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 to 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 the structure is isometric-hexoctahedral. Its density is around 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 kamask (lath or beam). 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 centimetres (36.2×21.3×9.1 in).^[2]

Physical properties

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

Identification

Kamacite is opaque, and generally is dark gray in color as well as streak. Kamacite has a metallic luster. Kamacite can vary in hardness based on the extent of shock it has gone under but commonly it is said to rank a four on the mohs hardness scale. Shock increases kamacite hardness, but this is not 100% reliable in determining shock histories as there is a myriad of other reasons the hardness of kamacite could increase ^[3]

Kamacite is extremely dense having a measured density of 7.9. It has a massive crystal habit but normally individual crystals are indistinguishable in natural occurrences. There is no planes of cleavage present in kamacite which gives it a hackly fracture. Kamacite is magnetic, and isometric which makes it behave optically isometrically.

Look-A-likes

Kamacite is very hard to tell apart from its other phase taenite and often occurs with taenite. It is amazing that the two phases are easiest and most reliably told apart by checking hardness as kamacite has a hardness of four and taenite is never less than five. When kamacite and taenite occur side by side bands of each make intricate lines and can be easily spotted and identified as different minerals. When prepared properly in these bands kamacite can be normally told apart as being the darker mineral. Plessite is the name of a fine grained mixture of kamacite and taenite that (as the name would have you expect from the Greek word "plythos" which means “filling") occurs as a filling in between the two. Kamacite being gray with a metallic luster makes kamacite visually similar to a great deal of metallic minerals such as tenanted and platinum. Kamacite’s magnetism, high specific gravity and hardness are its easiest most distinguishing features. Even with these features it would be hard to tell it apart from a few other minerals. If you are able to reliably ascertain that your sample fell from the sky and it fits these criteria it is a safe bet that it is kamacite.

Other Phases

Iron and nickel (kamacite) have another phase, taenite. Kamacite actually exists in three phases; kamacite, taenite, and a mixed area of kamacite and taenite (sometimes just referred to as plessite).^[4] Kamacite also has a few alterations including tochilinite which will be discussed later in section 4.4. 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 ^[5]

Tetragonal phase

There is evidence of a tetragonal phase, has appeared 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”.^[6] It has been speculated to be e-iron a hexagonal polymorph of iron. This is the only mention of this phase found in my research.

Thomson structures

Thomson structures, sometimes called Widmanstätten patterns are textures often seen in meteorites that contain kamacite. These are bands which are usually alternating between kamacite and taenite. G. Thomson (his first name is seemingly lost to history) 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 scientist, 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.^[7] 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.^[8]

Optical properties

Kamacite is unfortunately opaque and can only be observed in reflected light microscopy. It is isometric and therefore behaves isotropically.

Magnetism

Kamacite dominates meteorites magnetic properties. The taenite phase has no magnetism and kamacite is magnetic, as the meteorite cools below 750 °C iron becomes magnetic as it moves in to 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 that 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 (main source of meteorites on earth) magnetic field which has been shown to be as strong as .4 Os. To research is this Thermoremanent Magnetization or Chemical Remanent Magnetization in meteorites is of interest because it could not have been caused isothermally.^[9]

Crystallography

Kamacite is an isometric mineral with a body centered unit cell. Kamacite is usually not found in large crystals However the anomousaly largest kamacite crystal found and documented measured 92×54×23 centimeters.^[10] 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, hex octahedral crystals this causes the crystals to have many symmetry elements. Kamacite falls under the 4/m 3¯ 2/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 F m3m.

Unit cell

Kamacite is made up of a repeating unit of α-(Fe, Ni); Fe0+0.9Ni0.1 which makes up cell dimensions of a = 8.603, Z = 54; V = 636.72. The interatomic magnetic and quantum interactions of the iron atoms interacting with each other causes kamacite to have a body centered lattice.

Chemistry

Chemistry has been touched upon many times above. Kamacite is an iron nickel alloy, this next section will shed light on the chemistry element on this mineral.

Formula and dominant elements

Kamacite is made up of a repeating unit of α-(Fe, Ni); Fe0+0.9Ni0.1. Neglecting trace elements for a moment, it is normally considered to be made up of 90% iron and 10% nickel but can have a ratio of 95% iron and 5% nickel. This makes iron the dominant element in any sample of kamacite. It is grouped with the native elements in both Dana and Nickel-Strunz classification systems.^[11]

Conditions of formation

Kamacite starts to form around 723˚C this is 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.

