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

Abstract

<jats:title>Abstract</jats:title><jats:p>The results of a HRTEM and electron diffraction investigation of a Cu- and Ag-rich cosalite, Cu<jats:sub>1.2</jats:sub>Ag<jats:sub>0.9</jats:sub>Pb<jats:sub>6.7</jats:sub>Bi<jats:sub>8.2</jats:sub>S<jats:sub>20</jats:sub>, from Ivigtut, Greenland, are presented. Electron diffraction patterns down [001] show pairs of satellite reflections at G<jats:sub>p</jats:sub>±~1/8[140]* and<jats:bold>G</jats:bold><jats:sub>p</jats:sub>±~1/8[<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0026461X00045771_inline1.png" />40]* in addition to the strong Bragg reflections of the underlying average structure. The superlattice corresponds to a compositional and displacive modulation<jats:bold>G</jats:bold>±<jats:bold>q</jats:bold><jats:sub>prim</jats:sub>where<jats:bold>q</jats:bold><jats:sub>prim</jats:sub>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 (Pb<jats:sub>3</jats:sub>Bi<jats:sub>2</jats:sub>S<jats:sub>6</jats:sub>–PbAgBi<jats:sub>3</jats:sub>S<jats:sub>6</jats:sub>). 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]<jats:sub>cosalite</jats:sub>.</jats:p>