www.jgeosci.org Journal of Geosciences, 55 (2010), 161–167 DOI: 10.3190/jgeosci.067 Original paper The role of silver in the crystal structure of pyrargyrite: single crystal X-ray diffraction study František LauFek 1* , Jiří SeJkOra 2 , Michal Dušek 3 1 Czech Geological Survey, Geologická 6, 152 00 Prague 5, Czech Republic; frantisek.laufek@geology.cz 2 Department of Mineralogy and Petrology, National Museum, Václavské nám. 68, 115 79 Prague 1, Czech Republic 3 Institute of Physics AS CR, v.v.i., Na Slovance 2, 182 21 Prague 8, Czech Republic * Corresponding author The crystal structure of pyrargyrite, Ag 3 SbS 3 , from Příbram–Háje (Czech Republic) was reined from single-crystal X-ray diffraction data in the space group R3c to an R factor of 0.0112. Unit-cell parameters are a = 11.0464(3) Å, c = 8.7211(2) Å, V = 921.60(4) Å 3 , Z = 6. In order to mimic the spread of electron density of silver, the non-harmonic Gram–Charlier development of the silver atomic displacement parameters was applied. A reasonable triangular shape of the electron density maximum centred in Ag position was obtained. The analysis of potential barriers between Ag sites reveals that silver transfer is equally probable via additional P site not only within the Ag–S–Ag spirals and the Ag–Ag chains, but also between these spirals and chains. Keywords: pyrargyrite, crystal structure, non-harmonic reinement, Ag diffusion, atomic displacement parameters Received: 26 April 2010; accepted: 7 July 2010; handling editor: R. Skála 1. Introduction Pyrargyrite, Ag 3 SbS 3 , is Ag-rich sulphosalt rather com- mon in epithermal polymetallic base-metal sulphide deposits of the issure-vein type (Harlov and Sack 1995). Together with its low temperature modiication – pyrostil- pnite – pyrargyrite belongs to the proustite–xanthoconite group (Strunz and Nickel 2002). Pyrargyrite represents Sb end-member of solid solution with isostructural proustite, Ag 3 AsS 3 . This solid solution is believed to be continuous down to 90 °C (Ghosal and Sack 1995). From the chemical point of view, these two mineral phases can be considered as ones of the simplest Ag-bearing sulphosalts. As was mentioned by Bindi and Evain (2007), disorder is commonly observed during the structure solution and reinement of Ag + and Cu + bearing sulphosalts. Gaudin et al (2001) reported that Ag + and Cu + ions with electron coniguration d 10 easily adopt various complex asym- metric coordinations. The main factors inluencing the coordination of Ag + and Cu + ions in chalcogenides are the metal s/d orbital mixing and polarization factors (Gaudin et al. 2001). Then, Ag + and Cu + ions can occur in differ- ent, but overlapping sites and the crystal structure appears as disordered. A classical approach to address disordered materials is the application of a split atom model. Bindi and Evain (2007) demonstrated that the non-harmonic approach based on Gram–Charlier development of the atomic displacement factors (Kuhs 1992) represents an alternative and more effective way to determine and describe the structure of such materials. The eficiency of the non-harmonic approach has been recently proven by successful application to solution and reinement of various Ag-bearing sulphides and sulphosalts, e.g. sam- sonite, Ag 4 MnSb 2 S 6 (Bindi and Evain 2007), stephanite, Ag 5 SbS 4 (Leitl et al. 2009), pearceite–polybasite group minerals, [(Ag,Cu) 16 MS 11 where M = Sb, As] (Bindi et al. 2006, 2007a, b), and many others. This approach has also been applied to crystal structure studies of Ag-based ionic conductors from the argyrodite group, Ag 8 MX 6 where M = Ge, Sn, Si, P; X = S, Se, Te (e.g. Boucher et al. 1993; Gaudin et al. 2000). The crystal structure of pyrargyrite was determined for the irst time by Harker (1936) using the three-dimension- al Patterson method. Later, Engel and Nowacki (1966) reined the pyrargyrite structure to R = 6.9% with all atoms described as anisotropic, however exhibiting rather high values of displacement parameters for the Ag atoms in comparison to Sb and S atoms (B iso values of 4.67, 1.14 and 1.16 Å 2 for Ag, Sb and S atoms, respectively). Gagor et al. (2009) recently reined the crystal struc- ture of synthetic proustite using non-harmonic approach. Pyrargyrite and proustite are not only isostructural, they also display similar physical properties and crystal habit. Both mineral phases were investigated using impedance spectroscopy as possible ionic conductors by Schönau and Redfern (2002). In such Ag-bearing ionic conductors, Ag + cations can move easily above a transition temperature, giving a liquid-like structure in an open framework. The migration of silver is facilitated when there exists a low activation energy for atomic jumps (Bindi et al. 2006). The distribution of silver ions in the crystal structure may