Widmanstätten patterns, also known as Thomson structures, are figures of long
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow t ...
–
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
crystals, found in the
octahedrite iron meteorite
Iron meteorites, also known as siderites, or ferrous meteorites, are a type of meteorite that consist overwhelmingly of an iron–nickel alloy known as meteoric iron that usually consists of two mineral phases: kamacite and taenite. Most i ...
s and some
pallasite
The pallasites are a class of stony–iron meteorite.
Structure and composition
It consists of centimetre-sized olivine crystals of peridot quality in an iron-nickel matrix. Coarser metal areas develop Widmanstätten patterns upon etching. Min ...
s. They consist of a fine interleaving of
kamacite and
taenite
Taenite is a mineral found naturally on Earth mostly in iron meteorites. It is an alloy of iron and nickel, with a chemical formula of and nickel proportions of 20% up to 65%.
The name is derived from the Greek ταινία for "band, ribbon" ...
bands or ribbons called ''
lamellae''. Commonly, in gaps between the lamellae, a fine-grained mixture of kamacite and taenite called
plessite can be found. Widmanstätten patterns describe features in modern steels, titanium, and zirconium alloys.
Discovery
In 1808, these figures were observed by
Count Alois von Beckh Widmanstätten
Count (feminine: countess) is a historical title of nobility in certain European countries, varying in relative status, generally of middling rank in the hierarchy of nobility. Pine, L. G. ''Titles: How the King Became His Majesty''. New Yor ...
, the director of the Imperial Porcelain works in
Vienna
en, Viennese
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. While flame heating
iron meteorite
Iron meteorites, also known as siderites, or ferrous meteorites, are a type of meteorite that consist overwhelmingly of an iron–nickel alloy known as meteoric iron that usually consists of two mineral phases: kamacite and taenite. Most i ...
s, Widmanstätten noticed color and
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 von Schreibers, director of the Vienna Mineral and Zoology Cabinet, who named the structure after Widmanstätten.
[John G. Burke. ''Cosmic Debris: Meteorites in History''. University of California Press, 1986. ]
However, it is now believed that the discovery of the metal crystal pattern should be assigned to the English mineralogist
William (''Guglielmo'') Thomson, as he published the same findings four years earlier.
[O. Richard Norton. ''The Cambridge Encyclopedia of meteorites''. Cambridge, Cambridge University Press, 2002. .]
Working in Naples in 1804, Thomson treated a
Krasnojarsk
Krasnoyarsk ( ; rus, Красноя́рск, a=Ru-Красноярск2.ogg, p=krəsnɐˈjarsk) (in semantic translation - Red Ravine City) is the largest types of inhabited localities in Russia, city and administrative center of Krasnoyarsk Kr ...
meteorite
A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or moon. When the original object ...
with
nitric acid
Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...
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.
[ As a result, in 1804, his findings were only published in French in the '']Bibliothèque Britannique
The ''Bibliothèque Britannique'' was a monthly journal of the sciences and the arts published in Geneva by Marc-Auguste Pictet, his younger brother Charles, and their friend Frédéric-Guillaume Maurice.
Created in 1796, it covered a wide ran ...
''. [Gian Battista Vai, W. Glen E. Caldwell]
origins of geology in Italy''
Geological Society of America, 2006, [F. A. Paneth. ''The discovery and earliest reproductions of the Widmanstatten figures''. Geochimica et Cosmochimica Acta, 1960, 18, pp.176–182] At the beginning of 1806, Napoleon
Napoleon Bonaparte ; it, Napoleone Bonaparte, ; co, Napulione Buonaparte. (born Napoleone Buonaparte; 15 August 1769 – 5 May 1821), later known by his regnal name Napoleon I, was a French military commander and political leader wh ...
invaded the Kingdom of Naples
The Kingdom of Naples ( la, Regnum Neapolitanum; it, Regno di Napoli; nap, Regno 'e Napule), also known as the Kingdom of Sicily, was a state that ruled the part of the Italian Peninsula south of the Papal States between 1282 and 1816. It was ...
and Thomson was forced to flee to Sicily
(man) it, Siciliana (woman)
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[ and in November of that year, he died in ]Palermo
Palermo ( , ; scn, Palermu , locally also or ) is a city in southern Italy, the capital of both the autonomous region of Sicily and the Metropolitan City of Palermo, the city's surrounding metropolitan province. The city is noted for its ...
