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

AbstractThe results of a HRTEM and electron diffraction investigation of a Cu- and Ag-rich cosalite, Cu1.2Ag0.9Pb6.7Bi8.2S20, from Ivigtut, Greenland, are presented. Electron diffraction patterns down [001] show pairs of satellite reflections at Gp±~1/8[140]* and Gp±~1/8[40]* in addition to the strong Bragg reflections of the underlying average structure. The superlattice corresponds to a compositional and displacive modulation G±qprim where qprim 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.