pearceite–polybasite group.

Introduction

The minerals of the pearceite–polybasite group, general formula [M ₆T ₂X ₇][M ₁₀X ₄] with M = Ag and Cu, T = As and Sb, and X = S, Se and Te, are fairly common in Nature. Eight members are known at present: argentopearceite (Sejkora et al., 2020), argentopolybasite (Števko et al., 2023), benleonardite (Bindi et al., 2015), cupropearceite (Bindi et al., 2007a, 2007c), cupropolybasite (Bindi et al., 2007a, 2007c), pearceite (Bindi et al., 2007a), polybasite (Bindi et al., 2007a), and selenopolybasite (Bindi et al., 2007a, 2007b).

Their crystal structure can be described as a sequence, along the c axis, of two alternating layers: a [M ₆T ₂X ₇]^2– A layer and a [M ₁₀X ₄]²⁺ B layer (Bindi et al., 2007a and references therein). As documented by Bindi et al. (2007a), these minerals can exhibit three different polytypes: the high-temperature fast-ion conductivity form (Tac polytype), the partially ordered form (T2ac polytype), and the low-temperature fully ordered form (M2a2b2c polytype).

During an ongoing structural study of complex Ag(Cu)-bearing minerals in mineralogical collections from various museums, we found specimens containing Cu-free ‘polybasite’ (Bindi et al., 2007a) from two different localities in North America. The same mineral has been recently approved by the International Mineralogical Association (IMA) as a new species with the name argentopolybasite (T2ac polytype) and the simplified formula Ag₁₆Sb₂S₁₁ (corresponding to the structural formula [Ag₆Sb₂S₇][Ag₉AgS₄]; Števko et al., 2023). Interestingly, the As-dominant silver end-member argentopearceite (Ag₁₆As₂S₁₁; T2ac polytype), has also been recently discovered and described from the Lehnschafter mine, Czech Republic (Sejkora et al., 2020). Those discoveries, in particular that related to argentopearceite, were surprising as it seemed that copper in the B layer was mandatory to stabilise these phases, especially when occurring in their disordered or partially ordered polytypes (Bindi et al., 2006a, 2006b; 2013).

Given the presence of ubiquitous twinning, mobile Ag⁺ and Cu⁺ cations, satellite reflections due to commensurately modulated structures (giving rise to different polytypes) or to composite modulated structures, partially occupied sites and strong pseudo-symmetries, it is mandatory to critically evaluate the structural models obtained. The existence of two Cu- and Se-free polybasite samples with excellent diffraction quality gives us the opportunity to critically elucidate this unusual crystal structure and the effect of Ag, Sb and Se on the B sites of the B layer.

Occurrence and chemical composition

The museum specimen (Fig. 1) used in the current work (Royal Ontario Museum, Canada; catalogue number M27183) is from Gowganda, Timiskaming District, Ontario, Canada, a well-established source of silver minerals. The geology of the deposit was studied and described by Andrews et al. (1986). Argentopolybasite occurs as black anhedral grains, up to 60 μm in length, with a black streak, associated closely with polybasite, calcite and chalcopyrite. An Ag-rich variety of polybasite from the same museum specimen was studied by Bindi and Menchetti (2009), who did not realise the presence of argento-polybasite at that time. Argentopolybasite and polybasite show no macroscopic/microscopic differences.

Figure 1. Argentopolybasite-M2a2b2c-bearing specimen (1.5 × 0.5 × 0.5 cm) from Gowganda, Timiskaming District, Ontario (Canada), ROM accession number M27183; the specimen also contains polybasite, calcite and chalcopyrite. Courtesy of ROM (Royal Ontario Museum), Toronto, Canada. ©ROM. Photograph by Tina Weltz.

A second specimen containing argentopolybasite (Fig. 2) was collected independently at the IXL mine, Silver Mountain mining district, Alpine County, California (Clark and Evans, 1977), by Kyle Beucke and given for study to one of the authors (FNK). Associated minerals in the specimen include pyrite, argento-pearceite and acanthite. Both argentopolybasite and argentopearceite occur as black anhedral grains in a quartz gangue.

