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Fulgurites (), commonly called " fossilized lightning", are natural tubes, clumps, or masses of sintering, vitrification, or fused soil, sand, rock, organic debris and other sediments that sometimes form when lightning discharges into ground. When composed of silica, fulgurites are classified as a variety of the mineraloid lechatelierite.
When ordinary negative polarity cloud-ground lightning discharges into a grounding substrate, greater than 100 million volts (100 MV) of potential difference may be bridged. Such current may propagate into silica-rich sand, mixed soil, clay, or other sediments, rapidly vaporizing and melting resistant materials within such a common dissipation regime. This results in the formation of generally hollow and/or vesicular, branching assemblages of glassy phase tubes, crusts, and clumped masses. Fulgurites have no fixed composition because their chemical composition is determined by the physical and chemical properties of whatever material is being struck by lightning.
Fulgurites are structurally similar to Lichtenberg figures, which are the branching patterns produced on surfaces of insulators during dielectric breakdown by high-voltage discharges, such as lightning.
Material properties (size, color, texture) of fulgurites vary widely, depending on the size of the lightning bolt and the composition and moisture content of the surface struck by lightning. Most natural fulgurites fall on a spectrum from white to black. Iron is a common impurity that can result in a deep brownish-green coloration. Lechatelierite similar to fulgurites can also be produced via controlled (or uncontrolled) arcing of artificial electricity into a medium. Downed high voltage power lines have produced brightly colored lechatelierites, due to the incorporation of copper or other materials from the power lines. Brightly colored lechatelierites resembling fulgurites are usually synthetic and reflect the incorporation of synthetic materials. However, lightning can strike man-made objects, resulting in colored fulgurites.
The interior of Type I (sand) fulgurites normally is smooth or lined with fine bubbles, while their exteriors are coated with rough sedimentary particles or small rocks. Other types of fulgurites are usually vesicular, and may lack an open central tube; their exteriors can be porous or smooth. Branching fulgurites display fractal-like self-similarity and structural scale invariance as a macroscopic or microscopic network of root-like branches, and can display this texture without central channels or obvious divergence from morphology of context or target (e.g. sheet-like melt, rock fulgurites). Fulgurites are usually fragile, making the field collection of large specimens difficult.
Fulgurites can exceed 20 centimeters in diameter and can penetrate deep into the subsoil, sometimes occurring as far as below the surface that was struck, although they may also form directly on a sedimentary surface. One of the longest fulgurites to have been found in modern times was a little over in length, found in northern Florida. The Yale University Peabody Museum of Natural History displays one of the longest known preserved fulgurites, approximately in length. Charles Darwin in The Voyage of the Beagle recorded that tubes such as these found in Drigg, Cumberland, UK reached a length of . Fulgurites at Winans Lake, Livingston County, Michigan, extended discontinuously throughout a 30 m range and arguably include the largest reported fulgurite mass ever recovered and described: its largest section extending approximately 16 ft (4.88 m) in length by 1 ft in diameter (30 cm).
Many high-pressure, high-temperature materials have been observed in fulgurites. Many of these minerals and compounds are also known to be formed in extreme environments such as nuclear weapon tests, hypervelocity impacts, and cosmic dust. Shocked quartz was first described in fulgurites in 1980. Other materials, including highly reduced silicon-metal alloys (silicides), the fullerene allotropes C60 (buckminsterfullerenes) and C70, as well as high-pressure polymorphs of SiO2, have since been identified in fulgurites. Reduced phosphides have been identified in fulgurites, in the form of schreibersite ( and ), and titanium(III) phosphide. These reduced compounds are otherwise rare on Earth due to the presence of oxygen in Earth's atmosphere, which creates oxidizing surface conditions.
In 1805 the true process of forming fulgurites by lightning strikes to the ground was identified by agriculturist Hentzen and mineralogist and mining engineer Johann Karl Wilhelm Voigt. In 1817 mineralogist and mining engineer Karl Gustav Fiedler published and comprehensively documented the phenomenon in the Annalen der Physik.
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