Impact-induced high-temperature formation of metallic copper and bornite in Chang’e-6 lunar soils

Impacts play a critical role in modifying the chemical and structural characteristics of materials and in facilitating the migration of moderately volatile elements on airless bodies. In this study, a comprehensive SEM–FIB–TEM analysis of a Cu-rich grain from the recently returned Chang’e-6 lunar soil demonstrates that an assemblage of metallic copper, metallic iron, and Fe3+–Cu sulfides (i.e., bornite, Cu5FeS4) was formed under impact-induced high-temperature conditions. This finding provides evidence for the presence of bornite in lunar soils. The relatively homogeneous distribution of bornite, confined to the topmost surface of the Cu-rich grain coating, indicates a vapor-deposition origin. According to the thermodynamic modeling, the observed assemblages of metallic copper, metallic iron, and FeS within the grain interior can be explained by equilibrium thermodynamic behavior of the Cu–Fe–S system at elevated temperatures. The complex mineralogy involving Fe3+-bearing bornite, metallic iron, and metallic copper in the lunar soil further indicates that impact-induced melting and vaporization processes substantially perturb the valence states of metallic elements in the Cu–Fe–S system, thereby enabling the generation and stabilization of metallic phases and oxidized copper sulfides in lunar materials. Such natural high-temperature metallurgical processes provide critical insights into understanding extraterrestrial metal migration and enrichment under impact conditions on airless bodies.