Argentopolybasite, Ag₁₆Sb₂S₁₁, a new member of the polybasite group

Abstract

cotype locality). At the Kremnica deposit argentopolybasite was found as discrete, well-developed (pseudo)hexagonal tabular crystals up to 4 mm in size or as complex crystalline aggregates and groups up to 5 mm in size in cavities of quartz. It is associated with pyrargyrite, polybasite, stephanite, miargyrite, rozhdestvenskayaite-(Zn), argentotetrahedrite-(Zn), naumannite, gold and pyrite.Argentopolybasite is dark grey to black, with a black streak and metallic to opaque lustre. The Mohs hardness is about 3. It is brittle without any observable cleavage and with a conchoidal fracture. The calculated density is 6.403 g•cm -3 . In reflected light, argentopolybasite is grey, without any observable bireflectance and very weak pleochroism. It shows moderate anisotropy in crossed polarisers with weak greenish and green-blue tints. The reflectance values for wavelengths recommended by the Commission on Ore Mineralogy of the IMA are

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

“…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- T 2 ac 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.…”

“…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 T 2 ac 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.…”

“…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) Å 3 , indicating the T 2 ac 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- M 2 a 2 b 2 c from Gowganda.…”

“…The minerals of the pearceite–polybasite group, general formula [ M 6 T 2 X 7 ][ M 10 X 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).…”

“…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 ( T 2 ac polytype) and the simplified formula Ag 16 Sb 2 S 11 (corresponding to the structural formula [Ag 6 Sb 2 S 7 ][Ag 9 AgS 4 ]; Števko et al , 2023). Interestingly, the As-dominant silver end-member argentopearceite (Ag 16 As 2 S 11 ; T 2 ac polytype), has also been recently discovered and described from the Lehnschafter mine, Czech Republic (Sejkora et al , 2020).…”