ARN's Meterorite Classification List
© 1996, to 2006 ARN
Iron Meteorites Chart
All Iron meteorites contain FeNi with the nickel content have a minimum of 3 wt% .
The iron ( FeNi ) is always magnetic ! These are simple things but are absolute .
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 (described below) 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
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 iron core cooled down. It is easily seen in the following table that the structural class is dependent on the percentage of nickel available.
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 know 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, for the most 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 several 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 Mundrabilla 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.
NI 6.5-60.8 wt%
Ga 100-2 ppm
Ge 520-2 ppm
Ir 6-0.02 ppm
Bandwith Og-D 3.1-0.01 mm
Main Features Angular silicates common , unfractionated triolite/graphite/ Schreibersite nodules
Distinguishing Features Graphite and carbides common, Schreibersite cm-sized , carlsbergite absent
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
NI 6.1-6.8 wt%
Ga 55-49 ppm
Ge 247-212 ppm
Ir 2.1-0.07 ppm
Bandwith Og < 3mm
Main Features Cohenite abundant
Distinguishing Features Silicates, haxonite absent.
The IIAB group meteorites are
Hexahedrites or coarsest
Octahedrites, which consist of large Kamacite crystals with minor
Taenite. 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.
NI 5.3-6.4 wt%
Ga 62-46 ppm
Ge 185-107 ppm
Ir 0.9-0.01 ppm
Bandwith H-Ogg >50-5 mm
Main Features Hexahedrites have euhedral rhabdites. Schreibersite increase in Ogg , cm-sized
Distinguishing Features Daubreelite common to rare
The meteorites in this group are primarily
fine-grained (Of - Opl) structured irons. The Kamacite crystals are under
.2mm 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.
NI 9.3-11.5 wt%
Ga 37-39 ppm
Ge 88-114 ppm
Ir 11-4 ppm
Bandwith Opl 0.07-0.06 mm
Main Features Schreibersite common in matrix
Distinguishing Features Graphite / carbides are absent .
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
NI 9.6-11.3 wt%
Ga 70-83 ppm
Ge 82-98 ppm
Ir 18-3.5 ppm
Bandwith Om-Of 1.8-0.4 mm
Main Features Triolite less abundant than in !!!AB
Distinguishing Features Schreibersite common- ubiqjuitious , cm-sized
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.
NI 7.5-9.7 wt%
Ga 28-21 ppm
Ge 75-62 ppm
Ir 8-1 ppm
Bandwith Anomalous Ogg, Om, etc.
Main Features Silicate droplets common, chondritic to fractionated for example , K-rich
Distinguishing Features Graphite / carbides / daubreelite are all absent
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.
NI 10.6-14.3 wt%
Ga 8.9-11.6 ppm
Ge 99-193 ppm
Ir 23-0.75 ppm
Bandwith Opl 0.05-0.2 mm
Main Features Hi Ge/Ga High Co apx 0.7 wt %
Distinguishing Features Schreibersite within Kamacite spindles
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
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 come from the same 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.
NI 7.1-10.5 wt%
Ga 23-16 ppm
Ge 47-27 ppm
Ir 20-0.01 ppm
Bandwith Om 1.3-0.6 mm
Main Features Phosphide / triolite correlate with Ni; phosphate / chromite may be present
Distinguishing Features Haxonite absent ; graphite and cohenite rare-absent; Kamacite shock-hatched or recrystallized
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)
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.
NI 8.2-9.0 wt%
Ga 19-17 ppm
Ge 37-34 ppm
Ir 6-0.01 ppm
Bandwith Og 1.6-1.3 mm
Main Features Haxonite, or its decay product graphite is abundant in pessite .
Distinguishing Features Kamacite rarely is shock-hatched .
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.
NI 6.8-8.5 wt%
Ga 7.3-6.3 ppm
Ge 1.1-0.7 ppm
Ir 7.9-0.006 ppm
Bandwith Og-Om-Ogg 1.5-0.5 mm
Main Features Microscopic daubreelite is abundant .
Distinguishing Features Graphite and carbides are absent .
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. Apparently, this group originates in a
small-differentiated asteroid that was fragmented in a collision,
re-accreted, to be disrupted once again by a second collision.
NI 7.4-9.4 wt%
Ga 1.6-2.4 ppm
Ge 0.09-0.14 ppm
Ir 4-0.4 ppm
Bandwith Of 0.45-0.25 mm
Main Features Daubreelite abundant ; silicates 9 tridymite 0 may be present .
Distinguishing Features All of irons with 10 wt% are IVA; carbides and carlsbergite all absent .
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.
NI 16-18 wt%
Ga 0.17-0.27 ppm
Ge 0.003-0.07 ppm
Ir 38-13 ppm
Bandwith D <0.03 mm
Main Features Iron with ataxite structure
Distinguishing Features mKamacite spindles and grains in fine black plessite; graphite , carbide, carlsbergite and silicates all absent .
Edited and added to by Ken Regelman
Astronomical Research Network
Basic text contributed by Phil Dunton
P.O. Box 90701
Tucson, Arizona 85752