Iron meteorite

Origin

Iron meteorites have been linked to M-type asteroids because both have similar spectral characteristics in the visible and near-infrared. Iron meteorites are thought to be the fragments of the cores of larger ancient asteroids that have been shattered by impacts. The heat released from the radioactive decay of the short-lived nuclides ²⁶Al and ⁶⁰Fe is considered as a plausible cause for the melting and differentiation of their parent bodies in the early Solar System. Melting produced from the heat of impacts is another cause of melting and differentiation. The IIE iron meteorites may be a notable exception, in that they probably originate from the crust of S-type asteroid 6 Hebe.

Chemical and isotope analysis indicates that at least about 50 distinct parent bodies were involved. This implies that there were once at least this many large, differentiated, asteroids in the asteroid belt – many more than today.

Composition

The overwhelming bulk of these meteorites consists of the FeNi-alloys kamacite and taenite. Minor minerals, when occurring, often form rounded nodules of troilite or graphite, surrounded by schreibersite and cohenite. Schreibersite and troilite also occur as plate shaped inclusions, which show up on cut surfaces as cm-long and mm-thick lamellae. The troilite plates are called Reichenbach lamellae.

The chemical composition is dominated by the elements Fe, Ni and Co, which make up more than 95%. Ni is always present; the concentration is nearly always higher than 5% and may be as high as about 25%. A significant percentage of nickel can be used in the field to distinguish meteoritic irons from human-made iron products, which usually contain lower amounts of Ni, but it is not enough to prove meteoritic origin.

Use

Iron meteorites were historically used for their meteoric iron, which was forged into cultural objects, tools or weapons. With the advent of smelting and the beginning of the Iron Age the importance of iron meteorites as a resource decreased, at least in those cultures that developed those techniques. In Ancient Egypt and other civilizations before the Iron Age, iron was as valuable as gold, since both came from meteorites, for example Tutankhamun's meteoric iron dagger. The Inuit used the Cape York meteorite for a much longer time. Iron meteorites themselves were sometimes used unaltered as collectibles or even religious symbols (e.g. Clackamas worshiping the Willamette meteorite). Today iron meteorites are prized collectibles for academic institutions and individuals. Some are also tourist attractions as in the case of the Hoba meteorite.

Classification

Two classifications are in use: the classic structural classification and the newer chemical classification.

Structural classification

The older structural classification is based on the presence or absence of the Widmanstätten pattern, which can be assessed from the appearance of polished cross-sections that have been etched with acid. This is connected with the relative abundance of nickel to iron. The categories are:

• Hexahedrites (H): low nickel, no Widmanstätten pattern, may present Neumann lines;
• Octahedrites (O): average to high nickel, Widmanstätten patterns, most common class. They can be further divided up on the basis of the width of the kamacite lamellae from coarsest to finest.
  • Coarsest (Ogg): lamellae width > 3.3 mm
  • Coarse (Og): lamellae width 1.3–3.3 mm
  • Medium (Om): lamellae width 0.5–1.3 mm
  • Fine (Of): lamellae width 0.2–0.5 mm
  • Finest (Off): lamellae width < 0.2 mm
• Plessitic (Opl): a transitional structure between octahedrites and ataxites
• Ataxites (D): very high nickel, no Widmanstätten pattern, rare.

Chemical classification

A newer chemical classification scheme based on the proportions of the trace elements Ga, Ge and Ir separates the iron meteorites into classes corresponding to distinct asteroid parent bodies. This classification is based on diagrams that plot nickel content against different trace elements (e.g. Ga, Ge and Ir). The different iron meteorite groups appear as data point clusters.

There were originally four of these groups designated by the Roman numerals I, II, III, IV. When more chemical data became available these were split, e.g. Group IV was split into IVA and IVB meteorites. Even later some groups got joined again when intermediate meteorites were discovered, e.g. IIIA and IIIB were combined into the IIIAB meteorites.

In 2006 iron meteorites were classified into 13 groups (plus one for uncategorized irons):

• IAB
  • IA: Medium and coarse octahedrites, 6.4–8.7% Ni, 55–100 ppm Ga, 190–520 ppm Ge, 0.6–5.5 ppm Ir, Ge-Ni correlation negative.
  • IB: Ataxites and medium octahedrites, 8.7–25% Ni, 11–55 ppm Ga, 25–190 ppm Ge, 0.3–2 ppm Ir, Ge-Ni correlation negative.
• IC: 6.1–6.8% Ni. The Ni concentrations are positively correlated with As (4–9 μg/g), Au (0.6–1.0 μg/g) and P (0.17–0.40%) and negatively correlated with Ga (54–42 μg/g), Ir (9–0.07 μg/g) and W (2.4–0.8 μg/g).
• IIAB
  • IIA: Hexahedrites, 5.3–5.7% Ni, 57–62 ppm Ga, 170–185 ppm Ge, 2–60 ppm Ir.
  • IIB: Coarsest octahedrites, 5.7–6.4% Ni, 446–59 pm Ga, 107–183 ppm Ge, 0.01–0.5 ppm Ir, Ge-Ni correlation negative.
• IIC: Plessitic octahedrites, 9.3–11.5% Ni, 37–39 ppm Ga, 88–114 ppm Ge, 4–11 ppm Ir, Ge-Ni correlation positive
• IID: Fine to medium octahedrites, 9.8–11.3%Ni, 70–83 ppm Ga, 82–98 ppm Ge, 3.5–18 ppm Ir, Ge-Ni correlation positive
• IIE: octahedrites of various coarseness, 7.5–9.7% Ni, 21–28 ppm Ga, 60–75 ppm Ge, 1–8 ppm Ir, Ge-Ni correlation absent
• IIIAB: Medium octahedrites, 7.1–10.5% Ni, 16–23 ppm Ga, 27–47 ppm Ge, 0.01–19 ppm Ir
• IIICD: Ataxites to fine octahedrites, 10–23% Ni, 1.5–27 ppm Ga, 1.4–70 ppm Ge, 0.02–0.55 ppm Ir
• IIIE: Coarse octahedrites, 8.2–9.0% Ni, 17–19 ppm Ga, 3–37 ppm Ge, 0.05–6 ppm Ir, Ge-Ni correlation absent
• IIIF: Medium to coarse octahedrites, 6.8–7.8% Ni,6.3–7.2 ppm Ga, 0.7–1.1 ppm Ge, 1.3–7.9 ppm Ir, Ge–Ni correlation absent
• IVA: Fine octahedrites, 7.4–9.4% Ni, 1.6–2.4 ppm Ga, 0.09–0.14 ppm Ge, 0.4–4 ppm Ir, Ge-Ni correlation positive
• IVB: Ataxites, 16–26% Ni, 0.17–0.27 ppm Ga, 0,03–0,07 ppm Ge, 13–38 ppm Ir, Ge–Ni correlation positive
• Ungrouped meteorites. This is actually quite a large collection (about 15% of the total) of over 100 meteorites that do not fit into any of the larger classes above, and come from about 50 distinct parent bodies.

Additional groups and grouplets are discussed in the scientific literature:

• IIG: Hexahedrites with coarse schreibersite. Meteoric iron has low nickel concentration.

Magmatic and nonmagmatic (primitive) irons

The iron meteorites were previously divided into two classes: magmatic irons and non magmatic or primitive irons. Now this definition is deprecated.

Stony–iron meteorites

There are also specific categories for mixed-composition meteorites, in which iron and 'stony' materials are combined.

• Stony–iron meteorites
  • Pallasites
    • Main group pallasites
    • Eagle station pallasite grouplet
    • Pyroxene Pallasite grouplet
  • Mesosiderite group