Crystal structure of argentopyrite, AgFe₂S₃, and its relationship with cubanite

The structure of argentopyrite, AgFe₂S₃, was determined for the first time with single-crystal X-ray diffraction. In contrast to the previously reported orthorhombie symmetry, our data show that argentopyrite is monoclinic with space group P112₁/n (non-standard setting) and unit-cell parameters a = 6.6902(2), b = 11.4497(4), c = 6.4525(2) Å, γ = 90.2420(8)°, and V= 494.26(3) ų. Similar to cubanite (CuFe₂S₃), the structure of argentopyrite is also based on approximately hexagonal close-packed S atoms, with cations ordered over one half of the tetrahedral sites, forming corner-shared AgS₄ and FeS₄ tetrahedral sheets parallel to (001). The two structures differ chiefly in the linkage between the two adjacent tetrahedral sheets and the ordering patterns of cations within a tetrahedral sheet. Topologically, the structure of argentopyrite can be obtained by a displacement of a tetrahedral sheet in the cubanite structure along the (a/2 + b/6) direction relative to the sheet beneath, giving rise to a cluster of four edge-shared FeS₄ tetrahedra in argentopyrite, as compared to two in cubanite. There are two distinct Fe sites (Fel and Fe2) in argentopyrite, rather than only one, as in other MFe₂S₃ sulfide minerals (M = monovalent cations). Together with published Mössbauer data, we suggest that there exists some degree Of Fe²⁺-Fe³⁺ order-disorder in argentopyrite, with Fe²⁺ favoring the more distorted Fe2 tetrahedral site. Argentopyrite appears to possess all the features proposed by Putnis (1977) for a high-temperature ordered form, of cubanite.