METEORS & METEORITES A  B  C  D  E  F  G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z                    HOME ABOUT CATEGORIES SITE MAP COPYRIGHT ADVERTISE CONTACT entire Web this site iron meteorite A meteorite composed mainly of iron (Fe) and nickel (Ni) in the form of two nickel-iron alloys, kamacite and taenite. Iron meteorites are easy for even a layperson to tell apart from ordinary Earth rocks because of their metallic appearance and high density. They also tend to be bigger than either stony meteorites or stony-iron meteorites, because they usually survive passage through the atmosphere intact and suffer much less from the effects of ablation. All known iron meteorites have a combined mass of more than 500 tons – about 86% of the mass of all known meteorites. Yet they are quite rare, accounting for about one in 20 of observed falls. Two ways have been devised to classify iron meteorites. The older, structural method is based on characteristic crystalline features that show up when the meteorites are sectioned, etched, and polished. Octahedrites, the commonest type of iron, with less than 6% by weight of nickel, reveal a characteristic Widmanstätten pattern that results from lamellae (fine sheets) of kamacite intergrown with nickel-rich phases. The width of the kamacite lamellae allows classification into 5 structural groups: the coarsest, coarse, medium, fine and finest octahedrites. Plessitic octahedrites are transitional between octahedrites and ataxites. Ataxites are nickel-rich (> 20% nickel), and are mainly taenite. Hexahedrites contain less than 6% nickel and are comprised of kamacite only; both hexahedrites and ataxites lack a classic Widmansttten pattern. The newer chemical method of classifying iron meteorites involves measuring the amounts of trace elements such as germanium, gallium, or iridium present. The concentrations of these trace elements are then plotted against the overall nickel content on logarithmic scales. This technique has enabled 13 distinct groups, labeled by Roman numbers and letters, such as "IIIAB", to be identified. The members of each chemical group are thought to share a common origin on the same parent body. Iron meteorites come mostly from the cores of small differentiated asteroids that were broken apart by cataclysmic impacts shortly after their formation. Classification of iron meteorites Structural class Symbol Kamacite (mm) Nickel (%) Related chemical groups Hexahedrites H > 50 4.5 - 6.5 IIAB, IIG Coarsest octahedrites Ogg 3.3 - 50 6.5 - 7.2 IIAB, IIG Coarse octahedrites Og 1.3 - 3.3 6.5 - 8.5 IAB, IC, IIE, IIIAB, IIIE Medium octahedrites Om 0.5 - 1.3 7.4 - 10 IAB, IID, IIE, IIIAB, IIIF Fine octahedrites Of 0.2 - 0.5 7.8 - 13 IID, IIICD, IIIF, IVA Finest octahedrites Off < 0.2 7.8 -13 IIC, IIICD Plessitic octahedrites Opl < 0.2, spindles 9.2 - 18 IIC, IIF Ataxites D - > 16 IIF, IVB Largest iron meteorites Meteorite Country Found Structural class Group Mass (kg) Hoba Namibia 1920 ataxite IVB 60,000 Campo del Cielo Argentina 1990 octahedrite IAB 37,000 Cape York (Ahnighito) Greenland 1894 octahedrite IIIAB 31,000 Armanty China 1898 octahedrite IIIE 23,500 Bacubirito Mexico 1863 octahedrite UNG 22,000 Cape York (Agpalilik) Greenland 1963 octahedrite IIIAB 20,000 Mbosi Tanzania 1930 octahedrite UNG 16,000 Campo del Cielo Argentina 1570 octahedrite IAB 15,000 Williamette United States 1902 octahedrite IIIAB 14,900 Chupaderos Mexico 1854 octahedrite IIIAB 14,100 Mundrabilla Australia 1901 octahedrite IIICD 12,000 Morito Mexico 1600 octahedrite IIIAB 11,000 Related category    • METEORS AND METEORITES Recommended books       Also on this site: Encyclopedia of Alternative Energy & Sustainable Living Encyclopedia of History Transport Concepts & Designs (partner site) BACK TO TOP