Kenoargentotennantite-(Fe), Ag ₆ (Cu ₄ Fe ₂ )As ₄ S ₁₂ □, a new “keno” member of the tetrahedrite group

Kenoargentotennantite-(Fe), Ag 6 (Cu 4 Fe 2 )As 4 S 12 , has been discovered at the Pollone mine, Valdicastello Carducci, Pietrasanta, Apuan Alps, Tuscany (Italy), where it occurs in quartz baryte extension veins hosted in the pyrite baryte ore body as m-sized rims grown around crystals of Ag-bearing tennantite-(Zn), in association with argentotennantite-(Zn) and the notyet approved "kenoargentotennantite-(Zn)".Its colour is black and the luster is metallic.In reflected light microscopy, kenoargentotennantite-(Fe) is isotropic, grey with greenish tints in colour.Reflectance values for the four COM wavelengths in air are [ (nm): R (%)]: 470: 31.7;546: 30.6; 589: 29.8; 650: 28.5.Electron microprobe analysis (average of 6 spot analyses) gave (in wt%): Cu 20.39, Ag 27.21, Fe 3.90, Zn 1.45, Hg 4.28, As 11.99, Sb 8.28, S 22.21, total 99.71.The chemical formula, recalculated on the basis of Me = 16 atoms per formula unit, is Cu 5.61(6) Ag 4.41(7) Fe 1.22(3) Zn 0.39(5) Hg 0.37(2) (As 2.80(4) Sb 1.19(4) )S 12.12(9) .Kenoargentotennantite-(Fe) is cubic, I 4 3m, with a = 10.3740(5), V = 1116.45(16) 3 , Z = 2.The crystal structure of kenoargentotennantite-(Fe) has been refined by single-crystal Xray diffraction data to a final R 1 = 0.0404 on the basis of 374 reflections with F > 4 F and 23 refined parameters.Kenoargentotennantite-(Fe) is isotypic with other members of the tetrahedrite group.Silver is hosted at the triangularly coordinated M(2) site, whereas the tetrahedrally coordinated M(1) site is occupied by Cu and Fe, with minor Zn and Hg.The S(2) site is mainly vacant, counterbalanced by the metal cluster [Ag 6 ] 4+ , thus reflecting the keno-nature of this new member of the tetrahedrite group.Along with argentotennantite-(Fe) and argentotennantite-(Zn), kenoargentotennantite-(Fe) belongs to the arsenofreibergite series.