Iron-meteorites
Iron meteorites represent the largest group of all meteorite findings outside of the hot deserts of Africa and Asia and the ice of the Antarctica, as they can be 
recognized by layman by their metallic consistence and their heavy weight easily as meteorites. Moreover they weather slower than their stony relatives and are mostly much bigger, because they are compact and more stable and therefore rarely burst on their passage through the atmosphere as well as when they bounce on the earth. You find a list of the giants of the iron meteorites in the section Records.
In spite of this and the fact that all iron meteorites with a total weight of more than 300 tons come up to more than 80% of the total mass of all well known meteorites, they are however comparatively rare. Iron meteorites frequently are found and easily identified as meteorites, but nevertheless they make out merely 5,7% of all watched falls.
Concerning classification, iron meteorites are divided according to two completely different schemes into groups. The first scheme is a sort of relic from the times of classic meteorite science and distinguishes iron meteorites according to their structure and their prevailing mineral consistence, the second scheme is rather a modern attempt, dividing iron meteorites into chemical classes and thus trying to relate them to certain mother bodies.
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Octahedrites consist of a more or less fine synthetic growth of kamazite and taenite that has the structure of the octahedar. If such a meteorite is polished and its surface gets corroded with nitric acid, this synthetic growth becomes visible in form of the so called Widmanstätten patterns, fascinating plays of geometrically arranged volumes and beams. Corresponding to the width of the kamazite bands, one distinguishes between these lower groups: the nickel scarce, broad Octahedrites with a bandwidth of over 1,3mm, the middle Octahedrites with a bandwidth from 0,5mm to 1,3mm as well as the fine, more nickel rich Octahedrites with a bandwidth of less than 0,5mm. Often further groups are distinguished to describe still broader or finer structures. |
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Hexahedrites consist almost exclusively of the nickel scarce kamazite and show no Widmanstätten pattern if they are polished and corroded. Although in many Hexahedrites, after the corroding fine parallel lines become visible, the so-called Neuman lines, that represent structural shaping in the kamazite structure and are maybe the consequence of an impact event, the collision of the Hexahedrite-mother body with another asteroid or of the bounce of the meteorite on the Earth. |
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Also Ataxites show after the corroding no structures at all, but in contrast to the Hexahedrites, they consist almost totally of taenite and possess only microscopic small kamacite lamellas. Thus they count to the nickel richest meteorites in general (over 16% nickel content), but also to the rarest. Until now not one fall of an Ataxite was observed! Yet the world of the meteorites is a crazy world: the largest meteorite on this Earth, the Hoba-meteorite from Namibia, with a weight of more than 60 tons, belongs paradoxically to the rare class of the Ataxites. |
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Besides a specific iron and nickel content, Iron meteorites distinguish themselves by additional other minerals as well as by trace shares of precious and heavy metals e.g. germanium, gallium, iridium, arsen, tungsten or gold. The research of the ratio of trace metals to the nickel content made apparent the existence of certain chemical groups of Iron meteorites, whereby one assumes, that each of these chemical groups corresponds to one characteristic mother body. Here we will shortly show interest in thirteen established chemical groups, whereby one has to say, that about 15% of all well known Iron meteorites do not suit into this scheme and are summarized either into other smaller groups, that are not closely described here, or are very unique. Altogether these ungrouped Iron meteorites represent more than 90 other mother bodies whereby one must mention however, that the majority of these worlds probably today exist no more. Nevertheless most Iron meteorites - comparable with the nickel-iron core of the earth - represent the cores of differentiated asteroids or planetoids, who for getting released and arrive as a meteorite on the Earth, first of all had to be destroyed by catastrophic collisions and impact events!
