IAB irons
Irons with stony inclusions, from a partly melted asteroid. Canyon Diablo and Campo del Cielo.
- Classification
- Iron meteorites › IAB irons
- Share of witnessed falls
- about 1%
- In our catalogue
- 261 meteorites, 10 seen to fall
- Heaviest
- Campo del Cielo, 50 t
- Earliest recorded fall
- Mazapil, 1885
- Most recent fall
- Muzaffarpur, 1964
The IAB complex is the largest family of 'non-magmatic' irons: they did not form by slow crystallisation of a metal core. Most are medium to coarse octahedrites, and many contain silicate inclusions with the same oxygen isotopes as the winonaites, suggesting both formed on a partly melted asteroid that was later disrupted and reassembled.
The complex includes a main group and several subgroups defined by nickel and gold content (sLL, sLM, sLH, sHL, sHH). It includes the irons from Meteor Crater (Canyon Diablo) and the Campo del Cielo crater field.
Mineralogy and chemistry
IAB irons range widely in nickel, from about 6% to over 20%. The main group, with about 6.4% to 8.7% nickel, consists of coarse to medium octahedrites. Many IABs contain nodules of graphite and troilite, crystals of cohenite (an iron-nickel carbide) and schreibersite, and silicate inclusions of olivine, pyroxene and plagioclase whose chemistry and oxygen isotopes match the winonaites. The subgroups, labelled by low or high nickel and low, medium or high gold content, such as sLL, sLM, sHL and sHH, include irons once called IIICD, which are now part of the complex.
Trace elements in IAB irons do not follow the steep trends of a crystallising core. Instead they scatter, as expected for metal that melted only partly and mixed with unmelted rock.
Parent body and formation
The favoured explanation is that the IAB main group and the winonaites come from one asteroid that partly melted: metal and sulphide formed pools and veins but never gathered into a single core. A collision broke the hot body apart, molten metal mixed with fragments of silicate rock, and the debris reassembled and cooled slowly. Dating suggests the metal formed within about 5 to 15 million years of the first solids. Some subgroups may come from separate but similar bodies.
Abundance and famous examples
The IAB complex is one of the largest iron groups. Canyon Diablo is the iron of Meteor Crater in Arizona, a bowl about 1.2 km wide made roughly 50,000 years ago; about 30 tonnes of fragments are known. Its troilite has been the reference standard for sulphur isotopes and long served for lead isotopes, used to date Earth. Campo del Cielo, in northern Argentina, made a field of small craters about 4,000 years ago; its largest masses, such as El Chaco, weigh about 30 tonnes. Odessa in Texas, Toluca in Mexico and Nantan in China are other large finds. Falls include Mazapil, which fell in Mexico on 27 November 1885 during a shower of meteors, Bogou (Burkina Faso, 1962) and Udei Station (Nigeria, 1927).
How to recognise one
IABs are heavy, magnetic irons, often with rounded nodules of dark troilite and graphite or grey silicate showing on cut faces. Etched, the main group shows a coarse Widmanstätten pattern of broad kamacite bands. Telling them from other groups needs chemical analysis of trace elements.
Famous examples
- Canyon Diablo Iron, IAB-MG, United States, 1891 · 30 t
- Campo del Cielo Iron, IAB-MG, Argentina, 1575 · 50 t
- Odessa (iron) Iron, IAB-MG, United States, 1922 · 1.6 t
- Toluca Iron, IAB-sLL, Mexico, 1776 · 3 t
- Nantan Iron, IAB-MG, China, 1958 · 9.5 t
- Youndegin Iron, IAB-MG, Australia, 1884 · 3.8 t
- Cranbourne Iron, IAB-MG, Australia, 1854 · 8.6 t
- Mundrabilla Iron, IAB-ung, Australia, 1911 · 24 t
Related groups
- Winonaites Primitive achondrites related to the silicate inclusions in IAB irons.
- IIE irons Irons full of silicate fragments, possibly from asteroid Hebe.
- Ungrouped irons Irons that fit no known group: each may represent its own asteroid.
- Iron meteorites Almost pure iron-nickel metal, mostly from the cores of asteroids that melted and were later shattered.
Photos shared under Creative Commons licences (Wikimedia Commons).