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{{Short description|Crystal patterns found in some meteorites
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{{Use American English|date=June 2019}}
{{Use mdy dates|date=June 2019}}
[[File:TolucaMeteorite.jpg|thumb|300px|Segment of the [[Toluca (meteorite)|Toluca meteorite]], about 10 cm wide]]
{{Steels}}

'''Widmanstätten patterns''' {{IPAc-en|ˈ|v|ɪ|d|m|ɑː|n|ˌ|ʃ|t|eɪ|t|ɪ|n}} ({{respell|VID|man|shtay|tin}}), also known as '''Thomson structures''', are figures of long [[Phase (matter)|phases]] of [[nickel]]–[[iron]], found in the [[octahedrite]] shapes of iron meteorite crystals and some [[pallasite]]s. 

[[Iron meteorite]]s are very often formed from a single [[crystal]] of iron-nickel alloy, or sometimes a number of large crystals that may be many meters in size, and often lack any discernable crystal boundary on the surface. Large crystals are extremely rare in metals, and in meteors they occur from extremely slow cooling from a molten state in the vacuum of space when the [[Solar System]] first formed. Once in the solid state, the slow cooling then allows the [[solid solution]] to [[Precipitation (chemistry)|precipitate]]  a separate phase that grows within the [[crystal lattice]], which form at very specific angles that are determined by the lattice. In meteors, these [[interstitial defect]]s can grow large enough to fill the entire crystal with needle or ribbon-like structures easily visible to the naked eye, almost entirely consuming the original lattice. They consist of a fine interleaving of [[kamacite]] and [[taenite]] bands or ribbons called ''[[Lamella (materials)|lamellae]]''. Commonly, in gaps between the lamellae, a fine-grained mixture of kamacite and taenite called [[plessite]] can be found.<ref>''Encyclopedia of the Solar System'' by Tilman Spohn, Doris Breuer, Torrence V. Johnson -- Elsevier 2014 Page 632</ref> 

Widmanstätten structures describe analogous features in modern steels,<ref>Dominic Phelan and Rian Dippenaar: Widmanstätten Ferrite Plate Formation in Low-Carbon Steels, METALLURGICAL AND MATERIALS TRANSACTIONS A, VOLUME 35A, DECEMBER 2004, p. 3701</ref> titanium, and zirconium alloys, but are usually microscopic in size.

