Natural djurleite with refined composition Cu61.39S32 revealing disorder of some Cu sites

The djurleite phase with composition Cu61.39S32 was refined from a natural twinned specimen and shows disorder of some of its Cu sites.


Structure description
The Cu-S system has been the subject of structural research for nearly one century. Several well-defined compounds and their crystal structures have been reported, with high-chalcocite (Buerger & Wuensch, 1963), low-chalcocite and djurleite (Evans, 1979a,b) being the most prominent ones. Low-chalcocite and djurleite are difficult to distinguish, thus many samples labelled chalcocite represent in fact djurleite, or an intimately intergrown mixture of low-chalcocite and djurleite (Evans, 1981). The existence of djurleite was not suspected until it was discovered and suggested to be of orthorhombic symmetry by Djurle (1958). This phase was later assigned as an independent mineral species (Roseboom, 1962;Morimoto, 1962). Further studies of this mineral revealed that the previously supposed space group of Pmmm is affected by systematic twinning and that djurleite actually crystallizes in the monoclinic space group P2 1 /n (Takeda et al., 1967;Evans, 1979a,b). The final determination of the crystal structures of low-chalcocite and djurleite was accomplished by Evans (1979a). Lowchalcocite from Bristol, Connecticut, crystallizes in space group P2 1 /c and contains 48 formula units of Cu 2 S [a = 15.246 (4), b = 11.884 (2), c = 13.494 (3) Å , = 116.35 (1) ] while djurleite from the Ozark Lead Co. mine at Sweetwater, Missouri, contains eight formula units of Cu 31 S 16 in space group P2 1 /n [a = 26.897 (6), b= 15.745 (3), c= 13.565 (3) Å , = 90.13 (3) ; Evans, 1979a,b]. All atoms in the above structure models data reports were assumed to be fully occupied. However, there are some studies based on HRTEM revealing that there are vacancies of Cu in natural metadjurleite (Xu et al., 1991) or low-djurleite (Sun & Xue, 2001), but without a refined structure model. There are also some studies on synthetic copper-deficient copper sulfides, either considered as monoclinic djurleite (Yoon et al., 2015), or as cubic Cu 2 S (Zhou et al., 2016.;Zhang et al., 2021).
Since the crystal structure of djurleite has been discussed in detail in the original description (Evans, 1979a), here only the main differences are emphasized. For the present study, a crystal from a natural sample was used, revealing a refined composition of Cu 61.39 S 32 . In comparison with the original Cu 31 S 16 model where all sites are ordered and fully occupied, eight Cu sites are split over two positions and one Cu site shows partial occupancy. The crystal under investigation used for the present study was twinned whereas that used for the original study was not reported to be twinned. The reported temperature of formation for the latter crystal was below 370 K. Fig. 1 shows the overall atomic distribution of Cu 61.39 S 32 in the unit cell. For simplicity, two different coordination polyhedra around Cu sites are highlighted, with a criterion of the Cu-S distances being less than 3.0 Å . Representative for the principal environment of Cu sites, Cu1 shows a triangular coordination by three S atoms (Fig. 2). Another environment of Cu sites is represented by Cu48, where the surrounding four S atoms form a distorted tetrahedron, as shown in Fig. 3. The low-occupancy Cu12 site has the same environment as the Cu1 atom. The eight pairs of split Cu atoms have the same environment as Cu48, except the pairs Cu29A/B, Cu45A/B and Cu62A/B, which have the same environment as the Cu1 atom.

Synthesis and crystallization
Natural samples designated as chalcocite were purchased from Honghu Minerals (Hubei Province, China), Alibaba Taobao Co. Suitable single-crystal fragments were broken from a larger sample and glued on glass fibers for single-crystal X-ray diffraction experiments. Energy-dispersive X-ray spectroscopy measurements did not indicate the presence of elements other than Cu and S (see supporting information).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. Starting atomic coordinates and labels were adapted from the original structure investigation (Evans, 1979a). The crystal under investigation was twinned by pseudo-merohedry, revealing a twin ratio of 0.92:0.08. Eight pairs of split Cu sites were assigned, in all cases assuming full occupancy using the same anisotropic displacement parameters (EADP) for each pair: Cu29A/Cu29B [occupancy ratio 0.808 (12)/0.192 (12) The environment of Cu1. Displacement ellipsoids are drawn at the 90% probability level. [Symmetry code: (i) x, y, z À 1.]

Figure 1
The unit cell of Cu 61.39 S 32 , with sites Cu1 and Cu48 displayed with their different coordination environments as polyhedra.