Widmanstätten pattern

File:Widmanstatten-patterns-3.jpg

Meteorite cross-section, showing Widmanstätten patterns. The 60° angle between the sets of lamellæ indicates this plate to be cut approximately parallel to the octahedral form of the structure.

Widmanstätten patterns, called also 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

These figures are named after Count Alois von Beckh Widmanstätten, at the time director of the Imperial Porcelain works in Vienna, however an independent earlier discovery by G. Thomson is generally recognised.

Thomson had decided prior to 1804 to treat a Krasnojarsk meteorite with nitric acid with the purpose of removing the dull oxidation patina. Shortly after the contact of the metal with acid he noticed on the surface strange figures never seen before, which he detailed as described above. At the time Thomson was working in Naples, in the south of Italy. Due civil wars and political instability, he was having difficulty maintaining contact with his colleagues in England. In this period he referred for example of the miscarring of an important letter due the murder of its carrier. Probably due these problems, he published his findings in 1804 only in French on the Bibliothèque Britannique. At the beginning of 1806, after the Battle of Campo Tenese, Napoleon invaded the Kingdom of Naples (the second French invasion in 7 years), so Thomson was again forced to flee to Sicily. In November 1806 he died at Palermo at the age of 46. In 1808 Thomson's work was again published posthumously in Italian language (translated from the original English manuscript) in Atti dell'Accademia Delle Scienze di Siena. His peregrinations across Europe and Napoleonic wars obstacled his contacts with the scientific community and this, along with his early death, obscured his contributions for many years.

In 1808 Widmanstätten independently discovered the same pattern by flame heating some iron meteorites: the different iron alloys of meteorites oxidized at different rates during heating, causing colour and lustre zone differentiation. He did not publish his discovery, but claimed it only via oral communication with his colleagues. Nevertheless, he was acknowledged by Carl von Schreibers, director of the Vienna Mineral and Zoology Cabinet, who promptly named the structure after Widmanstätten.

The full credit of the discovery should be assigned to Thomson due chronological priority of publication.

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)
• 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.

Lamellæ formation mechanism

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°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 cristallographic 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 100 degrees per one million years. 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 these days, 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 neutralised 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 fine Widmanstätten pattern (lamellae width 0.3mm) of a Gibeon meteorite.

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

Octahedron

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 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

Different cuts produces different different Widmanstätten patterns

See also

• Meteorite
• G. Thomson
• Count Alois von Beckh Widmanstätten

Other projects

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External links

• Widmannstätten figures on the Gibeon Iron-Meteorite