Figure 2. Argentopolybasite-T2ac-bearing specimen (6 cm wide) from the IXL mine, Silver Mountain mining district, Alpine County, California. The specimen also contains pyrite, argentopearceite and acanthite.

The chemical composition of argentopolybasite from both occurrences was determined using wavelength-dispersive analysis (WDS) by means of a JEOL JXA-8200 (University of Florence, Italy) and a JEOL JXA-8530F (Natural History Museum of Vienna, Austria) electron probe micro-analyser. Major and minor elements were determined at a 25 kV accelerating voltage and a 20 nA beam current, with a spot size of 2 μm (no surface damage was observed when using these conditions). For the WDS analyses of the argentopolybasite from Gowganda the following lines were used: SKα, FeKα, CuKα, ZnKα, AsLα, SeLα, AgLα, SbLβ, TeLα, AuMα and PbMα. For the WDS analyses of the argentopolybasite from the IXL mine the following lines were used: SKα, CuKα, AsLα, SeKα, AgLα, SbLα and PbMα. The standards employed for Gowganda were: native elements for Cu, Ag, Au and Te; galena for Pb; pyrite for Fe and S; synthetic Sb₂S₃ for Sb; synthetic As₂S₃ for As; synthetic ZnS for Zn; and synthetic PtSe₂ for Se. The standards employed for the IXL mine were: lorándite for As; galena for Pb; chalcopyrite for Cu and S; native silver for Ag; stibnite for Sb; and synthetic Cu₂Se for Se. The detection limit for minor elements was 0.01 wt.%.

The argentopolybasite fragments from both specimens were found to be homogeneous within analytical error. The average chemical compositions (N = 5 and 4 for argentopolybasite from Gowganda and IXL mine, respectively) are reported in Table 1. On the basis of 29 atoms, the formula can be written as (Ag_16.00Cu_0.02)_Σ16.02(Sb_1.95As_0.04)_Σ1.99S_10.99 and (Ag_16.04Cu_0.02Pb_0.01)_Σ16.07(Sb_1.37As_0.56)_Σ1.93(S_10.46Se_0.54)_Σ11.00, for argentopolybasite from Gowganda and IXL mine, respectively.

Table 1. Mean analytical data (in wt.%) for argentopolybasite.

Note: n.a. = not analysed; b.d.l. = below detection limit

X-ray crystallography

Unit-cell parameters for argentopolybasite from Gowganda are: a = 26.384(2), b = 15.232(1), c = 24.148(2) Å, β = 90.03(1)° and V = 9704.6(1) ų, indicating the M2a2b2c polytype (Bindi et al., 2007a). On the contrary, hexagonal unit-cell parameters for argentopolybasite from IXL mine are: a = 15.061(2), c = 12.308(2) Å and V = 2417.8(2) ų, indicating the T2ac polytype (Bindi et al., 2007a). Given the much better diffraction quality and the fact that the trigonal polytype was already reported by Števko et al. (2023), we present here only the structural data for argentopolybasite-M2a2b2c from Gowganda.

