Trigonal Fe ₂ Si from the Blackville site, South Carolina, USA: occurrence, composition and determination of the crystal structure

Abstract Iron silicides, including hapkeite (Fe 2 Si), are rare minerals found in both terrestrial and extraterrestrial environments. In this study, we present the discovery of trigonal Fe 2 Si 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 Fe 2 Si, identifying it as a distinct phase from the cubic hapkeite. The high concentrations of V, Ti and P in trigonal Fe 2 Si and its co-occurrence with suessite suggest an impact-related origin or a lightning strike. Anthropogenic processes, although unlikely, cannot yet be completely ruled out. While this compound represents a new polymorph of Fe 2 Si, 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 Fe 2 Si was produced by natural processes or by anthropogenesis.