Iron meteorites, also called siderites or

ferrous In chemistry, iron(II) refers to the chemical element, element iron in its +2 oxidation number, oxidation state. The adjective ''ferrous'' or the prefix ''ferro-'' is often used to specify such compounds, as in ''ferrous chloride'' for iron(II ...

meteorites, are a type of

meteorite A meteorite is a rock (geology), rock that originated in outer space and has fallen to the surface of a planet or Natural satellite, moon. When the original object enters the atmosphere, various factors such as friction, pressure, and chemical ...

that consist overwhelmingly of an

iron–nickel alloy An iron–nickel alloy or nickel–iron alloy, abbreviated FeNi or NiFe, is a group of alloys consisting primarily of the elements nickel (Ni) and iron (Fe). It is the main constituent of the "iron" planetary cores and iron meteorites. In chemi ...

known as

meteoric iron Meteoric iron, sometimes meteoritic iron, is a native metal and early-universe protoplanetary-disk remnant found in meteorites and made from the elements iron and nickel, mainly in the form of the mineral phases kamacite and taenite. Meteoric ...

that usually consists of two

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

phases:

kamacite 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:ni ...

and

taenite Taenite is a mineral found naturally on Earth mostly in iron meteorites. It is an alloy of iron and nickel, with a chemical formula of and nickel proportions of 20% up to 65%. The name is derived from the Greek ταινία for "band, ribbo ...

. Most iron meteorites originate from cores of

planetesimal Planetesimals () are solid objects thought to exist in protoplanetary disks and debris disks. Believed to have formed in the Solar System about 4.6 billion years ago, they aid study of its formation. Formation A widely accepted theory of pla ...

s, with the exception of the IIE iron meteorite group. The iron found in iron meteorites was one of the earliest sources of usable iron available to

human Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

s, due to the malleability and ductility of the meteoric iron, before the development of

smelting Smelting is a process of applying heat and a chemical reducing agent to an ore to extract a desired base metal product. It is a form of extractive metallurgy that is used to obtain many metals such as iron-making, iron, copper extraction, copper ...

that signaled the beginning of the

Iron Age The Iron Age () is the final epoch of the three historical Metal Ages, after the Chalcolithic and Bronze Age. It has also been considered as the final age of the three-age division starting with prehistory (before recorded history) and progre ...

Occurrence

Although they are fairly rare compared to the

stony meteorite In meteoritics, a meteorite classification system attempts to group similar meteorites and allows scientists to communicate with a standardized terminology when discussing them. Meteorites are classified according to a variety of characteristics ...

s, comprising only about 5.7% of witnessed falls, iron meteorites have historically been heavily over-represented in

collections. This is due to several factors: * They are easily recognized as unusual, as opposed to stony meteorites. Modern-day searches for meteorites in deserts and Antarctica yield a much more representative sample of meteorites overall. * They are much more resistant to weathering. * They are much more likely to survive atmospheric entry, and are more resistant to the resulting

ablation Ablation ( – removal) is the removal or destruction of something from an object by vaporization, chipping, erosion, erosive processes, or by other means. Examples of ablative materials are described below, including spacecraft material for as ...

. Hence, they are more likely to be found as large pieces. * They can be found even when buried by use of surface metal-detecting equipment, due to their metallic composition. Because they are also denser than stony meteorites, iron meteorites also account for almost 90% of the mass of all known meteorites, about 500 tons. All the largest known meteorites are of this type, including the largest—the

Hoba meteorite The Hoba ( ) meteorite is named after the farm Hoba West, where it lies, not far from Grootfontein, in the Otjozondjupa Region of Namibia. It has been uncovered, but because of its large mass, has never been moved from where it fell. The main mass ...

