Phases in the tetrahedrite-tennantite solid solution series with the general composition (Cu,Ag)₁₀(Fe,Zn,Hg,Cu,…)₂(Sb,As)₄S₁₃ are among the most frequent complex sulfides in ore deposits. The crystal structures of the tetrahedrite-tennantite family can be derived from the  sphalerite structure type, where a part of the tetrahedra is replaced by other coordination polyhedra. Cu(1)S₄ tetrahedra form a tetrahedral framework with large cavities which host clusters of Cu(2) with three-fold coordination and lone electron pairs of Sb (As).  These metalloids are positioned as (Sb,As)S₃ pyramids in the cavity walls.

The four phases investigated here do not deviate from the ideal stochiometry

M₁₂(Sb,As)₄S₁₃,  yet some of the Cu atoms are substituted by Fe. Their compositions are Cu_11.4Fe_0.6Sb₄S₁₃, Cu_10.2Fe_1.8Sb₄S₁₃, Cu_11.9Fe_0.1As₄S₁₃, and Cu_10.8Fe_1.2As₄S₁₃. At low substitution, iron is incorporated as Fe³⁺. With increasing substitution, Fe³⁺ is converted into Fe²⁺, the conversion being complete at a composition of Cu₁₀Fe₂(Sb,As)₄S₁₃.

Rietveld refinement of x-ray diffraction data from low-Fe and high-Fe samples of both  compounds, obtained at SNBL/ESRF (Grenoble) (λ=0.3748 Å) at 25^o and 250^oC,  suggests that the coefficient of thermal expansion is highest for samples with low iron substitution. Splitting of triangular Cu(2) positions increases with temperature and the half-occupied Cu(2) positions from different coordination triangles approach each other, down to 2.70-2.75 Å in tennantite at 250^oC. There is insignificant residual electron density between the split Cu(2)-sites in tetrahedrite at 25^oC and its increase with temperature is moderate. The inter-site density is substantially higher in tennantite and increases considerably with temperature, especially in the low-Fe sample.

 F_obs and difference-Fourier maps (minima/maxima in eÅ^-3)  of  Cu_11.9Fe_0.1As₄S₁₃. Central point x(Cu2),0,0; distances in the three directions 2.5x2.5x2.5 Ǻ. Contour lines 2 (Fobs) and 0.1 (difference-Fourier).

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