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''. The Napoleonic wars
The Napoleonic Wars (1803–1815) were a series of major global conflicts pitting the French Empire and its allies, led by Napoleon I, against a fluctuating array of European states formed into various coalitions. It produced a period of Fre ...
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
Frederick Charles Leonard (March 12, 1896 – June 23, 1960) was an American astronomer. As a faculty member at the University of California, Los Angeles, he conducted extensive research on double stars and meteorites, largely shaping the univer ...
)
* ''Widmannstätten'' (used for example for the 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''.
Lamellae formation mechanism
Iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
and nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow t ...
form homogeneous
Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...
alloys
An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...
at temperatures below the melting point
The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depen ...
; these alloys are taenite
Taenite is a mineral found naturally on Earth mostly in iron meteorites. It is an alloy of iron and nickel, with a chemical formula of and nickel proportions of 20% up to 65%.
The name is derived from the Greek ταινία for "band, ribbon" ...
. At temperatures below 900 to 600 °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
A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or moon. When the original object ...
s have a nickel content intermediate between the norm for kamacite and taenite
Taenite is a mineral found naturally on Earth mostly in iron meteorites. It is an alloy of iron and nickel, with a chemical formula of and nickel proportions of 20% up to 65%.
The name is derived from the Greek ταινία for "band, ribbon" ...
; this leads under slow cooling conditions to the precipitation of kamacite and growth of kamacite plates along certain crystallographic planes in the taenite
Taenite is a mineral found naturally on Earth mostly in iron meteorites. It is an alloy of iron and nickel, with a chemical formula of and nickel proportions of 20% up to 65%.
The name is derived from the Greek ταινία for "band, ribbon" ...
crystal lattice
In geometry and crystallography, a Bravais lattice, named after , is an infinite array of discrete points generated by a set of discrete translation operations described in three dimensional space by
: \mathbf = n_1 \mathbf_1 + n_2 \mathbf_2 + n ...
.
The formation of Ni-poor kamacite proceeds by diffusion of Ni in the solid alloy at temperatures between 700 and 450 °C, and can only take place during very slow cooling, about 100 to 10,000 °C/Myr, with total cooling times of 10 Myr
The abbreviation Myr, "million years", is a unit of a quantity of (i.e. ) years, or 31.556926 teraseconds.
Usage
Myr (million years) is in common use in fields such as Earth science and cosmology. Myr is also used with Mya (million years ago). ...
or less. This explains why this structure cannot be reproduced in the laboratory.
The crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macro ...
line patterns become visible when the meteorites are cut, polished, and acid-etched, because taenite is more resistant to the acid.
The dimension of kamacite lamellae ranges from ''coarsest'' to ''finest'' (upon their size) as the nickel content increases. This classification is called '' 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 extraterrestrial origin of the material, and can be used to indicate if a piece of iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
may come from a meteorite
A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or moon. When the original object ...
.
Preparation
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
Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...
or ferric chloride
Iron(III) chloride is the inorganic compound with the formula . Also called ferric chloride, it is a common compound of iron in the +3 oxidation state. The anhydrous compound is a crystalline solid with a melting point of 307.6 °C. The col ...
, washed, and dried.
Shape and orientation
Cutting the meteorite along different planes affects the shape and direction of Widmanstätten figures because kamacite lamellae in octahedrites are precisely arranged. Octahedrites derive their name from the crystal structure paralleling an octahedron
In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at ea ...
