The occurrence of iron silicides in a fulgurite: Implications for fulgurite genesis

Abstract

Rapidly formed eutectic textures are observed in Fe silicides in a fulgurite from Michigan. The 14 cm-diameter fulgurite was formed in sandy glacial till in 2014 near Houghton Lake, Michigan. Spherical droplets of iron silicides up to ∼200 μm in diameter were found in the natural glass. Back-scattered electron images of some droplets show a eutectic intergrowth texture of two iron silicides with individual crystals up to ∼1 μm in maximum dimension. X-ray diffraction study showed the specimens to be an intergrowth of naquite (FeSi) and linzhiite (FeSi2) or naquite and xifengite (Fe5Si3). Droplets also contain minor native silicon, Fe-Ti silicides, and/or other Ti- rich phases which were discovered during TEM observations. It is important to note that the lower-temperature phase luobusaite (Fe3Si7) was not observed in any droplets, indicating rapid quenching of the fulgurite, consistent with a natural origin during a lightning strike as opposed to an artificial origin, e.g., resulting from a downed power line.