Fulgurite

Fulgurite

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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.

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Fulgurites (from the Latin fulgur, meaning "lightning") are natural tubes or masses of sintered soil, sand, and/or rock that form when lightning strikes the ground. They are classified as a variety of the mineraloid lechatelierite, although their absolute chemical composition is dependent on the physical and chemical properties of the material struck by lightning. They are commonly hollow and/or branching assemblages of glassy, protocrystalline, 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, or other sediments.^[1][2] 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.^[3][4]

Description

Fulgurites are formed when lightning strikes the ground, fusing and vitrifying mineral grains.^[5] The peak temperatures within a lightning channel are known to exceed 30,000 K, with sufficient pressure to produce planar deformation features, or "shock lamellae" in SiO₂ polymorphs.^[6][7][8] "Artificial fulgurites" can also be produced when the controlled arcing of electricity into a fusable medium. Downed high voltage power lines have produced blue, fulgurite-like lechatelierites, colored by copper from the power lines, themselves.^[9]

The color of fulgurites varies widely, depending on composition and chemical impurities. Most natural fulgurites all on a spectrum from white to black, but moderate iron content can result in a deep brownish-green coloration. More colorful variants are usually synthetic and reflect incorporation of synthetic materials. The interior of Type I (sand) fulgurites normally is very smooth or lined with fine bubbles, while other types are often both vesicular and dense or scoria-like; their exteriors generally can be coated with rough sedimentary particles and can be porous, smooth, or structurally complex. Branching fulgurites display fractal-like self-similarity and structural scale invariance as a macroscopic or microscopic network of root-like branches. Fulgurites formed in sand or loose soil are mechanically fragile, making the field collection of large specimens difficult.

The primary SiO₂ phase in fulgurites is lechatelierite, a silica glass. Because their groundmass is generally amorphous in structure, fulgurites are classified as mineraloids.

Fulgurites can exceed tens of centimeters in diameter and can penetrate deep into the subsoil, sometimes occurring as far as 15 m (49 ft) below the surface that was struck, but may form directly on appropriate sedimentary surfaces.^[10] One of the longest fulgurites to have been found in modern times was a little over 4.9 m (16 ft) in length, and was found in northern Florida.^[11] The Yale University Peabody Museum of Natural History displays one of the longest known preserved fulgurites, approximately 4 m (13 ft) in length.^[12] Charles Darwin in The Voyage of the Beagle recorded that tubes such as these found in Drigg, Cumberland, UK reached a length of 9.1 m (30 ft).^[13][14] The Winans Lake fulgurite[s] (Winans Lake, Livingston County, Michigan), extended discontinuously throughout a 30 m range, and arguably includes 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).^[2][15]

Classification

Fulgurites have been classified by Pasek et al. (2012)^[16] into five types related to the type of sediment in which the fulgurite formed, as follows:

• Type I - sand fulgurites with tubaceous structure; their central axial void may be collapsed
• Type II - soil fulgurites; these are glass-rich, and form in a wide range of sediment compositions, including clay-rich soils, silt-rich soils, gravel-rich soils, and loessoid; these may be tubaceous, branching, vesicular, irregular/slaggy, or may display a combination of these structures, and can produce exogenic fulgurites (droplet fulgurites)
• Type III - caliche or calcic sediment fulgurites, having thick, often surficially glazed granular walls with calcium-rich vitreous groundmass with little or no lechatelierite glass; their shapes are variable, with multiple narrow central channels common, and can span the entire range of morphological and structural variation for fulguritic objects
• Type IV - rock fulgurites, which are either crusts on minimally altered rocks, networks of tunneling within rocks, vesicular outgassed rocks (often glazed by a silicide-rich and/or metal oxide crust), or completely vitrified and dense rock material and masses of these forms with little sedimentary groundmass
• Droplet fulgurites (exogenic fulgurites), which are exogenous (e.g. spheroidal, botryoidal, filamentous, or aerodynamic),^[1][17] related by composition to Type II and Type IV fulgurites

Place in planetary processes and the geologic record

Many observations have been made in fulgurites of high-pressure, high-temperature materials more commonly assumed to be the exclusive to meteoritic sources, products of asteroid impacts, comet airbursts, or cosmic dust. Such materials - as a suite - formerly considered to be unique to hypervelocity impacts, have been identified in fulgurites, including highly reduced silicon-metal alloys (silicides), the fullerene allotropes C₆₀ (buckminsterfullerene) and C₇₀, as well as high-pressure polymorphs of SiO₂, in fulgurites.^{[2][6][15][18][19][20][21][22][23][24][25][26][27]}

Reduced phosphorus as phosphides and phosphites have been identified through quantitative analyses of a representative sample of 10 fulgurites recovered from most continents, in the form of schreibersite (Fe₃P, (Fe,Ni)₃P) - otherwise extremely rare on Earth, but common in meteorites, comets, interplanetary dust, and some planetary bodies - and TiP, which is unique to fulgurites.^[2][28][29]

In material culture

A fulgurite was found within the contents of the ash altar at the temple of Lykaian Zeus at Mount Lykaion in Greece. It may be associated with ritual activity performed there.^[30][31][32]

Fulgurites are appreciated by many for their value as tangible evidence of lightning strikes.^[33] Fulgurites also are popular among hobbyists and collectors of natural specimens.^[34]

Gallery

• 
  Type I (sand) fulgurite, Okeechobee, Florida.
• 
  Type I (sand) tubaceous fulgurites, Florida
• 
  Type I (sand) tubaceous fulgurites, Florida
• 
  Sand fulgurites, highly weathered (paleofulgurites) Algeria
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  Fulgurite, Mauritanian desert
• 
  Fulgurites, Mauritanian desert
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  Fulgurites, Senne, Westphalia, Germany
• 
  Tubaceous fulgurite, Iwamizawa, Japan
• 
  Type II (granitic soil, with abundant plagioclase clasts) tubaceous fulgurites, Coachella Valley, California
• 
  Type II (soil, clastic) tubaceous fulgurite, Lancaster, California

See also

• Trinitite

References

External links

Wikimedia Commons has media related to Fulgurites.

•  "Fulgurite". Encyclopædia Britannica. 11 (11th ed.). 1911. 
• The Bibliography of Fulgurites
• Petrified Lightning by Peter E. Viemeister (pdf)
• Interview (The Event: Petrified Lightning from Central Florida) with artist Allan McCollum along with an historical archive of sixty-six
• Mindat with location data
• W. M. Myers and Albert B. Peck, A Fulgurite from South Amboy, New Jersey, American Mineralogist, Volume 10, pages 152-155, 1925
• Vladimir A. Rakov, Lightning Makes Glass, 29th Annual Conference of the Glass Art Society, Tampa, Florida, 1999