Kamacite
A variety of Iron
What is Kamacite?
Kamacite is an alloy of iron and nickel, which is found on Earth only in meteorites. The proportion iron:nickel is between 90:10 and 95:5; small quantities of other elements, such as cobalt or carbon may also be present. The mineral has a metallic luster, is gray and has no clear cleavage although its crystal structure is isometric-hexoctahedral. Its density is about 8 g/cm and its hardness is 4 on the Mohs scale. It is also sometimes called balkeneisen. The name was coined in 1861 and is derived from the Greek root καμακ- "kamak" or κάμαξ "kamaks", meaning vine-pole. It is a major constituent of iron meteorites (octahedrite and hexahedrite types). In the octahedrites it is found in bands interleaving with taenite forming Widmanstätten patterns. In hexahedrites, fine parallel lines called Neumann lines are often seen, which are evidence for structural deformation of adjacent kamacite plates due to shock from impacts. At times kamacite can be found so closely intermixed with taenite that it is difficult to distinguish them visually, forming plessite. The largest documented kamacite crystal measured 92×54×23 cm (36.2×21.3×9.1 in).
Uses & Applications
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.
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Key Characteristics
Characteristics of Kamacite
Kamacite has many unique physical properties including Thomson structures and extremely high density.
Formation of Kamacite
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.
Composition of Kamacite
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.
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Quick Facts
Physical Properties
- Streak
- Grey
Chemical Properties
- Chemical Formula
- Fe
- Elements
- Fe
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