The mixed-layer structures of ikunolite, laitakarite, joséite-B and joséite-A

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Marco E. Ciriotti
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The mixed-layer structures of ikunolite, laitakarite, joséite-B and joséite-A

Messaggio da Marco E. Ciriotti » gio 26 ago, 2021 20:48

Referenza:
▪ Cook, N.J., Ciobanu, C.L:, Slattery, A.D., Wade, B.P., EHring, K. (2021): The Mixed-Layer Structures of Ikunolite, Laitakarite, Joséite-B and Joséite-A. Minerals, 11, 920.

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.

NdR: Io avevo accomunato nel gruppo della joséite (supergruppo della tetradimite) le seguenti specie:
Babkinite Pb2Bi2S3
Ikunolite Bi4S3
Joséite-A Bi4+xTe1-x-yS2+y (x = 0.08-0.33; y = -0.11-0.09)
Joséite-B Bi4+xTe2-xS (x = -0.04-0.11)
Laitakarite Bi4Se2S
Pilsenite Bi4Te3
Poubaite PbBi2Se4
Rucklidgeite PbBi2Te4

Pur non avendo avuto risultati di qualità dalla diffrazione delle diverse specie (cristalli troppo piccoli, sottili, per il diffrattometro) anche a fronte di ottimi dati sul chimismo delle singole specie e quindi non pubblicabili, non sono andato troppo lontano… e forse non è che gli autori hanno dimenticato qualcosa?
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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