Comparative modular analysis of two complex sulfosalt structures: sterryite, Cu(Ag,Cu)(3)Pb-19(Sb,As)(22)(As-As)S-56, and parasterryite, Ag4Pb20(Sb,As)(24)S-58

International audienceThe crystal structures of two very close, but distinct complex minerals of the lead sulfosalt group have been solved: sterryite, Cu(Ag,Cu)(3)Pb-19(Sb,As)(22)(As-As)S-56, and parasterryite, Ag4Pb20(Sb,As)(24)S-58. They are analyzed and compared according to modular analysis. The fundamental building block is a complex column centred on a Pb6S12 triangular prismatic core, with two additional long and short arms. The main chemical and topological differences relate to the short arm, which induces a relative a/4 shift (similar to 2 angstrom along the elongation parameter) of the constitutive rod layers, as illustrated by distinct cell settings within the same space group (P2(1)/n and P2(1)/c, respectively). Selection of the shortest (i.e. strongest) (Sb,As)-S bonds permitted to enhance the polymeric organization of (Sb, As) atoms with triangular pyramidal coordination. These two quasi-homeotypic structures are expanded derivatives of owyheeite, Ag3Pb10Sb11S28. The hierarchy of organization levels from zero- to three-dimensional entities is subordinated to building operators, which appear as the driving force for the construction of such complex structures. Minor cations (Ag, Cu) or the As-As pair in sterryite secure the final locking, which favours the formation of one or the other compound