Rozhdestvenskayaite Ag10Zn2Sb4S13 and argentotetrahedrite Ag6Cu4(Fe2+,Zn)2Sb4S13: two Ag-dominant members of the tetrahedrite group

We report the characterisation of two new tetrahedrite-group minerals, rozhdestvenskayaite, Ag₁₀Zn₂Sb₄S₁₃, and argentotetrahedrite, Ag₆Cu₄(Fe²⁺,Zn)₂Sb₄S₁₃, and discuss the structural chemistry of the Ag-rich tetrahedrite group in order to identify correlations between structure and composition. Argentotetrahedrite is the Sb-analogue of argentotennantite. Rozhdestvenskayaite is named for Irina Rodzhdestvenskaya in recognition of her important contributions to mineralogy, and to the crystal chemistry of the tetrahedrite group in particular. Rozhdestvenskayaite is cubic $I4¯3m$ with unit-cell parameter a = 10.9845(7) Å and V = 1325.37(15) ų, and has a Mohs’ hardness of 3.0. Argentotetrahedrite is cubic $I4¯3m$ with unit-cell parameter a = 10.6116(1) Å and V = 1194.92(2) ų, and has a Mohs’ hardness of 3.9. The five strongest peaks in the X-ray powder diffraction patterns ([(hkl), d_obs (Å), I/I_max (%)]) are: rozhdestvenskayaite [(222), 3.161, 100], [(004), 2.738, 35], [(044), 1.936, 24], [(226), 1.651, 19], [(134,015), 2.147, 18]; argentotetrahedrite [(222), 3.063, 100], [(044), 1.876, 35], [(004), 2.652, 28], [(226), 1.599, 25], [(134,015), 2.081, 19]. The large unit-cell volume of rozhdestvenskayaite is due to the high Ag content (70%) of the BS₄ tetrahedron. A major issue for understanding the crystal chemistry of the tetrahedrite group relates to the occupancy of the Z site in which sulfur is coordinated octahedrally to monovalent metals (Cu or Ag). It is shown that this site can be completely empty, as in freibergite, and that this vacancy leads to a marked reduction in the volume of the Z(A₆) octahedron from 16 Å³ in tetrahedrite with a fully occupied site to 11 Å³ in freibergite with a vacant Z site. In freibergite, the Ag–Ag distance of the Ag₆ octahedron is 2.84 Å and is almost the same as that of silver metal (2.85 Å), strongly suggesting the presence of metallic bonding in this octahedral group. In contrast, the corresponding Ag–Ag distances of argentotetrahedrite and rozhdestvenskayaite are 3.23 Å and 3.24 Å, respectively, indicative of no metallic bonding. An important consequence of the metallically bonded Ag₆ group of freibergite is that it has an aggregate formal charge of +4 (not +6), leading to a charge-balanced ideal formula (Ag₆)⁴⁺(Cu⁺)₄(Zn,Fe²⁺)₂Sb₄S₁₂.