Bindi Luca, Evain Michel, Menchetti Silvio
Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, 50121 Firenze, Italy.
Acta Crystallogr B. 2006 Apr;62(Pt 2):212-9. doi: 10.1107/S010876810600108X. Epub 2006 Mar 15.
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.
矿物硫砷铜银矿（(Ag,Cu)16(As,Sb)2S11）的晶体结构已在300 K、120 K和15 K温度下解析并精修。在室温下，硫砷铜银矿以三方对称结晶，空间群为P3m1；对该结构的精修得到1109个独立观测反射和92个变量的残余因子R = 0.0464。晶体结构由沿c轴堆叠的片层组成。砷原子形成孤立的(As,Sb)S3棱锥，这在硫盐中很常见，铜阳离子以线性配位连接两个硫原子，银阳离子处于完全占据的位置以及对应于扩散样路径最显著概率密度函数位置（模态）的各种位点。这些位置对应低配位（2、3和4）位点，这与银对这种环境的偏好一致。通过对原子位移因子的Gram-Charlier描述和分裂原子模型相结合的方法确定了d10银离子分布。为了分析银阳离子随温度变化的晶体化学行为，在120 K下进行了结构研究（R = 0.0450）。精修表明该矿物呈现出与室温结构相同的结构排列（空间群P3m1），并且表明银阳离子仍然高度无序。为了研究银阳离子可能的有序方案，在超低温（15 K）下进行了数据收集。发现结构骨架与室温及120 K原子结构相似，但采用五个银位置的完全分裂原子模型得到了最佳结果，对于651个观测反射和78个参数，R值为0.0449。尽管银阳离子密度凝聚成定义更明确的模态，但联合概率密度函数仍显示相邻位点有强烈的重叠。