Origin

Iron meteorites have been linked to

M-type asteroid M-type (metallic-type, aka M-class) asteroids are a spectral class of asteroids which appear to contain higher concentrations of metal phases (e.g. iron-nickel) than other asteroid classes, and are widely thought to be the source of iron meteorit ...

s because both have similar spectral characteristics in the visible and near-infrared. Iron meteorites are thought to be the fragments of the cores of larger ancient

asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...

s that have been shattered by impacts. The heat released from the radioactive decay of the short-lived nuclides ²⁶Al and ⁶⁰Fe is considered as a plausible cause for the melting and differentiation of their parent bodies in the early Solar System. Melting produced from the heat of impacts is another cause of melting and differentiation. The

IIE iron meteorite The iron meteorites of the IIE chemical type are octahedrites of various coarseness, most of which contain numerous inclusions of recrystallized stony silicates. Composition and origin They have mineral compositions and oxygen isotope Isot ...

s may be a notable exception, in that they probably originate from the crust of

S-type asteroid S-type (stony-type or silicaceous-type) asteroids are asteroids with a spectral type that is indicative of a siliceous (i.e. stony) mineralogical composition, hence the name. They have relatively high density. Approximately 17% of asteroids are ...

6 Hebe 6 Hebe () is a large main-belt asteroid, containing around 0.5% of the mass of the belt. However, due to its apparently high bulk density (greater than that of the Moon), Hebe does not rank among the top twenty asteroids by volume. This high bu ...

. Chemical and isotope analysis indicates that at least about 50 distinct parent bodies were involved. This implies that there were once at least this many large, differentiated, asteroids in the asteroid belt – many more than today.

Composition

The overwhelming bulk of these meteorites consists of the FeNi-alloys

and

. Minor minerals, when occurring, often form rounded nodules of

troilite Troilite () is a rare iron sulfide mineral with the simple formula of FeS. It is the iron-rich endmember of the pyrrhotite group. Pyrrhotite has the formula Fe(1−x)S (x = 0 to 0.2) which is iron deficient. As troilite lacks the iron deficiency ...

graphite Graphite () is a Crystallinity, crystalline allotrope (form) of the element carbon. It consists of many stacked Layered materials, layers of graphene, typically in excess of hundreds of layers. Graphite occurs naturally and is the most stable ...

, surrounded by

schreibersite Schreibersite is generally a rare iron nickel phosphide mineral, , though common in iron-nickel meteorites. It has been found on Disko Island in Greenland and Illinois. Another name used for the mineral is rhabdite. It forms tetragonal crystals ...

and

cohenite Cohenite is a naturally occurring iron carbide mineral with the chemical structure ( Fe, Ni, Co)3 C. This forms a hard, shiny, silver mineral which was named by E. Weinschenk in 1889 after the German mineralogist Emil Cohen, who first describe ...

Schreibersite Schreibersite is generally a rare iron nickel phosphide mineral, , though common in iron-nickel meteorites. It has been found on Disko Island in Greenland and Illinois. Another name used for the mineral is rhabdite. It forms tetragonal crystals ...

and

also occur as plate shaped inclusions, which show up on cut surfaces as cm-long and mm-thick lamellae. The

plates are called '' Reichenbach lamellae''. The chemical composition is dominated by the elements Fe, Ni and Co, which make up more than 95%. Ni is always present; the concentration is nearly always higher than 5% and may be as high as about 25%. A significant percentage of nickel can be used in the field to distinguish meteoritic irons from human-made iron products, which usually contain lower amounts of Ni, but it is not enough to prove meteoritic origin.

Use

Iron meteorites were historically used for their

, which was forged into cultural objects, tools or weapons. With the advent of smelting and the beginning of the

the importance of iron meteorites as a resource decreased, at least in those cultures that developed those techniques. In Ancient Egypt and other civilizations before the

, iron was as valuable as gold, since both came from meteorites, for example

Tutankhamun's meteoric iron dagger Tutankhamun's meteoric iron dagger, also known as Tutankhamun's iron dagger and King Tut's dagger, is an iron-bladed dagger from the tomb of the ancient Egyptian Pharaoh Tutankhamun (reigned c. 1334–1325 BC). As the blade composition and homo ...

