Widmanstätten pattern
Encyclopedia
Widmanstätten patterns, also called Thomson structures, are unique figures of long nickel
-iron
crystals, found in the octahedrite
iron meteorite
s and some pallasite
s. They consist of a fine interleaving of kamacite
and taenite
bands or ribbons called lamellæ. Commonly, in gaps between the lamellæ, a fine-grained mixture of kamacite
and taenite
called plessite
can be found.
, the director of the Imperial Porcelain works in Vienna
. While flame heating iron meteorite
s, Widmanstätten noticed color and lustre
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.
However, it is now believed that full credit for the discovery should actually be assigned to G. Thomson
as he published the same findings four years earlier.
Working in Naples in 1804, Thomson
treated a Krasnojarsk
meteorite
with nitric acid
in an effort 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 correspondance when its carrier was murdered. As a result, in 1804, his findings were only published in French in the Bibliothèque Britannique. At the beginning of 1806, Napoleon
invaded the Kingdom of Naples
and Thomson was forced to flee to Sicily
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. The Napoleonic wars
obstructed Thomson's contacts with the scientific community and his peregrinations across Europe, in addition to his early death, obscured his contributions for many years.
Moreover, due the discover priority of G. Thomson
, several authors suggested to call these figures Thomson structure or Thomson-Widmanstätten structure.
and nickel
form homogeneous alloys at temperatures below the melting point, these alloys are taenite. 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
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 700 and 450°C, and can only take place during very slow cooling, about 1 to 10000 degrees per one million years. 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. In the picture shown, the broad white bars are kamacite (dimensions in the mm-range), the thin line-like ribbons are taenite. The dark mottled areas are called plessite.
origin of the material and can be used to easily determine if a piece of iron
comes from a meteorite
.
solution (or more usually, ferric chloride solution). Since the Nickel
content of each meteorite varies, the time of etch also varies however 30 seconds to a minute are typical. Once the meteorite has been etched, it is usually neutralized in an alkali
(such as sodium carbonate
solution) to remove any remaining acid and then washed and dried, application of a light gun oil helps resist corrosion.
The dimension of kamacite
lamellæ ranges from coarsest to finest as the nickel content increases. Today iron meteorites are classified using the chemical classification, but originally they were classified measuring the width of these bands. It was called structural classification. Octahedrite
s can be divided in:
Iron meteorites without Widmanstätten bands:
. Opposite faces are parallel so, although an octahedron has 8 faces, there are only 4 sets of kamacite
plates. Iron and nickel-iron only form crystals with an external octahedral form very rarely, but these crystallographic orientations are still well defined 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:
.
Should be noted however that the appearance, the composition and the formation process of these terrestrial "Widmanstatten structures" are different from the characteristic structure of iron meteorites.
Also damascus steel
bears patterns but they are easily discernible from any Widmanstätten pattern.
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
-iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
crystals, found in the octahedrite
Octahedrite
Octahedrites are a class of iron meteorites within the structural classification. They are the most common class of iron meteorites.They are composed primarily of the nickel-iron alloys: taenite - high nickel content, and kamacite - low nickel content....
iron meteorite
Iron meteorite
Iron meteorites are meteorites that consist overwhelmingly of nickel-iron alloys. The metal taken from these meteorites is known as meteoric iron and was one of the earliest sources of usable iron available to humans.-Occurrence:...
s and some pallasite
Pallasite
A pallasite is a type of stony–iron meteorite.-Structure and composition:It consists of cm-sized olivine crystals of peridot quality in an iron-nickel matrix. Coarser metal areas develop Widmanstätten patterns upon etching...
s. They consist of a fine interleaving of kamacite
Kamacite
Kamacite is a mineral. It is an alloy of iron and nickel, usually in the proportions of 90:10 to 95:5 although impurities such as cobalt or carbon may be present. On the surface of Earth, it occurs naturally only in meteorites. It has a metallic luster, is gray and has no clear cleavage although...
and taenite
Taenite
Taenite is a mineral found naturally on Earth mostly in iron meteorites. It is an alloy of iron and nickel, with nickel proportions of 20% up to 65%.The name is derived from the Greek for "band". Taenite is a major constituent of iron meteorites...
