Fulgurite

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Fulgurite
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A tube fulgurite and a more irregular specimen.

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 natural, commonly hollow assemblages of glassy, protocrystalline, and heterogeneously-microcrystalline tubes, crusts, slags, vesicular masses, and clusters of dielectric and 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,humic sediments, conductive biomass (such as peat, water-saturated wood, or dung), or anthropogenic materials having similar compositions (e.g. concrete, brick, asphalt, tile, etc.).[1][2] Colloquially, they have been referred to as petrified lightning.[3] Fulgurites may also be instances of Lichtenberg figures, which are the branching patterns produced during dielectric breakdown by high-voltage discharges, such as lightning.

Description[edit]

Fulgurites are formed when lightning with a temperature of at least 1,800 °C (3,270 °F) melts silica or other common conductive and semiconductive minerals and substrates, fusing, vitrifying, oxidizing and reducing mineral grains and organic compounds;[4] the fulgurite mass is the rapidly-quenched product.[5] This process occurs over a timespan of the order of a single second,[6] and leaves evidence of the lightning path and its dispersion over the surface or into the earth.[7] Fulgurites can also be produced when the cables of a high voltage electrical distribution network break, and the wires fall onto a conductive surface beneath. It is a common and conceptually-limiting belief that is widely propagated that fulgurites are exclusively tubaceous, columnar structures forming in sand or sandy soil.

The primary SiO2 phase in ordinary Type I (sand) fulgurites, lechatelierite, also may be formed by meteorite impacts and during volcanic eruptions (often including a lightning-induced formation mechanism [8]), but the products of such events do not share the characteristic self-similar branching, bifurcated contours, and chaotically-deterministic external morphologies unique to fulgurites. Because their groundmass is generally amorphous in structure, fulgurite is classified as a mineraloid.

Fulgurites can exceed tens of centimeters in diameter and can penetrate deep into the soil, sometimes occurring as far as 15 m (49 ft) below the surface that was struck, but may form directly on appropriate sedimentary surfaces.[9] 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.[6] The Yale University Peabody Museum of Natural History displays the longest known preserved fulgurite, approximately 4 m (13 ft) in length.[10] 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).[citation needed] Geophysicist and petrologist Eric Essene, with Daniel Fisher, described and performed thermodynamic analysis on a morphologically-irregular and chemically-heterogeneous fulgurite - the Winans Lake fulgurite[s] - bearing chemically-reduced mineral and elemental phases, such as ferrosilicon and native silicon, recovered approximately 15 km north of Ann Arbor, Michigan, extending discontinuously throughout a 30 m range. Peculiarly, its spatial orientation conforms to true geographic North.[11]

Their color varies widely, depending on specific composition and degree of hydric saturation of sediments, rock, and/or biomass in which they formed, ranging from black or tan, to green, blue, metallic blue-grey, or a translucent white. 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 porous and scoria-like; their exteriors generally can be coated with rough sedimentary particles and can be porous, or smooth, or structurally-complex. Fulgurites display some degree of self-similarity and structural scale invariance as a macroscopic or microscopic diffusion limited aggregation network of rootlike branches and fractally-distributed voids. Fulgurites formed in sand or loose soil are mechanically fragile, making the field collection of large specimens difficult.

Fulgurites occasionally form as glazed tracks on rock, or as internal diffusion limited aggregation networks completely metamorphosing the target rock. Ejected droplets and irregular sub-rounded structures are associated with such rock (type IV) fulgurites, as well as type II (mixed soil [12]) and clay fulgurites; these ejected materials, often quenching into bizarre forms -often aerodynamic - and displaying structural complexity, are classified as exogenic fulgurites (type V), and can resemble many confirmed and putative crater glass forms, impactites, and tektites.[13][14]

Uses[edit]

Fulgurites are appreciated by many for their scientific value as permanent tangible evidence of transient lightning strikes.[15] For instance, the fact that fulgurites are abundant in the Sahara Desert demonstrates that lightning once was a frequent occurrence in that region.[16] Ancient fulgurites constitute the primary data source for paleolightning research and its applications in paleoclimatology.[17]

Fulgurites also are popular among hobbyists and collectors of natural specimens.[18]

