Widmanstätten pattern

Segment of theToluca meteorite,about 10 cm wide

Widmanstätten patterns,also known asThomson structures,are figures of longphasesofnickel–iron,found in theoctahedriteshapes of iron meteorite crystals and somepallasites.

Iron meteoritesare very often formed from a singlecrystalof 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 thesolid solutiontoprecipitatea separate phase that grows within thecrystal lattice,which form at very specific angles that are determined by the lattice. In meteors, theseinterstitial defectscan 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 ofkamaciteandtaenitebands or ribbons calledlamellae.Commonly, in gaps between the lamellae, a fine-grained mixture of kamacite and taenite calledplessitecan be found.

Widmanstätten structures describe analogous features in modern steels,titanium, and zirconium alloys, but are usually microscopic in size.

Discovery

Widmanstätten pattern in theStaunton meteorite

In 1808, these figures were observed byCount Alois von Beckh Widmanstätten,the director of the Imperial Porcelain works inVienna.While flame heatingiron meteorites,Widmanstätten noticed color andlusterzone 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 byCarl von Schreibers,director of the Vienna Mineral and Zoology Cabinet, who named the structure after Widmanstätten. However, it is now believed that the discovery of the metal crystal pattern should be assigned to the English mineralogistWilliam (Guglielmo) Thomson,as he published the same findings four years earlier.

Working in Naples in 1804, Thomson treated aKrasnojarskmeteoritewithnitric acidto 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 theBibliothèque Britannique.At the beginning of 1806,Napoleoninvaded theKingdom of Naplesand Thomson was forced to flee toSicilyand in November of that year, he died inPalermoat the age of 46. In 1808, Thomson's work was again published posthumously in Italian (translated from the original English manuscript) inAtti dell'Accademia Delle Scienze di Siena.TheNapoleonic warsobstructed 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 areWidmanstätten patternandWidmanstätten structure,however, there are some spelling variations:

• Widmanstetter(proposed byFrederick C. Leonard)
• Widmannstätten(used for example for theWidmannstätten lunar crater)
• Widmanstatten(Anglicized)

Due to the discover priority ofG. Thomson,several authors suggested to call these figuresThomson structureorThomson-Widmanstätten structure.

Lamellae formation mechanism

Phase diagram explaining how the pattern forms. Firstmeteoric ironis exclusively composed of taenite. When cooling off it passes a phase boundary wherekamaciteis exsolved from taenite.Meteoric ironwith less than about 6% nickel (hexahedrite) is completely changed to kamacite.

Widmanstätten pattern, metallographic polished section

Ironandnickelformhomogeneousalloysat temperatures below themelting point;these alloys aretaenite.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%).Octahedritemeteoriteshave a nickel content intermediate between the norm forkamaciteandtaenite;this leads under slow cooling conditions to the precipitation of kamacite and growth of kamacite plates along certaincrystallographic planesin thetaenitecrystal lattice.

The formation of Ni-poor kamacite proceeds by diffusion of Ni in the solid alloy at temperatures between 450 and 700 °C, and can only take place during very slow cooling, about 100 to 10,000 °C/Myr, with total cooling times of 10Myror less.This explains why this structure cannot be reproduced in the laboratory.

Thecrystallinepatterns become visible when the meteorites are cut, polished, and acid-etched, because taenite is more resistant to the acid.

The fine Widmanstätten pattern (lamellae width 0.3mm) of aGibeon meteorite.

The dimension ofkamacitelamellae ranges fromcoarsesttofinest(upon their size) as the nickel content increases. This classification is calledstructural 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 ofextraterrestrialorigin of the material, and can be used to indicate if a piece ofironmay come from ameteorite.

Preparation

Etched slice of aCanyon Diablo meteoriteshowing 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 asnitric acidorferric chloride,washed, and dried.

Shape and orientation

Octahedron

Different cuts produce different Widmanstätten patterns

Cutting the meteorite along different planes affects the shape and direction of Widmanstätten figures becausekamacitelamellaeinoctahedritesare precisely arranged. Octahedrites derive their name from the crystal structure paralleling anoctahedron.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 termWidmanstätten structureis also used on non-meteoritic material to indicate a structure with a geometrical pattern resulting from the formation of a newphasealong certaincrystallographic planesof the parent phase, such as the basketweave structure in somezirconium alloys.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 amartensiteor 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.

Widmanstätten structures tend to form within a certain temperature range, growing larger over time. Incarbon steel,for example, Widmanstätten structures form duringtemperingif the steel is held within a range around 500 °F (260 °C) for long periods of time. These structures form as a needle or plate-like growths ofcementitewithin 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 fromferritesometimes occur in carbon steel, if the carbon content is below but near theeutectoidcomposition (~ 0.8% carbon). This occurs as long needles of ferrite within thepearlite.

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 1,112 °F (600 °C) 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 thevacuumandmicrogravityofspace.Such patterns have also been seen inmulberry,a ternary uranium alloy, afteragingat or below400 °Cfor periods of minutes to hours produces amonoclinicɑ″ 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.

See also

• Acicular ferrite
• Count Alois von Beckh Widmanstätten
• Glossary of meteoritics
• Meteorite

External links

• Widmannstätten figures on the Gibeon Iron-Meteorite