. The Inuit used the

Cape York meteorite The Cape York meteorite, also known as the Innaanganeq meteorite, is one of the largest known iron meteorites, classified as a medium octahedrite in chemical group IIIAB meteorites, IIIAB. In addition to many small fragments, at least eight large ...

for a much longer time. Iron meteorites themselves were sometimes used unaltered as collectibles or even religious symbols (e.g. Clackamas worshiping the

Willamette meteorite The Willamette Meteorite, officially named WillametteMeteoritical Bulleti ...

). Today iron meteorites are prized collectibles for academic institutions and individuals. Some are also tourist attractions as in the case of the

Classification

Two classifications are in use: the classic structural classification and the newer chemical classification.

Structural classification

The older structural classification is based on the presence or absence of the

Widmanstätten pattern Widmanstätten patterns (), also known as Thomson structures, are figures of long Phase (matter), phases of nickel–iron, found in the octahedrite shapes of iron meteorite crystals and some pallasites. Iron meteorites are very often formed ...

, which can be assessed from the appearance of polished cross-sections that have been etched with acid. This is connected with the relative abundance of nickel to iron. The categories are: *

Hexahedrite Hexahedrites are a structural class of iron meteorite. They are composed almost exclusively of the nickel–iron alloy kamacite and are lower in nickel content than the octahedrites. The nickel concentration in hexahedrites is always below 5.8% ...

s (H): low nickel, no

, may present

Neumann lines Neumann lines, or Neumann bands, are fine patterns of parallel lines seen in cross-sections of many hexahedrite iron meteorites in the kamacite phase, although they may appear also in octahedrites provided the kamacite phase is about 30 micromet ...

; *

Octahedrite Octahedrites are the most common Iron meteorite#Structural classification, structural class of iron meteorites. The structures occur because the meteoric iron has a certain nickel concentration that leads to the exsolution of kamacite out of tae ...

s (O): average to high nickel,

s, most common class. They can be further divided up on the basis of the width of the kamacite lamellae from coarsest to finest. **Coarsest (Ogg): lamellae width > 3.3 mm **Coarse (Og): lamellae width 1.3–3.3 mm **Medium (Om): lamellae width 0.5–1.3 mm **Fine (Of): lamellae width 0.2–0.5 mm **Finest (Off): lamellae width < 0.2 mm * Plessitic (Opl): a transitional structure between octahedrites and ataxites *

Ataxite Ataxites (from Greek meaning "without structure") are a structural class of iron meteorites with a high nickel content and show no Widmanstätten patterns upon etching. Characteristics Ataxites are composed mainly of meteoric iron, a native meta ...

s (D): very high nickel, no

, rare.

Chemical classification

A newer chemical classification scheme based on the proportions of the trace elements Ga, Ge and Ir separates the iron meteorites into classes corresponding to distinct

parent bodies. This classification is based on diagrams that plot

nickel Nickel is a chemical element; it has symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive, but large pieces are slo ...

