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

摘要

The results of a HRTEM and electron diffraction investigation of a Cu- and Ag-rich cosalite, Cu_1.2Ag_0.9Pb_6.7Bi_8.2S₂₀, from Ivigtut, Greenland, are presented. Electron diffraction patterns down [001] show pairs of satellite reflections at G_p±~1/8[140]* and G_p±~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±q_prim where q_prim 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.