Iron Meteorite

MeteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and Click on Term to Read More composed mainly of iron (Fe) and nickel (Ni) in the form of two alloys, kamaciteMore common than taenite, both taenite and kamacite are Ni-Fe alloys found in iron meteorites. Kamacite, α-(Fe,Ni), contains 4-7.5 wt% Ni, and forms large body-centered cubic crystals that appear like broad bands or beam-like structures on the etched surface of a meteorite; its name is derived from the Greek word Click on Term to Read More and taeniteLess common than kamacite, both taenite and kamacite are Ni-Fe alloys found in iron meteorites. Taenite, γ-(Fe,Ni), has 27-65 wt% Ni, and forms small crystals that appear as highly reflecting thin ribbons on the etched surface of a meteorite; the name derives from the Greek word for "ribbon." Click on Term to Read More. Due to their metallic makeup and extraordinary weight, iron meteorites are easily distinguished from ordinary rocks. Also, because they rarely break up in the air and suffer much less from the effects of ablationGradual removal of the successive surface layers of a material through various processes. • The gradual removal and loss of meteoritic material by heating and vaporization as the meteoroid experiences frictional melting during its passage through the atmosphere. The resulting plasma ablates the meteor and, in cases where a meteor Click on Term to Read More during their passage through the atmosphere, they are usually much larger than stony or stony-iron meteorites. All known iron meteorites together have a mass of more than 500 tons, which is ~89% of the entire mass of all known meteorites. Yet they are comparatively rare, accounting for just 5.7% of witnessed falls.

There are two ways of classifying iron meteorites. The older, structural method is based on characteristic crystalline features that show up when the meteorites are sectioned, etched, and polished. This results in three subdivisions: hexahedrites (4–6 wt. % Ni), octahedrites (the commonest type: 6-12 wt. % Ni), and ataxites (> 12 wt. % Ni).

The newer chemical method is far more precise but depends on sophisticated instruments to determine abundances of trace elements such as Ge, Ga, and Ir. The concentrations of the trace elements are plotted against the overall Ni content on logarithmic scales to resolve well-defined chemical clusters, each representing a distinct chemical group. Iron meteorites of each chemical group formed on a common parent bodyThe body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early in the formation of our Solar System, while others like the asteroid 4-Vesta and Mars are still observable today. Click on Term to Read More. Iron meteorites come mostly from the cores of small differentiated asteroids that were disrupted by devastating impacts shortly after their formation. Short descriptions of the major groups follow.

