Temperature dependence of the silver distribution in the crystal structure of natural pearceite, (Ag,Cu)16(As,Sb)2S11.
The crystal structure of the mineral pearceite, (Ag,Cu)16(As,Sb)2S11, has been solved and refined at 300, 120 and 15 K. At room temperature pearceite crystallizes with trigonal symmetry, space group P3m1; the refinement of the structure leads to a residual factor of R = 0.0464 for 1109 independent observed reflections and 92 variables. The crystal structure consists of sheets stacked along the c axis. The As atoms form isolated (As,Sb)S3 pyramids, which typically occur in sulfosalts, copper cations link two S atoms in a linear coordination, and the silver cations are found in a fully occupied position and in various sites corresponding to the most pronounced probability density function locations (modes) of diffusion-like paths. These positions correspond to low-coordination (2, 3 and 4) sites, in agreement with the preference of silver for such environments. d10 silver-ion distribution has been determined by means of a combination of a Gram-Charlier description of the atomic displacement factors and a split-atom model. To analyse the crystal chemical behaviour of the silver cations as a function of temperature, a structural study was carried out at 120 K (R = 0.0450). The refinement indicates that the mineral exhibits the same structural arrangement as the room-temperature structure (space group P3m1) and shows that the silver cations are still highly disordered. In order to investigate a possible ordering scheme for the silver cations, a data collection at ultra-low temperature (15 K) was performed. The structural skeleton was found to be similar to that of the room-temperature and 120 K atomic structures, but the best solution was achieved with a fully split-atom model of five silver positions, giving an R value of 0.0449 for 651 observed reflections and 78 parameters. Although the silver cation densities condense into better defined modes, the joint probability density function still exhibits a strong overlapping of neighbouring sites.
Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, 50121 Firenze, Italy. lbindi@geo.unifi.it
Luca
L
Bindi
Michel
M
Evain
Silvio
S
Menchetti