bands or ribbons called lamellæ. Commonly, in gaps between the lamellæ, a fine-grained mixture of kamacite
Kamacite
Kamacite is a mineral. It is an alloy of iron and nickel, usually in the proportions of 90:10 to 95:5 although impurities such as cobalt or carbon may be present. On the surface of Earth, it occurs naturally only in meteorites. It has a metallic luster, is gray and has no clear cleavage although...
and taenite
Taenite
Taenite is a mineral found naturally on Earth mostly in iron meteorites. It is an alloy of iron and nickel, with nickel proportions of 20% up to 65%.The name is derived from the Greek for "band". Taenite is a major constituent of iron meteorites...
called plessite
Plessite
Plessite is a meteorite texture consisting of a fine-grained mixture of the minerals kamacite and taenite found in the octahedrite iron meteorites. It occurs in gaps between the larger bands of kamacite and taenite which form Widmanstätten patterns....
can be found.
Discovery
In 1808, these figures were named after Count Alois von Beckh WidmanstättenCount Alois von Beckh Widmanstätten
Count Alois von Beckh Widmanstätten was an Austrian printer and scientist. His name is sometimes given as Alois von Beckh-Widmannstätten or Aloys Beck, Edler von Widmannstätten.-Working life:...
, the director of the Imperial Porcelain works in Vienna
Vienna
Vienna is the capital and largest city of the Republic of Austria and one of the nine states of Austria. Vienna is Austria's primary city, with a population of about 1.723 million , and is by far the largest city in Austria, as well as its cultural, economic, and political centre...
. While flame heating iron meteorite
Iron meteorite
Iron meteorites are meteorites that consist overwhelmingly of nickel-iron alloys. The metal taken from these meteorites is known as meteoric iron and was one of the earliest sources of usable iron available to humans.-Occurrence:...
s, Widmanstätten noticed color and lustre
Lustre (mineralogy)
Lustre is a description of the way light interacts with the surface of a crystal, rock, or mineral. The word lustre traces its origins back to the Latin word lux, meaning "light", and generally implies radiance, gloss, or brilliance....
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
Carl Franz Anton Ritter von Schreibers
Carl Franz Anton Ritter von Schreibers was an Austrian naturalist who was a native of Pressburg, Hungary, Habsburg Empire . He earned his medical doctorate from Vienna in 1798, but also studied botany, mineralogy and zoology at the university...
, director of the Vienna Mineral and Zoology Cabinet, who named the structure after Widmanstätten.
However, it is now believed that full credit for the discovery should actually be assigned to G. Thomson
G. Thomson
G. Thomson was an English geologist. He died in Palermo at the early age of 46 years.-Name:His name is indicated only with G. because the full name is unknown. In several secondary sources he is called William Thomson or William Thompson, and sometimes is even confused with Lord Kelvin...
as he published the same findings four years earlier.
Working in Naples in 1804, Thomson
G. Thomson
G. Thomson was an English geologist. He died in Palermo at the early age of 46 years.-Name:His name is indicated only with G. because the full name is unknown. In several secondary sources he is called William Thomson or William Thompson, and sometimes is even confused with Lord Kelvin...
treated a Krasnojarsk
Krasnojarsk (meteorite)
-History:A mass of about 700 kg was detected in 1749 about 145 miles south of Krasnoyarsk. It was seen by P.S. Pallas in 1772 and then on his orders transported to Saint Petersburg....
meteorite
Meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
with nitric acid
Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
in an effort 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 correspondance when its carrier was murdered. As a result, in 1804, his findings were only published in French in the Bibliothèque Britannique. At the beginning of 1806, Napoleon
Napoleon I of France
Napoleon Bonaparte was a French military and political leader during the latter stages of the French Revolution.As Napoleon I, he was Emperor of the French from 1804 to 1815...
invaded the Kingdom of Naples
Kingdom of Naples
The Kingdom of Naples, comprising the southern part of the Italian peninsula, was the remainder of the old Kingdom of Sicily after secession of the island of Sicily as a result of the Sicilian Vespers rebellion of 1282. Known to contemporaries as the Kingdom of Sicily, it is dubbed Kingdom of...
and Thomson was forced to flee to Sicily
Sicily
Sicily is a region of Italy, and is the largest island in the Mediterranean Sea. Along with the surrounding minor islands, it constitutes an autonomous region of Italy, the Regione Autonoma Siciliana Sicily has a rich and unique culture, especially with regard to the arts, music, literature,...
and in November of that year, he died in Palermo
Palermo
Palermo is a city in Southern Italy, the capital of both the autonomous region of Sicily and the Province of Palermo. The city is noted for its history, culture, architecture and gastronomy, playing an important role throughout much of its existence; it is over 2,700 years old...
