What are iron meteorite chemical groups?
Iron meteorites are divided into chemical groups, such as IAB, IIAB, IIIAB and IVA, according to their nickel, gallium, germanium and iridium content.
Iron meteorites look similar to one another, but their chemistry varies. Since the 1960s, scientists have classified them into chemical groups by measuring trace elements, especially nickel, gallium, germanium and iridium.
The groups have names made of Roman numerals and letters, such as IAB, IC, IIAB, IIC, IID, IIE, IIF, IIIAB, IIIE, IIIF, IVA and IVB. The numerals originally referred to ranges of gallium and germanium concentration, from group I with the highest to group IV with the lowest. Letters were added as more groups were recognised, and some groups that turned out to be related were merged, such as IIIA and IIIB into IIIAB.
Each group is thought to come from a separate parent body. Within most groups, elements vary systematically. For instance, in magmatic groups such as IIIAB and IVA, iridium falls dramatically as nickel rises. This trend is what you would expect if the irons formed by fractional crystallisation in a single molten core: early-crystallising metal took up iridium, leaving later metal depleted.
Other groups, such as IAB and IIE, called non-magmatic, do not show simple trends and often contain silicate inclusions. They may have formed in pools of molten metal produced by impacts rather than in true cores.
The largest group is IIIAB, which includes Henbury and Cape York. IAB includes Canyon Diablo and Campo del Cielo. About 15 per cent of iron meteorites are ungrouped, each possibly from a separate body.