Iron Meteorite Classification
Structural Classification
Iron meteorites are formed of two distinct nickel-iron alloys. The most prevalent is Kamacite. In pure form Kamacite contains 89.54% iron and 10.46% nickel. Kamacite is soft for an iron alloy, only equaling the mineral Fluorite (CaF) at a Moh's hardness of 4. Kamacite forms cubic crystals and Hexahedrites are actually large cubic Kamacite crystals. The second nickel-iron alloy found is Taenite containing a higher content of nickel. Taenite in pure form will consist of 79.19% iron and 20.81% nickel. Taenite has the hardness expected of an iron alloy, measuring 5.5 on the Moh's hardness scale. This equates to the mineral Apatite (Calcium Fluoro/Chloro-Phosphate) or to that of an ordinary knife blade.
Iron meteorites are distinguished structurally by the size of the Kamacite crystals present. This physical attribute is dependent on the amount of nickel available to make up Taenite and the length of time the crystals had to grow as the nickel/iron melt cooled.
Chemical Classification
Iron meteorites are distinguished chemically by the total ratio of nickel to iron content and by the ratios of the trace elements that define distinct chemical groups. When the ratios of trace elements to nickel content for many iron meteorites is determined, we find that the meteorites belong to distinctive groups with similar content. We assume that each chemical group corresponds to one particular parent body. Currently there are 14 such groups. About 15% of known iron meteorites don't seem to belong to any of these groups and are classed as "Ungrouped." Present practice requires five example meteorites to form a new group. In the future we can expect that, as additional iron meteorites are found, some of the "ungrouped" meteorites will wind up in new iron meteorite chemical groups.
It is important to understand that the iron meteorites in part are remnants of the cores of planetary bodies that once existed in our solar system but were subsequently destroyed in colossal impacts after they were formed.
All structural classes of iron meteorites are found in this group but especially the medium and large crystal Octahedrites (Om-Og). Silicate rich iron meteorites that are seemingly closely related to a rare group of Achondrites, the Winonaites, are also found in this group. Possibly the Winonaites and IAB iron meteorites derive from the same parent body. IAB group meteorites frequently have bronze colored Troilite (iron sulphide) and black Graphite (elemental carbon) nodule inclusions. Small green hexagonal plates of natural Carborundum (CSi) have been found in the Canyon Diablo meteorites. The presence of the carbon suggests that there is a relationship between the IAB iron meteorites and Carbonaceous Chondrite meteorites. Similarity of trace element concentrations tends to confirm this conclusion. Recent trace element concentration evaluations place the previous IIICD group in the IAB Main Group. IAB also is divided into five sub-groups according to the ratios of gold and nickel found. Thus the Toluca meteorites are now in sub-group sLL which is a low-gold, low-nickel sub-group. Another example is that the Australian iron is paired with the a similar meteorite, the Waterville iron from Washington state, as the "Mundrabilla duo" and is considered to be a IAB meteorite but not in the Main Group.
Group IAB is a nonmagmatic group formed as impacts on a chondritic body caused local melting. These melts cooled rapidly as the heat was lost to the mainly silicate body. The IAB subgroups are the result of separate impact events on the same body at different locations or on different asteroids with similar compositions.
IC Group
IC iron meteorites are similar to the IAB group but much less common. They have smaller amounts of arsenic and gold as trace elements. Crystals of the iron carbide Cohenite are a frequent inclusion in this group while the silicate inclusions of the IAB group are missing.
IIAB Group
The IIAB group meteorites are Hexahedrites or coarsest Octahedrites, which consist of large Kamacite crystals with minor Taenite. The group formed through fractional crystallization of a slowly cooling magma. These are examples of the broadest (Ogg) known nickel-iron crystal structure, and generally have the lowest nickel content of the iron meteorites. The trace element concentration of these meteorites is similar to some Carbonaceous Chondrites and Enstatite Chondrites so they probably are fragments of a C-type asteroid.
