Widmanstätten pattern
From Wikipedia, the free encyclopedia
Widmanstätten patterns, named after Count Alois von Beckh Widmanstätten, are unique structures of long nickel-iron crystals, found in the octahedrite iron meteorites. They consist of a fine interleaving of kamacite and taenite bands or ribbons called lamellae.
Octahedrite meteorites have a nickel content intermediate between the norm for kamacite and taenite, which under slow cooling conditions leads to a separation of the nickel into the interleaving bands. The crystalline patterns become visible when the meteorites are cut, polished, and acid etched, because taenite is resistant to the acid.
Since nickel-iron crystals grow to lengths of several centimeters only when the molten metal cools slowly (over several million years), the presence of these patterns can be used to determine if a piece of iron is from a meteorite (as opposed to terrestrial in origin). This process was discovered in 1808 by Count Alois von Widmanstätten, who was the director of the Imperial Porcelain works in Vienna.
Commonly, in gaps between the lamellae, a fine-grained mixture of kamacite and taenite called plessite can be found.
[edit] References