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
Napoleonic Wars
The Napoleonic Wars were a series of wars declared against Napoleon's French Empire by opposing coalitions that ran from 1803 to 1815. As a continuation of the wars sparked by the French Revolution of 1789, they revolutionised European armies and played out on an unprecedented scale, mainly due to...
obstructed Thomson's contacts with the scientific community and his peregrinations 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. LeonardFrederick C. LeonardFrederick Charles Leonard was an astrophysicist at the UCLA who organised the UCLA's Department of Astronomy. He received his undergraduate degree from the University of Chicago in 1918 and his Ph.D. in astronomy from the University of California at Berkeley in 1921....
) - Widmannstätten (used for example for the Widmannstätten lunar craterWidmannstätten (crater)Widmannstätten is a lunar crater in the southern part of the Mare Smythii, near the eastern limb of the Moon. The rim of this crater has a wide gap along the western side, where it is joined to the larger Kiess. There is also a gap in the northern rim where the crater floor is joined to the...
) - Widmanstatten (Anglicized)
Moreover, due the discover priority of G. Thomson
G. Thomson
G. Thomson was an English geologist. He died in Palermo at the early age of 46 years.-Name:His name is indicated only with G. because the full name is unknown. In several secondary sources he is called William Thomson or William Thompson, and sometimes is even confused with Lord Kelvin...
, several authors suggested to call these figures Thomson structure or Thomson-Widmanstätten structure.
Lamellæ formation mechanism
IronIron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
and nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
form homogeneous alloys at temperatures below the melting point, these alloys are taenite. 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
Octahedrite
Octahedrites are a class of iron meteorites within the structural classification. They are the most common class of iron meteorites.They are composed primarily of the nickel-iron alloys: taenite - high nickel content, and kamacite - low nickel content....
meteorite
Meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
s have a nickel content intermediate between the norm for kamacite
Kamacite
Kamacite is a mineral. It is an alloy of iron and nickel, usually in the proportions of 90:10 to 95:5 although impurities such as cobalt or carbon may be present. On the surface of Earth, it occurs naturally only in meteorites. It has a metallic luster, is gray and has no clear cleavage although...
and taenite
Taenite
Taenite is a mineral found naturally on Earth mostly in iron meteorites. It is an alloy of iron and nickel, with nickel proportions of 20% up to 65%.The name is derived from the Greek for "band". Taenite is a major constituent of iron meteorites...
, 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 700 and 450°C, and can only take place during very slow cooling, about 1 to 10000 degrees per one million years. This explains why this structure cannot be reproduced in the laboratory.
The crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
line patterns become visible when the meteorites are cut, polished, and acid etched, because taenite
Taenite
Taenite is a mineral found naturally on Earth mostly in iron meteorites. It is an alloy of iron and nickel, with nickel proportions of 20% up to 65%.The name is derived from the Greek for "band". Taenite is a major constituent of iron meteorites...
is more resistant to the acid. In the picture shown, the broad white bars are kamacite (dimensions in the mm-range), the thin line-like ribbons are taenite. The dark mottled areas are called plessite.
Use
Since nickel-iron crystals grow to lengths of some centimetres only when the solid metal cools down at an exceptionally slow rate (over several million years), the presence of these patterns is the proof of the extraterrestrialOuter space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....
origin of the material and can be used to easily determine if a piece of iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
comes from a meteorite
Meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
.
Preparation
The methods used to reveal the Widmanstätten pattern on an iron meteorites vary, normally the slice is ground and polished first, then cleaned to remove any remaining polish and dirt, the slice is then placed into nitric acidNitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
solution (or more usually, ferric chloride solution). Since the Nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
content of each meteorite varies, the time of etch also varies however 30 seconds to a minute are typical. Once the meteorite has been etched, it is usually neutralized in an alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
(such as sodium carbonate
Sodium carbonate
Sodium carbonate , Na2CO3 is a sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate. Sodium carbonate is domestically well-known for its everyday use as a water softener. It can be extracted from the...
solution) to remove any remaining acid and then washed and dried, application of a light gun oil helps resist corrosion.