Many observations have been made in fulgurites of high-pressure, high-temperature materials more commonly assumed to be the exclusive products of asteroid impacts, comet airbursts, or cosmic debris influx. Such materials, formerly considered to be exclusive to hypervelocity impacts, have been identified in fulgurites, including fullerenes,[19] as well as high-pressure polymorphs of SiO2, in fulgurites.[20][21][22][23][24] Carbon-based fulgurites, produced directly on biomass, such as wood, or as scavenged biogenic (humic) carbon, are now known to exist; these are termed phytofulgurites.[25] In a phytofulgurite found in Russia - the type specimen - abundant non-racemic amino acids, with preference for left-handed chirality, and other organic compounds have been analytically-identified, although there is still uncertainty as to which fraction had been preserved or authigenically-synthesized from simpler molecules during a electrometamorphic event. Survivability of chiral organic compounds found in life has been demonstrated reliably in carbonaceous chondrites such as Murchison, a CM chondrite that fell 28 September 1969, in Murchison, Victoria, Australia.[26] Currently, no exhaustive classification scheme unequivocally-incorporates hybrid attributes common in surface-formed fulgurites occurring in organic-rich substrates;[27][28] indeed, no universally-applied classification scheme accounting for the recently-established diversity of fulgurites has yet been established.

References[edit]

  1. ^ http://scholarcommons.usf.edu/cgi/viewcontent.cgi?article=5289&context=etd
  2. ^ http://arizona.openrepository.com/arizona/handle/10150/144596
  3. ^ http://ira.usf.edu/CAM/exhibitions/1998_12_McCollum/supplemental_didactics/47.Petrified.pdf
  4. ^ http://www.sciencemag.org/content/234/4773/189.short
  5. ^ Carl Ege. "What are fulgurites and where can they be found?". geology.utah.gov. Retrieved 2009-03-21. 
  6. ^ a b Grapes, R. H. (2006). Pyrometamorphism. Springer. p. 28. ISBN 3-540-29453-8. 
  7. ^ Uman, Martin A. (2008). The Art and Science of Lightning Protection. Cambridge University Press. p. 212. ISBN 0-521-87811-X. 
  8. ^ http://geology.gsapubs.org/content/early/2015/02/26/G36255.1.abstract
  9. ^ Ripley, George; Charles Anderson Dana (1859). The New American Cyclopaedia. Appleton. p. 2. 
  10. ^ "New Peabody hall offering high-tech lessons about Earth and space". Yale Bulletin & Calendar (Yale University) 34 (30). June 9, 2006. Retrieved 2013-12-26. 
  11. ^ Science 10 October 1986: Vol. 234 no. 4773 pp. 189-193 DOI: 10.1126/science.234.4773.189
  12. ^ http://www.lpi.usra.edu/meetings/metsoc2010/pdf/5197.pdf
  13. ^ Exogenic fulgurites from Elko County, Nevada: a new class of fulgurite associated with large soil-gravel fulgurite tubes (Rocks & Minerals, Sep/Oct 2004, Vol. 79, No. 5.)
  14. ^ http://doi.org/10.1007/s00410-012-0753-5
  15. ^ Chambers's journal By William Chambers, Robert Chambers
  16. ^ Vladimir A. Rakov, Lightning Makes Glass, 29th Annual Conference of the Glass Art Society, 1999, University of Florida, Gainesville
  17. ^ http://quest.nasa.gov/projects/spacewardbound/docs/NavarroGonzalez_etal_2007.pdf
  18. ^ Patti Polk, Collecting Rocks, Gems & Minerals: Easy Identification - Values - Lapidary Uses, Krause, 2010, page 168 ISBN 978-1-4402-0415-9
  19. ^ http://www.ncbi.nlm.nih.gov/pubmed/17733026
  20. ^ http://ammin.geoscienceworld.org/content/100/7/1645.abstract
  21. ^ http://www.lpi.usra.edu/meetings/metsoc2010/pdf/5211.pdf
  22. ^ http://www.schweizerbart.de/content/papers_preview/download/78149
  23. ^ http://www.ncbi.nlm.nih.gov/pubmed/20229006
  24. ^ http://rsta.royalsocietypublishing.org/content/368/1922/3087
  25. ^ http://link.springer.com/article/10.1134%2FS1028334X06090212#page-1
  26. ^ http://www.nature.com/nature/journal/v389/n6648/abs/389265a0.html
  27. ^ http://link.springer.com/article/10.1134%2FS1028334X11030238#page-1
  28. ^ http://www.cprm.gov.br/33IGC/1207897.html

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