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)M 6 ′

[Ag₆]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 [Ag₆]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 Ag₆(Ag₄Fe₂)Sb₄S₁₂□ is a new tetrahedrite group mineral containing a natural [Ag₆]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 μm 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/cm³. The empirical formula calculated on the basis of cation = 16 apfu is ^M(2)Ag₆^M(1)(Ag_2.41Cu_1.20Fe_1.84Zn_0.71)_Σ6.16^X(3)(Sb_3.82As_0.01)_Σ3.83^S(1)S_11.60^S(2)□. It is cubic, with space group I 4 3m, a = 10.7119(6) Å, V = 1229.1(2) ų, and Z = 2. Since kenorozhdestvenskayaite-(Fe) is a new tetrahedrite group mineral containing a natural [Ag₆]⁴⁺ cluster, its structure is comparable to the synthetic ternary phosphide (Ag₆M₄P₁₂) M 6 ′ . 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. [ABSTRACT FROM AUTHOR]