Kenorozhdestvenskayaite-(Fe), Ag6(Ag4Fe2)Sb4S12□: A new tetrahedrite group mineral containing a natural [Ag6]4+ cluster and its relationship to the synthetic ternary phosphide (Ag6M4P12)M6′

[Ag-6](4+) clusters are extremely rare in nature (only found in Ag-rich tetrahedrite group minerals). Due to their remarkable structures and some promising applications, a few synthesis phases that contain octahedral [Ag-6](4+) clusters have been reported. However, the kinds of natural conditions that promote the formation of subvalent hexasilver clusters in tetrahedrite group minerals are still unclear. Kenorozhdestvenskayaite-(Fe), ideally Ag-6(Ag4Fe2)Sb4S12 square is a new tetrahedrite group mineral containing a natural [Ag-6](4+) cluster, found in the Yindongpo gold deposit, Weishancheng ore field, Henan Province, China. This new species occurs at the edges of galena crystals as anhedral grains of 2 to 20 mu m in size and is associated with pyrargyrite, pyrrhotite, and siderite. Kenorozhdestvenskayaite-(Fe) is black in color with metallic luster. It is brittle with conchoidal fracture and has a calculated density of 5.329 g/cm(3). The empirical formula calculated on the basis of cation = 16 apfu is Ag-M(2)(6)M(1)(Ag2.41Cu1.20Fe1.84Zn0.71)(Sigma 6.16)(X(3))(Sb3.82As0.01)(Sigma 3.83)S-S(1)(11.60)S(2)square. It is cubic, with space group I (4) over bar 3m, a = 10.7119(6) angstrom, V = 1229.1(2) angstrom(3), and Z = 2. Since kenorozhdestvenskayaite-(Fe) is a new tetrahedrite group mineral containing a natural [Ag-6](4+) cluster, its structure is comparable to the synthetic ternary phosphide (Ag6M4P12)M6 '. The presence of the unusual mineral assemblages, i.e., pyrrhotite and pyrargyrite, as well as the other keno-end-member tetrahedrites, indicates a low-f(S2) state for the mineralization stage, probably a result of the fluid boiling process in an open system that likely contributed to the formation of S-deficient tetrahedrites.