‘Basket-weave’ textures formed during cooling of natural bornite: a HAADF STEM study
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Date
2026
Authors
King, S.A.
Ciobanu, C.L.
Cook, N.J.
Slattery, A.D.
Ehrig, K.
Yao, J.
Campo Rodriguez, Y.T.
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Mineralogical Magazine, 2026; 90(1):93-113
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Samuel A. King, Cristiana L. Ciobanu, Nigel J. Cook, Ashley D. Slattery, Kathy Ehrig, Jie Yao, and Yuri T. Campo Rodriguez
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Abstract
Bornite (Cu₅FeS₄) and digenite (Cu₉₋ᵪFeᵪS₅; x = 0.4) have closely related cubic structures and are known for their range of superstructures derived from metal vacancies leading to larger unit cells expressed as n × a, where a = ∼5.5 Å and n is an integer. Such polymorphs can form during cooling from higher temperature bornite (Bn)–digenite (Dg) 1a solid solution (ss). The alleged basket-weave textures in natural bornite are investigated using high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) imaging and energy-dispersive X-ray spectrometry. These techniques, combined with crystal modelling and STEM simulations, are suitable for depicting changes in phases related to crystal-structural modularity as they collectively better reproduce atomic distributions in real space. Bornite associated with either chalcocite or chalcopyrite from the Olympic Dam Cu-U-Au-Ag deposit, South Australia has non-stoichiometric Cu/Fe ratios and displays nanoscale basket-weave textures between the main components Bn2a and anilite (Cu₇S₄); Dg1a is preserved throughout, albeit as a minor phase. Anilite is a derivative of digenite, whereby a = b = √2aDg and c = 2aDg. Two intermediate phases, Dg3a and Bn2a4a, are documented and an additional phase, Bn2a6a, is tentatively suggested to occur in Fe-rich nanodomains within Bn2a. Considering the epitaxial relationships between all phases, we infer that basket-weave textures record phase transitions via polymorphic transformations of parent Bn2a and Dg1a during cooling. Observed phase assemblages are thus linked to cooling of Bn–Dgₛₛ in the range 70–87 mol.% Bn along a Cu₆·₁₈Fe₁·₂₆S₅ – Cu₉.₁₂Fe₀·₈₉S₅ tie-line defined from measured compositions. We depict three associations: Bn2a + Dg1a, Bn2a4a + Dg3a, and Bn2a4a/Bn2a6a + anilite, formed during cooling. Polymorph associations like these are relevant for enrichment of critical/precious metals in copper ores because Bi, Pb, Ag, Te and, probably also Au, if dissolved in Bn–Dgss, could be incorporated into superstructures during Cu-Fe-sulfide phase transitions.
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© The Author(s), 2025. Published by Cambridge University Press on behalf of The Mineralogical Society of the United Kingdom and Ireland. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0), which permits unrestricted re-use, distribution and reproduction, provided the original article is properly cited.