The Mixed-Layer Structures of Ikunolite, Laitakarite, Joséite-B and Joséite-A

Abstract

We used high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) to image the crystal structures of four minerals in the Bi4X3 isoseries (X = Te, Se, S), a subgroup of the tetradymite homologous series: ikunolite (Bi4S3), laitakarite (Bi4Se2S), joséite-B (Bi4Te2S), and joséite-A (Bi4TeS2). The four minerals are isostructural and interpretable in terms of regular stacking of seven-atom packages: [Bi–S–Bi–S–Bi–S–Bi], [Bi–Se–Bi–S–Bi–Se–Bi], [Bi–Te–Bi–S–Bi–Te–Bi], and [Bi–S–Bi–Te–Bi–S–Bi], respectively. The four phases are mixed-layer structures representing the Bi2kTe3 (k = 2) module within the tetradymite series. Diffraction patterns confirm they are seven-fold superstructures of a rhombohedral subcell with c/3 = d~1.89–1.93 Å. Modulation along the d* interval matches calculations of reflection intensity using the fractional shift method for Bi4X3. Internal structures can be discerned by high-resolution HAADF STEM imaging and mapping. Paired bismuth atoms are positioned at the outside of each seven-atom layer, giving the minerals a modular structure that can also be considered as being composed of five-atom (X–Bi–X–Bi–X) and two-atom (Bi–Bi) sub-modules. The presence of mixed sites for substituting cations is shown, particularly for Pb. Moreover, Pb may be important in understanding the incorporation of Ag and Au in Bi–chalcogenides. Visualisation of crystal structures by HAADF STEM contributes to understanding relationships between phases in the tetradymite homologous series and will play an invaluable role in the characterization of potential additional members of the series.

Bi8Te3, the 11-Atom Layer Member of the Tetradymite Homologous Series

“…The crystal structure of Bi 8 Te 3 is shown on two projections (Figure 5B,C), both of which are relevant for the definition of stacking sequences for phases in the tetradymite group. This model is in good agreement with incremental layer expansion within the group via addition of n × mod2 (Bi-Bi pairs) to the mod5 slab common to all structures [1,2,13].…”

“…Whichever model is best-suited to describe the tetradymite series, recognition of layered compounds within a homologous series allows new structures to be accurately predicted from compositional data and the specific characteristics of electron diffractions, an intrinsic feature of mixed-layer compounds [21]. The data presented here further emphasize that Z-contrast imaging techniques such as HAADF STEM are optimally suited for characterization of mixed-layer compounds [13,20,[22][23][24][25].…”

“…Bi 8 Te 3 occurs within disseminations of bismuth minerals (dominantly joséite-B and native bismuth, with subordinate hedleyite and traces of joséite-A) in a hedenbergite skarn (Figures 1 and 2). Micron-to nano-scale compositional and structural characterization of joséite-A and -B is described by Cook et al [13]. In reflected light, Bi 8 Te 3 displays high reflectance, anisotropy, and is indistinguishable from hedleyite in air.…”

“…High-resolution images of the disordered sequences (Figure 10) show dark lines clearly separating individual layer units with central arrays of single or double Bi-Bi dumbbells (Figure 10A). Such dark lines are attributable to sulfur occurring in the middle part of joséite-B (7-atom layer [13]). The sequence can be identified using d7 and d9 layer widths of ~13.5 and ~17 Å.…”

“…In this scheme [2], tetradymite and isostructural Bi 2 X 3 phases, such as tellurobismuthite, paraguanajuatite (Bi 2 Se 3 ), kawazulite (Bi 2 Te 2 Se), and skippenite (Bi 2 Se 2 Te), are represented by simple repeats of a single 5-atom X-Bi-X-Bi-X layer. Members of the Bi 4 X 3 subgroup (ikunolite, laitakarite, pilsenite, joséite-A, and joséite-B) share 7-atom layers only [13]. Named phases in the Bi 3 X 3 (BiX) subgroup, tsumoite, ingodite, nevskite, and telluronevskite, are composed of a combination of 5-and 7-atom layers.…”