content against different trace elements (e.g. Ga, Ge and Ir). The different iron meteorite groups appear as data point clusters. There were originally four of these groups designated by the Roman numerals I, II, III, IV. When more chemical data became available these were split, e.g. Group IV was split into IVA and IVB meteorites. Even later some groups got joined again when intermediate meteorites were discovered, e.g. IIIA and IIIB were combined into the IIIAB meteorites. In 2006 iron meteorites were classified into 13 groups (one for uncategorized irons): * IAB ** IA: Medium and coarse octahedrites, 6.4–8.7% Ni, 55–100 ppm Ga, 190–520 ppm Ge, 0.6–5.5 ppm Ir, Ge-Ni correlation negative. ** IB: Ataxites and medium octahedrites, 8.7–25% Ni, 11–55 ppm Ga, 25–190 ppm Ge, 0.3–2 ppm Ir, Ge-Ni correlation negative. * IC: 6.1–6.8% Ni. The Ni concentrations are positively correlated with As (4–9 μg/g), Au (0.6–1.0 μg/g) and P (0.17–0.40%) and negatively correlated with Ga (54–42 μg/g), Ir (9–0.07 μg/g) and W (2.4–0.8 μg/g). * IIAB ** IIA: Hexahedrites, 5.3–5.7% Ni, 57–62 ppm Ga, 170–185 ppm Ge, 2–60 ppm Ir. ** IIB: Coarsest octahedrites, 5.7–6.4% Ni, 446–59 pm Ga, 107–183 ppm Ge, 0.01–0.5 ppm Ir, Ge-Ni correlation negative. * IIC: Plessitic octahedrites, 9.3–11.5% Ni, 37–39 ppm Ga, 88–114 ppm Ge, 4–11 ppm Ir, Ge-Ni correlation positive * IID: Fine to medium octahedrites, 9.8–11.3%Ni, 70–83 ppm Ga, 82–98 ppm Ge, 3.5–18 ppm Ir, Ge-Ni correlation positive * IIE: octahedrites of various coarseness, 7.5–9.7% Ni, 21–28 ppm Ga, 60–75 ppm Ge, 1–8 ppm Ir, Ge-Ni correlation absent * IIIAB: Medium octahedrites, 7.1–10.5% Ni, 16–23 ppm Ga, 27–47 ppm Ge, 0.01–19 ppm Ir * IIICD: Ataxites to fine octahedrites, 10–23% Ni, 1.5–27 ppm Ga, 1.4–70 ppm Ge, 0.02–0.55 ppm Ir * IIIE: Coarse octahedrites, 8.2–9.0% Ni, 17–19 ppm Ga, 3–37 ppm Ge, 0.05–6 ppm Ir, Ge-Ni correlation absent *

IIIF The International Image Interoperability Framework (IIIF, spoken as 'triple-I-eff') defines several application programming interfaces that provide a standardised method of describing and delivering images over the web, as well as "presentation ...

: Medium to coarse octahedrites, 6.8–7.8% Ni,6.3–7.2 ppm Ga, 0.7–1.1 ppm Ge, 1.3–7.9 ppm Ir, Ge–Ni correlation absent * IVA: Fine octahedrites, 7.4–9.4% Ni, 1.6–2.4 ppm Ga, 0.09–0.14 ppm Ge, 0.4–4 ppm Ir, Ge-Ni correlation positive * IVB: Ataxites, 16–26% Ni, 0.17–0.27 ppm Ga, 0,03–0,07 ppm Ge, 13–38 ppm Ir, Ge–Ni correlation positive * Ungrouped meteorites. This is actually quite a large collection (about 15% of the total) of over 100 meteorites that do not fit into any of the larger classes above, and come from about 50 distinct parent bodies. Additional groups and grouplets are discussed in the scientific literature: * IIG:

s with coarse

Meteoric iron Meteoric iron, sometimes meteoritic iron, is a native metal and early-universe protoplanetary-disk remnant found in meteorites and made from the elements iron and nickel, mainly in the form of the mineral phases kamacite and taenite. Meteoric ...

has low nickel concentration.

Magmatic and nonmagmatic (primitive) irons

The iron meteorites were previously divided into two classes: magmatic irons and non magmatic or primitive irons. Now this definition is deprecated.

Stony–iron meteorites

There are also specific categories for mixed-composition meteorites, in which iron and 'stony' materials are combined. * Stony–iron meteorites **

Pallasite The pallasites are a Meteorite classification#Terminology, class of stony–iron meteorite. They are relatively rare, and can be distinguished by the presence of large olivine crystal inclusions in the ferro-nickel matrix. These crystals represe ...

s *** Main group pallasites *** Eagle station pallasite grouplet *** Pyroxene Pallasite grouplet **

Mesosiderite Mesosiderites are a class of stony–iron meteorites consisting of about equal parts of metallic nickel-iron and silicate. They are breccias with an irregular texture; silicates and metal occur often in lumps or pebbles as well as in fine-grained ...

group

Gallery

File:Hoba Meteorite sire.jpg, The

, the biggest known iron meteorite. It lies in

Namibia Namibia, officially the Republic of Namibia, is a country on the west coast of Southern Africa. Its borders include the Atlantic Ocean to the west, Angola and Zambia to the north, Botswana to the east and South Africa to the south; in the no ...

and weighs about 60 tons. File:Willamette Meteorite AMNH.jpg, The

Willamette Meteorite The Willamette Meteorite, officially named WillametteMeteoritical Bulleti ...

on display at the

American Museum of Natural History The American Museum of Natural History (AMNH) is a natural history museum on the Upper West Side of Manhattan in New York City. Located in Theodore Roosevelt Park, across the street from Central Park, the museum complex comprises 21 interconn ...

