Iron meteorites come from the core of dead asteroids. Heavy metallic elements sank towards the center of these asteroids due to gravity. Collisions with other bodies then destroyed these asteroids, releasing their metallic cores to space. Iron meteorites are alloys, mainly of iron and nickel, although they also contain trace elements such as iridium. Iron meteorites account for only 4% of all observed meteorite falls, by number. However, due to their high density, the total weight of iron meteorites accounts for almost 50% of observed meteorite falls. This percentage increases up to 83% if the total weight of meteorite finds is included as well. Current scientific classification for iron meteorites is complex and exclusively dependent on the meteorite’s chemical composition. It includes 15 groups as follows: IAB, IC, IIAB, IIC, IID, IIE, IIF, IIG, IIIAB, IIICD, IIIE, IIIF, IVA, IVB, and Ungrouped irons. This classification is necessary for scientists as it is precise. There is a more visually intuitive classification that some collectors follow that is based on the internal structure. According to their structures, iron meteorites are classified into 3 types: octahedrites, hexahedrites and ataxites.
Iron meteorite
1. Octahedrites
They display lines (most easily visible after polishing and etching) which are called Widmanstatten pattern. The Widmanstatten pattern reflects light from different angles due to differences in the crystallization of kamacite and taenite minerals. This is the result of the cooling-down of an asteroid’s core at about 1 degree Celsius every million years. The resulting lines of crystallization form an octahedron (a 3-dimensional shape), from which this type’s name is derived. Some types of octahedrites also display Neumann lines although they are more characteristic of hexahedrites.
Iron octahedrite
2. Hexahedrites
They display a feature called Neumann lines. These lines are very fine and result from a strong mechanical shock in space due to an impact. Hexahedrites are lower in nickel than octahedrites, usually around 5.5%. When crystallized, kamacite forms right-angled cubic crystals with six sides.
3. Ataxites
They are a type of iron meteorite that do not show lines upon etching. Their name means ‘without structure’. Ataxites have >18% nickel content and most of the meteoric iron is kamacite with minor amounts of taenite, so no structural difference can be seen when they are cut and etched.
Iron meteorite classification
3. Stony-iron
Stony iron meteorites are thought to have originated at the core-mantle boundary of differentiated asteroids, where the metallic core meets silicated rocks. They were then shattered into space as the result of another meteorite impact, where they remained until they fell upon Earth. The proportion of silicates to metal is about 50% for each. However, it is not uncommon to randomly see stony-iron meteorites with a higher content of one than the other.
3.1. Pallasite
Pallasites are considered by many to be the most beautiful type of meteorite. They are thought to have originated at the core-mantle boundary of differentiated asteroids, where the metallic core meets lighter silicated rocks (olivines). They were then shattered into space as the result of another meteorite impact, where they remained until they fell upon Earth. There are 3 sub classes. Main group (90% of pallasites), Ungrouped (6%) and Eagle Station group (4%). Pallasites consist of a metallic matrix with a high quantity of olivine inclusions. Due to their aesthetics when cut, they are often used in jewelry. They also make attractive pieces when cut in half or sliced.
Pallasite meteorite
3.2. Mesosiderite
Mesosiderites are thought to have originated at the core-mantle boundary of a differentiated asteroid, where the metallic core meets the lighter rocks. They were then shattered into space as the result of another meteorite impact, where they remained until they fell upon Earth. Mesosiderites have similar parts of metal to rock. It is a chaotic mixture, some parts being rock with metal inclusions and others being metal with rock inclusions. There are 3 sub classes (A, B and C) which are based on textural and mineralogical differences. Asteroid 16 Psyche is considered the main candidate as the parent body for mesosiderites. It orbits the Sun between Mars and Jupiter in the asteroid belt.
