Argentotennantite-(Fe), Ag ₆ (Cu ₄ Fe ₂ )As ₄ S ₁₃ , a new member of the tetrahedrite group from the San Genaro mine, Peru: occurrence and crystal structure

Argentotennantite-(Fe), Ag6(Cu4Fe2)As4S13, has been approved as a new mineral species (IMA 2023-126) by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (CNMNC-IMA) using a sample of quartz gangue with argentotetrahedrite-(Zn), pyrargyrite, baryte, and siderite from the San Genaro mine, Castrovirreyna Province, Huancavelica, Peru. Argentotennantite-(Fe) occurs as anhedral grains up to 100 µm in size. More commonly, it forms micrometer-sized rims around crystals of argentotetrahedrite-(Zn) or replaces the latter around cavities and fissures, along with Ag-rich tennantite-(Fe). Argentotennantite-(Fe) is opaque and dark gray in color, with a metallic luster and black streak. In reflected light, it is isotropic and pale gray, with a brownish shade. The reflectance values for wavelengths recommended by the Commission on Ore Mineralogy of the IMA, measured in air, are as follows [λ (nm, R (%)]: 470, 29.9; 546, 30.1; 589, 29.8; and 650, 28.9. The chemical formula of the grain used for the crystal structure study, recalculated on the basis of ΣMe = 16 atoms per formula unit, is (Ag3.36Cu2.70)Σ6.06(Cu4.05Fe1.88Zn0.07)Σ6.00(As2.29Sb1.66)Σ3.95S12.82. Argentotennantite-(Fe) is cubic, I4‾3m, with a=10.4365(5) Å, V=1136.75(17) Å3, and Z=2. Its crystal structure has been refined by single-crystal X-ray diffraction data to a final R1=0.0384 on the basis of 246 unique reflections with F>4σF and 20 refined parameters. Argentotennantite-(Fe) is isotypic with other members of the tetrahedrite group. Structural relationships between argentotennantite-(Fe) and selected members of the tetrahedrite-group minerals are discussed, and previous findings relating to this species are briefly reviewed.