Chemical Classification of Iron Meteorites
Modern meteoritics classifies iron meteorites according to a chemical classification system using nickel and the trace elements gallium, germanium, and iridium, to define distinct chemical groups. Other trace elements used to resolve groups are antimony, arsenic, cobalt, copper, gold, thallium, and tungsten. The concentrations of the trace elements are plotted against the overall nickel content on logarithmic scales to resolve well-defined chemical clusters, each representing a distinct chemical group. Fourteen groups, designated by Roman numbers and letters, such as "IAB", have been recognized so far, with each group comprising five or more members. It is believed that the iron meteorites of each chemical group share the same origin and formed on a common parent body.
In the following, we will briefly discuss each chemical group, its primary properties, its relationship to certain structural classes, and its most famous members. However, we have to take into account that over 15% of all iron meteorites don't fit easily into the existing classification scheme. These irons are designated as ungrouped, probably representing more than 50 different parent bodies. We also have to consider that we won't be able to identify these parent bodies because most of them must have been destroyed in order to become a source for iron meteorites. Most iron meteorites were formed in the cores of small differentiated asteroids that were disrupted by devastating impacts shortly after their formation. They are true remnants of other worlds that once existed in the early solar system.
This well represented group contains about 125 members if we exclude all probable pairings. Some of them are downright famous, as for example, the meteorites of Toluca, Campo del Cielo, Odessa, or Canyon Diablo. The latter one created the well-known Meteor Crater in Arizona, USA. Other members are renowned for their abundant, beautiful silicate inclusions, e.g., Caddo County, Landes, and Zagora, just to name a few. Most IAB irons are coarse to medium octahedrites, although other structural classes do occur. The meteorites of this group often contain abundant inclusions of troilite, graphite, and cohenite, and various silicates that are closely related to a group of primitive achondrites, the winonaites. Recent research suggests that both the winonaites and IAB irons originated on the same parent body - a partially differentiated asteroid that was disrupted just as it began to form an iron core and silicate-rich crust. The disrupting impact mixed silicates into molten nickel-iron forming the silicated IAB irons, and mixed olivine-rich residues of partial melts into unmelted silicates, forming the winonaites.
The eleven meteorites of this small group are quite similar to the members of the IAB group. Most of them are coarse octahedrites, although other structural classes are represented in some members. Most IC irons contain abundant dark inclusions of the iron-carbide cohenite, although silicate inclusions are missing. Regarding elemental abundances, the IC irons display lower values of the trace elements arsenic and gold, and it is thought that they formed on a separate parent body. Well-known, representative members of the IC group include the meteorites of Arispe, Bendegó, and Mount Dooling.
This is another well-represented group of iron meteorites consisting of 106 members. Structurally, the IIAB irons are classified as hexahedrites or coarsest octahedrites, making them some of the most nickel-poor iron meteorites known. Famous IIAB hexahedrites are Braunau, a meteorite that fell in Bohemia in 1847, and North Chile, a find from 1875, both displaying abundant Neumann lines. When considering the coarsest octahedrites, we have to mention Lake Murray, one of the oldest meteorites known. It fell about 110 million years ago and has been preserved up to this day imbedded in some ancient sandstone. Finally, we have to mention the downright notorious Sikhote-Alin, Russia, a witnessed fall from 1947. Several thousand individuals with a total weight of more than 70 tons have been recovered, and this is surely one of the most beautiful and more affordable iron meteorites on the collector's market. Trace element abundances suggest that the IIAB irons formed in the core of a differentiated C-type asteroid that was disrupted by several impact events.
Group IIC consists of just eight members, most of them plessitic octahedrites with kamacite bandwidths below 0.2 mm. Plessite is a fine intergrowth of taenite and kamacite, and it is also found in other octahedrites filling spaces between kamacite bands and taenite ribbons. However, in plessitic octahedrites it is the primary mineral. With regards to elemental abundances, the IIC irons are known for their high thallium content, and it is thought that they formed in the core of a small, differentiated asteroid. Though not famous, the iron meteorites of Ballinoo, Salt River, and Unter-Mässing are typical members of the IIC group.
