Chemical Composition and Crystal Structure of Kenoargentotetrahedrite-(Fe), Ag6Cu4Fe2Sb4S12, from the Bajiazi Pb-Zn Deposit, Liaoning, China

Abstract

Kenoargentotetrahedrite-(Fe) is observed as greenish-grey anhedral grains, 50–150 μm in size, in association with galena, sphalerite and chalcopyrite in the Bajiazi Pb-Zn deposit of magmatic-hydrothermal type, Liaoning, China. The empirical formula from electron microprobe analyses is Ag5.50Cu4.17Fe1.75Zn0.31Sb3.96As0.04S12.08, corresponding to the ideal formula Ag6Cu4Fe2Sb4S12. The crystal structure of kenoargentotetrahedrite-(Fe) has been determined and refined by single-crystal X-ray diffraction with R1 = 0.0192 for 1866 (404 unique) reflections. It is cubic, space group I4¯3m with unit cell parameters a = 10.4928(2) Å, V = 1155.26(7) Å3 and Z = 2. The structure of kenoargentotetrahedrite-(Fe) is characterized by a poor occupancy of 0.05 of the octahedral S(2) site with the S(2)-M(2) bonding length of 1.9994(8) Å. The six Ag atoms at M(2) around S(2) form an octahedron cluster (Ag6)4+ with the valence state of +4 and Ag-Ag distance of 2.8276(1) Å. The structure is identical to that by Rozhdestvenskaya et al., being composed of a collapsed sodalite-like framework of corner-connected M(1)S4 tetrahedron forming cages containing M(2)6-octahedron cluster, encircled by four SbS3 trigonal pyramids. It is related to the tetrahedrite group minerals with the existence of the (Ag6)4+ cluster replacing the S(2)-centered Ag6 octahedron according to the substitution mechanism 6M(2)Ag+ + S(2)S2−=M(2)(Ag6)4+ + S(2) S.