==Discovery==
[[File:Widmanstätten pattern Staunton meteorite.jpg|thumb|right|Widmanstätten pattern in the [[Staunton meteorite]]{{efn-lr|The [[Staunton meteorite]] was found near [[Staunton, Virginia]] in the mid-19th century. Six pieces of nickel-iron were located over a period of some decades, with a total weight of 270&nbsp;lb.<ref>{{Cite journal
  |title=Meteorites of Virginia
  |first=F.B.
  |last=Hoffer
  |date=August 1974
  |journal=Virginia Minerals
  |volume=20
  |issue=3
  |url=https://www.dmme.virginia.gov/commercedocs/VAMIN_VOL20_NO03.PDF
  |access-date=October 8, 2019
  |archive-date=September 18, 2021
  |archive-url=https://web.archive.org/web/20210918194736/https://www.dmme.virginia.gov/commercedocs/VAMIN_VOL20_NO03.PDF
  |url-status=live
  }}</ref> }}]]
In 1808, these figures were observed by [[Count Alois von Beckh Widmanstätten]], the director of the Imperial Porcelain works in [[Vienna]]. While flame heating [[iron meteorite]]s,<ref>O. Richard Norton. ''Rocks from Space: Meteorites and Meteorite Hunters''. Mountain Press Pub. (1998) {{ISBN|0-87842-373-7}}</ref> Widmanstätten noticed color and [[Lustre (mineralogy)|luster]] zone differentiation as the various iron alloys oxidized at different rates. He did not publish his findings, claiming them only via oral communication with his colleagues. The discovery was acknowledged by [[Carl Franz Anton Ritter von Schreibers|Carl von Schreibers]], director of the Vienna Mineral and Zoology Cabinet, who named the structure after Widmanstätten.<ref>{{cite book|last1=Schreibers|first1=Carl von|title=Beyträge zur Geschichte und Kenntniß meteorischer Stein und Metalmassen, und Erscheinungen, welche deren Niederfall zu begleiten pflegen|trans-title=Contributions to the history and knowledge of meteoric stones and metallic masses, and phenomena which usually accompany their fall|date=1820|publisher=J.G. Heubner|location=Vienna, Austria|pages=70–72|url=https://books.google.com/books?id=WONQAAAAcAAJ&pg=PA70|language=German}}</ref><ref name=burke>John G. Burke. ''Cosmic Debris: Meteorites in History''. University of California Press, 1986. {{ISBN|0-520-05651-5}}</ref>{{rp|124}}
However, it is now believed that the discovery of the metal crystal pattern should be assigned to the English mineralogist [[William Thomson (mineralogist)|William (''Guglielmo'') Thomson]], as he published the same findings four years earlier.<ref>Thomson, G. (1804) "Essai sur le fer malléable trouvé en Sibérie par le Prof. Pallas" (Essay on malleable iron found in Siberia by Prof. Pallas), ''Bibliotèque Britannique'', '''27''' :  [https://babel.hathitrust.org/cgi/pt?id=umn.319510009686531;view=1up;seq=137 135–154] {{Webarchive|url=https://web.archive.org/web/20191215220419/https://babel.hathitrust.org/cgi/pt?id=umn.319510009686531;view=1up;seq=137 |date=December 15, 2019 }} ; [https://babel.hathitrust.org/cgi/pt?id=umn.319510009686531;view=1up;seq=213 209–229.] {{Webarchive|url=https://web.archive.org/web/20191215220419/https://babel.hathitrust.org/cgi/pt?id=umn.319510009686531;view=1up;seq=137 |date=December 15, 2019 }} (in French)</ref><ref name=burke /><ref name=vai /><ref name=cambridge>O. Richard Norton. ''The Cambridge Encyclopedia of meteorites''. Cambridge, Cambridge University Press, 2002. {{ISBN|0-521-62143-7}}.</ref>

Working in Naples in 1804, Thomson treated a [[Krasnojarsk (meteorite)|Krasnojarsk]] [[meteorite]] with [[nitric acid]] to remove the dull patina caused by oxidation. Shortly after the acid made contact with the metal, strange figures appeared on the surface, which he detailed as described above. Civil wars and political instability in southern Italy made it difficult for Thomson to maintain contact with his colleagues in England. This was demonstrated in his loss of important correspondence when its carrier was murdered.<ref name=vai/> As a result, in 1804, his findings were only published in French in the ''[[Bibliothèque Britannique]]''.<ref name=burke />{{rp|124–125}} <ref name=vai>Gian Battista Vai, W. Glen E. Caldwell. [https://books.google.com/books?id=rmrGS9s-KewC&pg=PA184&dq=%22Biblioth%C3%A8que+Britannique%22+thomson&sig=ZXbXj71Nt1i5rVbfazzBGUrLftE#PPA184,M1''The origins of geology in Italy'']. Geological Society of America, 2006, {{ISBN|0-8137-2411-2}}</ref><ref name=Paneth>F. A. Paneth. ''The discovery and earliest reproductions of the Widmanstatten figures''. Geochimica et Cosmochimica Acta, 1960, 18, pp.176–182</ref> At the beginning of 1806, [[Napoleon I of France|Napoleon]] invaded the [[Kingdom of Naples]] and Thomson was forced to flee to [[Sicily]]<ref name=vai/> and in November of that year, he died in [[Palermo]] at the age of 46. In 1808, Thomson's work was again published posthumously in Italian (translated from the original English manuscript) in ''Atti dell'Accademia Delle Scienze di Siena''.<ref name=siena>{{cite journal|last1=Thomson|first1=G.|title=Saggio di G.Thomson sul ferro malleabile trovato da Pallas in Siberia|journal=Atti dell'Accademia delle Scienze di Siena|date=1808|volume=9|pages=37–57|url=https://books.google.com/books?id=mv8EAAAAQAAJ&pg=PA37|trans-title=Essay by G. Thomson on malleable iron found by Pallas in Siberia|language=Italian}}</ref> The [[Napoleonic wars]] obstructed Thomson's contacts with the scientific community and his travels across Europe, in addition to his early death, obscured his contributions for many years.

