Chessboard structures: Atom-scale imaging of homologs from the kobellite series

Abstract

Abstract High-angle annular dark-field scanning transmission electron microscopy is a powerful Z-contrast technique able to depict the structural motifs in Pb-(Bi-Sb)-sulfosalts. Using two homologs from the kobellite homologous series, a group of “chessboard derivative structures,” represented by Bi-, and Sb-rich pairs of natural phases (the kobellite-tintinaite isotypic series and giessenite-izoklakeite homeotypic series), we visualize the slabs underpinning crystal structural modularity for the N = 2 homolog kobellite and the N = 4 homolog, in this case a Bi-rich izoklakeite [Sb/(Sb+Bi) = 0.35]. The homolog number, N, can be readily calculated as N = n1/6 – 1 and N = n2/4, where n1 and n2 are the numbers of atoms in the PbS- and SnS-motifs, respectively. Atom-scale imaging of thinned foils extracted in situ from samples for which compositional data are available also reveals syntactic unit-cell scale intergrowths on [001] zone axis with akobeiiite ∥ bizoidakefa. These are as small as half-unit cells of bizokiakeite and one-unit cell akobellite. Replacement relationships are also observed as irregular slabs of kobellite “intruding” into izoklakeite. Both banded and irregular intergrowths account for the compositional fields measured at the micrometer scale.