Widmanstätten pattern

Widmanstätten patterns, also known as Thomson structures, are figures of long phases of nickel–iron, found in the octahedrite shapes of iron meteorite crystals and some pallasites.

Iron meteorites are very often formed from a single crystal of 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 the solid solution to precipitate a separate phase that grows within the crystal lattice, which form at very specific angles that are determined by the lattice. In meteors, these interstitial defects can 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 of kamacite and taenite bands or ribbons called lamellae. Commonly, in gaps between the lamellae, a fine-grained mixture of kamacite and taenite called plessite can be found.

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