What is the Widmanstätten pattern?
The Widmanstätten pattern is the criss-cross arrangement of metal bands seen when many iron meteorites are cut, polished and etched. It forms from two iron-nickel alloys that separated as the metal cooled extremely slowly.
Slice through most iron meteorites, polish the surface to a mirror shine and treat it with a weak acid solution, and an extraordinary design appears: long, interlocking bands crossing at different angles, with fine-grained patches in between. This is the Widmanstätten pattern, and it is one of the most recognisable sights in meteorite science.
The bands are made of kamacite, an alloy low in nickel. Between them run thin, bright ribbons of taenite, which is richer in nickel. The spaces left over are filled with plessite, a fine mixture of the two. Because the alloys react differently to the etching solution, they reflect light differently, so the pattern stands out. Depending on how a meteorite is cut relative to the original crystal, the bands may form triangles, parallel stripes or a lattice.
The pattern is named after Count Alois von Beckh Widmanstätten of Vienna, who in 1808 observed it by heating slices of the Hraschina iron, which fell in what is now Croatia in 1751, so that the alloys took on different colours. An English scientist working in Naples, William Thomson, had seen it a few years earlier after treating a meteorite with acid, so it is sometimes called the Thomson structure.
Meteorites that show the pattern are called octahedrites, and the width of the bands is used to classify them, from coarse to fine. Gibeon from Namibia is a much-admired fine example. Etching uses hazardous chemicals and permanently alters a specimen, so it is a job for experienced preparators, not something to try at home and never an activity for children. Many museums display etched slices so everyone can enjoy the view safely.