==Name==
The most common names for these figures are ''Widmanstätten pattern'' and ''Widmanstätten structure;'' however, there are some spelling variations:
* ''Widmanstetter'' (proposed by [[Frederick C. Leonard]])<ref>O. Richard Norton, ''[http://meteoritemag.uark.edu/614.htm Personal Recollections of Frederick C. Leonard] {{webarchive|url=https://web.archive.org/web/20080705144307/http://meteoritemag.uark.edu/614.htm |date=2008-07-05 }}, Meteorite Magazine – Part II''</ref>
* ''Widmannstätten'' (used for example for the [[Widmannstätten (crater)|Widmannstätten lunar crater]])
* ''Widmanstatten'' (Anglicized)

Due to the discover priority of [[G. Thomson]], several authors suggested to call these figures ''Thomson structure'' or ''Thomson-Widmanstätten structure''.<ref name=burke /><ref name=vai /><ref name=cambridge />

==Lamellae formation mechanism==
[[File:Meteoric iron phase diagram taenite kamacite.svg|thumbnail|right| Phase diagram explaining how the pattern forms. First [[meteoric iron]] is exclusively composed of taenite. When cooling off it passes a phase boundary where [[kamacite]] is exsolved from taenite. [[Meteoric iron]] with less than about 6% nickel ([[hexahedrite]]) is completely changed to kamacite.]]
[[File:Widmannstaetten.png|thumb|Widmanstätten pattern, metallographic polished section]]
[[Iron]] and [[nickel]] form [[homogeneous]] [[alloys]] at temperatures below the [[melting point]]; these alloys are [[taenite]]. At temperatures below 900 to 600&nbsp;°C (depending on the Ni content), two alloys with different nickel content are stable: kamacite with lower Ni-content (5 to 15% Ni) and taenite with high Ni (up to 50%). [[Octahedrite]] [[meteorite]]s have a nickel content intermediate between the norm for [[kamacite]] and [[taenite]]; this leads under slow cooling conditions to the precipitation of kamacite and growth of kamacite plates along certain [[crystallographic planes]] in the [[taenite]] [[crystal lattice]].

The formation of Ni-poor kamacite proceeds by diffusion of Ni in the solid alloy at temperatures between 450 and 700&nbsp;°C, and can only take place during very slow cooling, about 100 to 10,000&nbsp;°C/Myr, with total cooling times of 10 [[Myr]] or less.<ref>{{citation|title=Iron meteorites: Crystallization, thermal history, parent bodies, and origin|journal=Chemie der Erde – Geochemistry|volume=69|issue=4|pages=293–325|doi=10.1016/j.chemer.2009.01.002|year=2009|last1=Goldstein|first1=J.I|last2=Scott|first2=E.R.D|last3=Chabot|first3=N.L|bibcode=2009ChEG...69..293G}}</ref> This explains why this structure cannot be reproduced in the laboratory.

The [[crystal]]line patterns become visible when the meteorites are cut, polished, and acid-etched, because taenite is more resistant to the acid.

[[File:Widmanstätten pattern kevinzim.jpg|thumb|The fine Widmanstätten pattern (lamellae width 0.3mm) of a [[Gibeon (meteorite)|Gibeon meteorite]].]]
The dimension of [[kamacite]] lamellae ranges from ''coarsest'' to ''finest'' (upon their size) as the nickel content increases. This classification is called ''[[Iron meteorite#Structural classification|structural classification]]''.