Trace elements

There has been a great deal of research into kamacite’s trace elements. The most notable trace elements in kamacite are gallium, germanium, cobalt, copper, and chromium. Colbalt is the most notable of these where the nickel content varies from 5.26% to 6.81% and the cobalt content can be from 0.25% to 0.77%.^[12] All of these trace elements are metallic and their appearance near the kamacite taenite border can give important clues to the environment the meteorite was formed in. Mass spectroscopy has revealed kamacite to contain considerable amounts of: Platinum to be an average of 16.31 (µg/g), iridium to be an average of 5.40 (µg/g), osmium to be an average of 3.89 (µg/g), tungsten to be an average of 1.97 (µg/g), gold to be an average of 0.75 (µg/g), rhenium to be an average of 0.22 (µg/g),.^[13] The considerable amounts of cobalt and platinum are the most notable.

Important minor elements, substitutions, solid solutions

Kamacite sulfurization has been done experimentally in lab conditions. Sulfurization resulted in three distinct phases: a mono-sulfide solid solution (Fe,Ni,Co)1-xS, a pentlandite phase (Fe,Ni,Co)9-xS8, as well as a P-rich phase. This was done in a lab to construct conditions concurrent with that of the solar nebula. With this information it would be possible to extract information about the thermodynamic, kinetic, and physical conditions of the early solar system. This still remains speculatory as many of the sulfides in meteorites are unstable and have been destroyed.^[14] Kamacite also alters to tochilinite (Fe2+5-6(Mg, Fe2+)5S6(OH)10). This is useful for giving clues as to how much the meteorite as a whole has been altered. Kamacite to tochilinite alteration can be seen in petrologic microscopes, scanning electron microscope, and electron microprobe analysis. This can be used to allow researchers to easily index the amount of alteration that has taken place in the sample. This index can be later referenced when analyzing other areas of the meteorite where alteration is not as clear.^[15]

Relationship with taenite

Taenite is the nickel rich end member of the kamacite - taenite solid solution. Taenite is naturally occurring on earth whereas kamacite is only found on earth when it comes from space. Kamacite forms taenite as it forms and expels nickle to the sourounding area, this area forms taenite. Due to the face centered nature of the kamacite lattice and the body centered nature of the nickel lattice the two make intricate angles when they come in contact with each other. These angles revile themselves macroscopically in the Thomson structure. Also due to this relationship we get the terms ataxite, hexahedrites and octahedrite. Ataxite refers to meteorites that do not show a grossly hexahedral or octahedral structure. Meteorites composed of 6 wt% or less nickel are often referred to as hexahedrites due to the crystal structure of kamacite being isometric and causing the meteorite to be cubic. Likewise if the meteorite is dominated by the face centered taenite it is called an octahedrite as kamacite will exsolve from the octahedral crystal boundaries of taenite making the meteorite appear octahedral. Both hexahedrites and octahedrite only appear when the meteorite breaks along crystal planes or when prepared to excentuate the Thomson structures therefore many are mistacklen called ataxites ar first.^[16][17]

Chemical explanation of heat

Trace elements have been analyzed in the formation of kamacite at different temps but the trace elements in taenite seem better suited to give clues of the formation temperature of the meteorite. As the meteorite cools and taenite and kamacite are sorting out of each other some of the trace elements will prefer to be located in taenite or kamacite. Analyzing the taenite kamasite boundary can give clues to how quickly coolong occurred as well as a meryad of other condisions during formation by the final location of the trace elements.

Stability range

Kamacite is only stable at temperatures below 723˚C ^[18] or 600 °C (Stacey and Banerjee, 2012). as that is where iron becomes cool enough to arrange in a body centered arrangement. Kamacite is also only stable at low pressures as can be assumed as it only forms in space.^[19]

Effect of shock

Metallographic and X-ray diffraction can be used on kamacite to determine the shock history of a meteorite. Using hardness to determine shock histories has been experimented with but was found to be too unreliable. Vickers hardness test was applied to a number of kamacite samples and shocked meteorites were found to have values of 160–170 kg/mm and non-shocked meteorites can have values as high as 244 kg/mm.^[20] Shock causes a unique iron transformation structure that is able to be measured using metallographic and X-ray diffraction techniques. After using metallographic and X-ray diffraction techniques to determine shock history it was found that 49% of meteorites found on earth contain evidence of shock.