This group comprises 17 members, structurally representing medium to fine octahedrites. The IID irons often contain abundant inclusions of schreibersite, and they display high amounts of gallium and germanium - facts that indicate a formation in the core of a larger asteroid. Alt Bela, Carbo, or Hraschina are typical IID irons. A famous member of this group is the so-called "bewitched burgrave" of Elbogen, Bohemia, a witnessed fall from 1400. Not only is Elbogen one of the oldest recorded falls, but it is also the very iron that, in 1808, Alois von Widmanstätten viewed when he discovered the internal structure hidden within. Interestingly, he did not etch the iron to reveal these structures. Rather, as he heated a slab of the Elbogen iron in the flame of a Bunsen burner, the figures that would be named for him were revealed.
The 18 members of this group are coarse to medium octahedrites, and most of them contain abundant, iron-rich silicate inclusions. These IIE silicates often occur in the form of congealed droplets rather than in the form of the more undifferentiated silicate clasts found in IAB irons. The IIE members, Miles and Watson, are renowned for their gem-like silicates, making them some of the most attractive silicated irons. Recent research suggests that the IIE irons did not form in the core of an asteroid, but instead, are products of partial melting and heating induced by impact events. Mineral and oxygen isotopic compositions of IIE irons suggest a close relationship exists with the ordinary chondrites of the H group. It is possible that both groups originated on the same parent body - the main belt asteroid 6 Hebe.
Group IIF consists of just five members, structurally representing plessitic octahedrites and ataxites. They are nickel-rich and contain high amounts of gallium, germanium, copper, and cobalt, indicating that formation occurred in the core of a differentiated asteroid. Renowned members are the meteorites of Del Rio, Monahans (1938), and Repeev Khutor, the latter representing the only witnessed fall of the group that occurred in Russia in 1933. The oxygen isotopic compositions of the IIF irons are similar to those of the
Eagle Station pallasites, and probably both groups share a common parent body. A similar oxygen isotopic composition is also displayed by the carbonaceous chondrites of the CO/CV clan, suggesting a formation of the IIF/Eagle Station pallasite parent body in the outer regions of the asteroid belt.
This is a brand-new group of iron meteorites, recently under publication. Formerly known as the Bellsbank grouplet, it consists of just five members: Bellsbank, La Primitiva, Tombigbee River, Twannberg, and the recently discovered Guanaco, a meteorite that was found in the Atacama Desert, Chile, in 2000. Structurally, the irons of the IIG group are hexahedrites or coarsest octahedrites. In their structural and elemental compositions they resemble the iron meteorites of group IIAB, but they contain even less nickel and unusual, abundant ribbons of the iron-phosphide schreibersite. These ribbons often cover 15% of the etched surface of a IIG iron. This suggests that the formation of the IIG irons occurred in the outer regions of the core of a differentiated asteroid, probably distinct from the IIAB parent body.
With about 233 members, group IIIAB is the best-represented class of iron meteorites in our collections. Compared to the members of the IIIA subgroup, which have mostly coarse octahedrite textures, the IIIB irons usually display medium textures. Still, the members of both subgroups form a continuous sequence in structural and elemental compositions that suggests a common origin, probably representing different regions of an asteroid's core. Group IIIAB contains several prominent members representing some of the largest irons ever found. Just to name a few, there are the giants of Cape York, Chupaderos, Morito, and Willamette - have a look at our
charts. Some IIIAB members contain large nodules of troilite and graphite, but silicate inclusions are rare. Despite this fact, recent research suggests a close relationship exists between the IIIAB irons and the silicate-rich
main group pallasites, some of the most attractive stony-iron meteorites known. Both groups probably formed on the same parent body, a differentiated asteroid that was disrupted by a single impact event. The IIIAB iron meteorites represent fragments of the core, while the main-group pallasites are samples of the core/mantle boundary of this common parent body.
