Classification and properties of iron meteorites

Most (86%) iron meteorites can be assigned to one of 12 genetic groups on the basis of systematic variations in their chemical, mineralogical, and structural properties; the remaining 14% are termed anomalous. The groups are best resolved on Ga‐Ni or Ge‐Ni plots, but they may also be defined using other elements, the distribution and morphology of characteristic minerals, and very often kamacite bandwidths. The power of this classification to reveal correlations of numerous and diverse properties within these groups and systematic variations between groups emphasizes its validity. Its use is essential for understanding the formation of iron meteorites. A comparison of the 12 groups suggests that there are two types with very different histories: (1) the major groups IIAB, IIIAB, and IVA (11, 32, and 8% of all irons, respectively), probably IIC, IID, and IVB, and possibly also IC, IIIE, and IIIF; within these groups, most properties are correlated and chemical and mineralogical trends closely similar and (2) the large group IAB (19% of all irons), IIICD, and probably IIE, in which correlations between properties are generally much weaker and the observed trends distinctly different from those in groups of the former type. Each group very probably formed in its own parent body. Groups of the second type seem to have diverse formational histories, but we believe that, unlike the first type, they were not once part of molten cores. This study is based on results from nearly 500 different iron meteorites, which are listed with their classification together with 70 other paired irons. We present a comparative study of the properties of the 12 groups without attempting to fit these data to detailed models for their formation.