Iron meteorite

The iron meteorites or nickel-iron meteorites make up about five percent of all meteorites and consist of an alloy of iron and about 5 to 20 percent by weight of nickel . Its interior is colored metallic gray and surrounded by a brown-black crust. It usually consists of two different minerals , kamacite and taenite , which form characteristic Widmanstätten structures . Iron meteorites often contain inclusions of the mineral troilite (iron sulfide). The largest meteorite found on Earth, the Hoba meteorite , is an iron meteorite.

Origin and Composition

Widmanstätten structure in meteorite iron

Iron meteorite with structure, melts, inclusions

Iron meteorite from Campo del Cielo , Argentina

Iron meteorites probably originate from the core of former asteroids , during the formation of which the heavy elements iron and nickel are deposited inside. They are often viewed as a model for the composition of the Earth's core . Inside the asteroids, the metals were completely melted and mixed, they cooled down very slowly - about 1 K in a thousand years. The melt initially crystallized as a homogeneous iron-nickel alloy, which upon further cooling disintegrated into two minerals with different nickel contents, the low-nickel kamacite (less than 6 percent nickel content), which is also known as bar iron, and the nickel-rich taenite (6 to 15 Percent nickel), also known as a band iron.

In addition to a specific iron and nickel content, the iron meteorites contain minerals such as cohenite (iron carbide), Schreibersite (nickel-iron phosphide), troilite (iron sulfide) and carbon in the form of graphite . They also contain trace amounts of precious and heavy metals such as germanium , gallium , iridium , arsenic , tungsten and gold .

classification

The nickel-iron meteorites are divided into hexaedrites, octahedrites and ataxites based on their composition and structure:

Hexaedrites were not heated above 800 ° C during their formation and consist almost exclusively of the mineral kamacite. The nickel content is 4–7.5%. They do not show any Widmanstatt structures like the octahedrites. The name refers to their cleavability according to the faces of a cube, or hexahedron. However, some of the meteorites show parallel lines after etching, the Neumann lines . They are deformations of the crystal structure that are obviously the result of an impact event and could have occurred when the original body collided with another asteroid or when it hit the earth.

Octahedrites were exposed to temperatures above 800 ° C during their formation. They are made up of a mixture of Kamacite and Taenit. If these meteorites are polished and etched, the typical Widmanstatt structures of kamacite beams and taenite lamellas are revealed. They are arranged parallel to the faces of an octahedron, hence the name. According to the width of the Kamacit bars, the octahedrites are divided into structural classes. There is a correlation with the nickel content: the more nickel, the finer the structure. The reason for this lies in the fact that in the iron-nickel system, the precipitation of kamacite from the initially homogeneous taenite takes place at a lower temperature, the higher the nickel content, so that only narrow bars can then form due to the slower diffusion .

A distinction is made between the following classes:

Largest octahedrites (Ogg), bar width more than 3.3 mm, 5–9% Ni
Coarse octahedrites (Og), bars 1.3 to 3.3 mm, 6.5-8.5% Ni
Medium octahedrites (Om), bars 0.5 to 1.3 mm, 7-13% Ni
Fine octahedrites (Of), bars 0.2-0.5 mm, 7.5-13% Ni
Finest octahedrites (off), bars smaller than 0.2 mm, 17–18% Ni

In addition, the octahedrites can be divided into chemical groups I to IV according to their content of the trace elements Ga, Ge and Ir. There are also a number of octahedrites that have not yet been assigned to any of these groups. Well-known representatives of the octahedrite group are the Gibeon meteorite, Sikhote-Alin meteorite , Campo del Cielo meteorite , Canyon Diablo meteorite , Nantan meteorite , the Mundrabilla meteorite and the meteorite Toluca .

The ataxites (the name means "without structure") have nickel contents of more than 15 percent. Only the mineral taenite is present in these meteorites; there are no Widmanstatt structures . The ataxites include, for example, the Chinga meteorites and Dronino meteorites, as well as the 60-ton meteorite Hoba .