This medium-sized group comprises 42 members, mostly belonging to the structural classes of fine and finest octahedrites, or ataxites. Several members of group IIICD contain abundant silicate inclusions, similar to the inclusions in IAB irons, and there are additional similarities in elemental compositions suggesting a close relationship exists between IIICD and IAB irons. Perhaps both groups share a common origin on a partially differentiated asteroid, one also thought to be the source of a class of rare primitive achondrites - the winonaites. However, the irons of group IIICD display some unique features that clearly distinguish them from the IAB irons and other groups. For example, the presence of the carbide haxonite is quite characteristic for the meteorites of group IIICD. Renowned members are Carlton, Morasko, the highly silicated Maltahöhe, and the anomalous, troilite-rich Mundrabilla, one of the largest iron meteorites ever found.
The 13 members of this small group are quite similar to the iron meteorites of group IIIAB, but they can be easily distinguished by several characteristic features. Structurally belonging to the class of the coarse octahedrites, the IIIE irons are characterized by short and "swollen" kamacite bands, and abundant inclusions of haxonite. This strange carbide is white in color, very hard, and a challenge to every saw when it comes to cutting a IIIE iron. The elemental compositions of the IIIE irons are similar to those of the IIIAB irons, but distinct enough to justify the establishment of a separate group. Recent research suggests that the IIIE irons are fragments of the core of a distinct parent body that formed in close proximity to the IIIAB parent body, within the same region of the early solar nebula. The most famous member of group IIIE is Armanty, a real giant from China weighing more than 23 tons.
This small group counts just eight members. Most IIIF irons are medium to fine octahedrites with a relatively low nickel content. With regards to their elemental compositions, they display high values for chromium, and low values for germanium, cobalt, and phosphorus. Consequently, IIIF irons rarely contain inclusions of phosphides (e.g. schreibersite), and troilite is equally rare or absent. These data suggest the formation of the IIIF irons occurred in the core of a small, differentiated asteroid. The iron meteorites of Klamath Falls, Nelson County, and St. Genevieve County are typical, although not famous, members of this group.
The 65 members of this well-represented group are mostly fine octahedrites, and they display a unique trace element pattern of extraordinarily low germanium and low gallium values. Some IVA irons contain sparsely distributed small nodules of troilite and graphite, although silicate inclusions are rare to absent in most members. Recent research suggests that the IVA irons formed in the core of a small, differentiated asteroid that was disrupted by a major impact shortly after its formation. After the asteroid was reaccreted, it was again disrupted about 450 million years ago. The famous meteorite Gibeon is a rather typical member of this group, and more than 30 tons of this IVA iron have been recovered from its large, prehistoric strewn field in Namibia. However, one anomalous, silicate-rich IVA member, Steinbach, is of major scientific interest. This historic German find consists of nearly equal parts of a IVA nickel-iron matrix and reddish silicates; these silicates are a mixture of pyroxenes and the rare mineral tridymite. It is still heavily debated whether this beautiful silicated iron represents a IVA analog to the pallasites, those true stony-iron meteorites that formed at the core/mantle boundary of their parent body, or if Steinbach is just a secondary product, formed during the reaccretion of the IVA parent body following the first disrupting impact.
The 13 members of this small group are extraordinarily nickel-rich, and consequently, all IVB irons belong to the structural class of the ataxites. However, if viewed under magnification, it becomes obvious that the IVB irons have a plessitic composition and represent a microscopic intergrowth of kamacite and taenite. Inclusions are rare and silicates are virtually absent. The IVB irons display low values for the trace elements gallium and germanium, consistent with the supposed formation of the IVB irons in the core of a small, differentiated asteroid. Famous IVB members include Hoba, the largest meteorite on Earth, and Cape of Good Hope, both of these representing South African finds. Parts of the latter iron were removed and forged before 1811, and one piece was forged into a legendary sword that was presented to Czar Alexander I, the Russian Emperor.
More than 110 iron meteorites have never been chemically classified, while another 95 have been classified as ungrouped irons. The latter don't fit into any of the existing 14 chemical groups, and display unique structural and elemental compositions. Some of these ungrouped irons have similar compositions to others, and they have been provisionally placed into several grouplets comprising less than five members each, e.g., the Prambanan grouplet, and the Cambria grouplet. The remaining ungrouped irons are unique, and they probably represent singular samples of their parent bodies. Some of the largest irons have been classified as ungrouped, e.g., Bacubirito, and Mbosi - have a look at our