15 - Iron meteorites

Summary

Introduction

The most comprehensive systematic study of iron meteorites was published in 1975, and still remains the most authoritative and best illustrated text on the subject [15.1], despite advances in classification and understanding of chemical composition [15.2]. Only a few iron meteorites with representative textures are included here. Although [15.1] is out of print, it is available on-line (evols.library.manoa.hawaii. edu/handle/10524/33750). The number of iron meteorites listed in the Meteoritical Bulletin (June 2014), and not accounting for pairing, was 1097 (www.lpi.usra.edu/meteor/metbull.php). Only 49 of these (∼5%) are observed falls. Iron meteorites are now classified into 11 groups on the basis of composition [15.2], with a large number of specimens (∼20%) that are not affiliated to any specific group. An additional 292 meteorites were classified as iron meteorites but are now regarded as related to either primitive achondrites or ordinary chondrites (see Table 15.1).

Iron meteorites consist of metallic iron with variable nickel contents (typically between 5 and 15 wt.%), although there are a few meteorites with up to 60 wt.% Ni [15.1]. Iron meteorites are the biggest and heaviest examples of extraterrestrial material that fall to Earth. Because they are so different in appearance from terrestrial rocks, they are readily identified and collected. Prior to the start of regular collection trips to deserts (which have returned several thousand stone meteorite finds), iron meteorites dominated meteorite finds, although accounting for only about 5% of witnessed falls [15.3]. A single iron meteorite can weigh several tonnes. As noted above, one of the most important sources of information about iron meteorites is The Handbook of Iron Meteorites by Buchwald [15.1], from which much of the information given here about the structural classification of iron meteorites is drawn. Recent reviews of iron meteorites [15.4–15.7] have also been valuable sources of information.

Classification of iron meteorites

Structural classification

Iron meteorites are classified into eight subgroups on the basis of metallic structure, an intergrowth of two metals of Fe with Ni. The structure is shown when an iron is polished, and then etched with nital, a solution of nitric acid in alcohol. One metal (kamacite; α-Fe-Ni, also known as ferrite) is body-centred cubic iron with <6 wt.% nickel. The second is taenite (γ-Fe-Ni, or austenite), face-centred cubic iron with >25 wt.% nickel.