HRTEM observations of structural and chemical modulations in cosalite and its relationship to the lillianite homologues

The results of a HRTEM and electron diffraction investigation of a Cu- and Ag-rich cosalite, pairs of satellite reflections at G_p ± ~ 1 8[140]* and G_p ± ~ 1 8[1̄40]* in addition to the strong Bragg reflections of the underlying average structure. The superlattice corresponds to a compositional and displacive modulation G ± q_pnm where q_pnm is the primary modulation vector 1 8[140]*. The superlattice is attributed to coupled Ag Pb Bi and Cu □ ordering and associated structural relaxation. The HRTEM images show a complex moire texture due to the structural modulations. Cosalite from Ivigtut, Greenland, also forms coherent intergrowths with members of the lillianite gustavite series (Pb₃Bi₂S₆ PbAgBi₃S₆). A model for the intergrowth between the structures is presented based on sharing both S and Pb atoms in the PbS-like slabs in both structures along [010] _cosalite.