Geologic occurrences

Kamacite has been found on every continent on earth and it has also been found on Mars, but it is most notable in meteorites.

Meteorites

Kamacite is primarily associated with meteorites because it needs high temperatures, low pressures and few other more reactive elements like oxygen. Chondrites containing meteorites can be split into groups based on the chondrites present. There are three major types’ enstatite chondrites, carbonaceous chondrites and ordinary chondrites. Ordinary chondrites are the most abundant type of meteorite found on earth making up 85% of all meteorites recorded.^[21] Ordinary chondrites are thought to have all originated from three different sources thus they come in three types LL, L, and H; LL stands for Low iron, Low metal, L stands for Low iron abundance, and H is High iron content. All ordinary chondrites contain kamacite in decreasing abundance as you move from H to LL chondrites.^[22] Kamacite is also found in many of the less common meteorites mesosiderites and E chondrites. E chondrites are chondrites which are made up of primarily enstatite and only account for 2% of metorites that fall on earth. E chondrites have an entirely different source rock than that of the ordinary chondrites.^[23] In analysis of kamacite in E chondrites it was found that they contain generally less nickel then average.^[24]

Abundance

Since kamacite is only formed in space and is only found on Earth in meteorites, it has very low abundance on Earth. Its abundance outside our solar system is difficult to determine. Iron is thought to be the most abundant element in the universe so it is likely to be comparatively quite abundant in the universe.^[25]

Associated minerals

As mentioned in previous sections taenite, and tochilinite are minerals that are commonly associated with kamacite.

Specific examples

Meteor crater Arizona

Kamacite has been founded and studied in Meteor Crater, Arizona. Meteor Crater was the first confirmed meteor impact site on the planet, and was not universally recognized as such till the 1950s. In the 1960s United States Geological Survey discovered kamacite in specimens gathered from around the site tying the mineral to meteorites.^[26]

Planets

Kamacite primarily forms on meteorites but has been found on extraterrestrial bodies such as Mars. This was discovered by The Mars Exploration Rover (MER) Opportunity. The kamacite did not originate on Mars but was put there by a meteorite. This was particularly of interest because the meteorite fell under the lesser known class of mesosiderites. Mesosiderites are very rare on Earth and its occurrence on Mars gives clues to the origin of its larger source rock.^[27]

Uses

7.1. Use to date and infer formation conditions of meteorites 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.

Museums, University, and Photo specimen preparation

Due to the rareness and the generally dull appearance of kamacite it is not popular among private collectors. However many museums and universities have samples of kamacite in their collection. Normally kamacite samples are prepared using polish and acid to show off the Thomson structures such as the photos in figure 5 an etched polished meteorite showing the Thomson structures. Preparing specimens to look like this involves washing them in a solvent, such as Thomson did with nitric acid to bring out the Thomson structures. Then they are heavily polished so they look shiny. Genrally the kamacite can be told apart from taenite easily as after this process the kamacite looks slightly darker than the taenite.^[28]

Looking to the future

Kamacite and taenite both have the potential to be economically valuable in the near-ish future. An option that would make asteroid mining more profitable would be to gather the trace elements. The trick would be to send something that would refine elements such as platinum and gold. Considering platinum is 12,394.00 US$/Kg and 16.11 µg/g kamacite and gold is 12,346.00 US$/Kg and 0.52 µg/g in kamacite. However even using quick calculations this would be a costly endeavor and likeliness of a profitable return is fairly slim.^[29] While it would be hard to argue a way to extract these minerals from space for use on earth but already being in abundance in space makes them valuable for use in space. It is easier to imagine a scenario where the iron is used in space to build things in space as transport of material from earth is the costly part of the operation. Similar to current plans of reusing the modules of the International Space Station in other missions an iron meteorite could be used to build space craft in space. The National Aeronautics and Space Administration (NASA) has put forward preliminary plans to do just that, build a space ship in space.^[30]

See also

• Glossary of meteoritics

References

• Mason B., 1962: Meteorites. J. Wiley & Sons, New York

Kamacite and taenite after taenite, exhibiting the octahedral structure of taenite, Nantan (Nandan) iron meteorite, Nandan County, Guangxi Zhuang Autonomous Region, China. Size: 4.8×3.0×2.8 cm. The Nantan irons, a witnessed fall in 1516, have a composition of 92.35% iron and 6.96% nickel.

Wikimedia Commons has media related to Kamacite.