Next Article in Journal
The Effect of Surface Charge on the Separation of Pyrite from Serpentine by Flotation
Next Article in Special Issue
Editorial for Special Issue “Minerals Down to the Nanoscale: A Glimpse at Ore-Forming Processes”
Previous Article in Journal
Potential for Mineral Carbonation of CO2 in Pleistocene Basaltic Rocks in Volos Region (Central Greece)
Previous Article in Special Issue
Nanoscale Study of Titanomagnetite from the Panzhihua Layered Intrusion, Southwest China: Multistage Exsolutions Record Ore Formation
Open AccessArticle

Polytypism and Polysomatism in Mixed-Layer Chalcogenides: Characterization of PbBi4Te4S3 and Inferences for Ordered Phases in the Aleksite Series

1
School of Chemical Engineering and Advanced Materials, The University of Adelaide, Adelaide SA 5005, Australia
2
College of Zijin Mining, Fuzhou University, Fuzhou 350108, China
3
Adelaide Microscopy, The University of Adelaide, Adelaide SA 5005, Australia
4
Museum Victoria, Melbourne, P.O. Box 666, Melbourne VIC 3001, Australia
5
Department of Earth Sciences, The Natural History Museum, Cromwell Road, London SW7 5BD, UK
*
Author to whom correspondence should be addressed.
Minerals 2019, 9(10), 628; https://doi.org/10.3390/min9100628
Received: 9 August 2019 / Revised: 6 October 2019 / Accepted: 7 October 2019 / Published: 12 October 2019
(This article belongs to the Special Issue Minerals Down to the Nanoscale: A Glimpse at Ore-Forming Processes)
Bi-Pb-chalcogenides of the aleksite series represent homologous mixed-layer compounds derived from tetradymite (Bi2Te2S). Considering tetradymite as composed of five-atom (Bi2Te2S) layers, the named minerals of the aleksite homologous series, aleksite (PbBi2Te2S2) and saddlebackite, (Pb2Bi2Te2S3) have been considered as phases formed by regular stacking of longer seven- and nine-atom layers. High-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) imaging of thinned foils prepared in-situ on domains deemed homogeneous from electron probe microanalysis, STEM energy-dispersive X-ray spectrometry (EDS) element mapping and fast Fourier transforms (FFTs) from the images offer new insights into these structures. The hitherto-unnamed phase, PbBi4Te4S3, previously interpreted as regular intergrowths of five- and seven-atom layers, is characterized in terms of regular repeats of five- and seven-atom layers over distances of at least 350 nm, defining the (57), or 24H polytype. Imaging of other domains in the same lamella with identical composition at the electron microprobe scale reveals the presence of two additional polytypes: (5559), or 48H; and (557.559) or 72H. Unit cell dimensions for all three polytypes of PbBi4Te4S3 can be both measured and predicted from the HAADF STEM imaging and FFTs. STEM EDS mapping of each polytype confirm the internal structure of each layer. Lead and S occur within the centre of the layers, i.e., Te–Bi–S–Pb–S–Bi–Te in the seven-atom layer, Te–Bi–S–Pb–S–Pb–S–Bi–Te in the nine-atom layer, and so on. Polytypism is an intrinsic feature of the aleksite series, with each named mineral or unnamed phase, except the simple five-atom layer, defined by several alternative stacking sequences of different length, readily explaining the differing c values given in previous work. Homology is defined in terms of layer width and the stacking arrangement of those layers. Coherent structures of the same composition need not only be built of layers of adjacent size (i.e., five- and seven-atom layers) but, as exemplified by the (5559) polytype, may also contain non-adjacent layers, a finding not anticipated in previous work. New polysomes remain to be discovered in nature and each potentially exists as multiple polytypes. The present study further emphasizes the utility of HAADF STEM imaging and atomic-scale STEM EDS element mapping as an optimal tool for tracking stacking sequences and characterising structures in mixed-layer compounds. View Full-Text
Keywords: bismuth chalcogenides; aleksite series; high-angle annular dark-field scanning transmission electron microscopy bismuth chalcogenides; aleksite series; high-angle annular dark-field scanning transmission electron microscopy
Show Figures

Figure 1

MDPI and ACS Style

Cook, N.J.; Ciobanu, C.L.; Liu, W.; Slattery, A.; Wade, B.P.; Mills, S.J.; Stanley, C.J. Polytypism and Polysomatism in Mixed-Layer Chalcogenides: Characterization of PbBi4Te4S3 and Inferences for Ordered Phases in the Aleksite Series. Minerals 2019, 9, 628.

Show more citation formats Show less citations formats
Note that from the first issue of 2016, MDPI journals use article numbers instead of page numbers. See further details here.

Article Access Map by Country/Region

1
Back to TopTop