05 Classification of.. - Department of Earth and Planetary Sciences

,85% of iron meteorites fall into one of 13
chemical groups: IAB, IC, IIAB, IIC, IID, IIE, IIF,
IIIAB, IIICD, IIIE, IIIF, IVA, and IVB. With two
exceptions, these groups fall within restricted areas
on a log (Ge) versus log (Ni) plot (Figure 26(a)),
whereas on log (Ir) versus log (Ni) plot the
same samples form narrow, elongated fields
spanning a considerable range in iridium contents
(Figure 26(b)). It is customary to refer to these
clusters as groups as if they contain at least five
members. Individual samples that fall outside
defined chemical groups are called ungrouped
or anomalous iron meteorites, and some of these
form grouplets of two to four individuals.
The taxonomic significance of gallium and
germanium derives from the fact that they are the
most volatile siderophile elements (Wasson and
Wai, 1976; Wai and Wasson, 1979) and tend to
be strongly fractionated between different iron
groups. Early determinations of these elements
(Lovering et al., 1957) resolved only four
groups, designated by Roman numerals I–IV in
order of decreasing content of gallium and
Classification of Nonchondritic Meteorites 115
germanium. Letters were added later to distinguish
additional groups. Not all of these groups
proved to be independent, leading to the current
(confusing) nomenclature, in which some groups
have a combination of two letters after their
Roman numeral.
Chemical evidence indicates that magmatic
iron meteorite groups formed by fractional
crystallization, most likely in the cores of
differentiated asteroids.
Of the 13 major iron meteorite groups, only the
IAB and IIICD groups have broad ranges in nickel
and some trace elements. These ranges are difficult
to explain by simple fractional crystallization or a
metallic core. In addition, many members of these
groups contain silicate inclusions, which are
roughly chondritic in mineralogy and chemical
compositions, but the textures are achondritic,
exhibiting recrystallized textures. Silicate
inclusions in IAB irons are linked through oxygen
isotopic compositions and mineral compositions
to winonaites and may be related to IIICD irons
(Figure 17).
About 15% of all known iron meteorites do not
belong to one of the above defined 13 chemical
groups (Wasson and Kallemeyn, 1990). If each
ungrouped individual or grouplet represents a
separate parent body, then iron meteorites sample
between 30 and 50 asteroids (Scott, 1979; Wasson,
1990). Mineralogy of several ungrouped iron
meteorites with silicate inclusions and known
oxygen isotopic compositions are listed in
Table 8, which also contains information about
several ungrouped nonchondritic meteorites.
Figure 26 Plots of: (a) Ni versus Ge and (b) Ni versus
Ir showing the fields for the 13 iron meteorite groups
(source Scott and Wasson, 1975).
1.05.4.5.2 Structural groups
The metallic phase of many iron meteorites
shows a texture called the Widmanstätten pattern,
an oriented intergrowth of body-centered cubic
a-FeNi (kamacite), and high-nickel regions
composed of several phases (a tetragonal FeNi
phase (tetrataenite), a body-centered cubic FeNi
phase (a 2 -FeNi), and awaruite (FeNi 3 )). Kamacite
forms lamellae with a nickel content of ,6%.
The width of the kamacite lamellae is a function
of the cooling rate and, hence, is approximately
constant in a given iron meteorite, but
varies widely between different meteorites.
Because kamacite lamellae are oriented along
octahedralplanes, irons which show this structure
are called octahedrites (Figures 27(a)–(e)).
According to the width of the kamacite lamellae,
octahedrites are further subdivided into
coarsest (Ogg, bandwidths .3.3 mm), coarse
(Og, 1.3–3.3 mm), medium (Om, 0.5–1.3 mm),
fine (Of, 0.2–0.5 mm), and finest (Off, ,0.2 mm).
Plessitic octahedrites (Opl) are transitional to
ataxites (D).