• IAB group irons include the famous Toluca, Campo del Cielo, Odessa, and Canyon Diablo meteorites. Most IAB irons are coarse to medium octahedrites (although other structural classes do occur) and often contain abundant inclusions of troiliteBrass colored non-magnetic mineral of iron sulfide, FeS, found in a variety of meteorites. Click on Term to Read More, graphiteOpaque form of carbon (C) found in some iron and ordinary chondrites and in ureilite meteorites. Each C atom is bonded to three others in a plane composed of fused hexagonal rings, just like those in aromatic hydrocarbons. The two known forms of graphite, α (hexagonal) and β (rhombohedral), have Click on Term to Read More, and coheniteFe-Ni-Co carbide, (Fe,Ni,Co)3C, that occurs as an accessory constituent in several iron meteorites, and coarse octahedrites with < 7 wt. % Ni. Click on Term to Read More, and various silicates. Recent research suggests that both winonaites and IAB irons originated on the same parent body – a partially differentiated asteroid that was disrupted just as it began to form an Fe coreIn the context of planetary formation, the core is the central region of a large differentiated asteroid, planet or moon and made up of denser materials than the surrounding mantle and crust. For example, the cores of the Earth, the terrestrial planets and differentiated asteroids are rich in metallic iron-nickel. Click on Term to Read More and silicate-rich crustOutermost layer of a differentiated planet, asteroid or moon, usually consisting of silicate rock and extending no more than 10s of km from the surface. The term is also applied to icy bodies, in which case it is composed of ices, frozen gases, and accumulated meteoritic material. On Earth, the Click on Term to Read More. The impact mixed silicates into molten Ni-Fe metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn Click on Term to Read More forming the silicated IAB irons, and mixed olivine-rich residues of partial melts into unmelted silicates, forming the winonaites.
• IIAB Group irons are classified as hexahedrites or coarsest octahedrites, making them some of the most Ni-poor iron meteorites known. Perhaps the most famous IIAB iron is Sikhote-Alin, a witnessed fallMeteorite seen to fall. Such meteorites are usually collected soon after falling and are not affected by terrestrial weathering (Weathering = 0). Beginning in 2014 (date needs confirmation), the NomComm adopted the use of the terms "probable fall" and "confirmed fall" to provide better insight into the meteorite's history. If Click on Term to Read More from Russia in 1947 (commemorative stamp and meteorite shown below). Several thousand pieces with a total weight of over 70 tons have been recovered. Trace elementSubstance composed of atoms, each of which has the same atomic number (Z) and chemical properties. The chemical properties of an element are determined by the arrangement of the electrons in the various shells (specified by their quantum number) that surround the nucleus. In a neutral atom, the number of Click on Term to Read More abundances suggest that this group formed in the core of a differentiated C-type asteroid that was disrupted by several impact events.
• IIIAB Group irons consist of two closely related groups. The IIIA subgroup has mostly coarse octahedriteMost Common type of iron meteorite, composed mainly of taenite and kamacite and named for the octahedral (eight-sided) shape of the kamacite crystals. When sliced, polished and etched with an acid such as nitric acid, they display a characteristic Widmanstätten pattern. Spaces between larger kamacite and taenite plates are often Click on Term to Read More textures, whereas IIIB irons usually display medium textures. However, the two subgroups form a continuous sequence in structure and elemental compositions consistent with a common origin (the subgroups probably represent different parts of an asteroid’s core). Group IIIAB includes some of the largest irons ever found (Cape York, Chupaderos, Morito, and Willamette). Some IIIAB members contain large nodules of troilite and graphite, but silicateThe most abundant group of minerals in Earth's crust, the structure of silicates are dominated by the silica tetrahedron, SiO44-, with metal ions occurring between tetrahedra). The mesodesmic bonds of the silicon tetrahedron allow extensive polymerization and silicates are classified according to the amount of linking that occurs between the inclusions are rare. Recent research suggests a close relationship between the IIIAB irons and the silicate-rich main group pallasites: both groups probably formed on the same parent body – a differentiated asteroid that was disrupted by a single impact event. IIIAB iron meteorites represent fragments of the core, whereas main-group pallasites are samples of the core-mantle boundary.
• IIICD Group irons mostly belong to the structural classes of fine and finest octahedrites, or ataxites. Several IIICD meteorites contain abundant silicate inclusions, similar to the inclusions in IAB irons. There are additional similarities in elemental compositions suggesting a close relationship between IIICD and IAB irons. However, group IIICD displays some unique features that clearly distinguish them from the IAB irons, e.g., presence of haxoniteIron-nickel carbide, (Fe,Ni)23C6, found in iron meteorites. Click on Term to Read More. This group includes the anomalous, troilite-rich Mundrabilla, one of the largest iron meteorites ever found.
• IVA Group irons are mostly fine octahedrites, and have extraordinarily low Ge and Ga abundances. Some IVA irons contain sparsely distributed small nodules of troilite and graphite (black spots in image below), although silicate inclusions are rare to absent in most members. Recent research suggests that the IVA irons formed in the core of a small, differentiated asteroid disrupted by a major impact shortly after its formation. After re-accretion, the asteroid was again disrupted ~450 Ma ago. The famous meteorite Gibeon (below) is a typical member of this group; over 30 tons have been recovered from its large, prehistoric strewn fieldArea on the surface containing meteorites and fragments from a single fall. Also applied to the area covered by tektites, which are produced by large meteorite impacts. Strewnfields are often oval-shaped with the largest specimens found at one end. Given that the largest specimens go the greatest distance, a meteoroid's in Namibia.
• UngroupedModifying term used to describe meteorites that are mineralogically and/or chemically unique and defy classification into the group or sub-group they most closely resemble. Some examples include Ungrouped Achondrite (achondrite-ung), Ungrouped Chondrite (chondrite-ung), Ungrouped Iron (iron-ung), and Ungrouped Carbonaceous (C-ung). Click on Term to Read More A large number of ungrouped irons don’t fit into any of the existing 14 chemical groups, and display unique structural and elemental compositions. Some have compositions similar to other ungrouped irons, and have been provisionally placed into several grouplets comprising less than five members each. The remaining ungrouped irons are unique, and probably represent single samples of their parent bodies.