. 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 is also used on non-meteoritic material to indicate a structure with a geometrical pattern resulting from the formation of a new phase along certain crystallographic planes of the parent phase, such as the basketweave structure in some zirconium alloy
Zirconium alloys are solid solutions of zirconium or other metals, a common subgroup having the trade mark Zircaloy. Zirconium has very low absorption cross-section of thermal neutrons, high hardness, ductility and corrosion resistance. One of th ...
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
Martensite is a very hard form of steel crystalline structure. It is named after German metallurgist Adolf Martens. By analogy the term can also refer to any crystal structure that is formed by diffusionless transformation.
Properties
M ...
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.[''Metallography and Microstructure in Ancient and Historic Metals'' By David A. Scott – J. Paul Getty Trust 1991 Page 20–21]
Widmanstätten structures tend to form within a certain temperature range, growing larger over time. In carbon steel
Carbon steel is a steel with carbon content from about 0.05 up to 2.1 percent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states:
* no minimum content is specified or required for chromium, coba ...
, for example, Widmanstätten structures form during tempering if the steel is held within a range around for long periods of time. These structures form as a needle or plate-like growths of cementite
Cementite (or iron carbide) is a compound of iron and carbon, more precisely an intermediate transition metal carbide with the formula Fe3C. By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure. It is a hard, ...
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 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.
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 for long periods. 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
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often ...
and microgravity
The term micro-g environment (also μg, often referred to by the term microgravity) is more or less synonymous with the terms '' weightlessness'' and ''zero-g'', but emphasising that g-forces are never exactly zero—just very small (on the ...
of space
Space is the boundless three-dimensional extent in which objects and events have relative position and direction. In classical physics, physical space is often conceived in three linear dimensions, although modern physicists usually consi ...
. Such patterns have also been seen in mulberry
''Morus'', a genus of flowering plants in the family Moraceae, consists of diverse species of deciduous trees commonly known as mulberries, growing wild and under cultivation in many temperate world regions. Generally, the genus has 64 ident ...
, a ternary uranium alloy, after aging
Ageing ( BE) or aging ( AE) is the process of becoming older. The term refers mainly to humans, many other animals, and fungi, whereas for example, bacteria, perennial plants and some simple animals are potentially biologically immortal. In ...
at or below for periods of minutes to hours produces a monoclinic
In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic ...
ɑ phase.
However, the appearance, the composition, and the formation process of these terrestrial Widmanstätten structures are different from the characteristic structure of iron meteorites.
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.[''Iron and Steel in Ancient Times'' by Vagn Fabritius Buchwald -- Det Kongelige Danske Videnskabernes Selskab 2005 Page 26] Pattern-welded
Pattern welding is the practice in sword and knife making of forming a blade of several metal pieces of differing composition that are forge-welded together and twisted and manipulated to form a pattern. Often mistakenly called Damascus steel, ...
steels such as Damascus steel also bear patterns, but they are easily discernible from any Widmanstätten pattern.
See also
* Acicular ferrite
Acicular ferrite is a microstructure of ferrite in steel that is characterised by needle-shaped crystallites or grains when viewed in two dimensions. The grains, actually three-dimensional in shape, have a thin lenticular shape. This microstructu ...
* Count Alois von Beckh Widmanstätten
Count (feminine: countess) is a historical title of nobility in certain European countries, varying in relative status, generally of middling rank in the hierarchy of nobility. Pine, L. G. ''Titles: How the King Became His Majesty''. New Yor ...
* Glossary of meteoritics
This is a glossary of terms used in meteoritics, the science of meteorites.
#
* 2 Pallas – an asteroid from the asteroid belt and one of the likely parent bodies of the CR meteorites.
* 4 Vesta – second-largest asteroid in the asteroid b ...
* Meteorite
A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or moon. When the original object ...
References
External links
Widmannstätten figures on the Gibeon Iron-Meteorite
{{DEFAULTSORT:Widmanstatten Pattern
Meteorite mineralogy and petrology
Patterns