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[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)Ag6M(1)(Ag2.41Cu1.20Fe1.84Zn0.71)Σ6.16X(3)(Sb3.82As0.01)Σ3.83S(1)S11.60S(2)□. It is cubic, with space group I 4 3m, a = 10.7119(6) Å, V = 1229.1(2) Å3, 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-fS2 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]

The new mineral kenoargentotetrahedrite-(Zn), [Ag 6 ] 4+ (Cu 4 Zn 2) Sb 4 S 12□ , was discovered at the Yindongpo Au deposit, Henan Province, China. It occurs as black metallic anhedral grains or equant crystals up to 40 µ m in size. It is opaque in transmitted light and shows a greenish-grey colour in reflected light. Electron microprobe analysis for the studied material gave the empirical formula (on the basis of total cations being equal to 16 atoms per formula unit, apfu): M(2) (Ag 3.75 Cu 2.25) Σ6 M(1) [Cu 3.90 (Zn 1.18 Fe 0.69 Cd 0.26) Σ2.13 ] Σ6.03 X(3) (Sb 3.69 As 0.27) Σ3.96 S(1) S 11.94 S(2)□. Kenoargentotetrahedrite-(Zn) is cubic, I4‾ 3 m (no. 217), with a = 10.4624(4) Å, V= 1145.23(13) Å 3 , and Z= 2. The crystal structure has been refined to a final value of R1= 0.0247 on the basis of 274 independent reflections (Fo > 4 σ (Fo)) by using single-crystal X-ray diffraction data. The seven strongest X-ray powder diffraction lines (d in Å (I) (hkl)) are the following: 3.010 (100) (222), 1.844 (30) (044), 2.606 (22) (004), 1.572 (15) (226), 2.046 (9) (134), 7.35 (6) (011), and 1.909 (5) (125). Kenoargentotetrahedrite-(Zn) is isostructural with other keno-member tetrahedrites with the [Ag 6 ] 4+ cluster. The structure refinement result confirms the coupling between the site occupancy factor of subvalent hexasilver clusters at the M (2) site and that of the vacancy at the S (2) site. This relationship further substantiates the charge balance substitution mechanism of S-deficiency tetrahedrites: 6 M(2) Ag ++S(2) S 2-= M(2) [Ag 6 ] 4++S(2)□. [ABSTRACT FROM AUTHOR]