On the anomalous shapes of native copper crystals from the Michigan Copper Country

Boulliard, Jean-Claude; Aléon, Jérôme; Gaillou, Eloïse

For over a century, the anomalous shapes of Michigan copper crystals from the Michigan Copper Country have been acknowledged. They are well known by mineral collectors and curated in museums from all around the world; still, their particular habits remain enigmatic. These natural crystals do not seem to follow crystal shape theories, based on the internal three-dimensional crystal structure. In this article, we offer a unique perspective on the formation of Michigan copper crystals.

Firstly, we review the most common theories of crystal shapes. Then, taking into account the surface reconstructions induced by adsorbed oxygen, detected by ultra-high vacuum techniques, we present evidence of a strong correlation between these oxygen-induced surface reconstructions and the anomalous shapes. Finally, in order to understand why these shapes are not found in copper at other localities, oxygen dosing was performed using NanoSIMS on different natural copper crystals as a preliminary investigation. The higher oxygen content found in the Michigan copper crystal studied compared to others supports the influence of adsorbed oxygen on the anomalous crystal shapes. This result shows which mechanisms could modify crystal shapes and allow the development of strategies to monitor them, due to the presence of oxygen impurities. This new find is of great importance in shape-dependent catalysis, sensor characteristics, or other properties of material such as nanocrystals.

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Boulliard, Jean-Claude / Aléon, Jérôme / Gaillou, Eloïse: On the anomalous shapes of native copper crystals from the Michigan Copper Country. 2021. Copernicus Publications.

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