Dimensions
Kamacite
Kamacite is a mineral. It is an alloy of iron and nickel, usually in the proportions of 90:10 to 95:5 although impurities such as cobalt or carbon may be present. On the surface of Earth, it occurs naturally only in meteorites. It has a metallic luster, is gray and has no clear cleavage although...
lamellæ ranges from coarsest to finest as the nickel content increases. Today iron meteorites are classified using the chemical classification, but originally they were classified measuring the width of these bands. It was called structural classification. Octahedrite
Octahedrite
Octahedrites are a class of iron meteorites within the structural classification. They are the most common class of iron meteorites.They are composed primarily of the nickel-iron alloys: taenite - high nickel content, and kamacite - low nickel content....
s can be divided in:
- Coarsest octahedrites: bands between 3.3 and 50 mm
- Coarse octahedrites: bands between 1.3 and 3.3 mm
- Medium octahedrites: bands between 0.5 and 1.3 mm
- Fine octahedrites: bands between 0.2 and 0.5 mm
- Finest octahedrites: bands finer than 0.2 mm
Iron meteorites without Widmanstätten bands:
- but with Neumann lineNeumann lineNeumann lines, or Neumann bands, are fine patterns of parallel lines seen in cross-sections of many hexahedrite iron meteorites in the kamacite phase, although they may appear also in octahedrites provided the kamacite phase is about 30 micrometres wide. They can be seen after a polished meteorite...
s are called HexahedriteHexahedriteHexahedrites are a type of iron meteorite.They are composed almost exclusively of the nickel-iron alloy kamacite and are lower in nickel content than the octahedrites. The nickel concentration in hexahedrites is always below 5.8% and only rarely below 5.3%....
s - without any structure are called AtaxiteAtaxiteAtaxites are a class of iron meteorites. They are composed mainly of the alloy taenite, and also contain plessite, troilite, and microscopic lamellae of kamacite. They have no visible Widmanstätten pattern. Ataxites are the most nickel-rich meteorites known; they usually contain over 18% nickel...
s
Shape and orientation
Cutting the meteorite along different planes affects the shape and direction of Widmanstätten figures because kamacite lamellæ in octahedrites are precisely arranged. Octahedrites derive their name from the crystal structure paralleling an octahedronOctahedron
In geometry, an octahedron is a polyhedron with eight faces. A regular octahedron is a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex....
. Opposite faces are parallel so, although an octahedron has 8 faces, there are only 4 sets of kamacite
Kamacite
Kamacite is a mineral. It is an alloy of iron and nickel, usually in the proportions of 90:10 to 95:5 although impurities such as cobalt or carbon may be present. On the surface of Earth, it occurs naturally only in meteorites. It has a metallic luster, is gray and has no clear cleavage although...
plates. Iron and nickel-iron only form crystals with an external octahedral form very rarely, but these crystallographic orientations are still well defined 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 centre) : 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 "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 along certain crystallographic planes of the parent phase. For example the basketweave structure in ZircaloyZircaloy
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...
.
Should be noted however that the appearance, the composition and the formation process of these terrestrial "Widmanstatten structures" are different from the characteristic structure of iron meteorites.
Also damascus steel
Damascus steel
Damascus steel was a term used by several Western cultures from the Medieval period onward to describe a type of steel used in swordmaking from about 300 BCE to 1700 CE. These swords are characterized by distinctive patterns of banding and mottling reminiscent of flowing water...
bears patterns but they are easily discernible from any Widmanstätten pattern.
See also
- Count Alois von Beckh WidmanstättenCount Alois von Beckh WidmanstättenCount Alois von Beckh Widmanstätten was an Austrian printer and scientist. His name is sometimes given as Alois von Beckh-Widmannstätten or Aloys Beck, Edler von Widmannstätten.-Working life:...
- G. ThomsonG. ThomsonG. Thomson was an English geologist. He died in Palermo at the early age of 46 years.-Name:His name is indicated only with G. because the full name is unknown. In several secondary sources he is called William Thomson or William Thompson, and sometimes is even confused with Lord Kelvin...
- MeteoriteMeteoriteA meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
- Acicular ferriteAcicular ferriteAcicular ferrite is a microstructure of ferrite 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...