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IAB - Group A large part of all Iron meteorites belong to this group, where all structural classes are represented. But especially frequent among the IAB-irons are big and middle Octahedrites as well as silicate rich irons, Iron meteorites with more or less large inclusions out of different silicates. These silicates are chemically close related to the Winonaites, a rare group of Primitive Achondrites, so that one supposes, that both groups come from one and the same mother body. Often in IAB-Iron meteorites also bronze coloured inclusions of iron sulphite troilite as well as black graphite-nodules are found. Not only the occurrence of this rudimental form of the carbon hints to a a narrow relationship of the IAB-group with the Carbonaceous Chondrites; also the distribution of the trace elements allows such a conclusion. |
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IC - Group The much rarer IC-iron meteorites are quite similar to the IAB-group, with the difference, that they show slighter shares of the trace elements arsen and gold. Structurally they belong to the broad Octahedrites, although also differently structured IC-Irons are known. Typical for this group furthermore is the frequent occurrence of dark inclusions of iron carbide cohenite, while silicate inclusions are missing. |
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IIAB - Group The meteorites of this group are Hexahedrites, i.e. they consist of single, very large kamazite crystals, or broadest known Octahedrites. The distribution of the trace elements in the IIAB-Irons resembles the distribution some Carbonaceous Chondrites and Enstatite-Chondrites so that one can assume, that the IIAB-Irons originate from such a chondritic mother body. |
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IIC - Group The group of the IIC-iron meteorites consists of finest and plessitic Octahedrites with kamazite bands under 0,2mm. The so-called "fill iron" plessite, an especially fine synthesis of taenite and kamazite spindles, occurs also in other Octahedrites in the transition between taenite and kamazite, in the Iron meteorites of the group IIC it is however the main mineral element. |
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IID - Group The members of this group are mainly middle to fine Octahedrites, which distinguish themselves by a similar distribution of the trace elements and a very high share of gallium and germanium. Most IID-Iron meteorites contain numerous inclusions of the nickel iron phosphate schreibersite - an outspoken hard mineral which often makes the cutting of an IID-Iron very difficult. |
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IIE - Group The IIE-Iron meteorites structurally belong to the class of the broad to middle Octahedrites, and they quite often contain numerous inclusions of different iron rich silicates. Unlike in the IAB-Irons, the silicate inclusions however don't appear as undifferentiated clastes, but rather in form of hardened, often clear drops, that make IIE-Iron meteorites to the optically most attractive meteorites in general. Chemically the IIE-Irons seem to show some narrow relations to the H-Chondrites, and perhaps both groups come from one and the same mother body. |
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IIF - Group This little group consists of plessitic Octahedrites and Ataxites, which all have a high nickel amount as well as particularly high shares of the trace metals germanium and gallium. Some chemical relations exist both to the Pallasites of the Eagle-station-group and to the Carbonaceous Chondrites of the groups CO and CV. The Eagle-station-Pallasites maybe even come from the same mother body. |
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IIIAB - Group Next to the group of the IAB-Irons, the IIIAB-group has most members. Structurally they are wide to middle Octahedrites. Occasionally in these meteorites one finds inclusions of troilite and graphite, while silicate inclusions are quite seldom. Nevertheless, some narrow relations to the Pallasites of the main group exist, and one assumes today that both groups come from a common mother body. |
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IIICD - Group Structurally the meteorites of the IIICD-group are finest Octahedrites and Ataxites, while chemically some narrow relations to the meteorites of the IAB-group exist. Like these, the IIICD-Irons also frequently contain silicate inclusions, and one assumes today, that both groups come from a common mother body. Consequently also a very narrow connection to the Winonaites exists, a rare group of Primitive Achondrites. Typical for the IIICD-Iron meteorites is the occurrence of the nickel iron carbide haxonite, a mineral, that is exclusively found in certain meteorites. |
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IIIE - Group Structurally as chemically the IIIE-Iron meteorites are quite similar to the IIIAB-group, but differ by an unique distribution of the trace elements as well as by typical inclusions of the nickel iron carbide haxonite, a feature that they also share with the Iron meteorites of the IIICD-groups. It is therefore not entirely clear whether they arev an independent group in the sense of a separate mother body. Here perhaps in the future further investigations will bring new results. |
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IIIF - Group This little group encompasses structurally a spectrum of broadest to fine Octahedrites, however differs from other Iron meteorites both by a relatively slight nickel content and by the very low content and unique distribution of certain trace elements. |
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IVA - Group The members of the IVA-group structurally belong to the class of the fine Octahedrites and possess an unique distribution of their trace elements. There are inclusions of troilite and graphite, while silicate inclusions are quite rarely. Solely the anomalous meteorite Steinbach, a historic, German find, is a remarkable exception, because it consists almost to the half of reddish-brown pyroxenene that is imbedded in a nickel iron matrix of the type IVA. Whether it is the product of an impact on the IVA-mother body or a pendant to the Pallasites and therefore a real Stony-iron meteorite, is still violently discussed. |
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IVB - Group The Iron meteorites of the group IVB all possess a high nickel content of about 17% and structurally belong to the class of the Ataxites. Observed under the microscope, one notices however that they do not consist of pure taenite, but rather are of plessitic nature, i.e. are constructed out of a very narrow synthesis of kamazite and taenite . A typical example for an IVB-Iron is the largest meteorite of this Earth, Hoba from Namibia. |
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UNGR - Group With this symbol, that means something like "ungrouped", all meteorites are summarized, that cannot be incorporated in the chemical groups mentioned above. Although in the mean time, some of these ungrouped Irons were organized by researchers into about twenty different small groups, to be recognized as a new meteorite group however, usually it needs a number of at least five members - a request of the nomenclature committee of the Meteoritical Society that shall prevent that too quickly new groups get exclaimed, that afterwards possibly show themselves as a part or expansion of another group.
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