Abstract
Abstract. 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.