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

Nigel John Cook,Ashley D Slattery,Cristiana L Ciobanu,Kathy Ehrig,Benjamin P Wade

doi:10.3390/min11090920

Nigel John Cook, Ashley D Slattery + Show 3 more

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https://doi.org/10.3390/min11090920

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Journal: Minerals	Publication Date: Aug 25, 2021
Citations: 5	License type: CC BY 4.0

Affiliation: University of Adelaide, BHP (Australia)

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.

Highlights

The tetradymite homologous series is an extended family of named minerals, unnamed natural phases, experimentally synthesized products, and predicted mixed-layer structures with Bix Xy stoichiometry (X = S, Se, or Te) that can be related to the tetradymite (Bi2 Te2 S) archetype [1,2]
To derive a systematic homology for the series and predict structural arrangements for any stoichiometry, Ciobanu et al [2] presented a structural model drawing on earlier work by many authors ([3,4,5,6,7,8], among others), in which layer stacks of different size are combined in various proportions
The individual crystal structures could be better refined if using dynamical diffraction refinement or precession electron diffraction to eliminate dynamical effects, e.g., [33,34,35], such dedicated methods were not used for the present study, which is focused on imaging the structures and correlating such imaging with generic homology in the tetradymite series

Summary

Introduction

To derive a systematic homology for the series and predict structural arrangements for any stoichiometry, Ciobanu et al [2] presented a structural model drawing on earlier work by many authors ([3,4,5,6,7,8], among others), in which layer stacks of different size are combined in various proportions This crystal-structural model in turn provides a crystal-structural framework for the series that can assist in the characterisation of new natural or synthetic phases without the requirement of single crystal diffraction studies, which are often impossible, or at least impractical, due to the characteristic nanoscale intergrowths and disordering among members of the series. Natural ikunolite typically contains some Se, suggesting partial or complete solid solution series between Bi4 S3 and Bi4 S2 Se

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