Investigations of the respective ratio of the trace metals gallium, germanium, cobalt, chromium and copper to the nickel content in nickel-iron meteorites by JF Lovering et al. (1957) led, in addition to the structural classification, to the introduction of chemical groups I to IV. This classification was expanded in 1967 by JT Wasson and J. Kimberlin to a total of 13 groups, which are differentiated by adding letters to the group number. It is assumed that each of these chemical groups corresponds to its own original body. After all, around 10 percent of iron meteorites do not fit into any of these 13 groups and are referred to as ungrouped (UNGR). Nickel-iron meteorites can also be divided into magmatic and non-magmatic. The former emerged from a melt that had completely melted, while the non-magmatic meteorites were probably not completely melted and may have been formed during an impact.

The majority of the nickel-iron meteorites belong to the IAB group. They are coarse and medium octahedrites with clearly defined Widmanstatt structures. They contain inclusions of various silicates that are chemically closely related to primitive achondrites . It is assumed that both groups of meteorites come from the same body of origin. The IAB iron meteorites often contain inclusions of the iron sulfide troilit and black graphite nodules. The presence of this elementary form of carbon and the distribution of the trace elements give an indication of the relationship between the IAB iron meteorites and the carbonaceous chondrites .

The meteorites of group IIAB are hexaedrites, which are composed of individual, very large kamacite crystals. The distribution of trace elements is similar to that in some carbonaceous chondrites and enstatite chondrites . It is therefore assumed that the IIAB irons come from a chondritic body.

The group of IIC iron meteorites consists of octahedrites with a very fine crystal structure.

The group IID meteorites are medium to fine octahedrites that contain high proportions of gallium and germanium . They often contain inclusions of the nickel-iron phosphide writersite - an extremely hard mineral.

Group IIE meteorites are coarse to medium octahedrites that contain numerous inclusions of iron-rich silicates. There is a chemical relationship to the H- chondrites .

Group IIF meteorites are composed of octahedrites and ataxites. There is a chemical relationship to the pallasites and the carbonaceous chondrites of the groups CO and CV.

The group IIIAB represents the second large group of iron meteorites next to the IAB meteorites. IIIAB meteorites are coarse to medium octahedrites that are chemically related to the pallasites of the main group. Obviously, both groups come from a common body of origin.

The groups IIICD and IIIE are very fine octahedrites and ataxites with different proportions of trace elements.

The members of the IVA group are fine octahedrites. The distribution of their trace elements distinguishes them from all other groups.

Group IVB meteorites are ataxites with a nickel content of over about 17 percent.

Cultural history

Meteoritic iron was used for the manufacture of cult objects, tools and weapons even before the actual Iron Age . For example, in a small burial ground from the time from 3300 to 3000 BC. Found iron pearls with a nickel content of 7.5 percent near the Egyptian settlement of Gerzeh , which suggests the meteoritic origin. A dagger blade made of meteoritic iron was also found in the burial chamber of the pharaoh Tutankhamun (around 1340 BC).

The large iron meteorites from Cape York found in Greenland were used by the Eskimos to make metal harpoon tips and knives. Even today, because of its relative rarity, meteoritic iron is used to make jewelry or handmade knives.

Individual evidence

^ Vagn F. Buchwald: Handbook of Iron Meteorites . University of California Press, 1975.
^ John T. Wasson: Meteorites. Classification and Properties. Springer-Verlag 1974.
^ JC Waldbaum: The first archaeological appearance of iron and the transition to the iron age. In: The Coming of the Age of Iron . Yale University Press, 1980.
^ JK Bjorkman: Meteors and Meteorites in the Ancient Near East. Meteoritics 8 (1973) 91-132.
↑ John Ross: Voyage of Discovery in Baffin's Bay. London 1819.

Web links

Commons : Iron meteorites - Collection of images, videos and audio files

Iron meteorite in the WiKi mineral atlas

[1] Vagn F. Buchwald: Handbook of Iron Meteorites . University of California Press, 1975.

[2] John T. Wasson: Meteorites. Classification and Properties. Springer-Verlag 1974.

[3] JC Waldbaum: The first archaeological appearance of iron and the transition to the iron age. In: The Coming of the Age of Iron . Yale University Press, 1980.

[4] JK Bjorkman: Meteors and Meteorites in the Ancient Near East. Meteoritics 8 (1973) 91-132.

[5] John Ross: Voyage of Discovery in Baffin's Bay. London 1819.