Meteorites have fascinated mankind for centuries. Indeed, more than two dozen meteorites have been venerated by Indian tribes, aborigines, Arabs and other ancient peoples. The study of meteorites is part of the overall study of the origin of our solar system. There was a recent meteor explosion over the city of Chelyabinsk with up to 1000 injuries. Think what the damage would have been like if it hit a major city. Some asteroids are exceptionally large, and when they strike earth, they can make an immense crater. Some of these, as in Figure 1, are in arid climates and can be seen today. Such an impact near the Yucatan Peninsula has been claimed to have caused the extinction of dinosaurs.
There are three basic types of meteorites: stones, stoney-irons, and iron. The classification of meteorites is a complex subject. For the iron meteorites, classification is based upon chemical composition, macrostructure, and microstructure. Basically, iron meteorites “fall” (no pun intended!) into three categories – hexahedrites, octahedrites, and ataxites. Some, however, do not fully fit the requirements of these groups and are classed as anomalous. Displays of meteorites in museums generally consist of large, solid chunks of iron meteorites and of etched slices, as shown in Figures 2 to 6. These slices are ground smooth and then etched with a strong acid solution that brings out the growth structure. The octahedrites are commonly exhibited in this manner because they undergo a solid-state phase transformation where the kamacite (ferrite) nucleates and grows along the octahedral planes of the parent taenite (austenite) phase producing a beautiful etched pattern.