Kamacite is an alloy of iron and nickel, which is found on Earth only in meteorites. According to the International Mineralogical Association (IMA) it is considered a proper nickel-rich variety of the mineral native iron. 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/cm3 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).
Physical properties Kamacite has many unique physical properties including Widmanstätten pattern and extremely high density.
Identification Kamacite is opaque, and its surface generally displays varying shades of gray streaking, or "quilting" patterns. Kamacite has a metallic luster. Kamacite can vary in hardness based on the extent of shock it has undergone, but commonly ranks a four on the mohs hardness scale. Shock increases kamacite hardness, but this is not 100% reliable in determining shock histories as there are myriad other reasons that the hardness of kamacite could increase. Kamacite has a measured density of 7.9 g/cm3. It has a massive crystal habit but normally individual crystals are indistinguishable in natural occurrences. There are no planes of cleavage present in kamacite which gives it a hackly fracture. Kamacite is magnetic, and isometric which makes it behave optically isometrically. Kamacite occurs with taenite and a mixed area of kamacite and taenite referred to as plessite. Taenite contains more nickel (12 to 45 wt. % Ni) than kamacite (which has 5 to 12 wt. % Ni). The increase in nickel content causes taenite to have a face-centered unit cell, whereas kamacite's higher iron content causes its unit cell to be body centered. This difference is caused by nickel and iron having a similar size but different interatomic magnetic and quantum interactions.
Tetragonal phase There is evidence of a tetragonal phase, observed in X-ray powder tests and later under a microscope. When tested two meteorites gave d-values that could "be indexed on the basis of a tetragonal unit cell, but not on the basis of a cubic or hexagonal unit cell". It has been speculated to be e-iron, a hexagonal polymorph of iron.
Widmanstätten patterns
Widmanstätten pattern, sometimes referred to as Thomson pattern, are textures often seen in meteorites that contain kamacite. These are bands which are usually alternating between kamacite and taenite. In 1804, William Thomson stumbled upon these structures when he noticed unexpected geometric patterns after cleaning a specimen with nitric acid (HNO3). 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 discovered his work. It was not until 1808, four years later, that the same etching patterns were discovered by Count Alois von Beck 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. 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.
Optical properties Kamacite is opaque and can be observed only in reflected light microscopy. It is isometric and therefore behaves isotropically.
Magnetism As the meteorite cools below 750 °C iron becomes magnetic as it moves into the kamacite phase. During this cooling the meteorite takes on non-conventional thermoremanent magnetization. Thermoremanent magnetization on Earth gives iron minerals formed in the Earth's crust, a higher magnetization than if they were formed in the same field at room temperature. This is a non-conventional thermoremanent magnetization because it appears to be due to a chemical remanent process which is induced as taenite is cooled to kamacite. What makes this especially interesting is this has been shown to account for all of the ordinary chondrites magnetic field which has been shown to be as strong as 0.4 oersted (symbol Oe).
Crystallography Kamacite is an isometric mineral with a body cubic centered unit cell. Kamacite is usually not found in large crystals; however the anomalously largest kamacite crystal found and documented measured 92×54×23 centimeters. Even with large crystals being so rare, crystallography is extremely important to understand plays an important role in the formation of Thomson structures.
Symmetry Kamacite forms isometric, hexoctahedral crystals; this causes the crystals to have many symmetry elements. Kamacite falls under the 4/m32/m class in the Hermann–Mauguin notation meaning it has three fourfold axes, four threefold axes, and six twofold axes and nine mirror planes. Kamacite has a space group of Fm3m.
Unit cell Kamacite is made up of a repeating unit of α-(Fe, Ni), Fe0.9Ni0.1, which makes up cell dimensions of a = 8.603 Å, Z = 54 Å; V = 636.72 Å3. The interatomic magnetic and quantum interactions of the zerovalent iron (metallic Fe0) atoms interacting with each other causes kamacite to have a body centered lattice.
Chemistry
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