Fulgurite morphology: a classification scheme and clues to formation

Fulgurites are natural glasses formed by cloud-to-ground lightning. Several different morphologies of fulgurites have been reported in previous studies, including sand fulgurites, rock fulgurites, and clay fulgurites. Herein, we examine sand, clay, and caliche fulgurites and demonstrate that these differ systematically in their morphology. We further use morphological features to constrain properties of fulgurite-forming lightning strikes. We classify fulgurites into four types of morphologies with an additional minor type. Type I fulgurites are sand fulgurites consisting of thin, glass walls; type II fulgurites are clay fulgurites, consisting of thick, melt rich walls; type III fulgurites are caliche fulgurites, consisting of thick, glass poor walls; and type IV fulgurites are rock fulgurites, consisting of glasses with walls consisting of surrounding, unmelted rock. Fulgurite morphology shows that the energy of fulgurite-forming strikes is between 1 and 30 MJ/m of fulgurite formed, suggests heating rates in the order of 1,000 K/s, and lightning channel thicknesses of about 1 mm diameter. Lightning generates mixtures of at least two components in most fulgurites: an SiO2 glass identified as lechatelierite and a groundmass of more varied composition. In addition to these four primary types, a fifth type-droplet fulgurites-is morphologically dissimilar from the other types, but is compositionally related to the type II or IV fulgurites. Additionally, two fulgurites, both from York County, Pennsylvania, USA, showed the reduction of iron to iron metal with an assortment of Fe-Ti and Si-P compounds with stoichiometry that ranges from nearly pure Fe metal to FeSi. These metal silicides include stoichiometric Fe3Si, Fe2Si, and Fe5Si3, and possibly Fe8Si3 and Fe7Si3, and provide a terrestrial source for these phases, which are typically associated with extraterrestrial material.