IIC Group
The meteorites in this group are primarily fine-grained (Of - Opl) structured irons. The Kamacite crystals are under .2 mm in width. A fine-grained (Opl) mixture of Taenite and Kamacite is found as a fill between Kamacite crystals in most iron meteorites. In-group IIC this fine mixture of Taenite and Kamacite is the main ingredient. These meteorites have high thallium content. The IIC's are probably the remnants of a small-differentiated asteroid.
IID Group
This group consists of medium to fine Octahedrites (Om - Of). They are distinguished by the occurrence of relatively larger amounts of the trace elements gallium and germanium. Many IID meteorites contain inclusions of the very hard mineral Schreibersite (nickel-iron phosphide.) The trace element concentrations indicate the source of the IID group is the core of a large asteroid.
IIE Group
These meteorites generally have a broad crystal structure (Om - Ogg). They frequently contain clear iron rich silicate inclusions. The IIE irons seem to be chemically related to the H-Chondrites and may come from the same parent body, possibly asteroid 6 Hebe.
IIF Group
This group consists of plessitic Octahedrites and Ataxites (Opl). They all have high nickel content and high amounts of germanium, gallium, copper, and cobalt. Isotopic compositions of this group indicate a relationship with some Pallasites and the type CO/CV Carbonaceous Chondrites. These three groups of meteorites may have a single parent body that formed in the outer region of the asteroid belt.
IIG Group
This new group ranges from Hexahedrites to coarsest Octahedrites. They resemble group IIAB but have even lower nickel content. Abundant Schreibersite suggests the IIG meteorites come from the outer region of the core of a differentiated asteroid distinct from the IIAB parent body.
IIIAB Group
These are wide to medium Octahedrites (Om - Og.) There are occasional inclusions of Troilite, Graphite and minor silicates. The IIIA subgroup is mostly coarse octahedrite while the IIIB subgroup exhibits medium textures. There is a continuous set of elemental compositions indicating that the two subgroups result from fractional crystallization of a slowly cooling magma core of a differentiated asteroid. These irons show trace element relationships with the main group Pallasites. Probably, they come from a common parent body. The IIIAB group from the core and the Pallasites coming from the core/mantle boundary.
IIICD Group
This group is no longer considered valid as these irons are now included in the IAB main group but the group name remains included here as collectors will frequently still find the name used by dealers. Here is the old group description: "These are the finest octahedrites and ataxites. They seem to be chemically related to the IAB group, contain silicate inclusions like the IAB group, and are assumed today to have a common parent body. Therefore a presumption of a relationship with the Winonaites exists. Typically these meteorites contain Haxonite (iron, nickel, carbide) inclusions."
IIIE Group
Chemically similar to the IIIAB group, these meteorites differ in having a unique trace element distribution. They also can contain inclusions of the mineral Haxonite. Whether this group is truly an independent group is currently unsettled.
IIIF Group
This small group has a broad variety of structural classes from find to broadest (Of - Ogg.) They differ from other meteorites in having low nickel content and a unique trace element distribution. They have high amounts of chromium, and low amounts of germanium, cobalt, and phosphorus. Troilite and Schreibersite are generally absent. This is considered to be evidence that this group originated in the core of a small, differentiated asteroid.
IVA Group
The members of this group belong to the fine Octahedrite class (Of.) They have a distinctive distribution of trace elements and include nodules of Troilite and Graphite with rare silicates. The IVA group formed from a magma melt that cooled slowly. Apparently, this group originates in a small-differentiated asteroid that was fragmented in a collision, re-accreted, to be disrupted and melted once again by a second collision.
IVB Group
These meteorites all have around 17% nickel content. They are structurally Ataxites. While they appear to be pure Taenite, under a microscope they are seen to consist of a plessitic mixture of Taenite and Kamacite. The IVB group has low values of gallium and germanium supposedly consistent with formation in the core of a small-differentiated asteroid.
Updated October, 2006
