several authors suggested to call these figures Thomson structure or Thomson-Widmanstätten structure. Lamellae 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. 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. 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 may come from a meteorite. 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 or ferric chloride, 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. 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