RAPID FORMATION OF EUTECTIC TEXTURES IN FE-SI INTERMETALLIC COMPOUNDS IN A FULGURITE FROM CENTRAL LOWER MICHIGAN

Rapidly formed eutectic textures are observed in Fe silicides in a fulgurite from Michigan. A fulgurite is a natural glass that is formed when lighting strikes sand, soil, or rock. During the formation process it is estimated that temperatures can exceed 3000 K followed by rapid cooling (under 700 C in less than 1 minute). Extreme temperature combined with organic matter in the soil may result in highly reducing conditions. These extreme conditions effectively smelt the soil, producing native silicon, iron silicides and other compounds in a matter of seconds. A 14 cm diameter fulgurite was formed in sandy glacial till in 2014 near Houghton Lake, MI, and a part subsequently donated to the A. E. Seaman Mineral Museum at Michigan Tech. Spherical droplets of iron silicides to ~200 μm were found in the natural glass. Back-Scattered Electron images of some droplets showed what appeared to be a eutectic intergrowth texture of two iron silicides with individual grains to ~1 μm. Electron Diffraction in the TEM showed the specimen to be an intergrowth of naquite (FeSi) and an as yet unidentified iron silicide, approximately (Fe₃Si₇) with minor native silicon and Fe-Ti silicides. Other droplets contained higher iron silicides and/or Ti- rich silicides. Understanding the formation of these phases in a fulgurite may inform on questions as diverse as lightning hazards and the synthesis of an important semiconductor.