Comparative modular analysis of two complex sulfosalt structures: sterryite, Cu(Ag,Cu)3Pb19(Sb,As)22(As–As)S56, and parasterryite, Ag4Pb20(Sb,As)24S58

Abstract

The crystal structures of two very close, but distinct complex minerals of the lead sulfosalt group have been solved: sterryite, Cu(Ag,Cu)3Pb19(Sb,As)22(As–As)S56, and parasterryite, Ag4Pb20(Sb,As)24S58. 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 (∼ 2 Å along the elongation parameter) of the constitutive rod layers, as illustrated by distinct cell settings within the same space group (P21/n and P21/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.