CRYSTAL STRUCTURE OF COPPER-RICH UNSUBSTITUTED TENNANTITE, Cu12.5As4S13

Abstract

The crystal structure of a Cu-rich tennantite from Cerro Atajo, province of Catamarca, northwestern Argentina, has been refined using single-crystal data (MoKX-ray diffraction, CCD detector, R1 = 0.03). It crystallizes in space group I3m, with a = 10.1756(9) A. The X-ray-diffraction analysis led to the formula Cu12.5(As0.98Sb0.02)� 4S13, in full agreement with the electron- microprobe data and the unit-cell parameter. Cu-rich tennantite shows all structural features of tetrahedrite and tennantite, but it contains a partly occupied triangular Cu2A position, accompanied by a new site, Cu2B, coordinated tetrahedrally to three S and one As atom. This finding is in contrast to Cu-rich tetrahedrite, where partial occupancy and redistribution of Cu are associated with the Cu1 site. Atoms Cu2A and Cu2B form, respectively, an octahedron and a Laves polyhedron around the S2 site, a situation similar to Te3 coordination in � -Ag8SiTe6 and � -Ag8GeTe6. On the basis of bond lengths, we contend that the Cu 2+ necessary to balance the formula resides primarily at the Cu1 site. The electron density shows a dynamic disorder between closely separated Cu2A and Cu2B sites, which, together with available vacancies at these positions, suggests that Cu-rich tennantite is an ionic conductor even at room temperature. The Cu-rich tennantite is a rare case of formation from late hydrothermal solutions devoid of ubiquitous Fe, Zn and other divalent elements, which usually enter tennantite as electron donors.