Widmanstätten pattern
Widmanstätten patterns, also called Thomson structures, are unique figures of long nickel-iron crystals, found in the octahedrite iron meteorites and some pallasites. 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.
Discovery
In 1808, these figures were named after Count Alois von Beckh Widmanstätten, the director of the Imperial Porcelain works in Vienna. While flame heating iron meteorites,[1] 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.[2]
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.[2][3][4]
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 correspondence when its carrier was murdered.[3] As a result, in 1804, his findings were only published in French in the Bibliothèque Britannique.[2][3][5] At the beginning of 1806, Napoleon invaded the Kingdom of Naples and Thomson was forced to flee to Sicily[3] 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.[6] 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.
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)[7]
- Widmannstätten (used for example for the Widmannstätten lunar crater)
- Widmanstatten (Anglicized)
Moreover, due the discover priority of G. Thomson, several authors suggested to call these figures Thomson structure or Thomson-Widmanstätten structure.[2][3][4]
Lamellæ formation mechanism
Iron 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 meteorites 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 100 to 10,000 °C/Myr, with total cooling times of 10 Myr or less.[8]. This explains why this structure cannot be reproduced in the laboratory.
The crystalline 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.
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 extraterrestrial origin of the material and can be used to easily determine if a piece of iron comes from a meteorite.
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 acid 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.
Dimensions
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. Octahedrites 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 lines are called Hexahedrites
- without any structure are called Ataxites
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 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 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 Zircaloy.
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.
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Widmanstätten pattern observed in Zircaloy 4, βZr grain boundaries are still visible even though βZr has been transformed to Widmanstätten.
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Micrograph of the previous probe
See also
Notes
- ^ O. Richard Norton. Rocks from Space: Meteorites and Meteorite Hunters. Mountain Press Pub. (1998) ISBN 0878423737
- ^ a b c d John G. Burke. Cosmic Debris: Meteorites in History. University of California Press, 1986. ISBN 0520056515
- ^ a b c d e Gian Battista Vai, W. Glen E. Caldwell. The origins of geology in Italy. Geological Society of America, 2006, ISBN 0813724112
- ^ a b O. Richard Norton. The Cambridge Encyclopedia of meteorites. Cambridge, Cambridge University Press, 2002. ISBN 0521621437.
- ^ F. A. Paneth. The discovery and earliest reproductions of the Widmanstatten figures. Geochimica et Cosmochimica Acta, 1960, 18, pp.176-182
- ^ G.Thomson. Saggio di G.Thomson sul ferro Malleabile trovato da Pallas in Siberia. Atti dell'Accademia Delle Scienze di Siena, 1808, IX, p. 37.
- ^ O. Richard Norton, Personal Recollections of Frederick C. Leonard, Meteorite Magazine - Part II
- ^ J. Goldstein et al. (2009), [1] "Iron meteorites: Crystallization, thermal history, parent bodies, and origin"
External links