A small argentopolybasite fragment (0.055 × 0.040 × 0.032 mm in size) was extracted from the M27183 sample and mounted on a 5 μm diameter carbon fibre, which was, in turn, attached to a glass rod. As pearceite–polybasite minerals are usually twinned (Bindi et al., 2020 and references therein), a full diffraction sphere was collected at ambient temperature using an Oxford Diffraction Xcalibur 3 single-crystal diffractometer. Refined unit-cell parameters are: a = 26.384(2), b = 15.232(1), c = 24.148(2) Å, β = 90.03(1)° and V = 9704.6(1) ų (Z = 16). Intensity integration and standard Lorentz-polarisation corrections were done with the CrysAlis RED (Oxford Diffraction, 2006) software package. The program ABSPACK of the CrysAlis RED package was used for the multi-scan absorption correction. Subsequent calculations were conducted with the JANA2006 program suite (Petříček et al., 2006). The refinement of the structure was carried out in the space group C2/c starting from the atomic coordinates given by Bindi and Menchetti (2009) for the crystal structure of Ag-rich polybasite-M2a2b2c. Site-scattering values were refined using scattering curves (Wilson, 1992) for neutral species for the Sb sites (Sb vs As) and for the Cu sites (Ag vs Cu) of the B layer (hereafter labelled B sites). The Sb sites and the B sites were found to be fully occupied by antimony and silver, respectively, and their occupancies were fixed during subsequent refinement cycles. With the introduction of twinning by metric merohedry (see Evain et al., 2006) the refinement smoothly converged to R = 0.103 for observed reflections [2σ(I) level], including all the collected reflections in the refinement. Indeed, the peculiar geometry of the pseudo-orthorhombic unit cell (with a ≈ 3^½⋅b) makes a {110} twinning very probable. Based on this refinement, the analyses of the difference-Fourier synthesis maps suggested an additional twin law with a twofold axis, perpendicular to the previous threefold axis as a generator twin element, thus leading to a second-degree twin. The introduction of only three new parameters (the new twin volume ratios) dramatically lowered the R value to 0.055, although the new domains were rather small [volume percentages: 4.12(4)%, 3.05(2)% and 2.54(3)%].

A non-harmonic approach with a Gram–Charlier development of the Debye–Waller factors up to the third order (Johnson and Levy, 1974; Kuhs, 1984; Bindi and Evain, 2007) was then used to describe the electron density in the vicinity of four Ag atoms (i.e. Ag15, Ag22, Ag24 and Ag29) for which residues were found in the difference-Fourier maps. Using this approach, with anisotropic atomic displacement parameters for all the atoms and no constraints, the residual value settled at R = 0.0334 (Rw = 0.0915) for 11,867 independent observed reflections [2σ(I) level] and 443 parameters, and at R = 0.0380 (Rw = 0.0945) for all 18,461 independent reflections. Experimental details are given in Table 2. Atomic coordinates and bond distances are reported in Table 3 and 4, respectively. The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).

Table 2. Crystallographic data for the selected argentopolybasite-M2a2b2c crystal.

Table 3. Atoms, fractional atomic coordinates and equivalent isotropic displacement parameters (Ų) for the selected argentopolybasite-M2a2b2c crystal from Gowganda.

Table 4. Main interatomic distances (Å) for the selected argentopolybasite-M2a2b2c crystal from Gowganda.

Results and discussion

As illustrated in previous structural studies (Bindi et al., 2020 and references therein), the crystal structure of the monoclinic polytype (M2a2b2c) of argentopolybasite (Fig. 3) can be described as the sequence, along the c axis, of two alternating layers: a [M ₆T ₂X ₇]^2– A (or A') layer and a [M ₁₀X ₄]²⁺ B (or B') layer (A,B and A',B' being related by a c glide reflection).

Figure 3. Projection of the argentopolybasite-M2a2b2c structure along the monoclinic b axis, emphasising the succession of the [M ₆T ₂X ₇]^2– A (or A') and [M ₁₀X ₄]²⁺ B (B') layers. Grey, orange and yellow spheres indicate Ag, Sb and S atoms, respectively. Blue spheres indicate the linearly-coordinated B positions. The unit cell is outlined. A,B and A',B' are related by a c glide reflection. Drawn using VESTA (Momma and Izumi, 2011).