==Usage==
Since nickel-iron crystals grow to lengths of some centimeters only when the solid metal cools down at an exceptionally slow rate (over several million years), the presence of these patterns is strongly suggestive of [[Outer space|extraterrestrial]] origin of the material, and can be used to indicate if a piece of [[iron]] may come from a [[meteorite]].{{citation needed|date=October 2019}}

==Preparation==
[[File:Canyon Diablo meteorite, pattern.jpg|thumb|Etched slice of a [[Canyon Diablo meteorite]] showing a Widmanstätten pattern]]

The methods used to reveal the Widmanstätten pattern on iron meteorites vary. Most commonly, the slice is ground and polished, cleaned, etched with a chemical such as [[nitric acid]] or [[ferric chloride]], washed, and dried.<ref>{{cite web|last1=Harris|first1=Paul|last2=Hartman|first2=Ron|last3=Hartman|first3=James|title=Etching Iron Meteorites|url=http://www.meteorite-times.com/articles/etching-iron-meteorites/|publisher=Meteorite Times|access-date=October 14, 2016|date=November 1, 2002|archive-date=October 18, 2016|archive-url=https://web.archive.org/web/20161018202645/http://www.meteorite-times.com/articles/etching-iron-meteorites/|url-status=live}}</ref><ref>{{cite journal |author=Nininger, H.H. |title=Directions for the Etching and Preservation of Metallic Meteorites |journal=Proceedings of the Colorado Museum of Natural History |volume=15 |issue=1 |pages=3–14 |date=February 1936|bibcode=1945PA.....53...82N}}</ref>
{{-}}

==Shape and orientation==
[[File:120px-Octahedron-slowturn.gif|thumb|Octahedron]]
[[File:Cutting the octaedron.png|thumb|Different cuts produce different Widmanstätten patterns]]

Cutting the meteorite along different planes affects the shape and direction of Widmanstätten figures because [[kamacite]] [[lamella (materials)|lamellae]] in [[octahedrite]]s are precisely arranged. Octahedrites derive their name from the crystal structure paralleling an [[octahedron]]. Opposite faces are parallel so, although an octahedron has 8 faces, there are only 4 sets of kamacite plates. Iron and nickel-iron form crystals with an external octahedral structure only very rarely, but these orientations are still plainly detectable crystallographically without the external habit. Cutting an octahedrite meteorite along different planes (or any other material with octahedral symmetry, which is a sub-class of cubic symmetry) will result in one of these cases:
* perpendicular cut to one of the three (cubic) axes: two sets of bands at right angles each other
* parallel cut to one of the octahedron faces (cutting all 3 cubic axes at the same distance from the crystallographic center) : three sets of bands running at 60° angles each other
* any other angle: four sets of bands with different angles of intersection

==Structures in non-meteoritic materials==
The term {{em|Widmanstätten structure}} is also used on non-meteoritic material to indicate a structure with a geometrical pattern resulting from the formation of a new [[phase (matter)|phase]] along certain [[crystallographic plane]]s of the parent phase, such as the basketweave structure in some [[zirconium alloy]]s. The Widmanstätten structures form due to the growth of new phases within the grain boundaries of the parent metals, generally increasing the hardness and brittleness of the metal. The structures form due to the precipitation of a single crystal phase into two separate phases. In this way, the Widmanstätten transformation differs from other transformations, such as a [[martensite]] or ferrite transformation. The structures form at very precise angles, which may vary depending on the arrangement of the crystal lattices. These are usually very small structures that must be viewed through a microscope because a very long cooling rate is generally needed to produce structures visible to the naked eye. However, they usually have a great and often an undesirable effect on the properties of the alloy.<ref name="ReferenceA">''Metallography and Microstructure in Ancient and Historic Metals'' By David A. Scott – J. Paul Getty Trust 1991 Page 20–21</ref>

Widmanstätten structures tend to form within a certain temperature range, growing larger over time. In [[carbon steel]], for example, Widmanstätten structures form during [[tempering (metallurgy)|tempering]] if the steel is held within a range around {{convert|500|F|C|}} for long periods of time. These structures form as a needle or plate-like growths of [[cementite]] within the crystal boundaries of the martensite. This increases the brittleness of the steel in a way that can only be relieved by recrystallizing. Widmanstätten structures made from [[Allotropes of iron|ferrite]] sometimes occur in carbon steel, if the carbon content is below but near the [[eutectoid]] composition (~ 0.8% carbon). This occurs as long needles of ferrite within the [[pearlite]].<ref name="ReferenceA"/>