. It weighs about 14,500 kilograms (32,000 pounds). This is the largest meteorite ever found in the United States. File:Bendeg%C3%B3_meteorite,_front,_National_Museum,_Rio_de_Janeiro.jpg, The

Bendegó meteorite The Bendegó Meteorite (also known as Pedra do Bendegó or simply Bendengó) is a meteorite found in the interior of the state of Bahia, Brazil. It is the largest iron meteorite ever found on Brazilian soil, weighing , measuring over in length. ...

, weighing 5,360 kilograms (11,600 pounds), was found in 1784 and brought in 1888 to its current location at

National Museum of Brazil National may refer to: Common uses * Nation or country ** Nationality – a ''national'' is a person who is subject to a nation, regardless of whether the person has full rights as a citizen Places in the United States * National, Maryland, ...

in Rio de Janeiro. It is the largest meteorite ever found in Brazil. File:Meteoric iron 635 kg - foto Marco Busdraghi.JPG, The Otumpa mass, meteoric iron weighing 635 kilograms (1,400 pounds), from the

Campo del Cielo Campo del Cielo ("Field of Heaven" or "Field of the Sky" in English) refers to a group of iron meteorites and the area in Argentina where they were found. The site straddles the provinces of Chaco and Santiago del Estero, located north-northwes ...

, exhibited in the

Natural History Museum, London The Natural History Museum in London is a museum that exhibits a vast range of specimens from various segments of natural history. It is one of three major museums on Exhibition Road in South Kensington, the others being the Science Museum (Lo ...

, found in 1783 in Chaco, Argentina. File:SikhoteAlinMeteorite.jpg, A individual meteorite from the 1947 Sikhote-Alin meteorite shower (coarsest

octahedrite Octahedrites are the most common Iron meteorite#Structural classification, structural class of iron meteorites. The structures occur because the meteoric iron has a certain nickel concentration that leads to the exsolution of kamacite out of tae ...

, class IIAB). This specimen is about wide. File:ChingaMeteorite.jpg, A individual Chinga iron meteorite (

, class IVB).Chinga meteorite
at Meteoritical Bulletin Database. This specimen is about 9 centimeters wide. File:Meteorite_fragment_from_the_Cañon_Diablo_Meteorite.jpg, Meteorite fragment from the Cañon Diablo Meteorite 90mm wide File:Gibeon Meteorite.jpg, The Gibeon meteorite: Year found: 1836, Country: Namibia, individual weighing 3986 grams. This specimen is in the private collection of

Howardite Howardites are achondritic stony meteorites that originate from the surface of the asteroid 4 Vesta, and as such are part of the HED meteorite clan. There are about 200 distinct members known. Characteristics They are a regolith breccia consist ...

meteorites. File:Murnpeowie meteorite.jpg, The Murnpeowie meteorite, with regmaglypts resembling thumbprints, discovered on

Murnpeowie Murnpeowie or Murnpeowie Station is a pastoral lease in outback South Australia. The pastoral lease once operated as a sheep station but now operates as a cattle station. The land occupying the extent of the pastoral lease was gazetted as a ...

Station,

South Australia South Australia (commonly abbreviated as SA) is a States and territories of Australia, state in the southern central part of Australia. With a total land area of , it is the fourth-largest of Australia's states and territories by area, which in ...

in 1910. File:Iron meteorite, 5cm, 77gm.jpg, Iron meteorite, 5 cm long, weighing 77 grams

Notes

References

External links

Meteorite articles, including discussions of iron meteorites, in Planetary Science Research Discoveries

from Meteorites Australia {{DEFAULTSORT:Iron Meteorite