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

Abstract

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.

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

“…This homology thus allows the derivation of a systematic group homology [1,2] and implies that each individual member of the series should be stoichiometric in composition. Lattice-scale intergrowths are, however, a common feature of the tetradymite group [e.g., [14]) and the related aleksite homologous series [15], leading to non-stoichiometric compositions being widely reported in the literature.…”

Ferro-tschermakite with polysomatic chain-width disorder identified in silician magnetite from Wirrda Well, South Australia: A HAADF STEM study

“…The occurrence of chain-width disorder during pyribole crystallization is most likely the rule rather than the exception in minerals that share crystal structural blocks. Recent HAADF STEM studies have also revealed intergrowths of single units or individual sub-structural motifs relating to stacking disorder recorded during crystallisation among members of diverse modular homologous series, e.g., REEfluorocarbonates (Ciobanu et al 2017), mixed-layer chalcogenides (Cook et al 2019), or Sb-Bi-sulfosalts with chessboard structures (Li et al 2019). Yin, S., Wirth, R., Ma, C.Q., and Xu, J.N.…”

“…In HAADF STEM mode, the image contrast is correlated with atomic mass, i.e., the atomic number Zdependence of the contrast is ~Z 2 , with application to the smallest U-bearing nanoparticles in environmental studies (e.g., Utsunomiya and Ewing 2003). Recent HAADF STEM studies of Pb-Bi sulfosalts, REE-fluorocarbonates, and Pb-Bi-or Bi-chalcogenides show how we can directly visualize structural blocks defining the homology of a mineral series and layer types in mixed-layer compounds (Medlin et al 2010(Medlin et al , 2013(Medlin et al , 2014(Medlin et al , 2019Ciobanu et al 2016Ciobanu et al , 2017Li et al 2019;Cook et al 2019).…”

Editorial for Special Issue “Minerals Down to the Nanoscale: A Glimpse at Ore-Forming Processes”

“…The mapping of trace elements in ore minerals using nanoSIMS is revolutionary in terms of its detection limits and spatial resolution, but also permits the visualization of transient radionuclides at ultra-trace concentrations [33]. Last but not least, the observation of multiple stacking sequences in mixed-layer compounds raises the issue of the identity of minerals [34].…”

“…In the final paper of this special issue, Cook et al [34] use HAADF STEM imaging and STEM EDS mapping to address polysomatism and polytypism in modular, mixed-layer Pb-Bi-chalcogenide minerals belonging to the aleksite group. Focusing on a nanoscale characterization of unnamed PbBi 4 Te 4 S 3 in a specimen from Clogau, Wales (UK), the authors show that this phase occurs as at least three distinct polytypes within the same sample and go on to demonstrate how additional polysomes in the aleksite group can be derived by combinations of layers of different sizes.…”