Part of Hall of Meteorites.
Crystals in Cross Section
The crystal structure in iron meteorites is three-dimensional, so the pattern looks different depending on how you slice it. These meteorites have been cut with a saw and polished flat to reveal the crystals in cross section.
The actual 3-D structure is made of numerous flat plates of the iron-nickel alloy kamacite. When molten iron and nickel first cool and harden in a planet's core, they form a different alloy, taenite. But as the solid taenite cools, plates of crystallized kamacite grow through it. After millions of years, when the metal cools so much that crystals stop growing, the finished pattern freezes in place.
Northen Territory, Australia
The Widmanstätten Structure
In the metal cores of partially molten asteroids, iron-nickel alloys crystallized in a distinctive pattern known as the Widmanstätten structure. This pattern is named after one of the first people to observe it some 200 years ago, Count Alois de Widmanstätten. The pattern forms only deep inside planetary bodies that take millions of years to cool. Iron never crystallizes this way on Earth's surface, so any metal showing this pattern on Earth is definitely from a meteorite.
The crystal patterns in iron meteorites range from very coarse to extremely fine. The thickness of the crystals in the pattern depends mainly on the amount of nickel they contain, and how slowly the metal cooled. With this information, one can estimate the size of the meteorite's parent body, because larger asteroids cool more slowly than small ones.