Kenoargentotetrahedrite-(Zn), [Ag₆]⁴⁺(Cu₄Zn₂)Sb₄S₁₂□, a new member of the tetrahedrite group from the Yindongpo Au deposit, China

Abstract

The new mineral kenoargentotetrahedrite-(Zn), [Ag6]4+(Cu4Zn2)Sb4S12□, was discovered at the Yindongpo Au deposit, Henan Province, China. It occurs as black metallic anhedral grains or equant crystals up to 40 µm 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)(Ag3.75Cu2.25)Σ6M(1)[Cu3.90(Zn1.18Fe0.69Cd0.26)Σ2.13]Σ6.03X(3)(Sb3.69As0.27)Σ3.96S(1)S11.94 S(2)□. Kenoargentotetrahedrite-(Zn) is cubic, I4‾3m (no. 217), with a = 10.4624(4) Å, V= 1145.23(13) Å3, 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 > 4σ (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 [Ag6]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: 6M(2)Ag++S(2)S2-= M(2)[Ag6]4++S(2)□.