Fulgurite Morphology: A Classification Scheme and Clues to Formation

Authors

Matthew A. Pasek, University of South FloridaFollow
Kristin Block, University of Arizona
Virginia Pasek, Free Radical Consulting

Document Type

Article

Publication Date

9-2012

Keywords

fulgurite, lechatelierite, contact metamorphism, silicides

Digital Object Identifier (DOI)

https://doi.org/10.1007/s00410-012-0753-5

Abstract

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.

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Was this content written or created while at USF?

Yes

Citation / Publisher Attribution

Contributions to Mineralogy and Petrology, v. 164, issue 3, p. 477-492

Scholar Commons Citation

Pasek, Matthew A.; Block, Kristin; and Pasek, Virginia, "Fulgurite Morphology: A Classification Scheme and Clues to Formation" (2012). School of Geosciences Faculty and Staff Publications. 617.
https://digitalcommons.usf.edu/geo_facpub/617