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CRYSTAL STRUCTURE OF COPPER-RICH UNSUBSTITUTED TENNANTITE, Cu_12.5As₄S₁₃

Emil Makovicky^1,, Ljiljana Karanovi^1,¶, Dejan Poleti^1,, Toni Bali-uni¹ and Werner H. Paar²

¹ Geological Institute, University of Copenhagen, Øster Voldgade 10, DK–1350 Copenhagen K, Denmark
² Department of Geography, Geology and Mineralogy, University of Salzburg, Hellbrunnerstrasse 34/III, A–5020 Salzburg, Austria

E-mail address: emilm{at}geol.ku.dk

The crystal structure of a Cu-rich tennantite from Cerro Atajo, province of Catamarca, northwestern Argentina, has been refined using single-crystal data [MoK X-ray diffraction, CCD detector, R₁ = 0.03]. It crystallizes in space group I3m, with a = 10.1756(9) Å. The X-ray-diffraction analysis led to the formula Cu_12.5(As_0.98Sb_0.02)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 Cu²⁺ 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.

Keywords: Cu-rich tennantite, crystal structure, tetrahedrite, copper disorder, solid electrolyte, bismuthian watanabeite, Cerro Atajo, Argentina.

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