Iron meteorite

Tamentit Iron Meteorite, found in 1864 in the Sahara Desert, weight about 500 kg. On display at Vulcania park in France.

The Willamette Meteorite on display at the American Museum of Natural History. It weighs about 32,000 pounds or 14.15 tonnes. This is the largest meteorite ever found in the United States.

Widmanstätten pattern as seen on an etched and polished slice of the Seymchan meteorite. Scale unknown.

A 1.7 kg individual meteorite from the 1947 Sikhote-Alin meteorite shower (coarsest octahedrite, class IIAB). This specimen is about 12 cm wide.

A 700 g individual Chinga iron meteorite (Ataxite, class IVB). This specimen is about 9 centimeters wide.

Iron meteorites are meteorites that consist overwhelmingly of nickel–iron alloys. The metal taken from these meteorites is known as meteoric iron and was one of the earliest sources of usable iron available to humans.

Occurrence

While they are fairly rare compared to the stony meteorites, comprising about 5.7% of witnessed falls, they have historically been heavily over-represented in meteorite collections. This is due to several factors:

• They are easily recognized as unusual even by laymen, as opposed to stony meteorites. Modern-day searches for meteorites in deserts and Antarctica yield a much more representative sample of meteorites overall.
• They are much more resistant to weathering.
• They are much more likely to survive atmospheric entry, and are more resistant to the resulting ablation. Hence they are more likely to be found as large pieces.

In fact, iron meteorites account for almost 90% of the mass of all known meteorites, about 500 tons. All the largest known meteorites are of this type, including the largest—the Hoba meteorite.

Origin

Iron meteorites have been linked to M-type asteroids since both types of objects have similar spectral characteristics in the visible and near-infrared wavelength regions. 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. The IIE chemical class 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 Fe,Ni-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 man-made products, which usually contain lower amounts of Ni, but it is not enough to prove the meteoritic origin (e.g. some US coins).

Use

The iron nickel alloy was used by several cultures for the manufacturing of tools and weapons. For example the Inuit used parts of the Cape York meteorite. Fragments from Gibeon were used for centuries by Nama people. There are also reports of their use for manufacture of various items in Tibet (see Thokcha).

Today meteoritic iron is used in niche jewellery and knife production, but most of it is used for research, educational or collecting purposes.

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 based on the proportions of the trace elements Ga, Ge and Ir separates the iron meteorites into classes corresponding to distinct asteroid parent bodies:

• 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 negativ.
• 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 negativ.
• IC:
• IIA: Hexahedrites, 5.3-5.7% Ni, 57-62 ppm Ga, 170-185 ppm Ge, 2-60ppm 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 negativ.
• IIC: Plessitic octahedrites, 9.3-11.5% Ni, 37-39 ppm Ga, 88-114 ppm Ge, 4-11 ppm Ir, Ge-Ni correlation positiv
• 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 positiv
• 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
• IIG:
• IIF:
• 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 positiv
• IVB: Ataxites, 16-26% Ni, 0.17-0.27 ppm Ga, 0,03-0,07 ppm Ge, 13-38 ppm Ir, Ge-Ni correlation positiv
• Ungrouped meteorites. This is actually quite a large collection (about 15% of the total) of over 100 meteorites which do not fit into any of the larger classes above, and come from about 50 distinct parent bodies.

Magmatic and primitive irons

The iron meteorites can be divided into two classes: magmatic irons and non magmatic or primitive irons.

Iron class	Groups
Primitive iron meteorites	IAB, IIE, Udei Station grouplet, Pitts grouplet, sLL, sLM, sLH, sHL, sHH
Magmatic iron meteorites	IC, IIAB, IIC, IID, IIF, IIG, IIIAB, IIIE, IIIF, IVA, IVB

See also

• Hraschina meteorite
• Meteoritics

External links

• Meteorite articles, including discussions of iron meteorites, in Planetary Science Research Discoveries
• Iron Meteorite images from Meteorites Australia