Structure of lengenbachite: a high-resolution transmission electron microscope study

High-resolution transmission electron microscopy and electron diffraction have been used to examine the structure of lengenbachite, of approximate composition Pb₆As₄(Ag,Cu)₂ S₁₃. The structure deduced consists of a semicommensurate intergrowth of a pseudotetragonal layer (T), a sheared four-atom-thick {100} PbS-like slice, and a slightly distorted five-atom-thick pseudo-orthohexagonal {111} M₂S₃ slice (O). Both layer types contain Pb and As atoms in cation sites, giving layer stoichiometries of T = (PB,As)₄S₄ and O = (As,Pb)₂S₃. The structure is in agreement with the cell geometries and compositions proposed by earlier workers. The relationships with the structures of cylindrite and "franckeites' are also discussed. -Authors