Kamacite

Kamacite has many unique physical properties including Thomson structures and extremely high density.

Kamacite starts to form around 723 °C, where iron splits from being face centered to body centered while nickel remains face centered. To accommodate this areas start to form of higher iron concentration displacing nickel to the areas around it which creates taenite which is the nickel end member.

Thomson structures, usually referred to as Widmanstätten patterns are textures often seen in meteorites that contain kamacite. These are bands which are usually alternating between kamacite and taenite. G. Thomson stumbled upon these structures in 1804 after cleaning a specimen with nitric acid he noticed geometric patterns. He published his observations in a French journal but due to the Napoleonic wars the English scientists, who were doing much of the meteorite research of the time, never saw his work. It was not until four years after in 1808 the same patterns were discovered by Count Alois von Beckh Widmanstätten who was heating iron meteorites when he noticed geometric patterns caused by the differing oxidation rates of kamacite and taenite. Widmanstätten told many of his colleagues about these patterns in correspondence leading to them being referred to as Widmanstätten patterns in most literature. Thomson structures or Widmanstätten patterns are created as the meteorite cools; at high temperatures both iron and nickel have face-centered lattices. When the meteorite is formed it starts out as entirely molten taenite (greater than 1500 °C) and as it cools past 723 °C the primary metastable phase of the alloy changes into taenite and kamacite begins to precipitate out. It is in this window where the meteorite is cooling below 723 °C where the Thomson structures form and they can be greatly affected by the temperature, pressure, and composition of the meteorite.

The primary research use of kamacite is to shed light on a meteorite's history. Whether it is looking at the shock history in the iron structures or the conditions during the formation of the meteorite using the kamacite-taenite boundary understanding kamacite is key to understanding our universe.