Prior to the age of professional meteorite hunting in hot deserts and their robotic recovery in the ice fields of Antarctica, most meteorite finds were irons. Due to their metallic composition and their extraordinary weight, even a layman can tell them from ordinary rocks, and they are easily recognized as foreign intruders. Moreover, most iron meteorites are quite resistant to terrestrial weathering, permitting them to be preserved much longer than any other type of meteorite. Finally, irons are usually much larger than stony or stony-iron meteorites. Irons rarely are fragmented upon entering the atmosphere and suffer much less from the effects of ablation during their passage through the atmosphere. In fact, the largest meteorites are irons; have a look at our
charts. All iron meteorites taken together comprise a total known weight of more than 500 tons, and they represent approximately 89.3% of the entire mass of all meteorites known. Despite these facts, iron meteorites are rare since they represent just 5.7% of all witnessed falls.
Iron meteorites are composed largely of nickel-iron metal, and most contain only minor accessory minerals. These accessory minerals often occur in rounded nodules that consist of the iron-sulfide troilite or graphite, often surrounded by the iron-phosphide schreibersite and the iron-carbide cohenite. Despite the fact that some iron meteorites contain silicate inclusions, most have fundamentally the same superficial appearance.
Presently, iron meteorites are classified under two established systems. Just a few decades ago, iron meteorites were exclusively classified according to the macroscopic structures revealed when their polished surface was etched with nitric acid. Depending on these structures, they were separated into three classes: octahedrites, hexahedrites, and ataxites. Beyond that, modern research employs very sophisticated tools such as electron microprobes and X-ray spectroscopes, devices that enable us to detect minute amounts of trace elements such as germanium, gallium, or iridium. Based on the specific concentrations of these trace elements and their correlation with the overall nickel content, iron meteorites are classified into several chemical groups, and each group is thought to represent a unique parent body. We will elaborate on the different classification schemes and groups of iron meteorites below.
Structural Classification of Iron Meteorites
Nickel-iron metal in iron meteorites occurs in the form of two distinct alloys. The most common alloy is kamacite, named for the Greek word for "beam". Kamacite contains 4 to 7.5% nickel, and it forms large crystals that appear like broad bands or beam-like structures on the etched surface of an iron meteorite. The other alloy is called taenite for the Greek word for "ribbon". Taenite contains 27 to 65% nickel, and it usually forms smaller crystals that appear as highly reflecting thin ribbons on the surface of an etched iron. Depending on the occurrence and the distribution of these nickel-iron alloys, etched iron meteorites display characteristic structures that are used to classify iron meteorites into octahedrites, hexahedrites, and ataxites.
The most common structure displayed on the etched surface of iron meteorites is a more or less fine intergrowth of kamacite and taenite lamellae that intersect one another at various angles. These fascinating patterns of crisscrossing bands and ribbons, called "Widmanstätten figures" for their discoverer, Alois von Widmanstätten, reveal an intergrowth of larger kamacite and taenite plates. This intergrowth has a spatial arrangement in the form of an octahedron, and thus, these iron meteorites are called octahedrites. Spaces between larger kamacite and taenite plates are often filled by a fine-grained mixture of kamacite and taenite called plessite, for the Greek word for "filling". The octahedrites are further divided into several subgroups based on the width of their kamacite lamellae, and each subgroup is associated with a particular chemical class of iron meteorites (see
Hexahedrites consist primarily of kamacite, and they are named for the way that the crystal structure of kamacite is arranged according to the spatial form of a hexahedron. Pure kamacite forms cubic crystals with six equal sides at right angles to each other, and hexahedrites are actually large, cubic kamacite crystals. Upon etching, hexahedrites don't display any Widmanstätten figures, but they often exhibit fine, parallel lines called "Neumann lines" for their discoverer, Franz Ernst Neumann, who first studied them in 1848. These lines represent a shock-induced, structural deformation of the kamacite plates, and they suggest an impact history for the hexahedrite parent body, at least for the hexahedrites related to chemical group IAB (see
Some iron meteorites reveal no obvious internal structure upon etching, and they are called ataxites, for the Greek word for "without structure". Ataxites consist primarily of nickel-rich taenite, and kamacite is found only in the form of microscopic lamellae and spindles. Consequently, ataxites represent the most nickel-rich meteorites known, and are among the most rare. Among the 50 witnessed iron meteorite falls, none has been an ataxite; all of the known ataxites are finds. Paradoxically, the largest meteorite known, Hoba, belongs to this rare structural class - a strange coincidence that that is hard to reconcile.
between structural and chemical groups of iron meteorites:
|| Kamacite mm
|| Nickel %
|| 4.5 - 6.5
|| 3.3 - 50
|| 6.5 - 7.2
|| 1.3 - 3.3
|| 6.5 - 8.5
||0.5 - 1.3
|| 7.4 - 10
|| 0.2 - 0.5
|| 7.8 - 13
|| < 0.2
|| 7.8 - 13
|| < 0.2, spindles
|| 9.2 - 18
|| > 16