Widmanstätten structures form in many other metals as well. They will form in brass, especially if the alloy has a very high zinc content, becoming needles of zinc in the copper matrix. The needles will usually form when the brass cools from the recrystallization temperature, and will become very coarse if the brass is annealed to {{convert|1112|F|C}} for long periods.<ref name="ReferenceA"/> [[Telluric iron]], which is an iron-nickel alloy very similar to meteorites, also displays very coarse Widmanstätten structures. Telluric iron is metallic iron, rather than an ore (in which iron is usually found), and it originated from the Earth rather than from space. Telluric iron is an extremely rare metal, found only in a few places in the world. Like meteorites, the very coarse Widmanstätten structures most likely develop through very slow cooling, except that the cooling occurred in the Earth's mantle and crust rather than in the [[vacuum (space)|vacuum]] and [[microgravity]] of [[outer space|space]].<ref>''Meteoritic Iron, Telluric Iron and Wrought Iron in Greenland'' By Vagn Fabritius Buchwald, Gert Mosdal -- Kommissionen for videnskabelige Undersogelse i Gronland 1979 Page 20 on page 20</ref> Such patterns have also been seen in [[mulberry (uranium alloy)|mulberry]], a ternary uranium alloy, after [[age hardening|aging]] at or below {{val|400|u=degC}} for periods of minutes to hours produces a [[monoclinic]] ɑ{{''}} phase.<ref name="Dean" >{{Cite web
 |title=A Study of the Time-Temperature Transformation Behavior of a Uranium=7.5 weight percent Niobium-2.5 weight percent Zirconium Alloy
 |first1=C.W.
 |last1=Dean
 |date=October 24, 1969
 |id=Oak Ridge Report Y-1694
 |publisher=Union Carbide Corporation, [[Y-12 Plant]], [[Oak Ridge National Laboratory]]
 |url=https://digital.library.unt.edu/ark:/67531/metadc1033722/m2/1/high_res_d/4749751.pdf
 |pages=53–54, 65
 |access-date=February 20, 2018
 |archive-date=July 24, 2018
 |archive-url=https://web.archive.org/web/20180724134105/https://digital.library.unt.edu/ark:/67531/metadc1033722/m2/1/high_res_d/4749751.pdf
 |url-status=live
 }}</ref>

However, the appearance, the composition, and the formation process of these terrestrial Widmanstätten structures are different from the characteristic structure of iron meteorites.<ref name=Buchwald/>

When an iron meteorite is forged into a tool or weapon, the Widmanstätten patterns remain but become stretched and distorted. The patterns usually cannot be fully eliminated by blacksmithing, even through extensive working. When a knife or tool is forged from meteoric iron and then polished, the patterns appear on the surface of the metal, albeit distorted, but they tend to retain some of the original octahedral shapes and the appearance of thin lamellae crisscrossing each other.<ref name=Buchwald>{{cite book|title=''Iron and Steel in Ancient Times''|author=Vagn Fabritius Buchwald -- Det Kongelige Danske Videnskabernes Selskab |date=2005|page =26|url=https://books.google.com/books/about/Iron_and_Steel_in_Ancient_Times.html?id=c947L8YJerUC}}</ref>

==See also==
* [[Acicular ferrite]]
* [[Count Alois von Beckh Widmanstätten]]
* [[Glossary of meteoritics]]
* [[Meteorite]]

== References ==
{{Notelist-lr}}
{{Reflist}}

==External links==
{{Commons category|Widmanstätten pattern}}
*[http://www.cvs.fi/ylinsivu61323.htm Widmannstätten figures on the Gibeon Iron-Meteorite]

{{Meteorites}}
{{Patterns in nature}}

{{DEFAULTSORT:Widmanstatten Pattern}}
[[Category:Meteorite mineralogy and petrology]]
[[Category:Patterns]]
[[Category:Ferrous alloys]]
[[Category:Nickel alloys]]