Fulgurite
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Parallel view ( )
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Cross-eye view ( )
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| Two Type I (arenaceous) fulgurites: a common tube fulgurite and a more irregular specimen. |
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| Two small Type I Saharan Desert fulgurites. In a planar view the specimen on the right has a blade-like morphology, but its tubular nature is dramatically shown in a stereo view. |
Fulgurites (from the Latin fulgur, meaning "lightning") are classified generically as a variety of the mineraloid lechatelierite, although their absolute chemical composition is dependent on the physical and chemical properties of target material affected by the discharge of cloud-ground lightning. They are commonly hollow and/or branching assemblages of glassy, , and heterogeneously microcrystalline tubes, crusts, slags, vesicular masses, and clusters of refractory materials that often form during the discharge phase of lightning strikes propagating into silica-rich quartzose sand, mixed soil, clay, caliche and other carbonate-rich sediments. Colloquially, they have been referred to as petrified lightning. Fulgurites are homologous to Lichtenberg figures, which are the branching patterns produced on surfaces of insulators during dielectric breakdown by high-voltage discharges, such as lightning.
Fulgurites are formed when lightning melts silica or other common conductive and semiconductive minerals and substrates, fusing, vitrifying, oxidizing and reducing mineral grains and organic compounds; the fulgurite mass is the rapidly quenched end-product. The temperature peak within a lightning channel, however, is known to exceed 30,000 K, with sufficient pressure to produce planar deformation features, or "shock lamellae" in SiO2polymorphs. It is assumed that the process of forming a fulgurite occurs over a timespan of the order of a single second, following the termination of the return stroke sequence, and leaves direct evidence of the dissipation path and its dispersion over the surface or into the earth. Artificial fulgurites can also be produced when the controlled arcing of electricity into a fusable medium. This has been documented in cases of downed high voltage power lines; current was discharged into the ground, producing blue fulgurite-like lechatelierites, colored by copper from the power line.
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Wikipedia
