Trigonal Fe2Si from the Blackville site, South Carolina, USA: occurrence, composition and determination of the crystal structure

Iron silicides, including hapkeite (Fe2Si), are rare minerals found in both terrestrial and extraterrestrial environments. In this study, we present the discovery of trigonal Fe2Si in glass from the Blackville site, South Carolina, USA, in a discrete, deeply buried layer where it is associated with suessite and other impact-related materials. The site has been previously studied for its rich assemblage of Fe–Si spherules, platinum, iridium and nanodiamonds. Using advanced micro-computed tomography, scanning electron microscopy, electron microprobe analysis, and single-crystal X-ray diffraction, we have characterised the crystal structure of trigonal Fe2Si, identifying it as a distinct phase from the cubic hapkeite. The high concentrations of V, Ti and P in trigonal Fe2Si and its co-occurrence with suessite suggest an impactrelated origin or a lightning strike. Anthropogenic processes, although unlikely, cannot yet be completely ruled out. While this compound represents a new polymorph of Fe2Si, we refrain at this time from proposing it as a new mineral species to the International Mineralogical Association (IMA) because of remaining uncertainties about its formation. Further research is needed to determine whether this trigonal Fe2Si was produced by natural processes or by anthropogenesis.