In the [M ₆T ₂X ₇]^2– A (or A') layer, each silver cation is three-fold coordinated by sulfur. The Sb atoms are the top of a trigonal pyramid with the three S atoms forming a base. In the [M ₁₀X ₄]²⁺ B (or B') layer, the 18 independent Ag atoms show different environments from quasi-linear to quasi-tetrahedral (Table 4). Argentopolybasite shows the presence of Ag at all the B structural positions of the B layer. The B positions [B1, B2 (4e) and B3 (8f)] are generally occupied by copper in pearceite–polybasite minerals in a nearly perfect linear coordination (see Bindi et al., 2020 and references therein). In argentopolybasite, the mean Ag–S bond distances at the B sites, i.e. 2.399, 2.398 and 2.397 Å, for B1, B2 and B3, respectively, are very close to those observed for the linearly-coordinated Ag atoms of the B layer (Table 4) and exhibit bond-valence sums close to 1.00 (0.94–0.95 valence units, Brese and O'Keeffe, 1991). In the trigonal T2ac polytype of argentopolybasite (Števko et al., 2023) the two B sites (labelled Ag11 and Cu12) show mean bond distances of 2.244 and 2.230 Å in the refined structure model of these authors (R = 0.0741; four-fold twinning was modelled). The two sites also show partial occupancy: Ag11 = 0.687(10)Ag and Cu12 = 0.82(3)Cu. This is bizarre because the B sites always act as a fully-occupied, ordered part surrounded by mobile cations in the B layer (Bindi et al., 2006a, 2020). Furthermore, the occupancy of the B sites is strictly correlated with the way the mobile part (liquid-like structure) has been modelled and the approach considered to deal with the pervasive twinning.

To try to elucidate this discrepancy, we carried out a multi-regression analysis on 67 samples of the pearceite–polybasite group which were studied by both electron probe micro-analyser and single-crystal X-ray diffraction (Bindi et al., 2020, references therein and unpublished data). The analysis allows modelling of the B–S distance of the linearly-coordinated Ag atom in the B layer on the basis of the Ag, Sb and Se contents, i.e. B–S (Å) = 2.101(8) + 0.213(5)Ag atoms per formula unit (apfu) + 0.037(5)Sb apfu + 0.012(2)Se apfu. The observed (derived from structure refinements) versus calculated (with the equation above) bond distances are shown in Fig. 4. Although there is not a large variation of the distribution of the B–S distances mainly due to the rarity of Cu-free members, the two values obtained by Števko et al. (2023) for holotype argentopolybasite-T2ac are clearly off the main trend. This could imply either that the crystal they used for the structural study is not that analysed by electron microprobe or that the site occupancy and/or twinning was not modelled properly. To corroborate the hypothesis of a likely problem with the structure refinement there is the fact that the unit-cell and the overall geometry of the mineral studied by Števko et al. (2023) perfectly fit the trend expected for Cu-free polybasite and argentopolybasite sensu stricto. In the structure of these minerals, disregarding the polytype, the B−S bond distances are mainly lined along the c axis (Fig. 3). This means that when Ag substitutes for Cu in these structural positions an increase in the c parameter and a consequent increase in the volume of the unit cell are expected. This is easily observable by plotting the unit-cell volume as a function of the copper content obtained by electron microprobe. To compare all the members belonging to the pearceite–polybasite group, the variation of the hexagonal subcell volume (i.e. a ≈ 7.5 and c ≈ 12 Å) was considered. Argentopolybasite of the present investigation and that studied by Števko et al. (2023) (green and red circles in Fig. 5) are an excellent fit for the general trend observed for the minerals of the pearceite–polybasite group.

Figure 4. Multi-regression analysis (see text) showing the combined effect of Ag, Sb and Se on the linearly-coordinated B–S distance of the B layer. Red circles refer to the B–S distances in holotype argentopolybasite-T2ac studied by Števko et al. (2023). Green circles refer to the B–S distances in argentopolybasite of the current study (Gowganda). Standard uncertainties are smaller than the size of the symbols.

Figure 5. Relationship between the unit-cell volume of the hexagonal subcell (ų) and the copper content obtained by electron probe micro-analysis [Cu_EPMA] (apfu) for the different members of the pearceite–polybasite group (from Bindi et al., 2006a, 2007a,b,c,d; Bindi and Menchetti, 2009). Green and red circles indicate argentopolybasite of the current study (Gowganda) and that studied by Števko et al. (2023), respectively. Standard uncertainties are smaller than the size of the symbols.

The discovery of argentopolybasite and argentopearceite demonstrates that there could be other surprises hidden in this complex group of minerals. It is critical to further study them for the advancement of the knowledge about processes relevant for Earth and to share that knowledge with others such as in environmental and material sciences.