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.
In 1808, these figures were observed by Count Alois von Beckh Widmanstätten, the director of the Imperial Porcelain works in Vienna.
The discovery was acknowledged by Carl von Schreibers, director of the Vienna Mineral and Zoology Cabinet, who named the structure after Widmanstätten.
[5][6]: 124 However, it is now believed that the discovery of the metal crystal pattern should be assigned to the English mineralogist William (Guglielmo) Thomson, as he published the same findings four years earlier.
[7][6][8][9] Working in Naples in 1804, Thomson treated a Krasnojarsk meteorite with nitric acid to remove the dull patina caused by oxidation.
Civil wars and political instability in southern Italy made it difficult for Thomson to maintain contact with his colleagues in England.
[6]: 124–125 [8][10] At the beginning of 1806, Napoleon invaded the Kingdom of Naples and Thomson was forced to flee to Sicily[8] and in November of that year, he died in Palermo at the age of 46.
In 1808, Thomson's work was again published posthumously in Italian (translated from the original English manuscript) in Atti dell'Accademia Delle Scienze di Siena.
[11] The Napoleonic wars obstructed Thomson's contacts with the scientific community and his travels across Europe, in addition to his early death, obscured his contributions for many years.
Since nickel-iron crystals grow to lengths of some centimeters only when the solid metal cools down at an exceptionally slow rate (over several million years), the presence of these patterns is strongly suggestive of extraterrestrial origin of the material, and can be used to indicate if a piece of iron may come from a meteorite.
Most commonly, the slice is ground and polished, cleaned, etched with a chemical such as nitric acid or ferric chloride, washed, and dried.
[14][15] Cutting the meteorite along different planes affects the shape and direction of Widmanstätten figures because kamacite lamellae in octahedrites are precisely arranged.
