Iron meteorites are the largest and the most spectacular of all meteorites. Lumps of iron weighing up to 60 tons have been found, such as the Hoba meteorite in Namibia. About 30 % of all known meteorites are of the type iron meteorites. They contain up to 98 % nickel-iron of which the nickel content very seldom exceeds 16 %, and most have between 7 and 13 % nickel. In addition to a few percent of the iron sulphate troilite, there are also minerals such as the iron-nickel phosphate schreibersite and graphite (carbon) in small amounts in these meteorites. In a very few of them diamonds have also been found.
Iron meteorites have their origin in the core of large asteroids or planets. They are divided into three main groups: octahedrites, hexahedrites and ataxites, together with 13 sub-groups. All iron meteorites rust easily, and they are often found as large clumps of rust.
These are the commonest iron meteorites, and contain between 7 and 15 % nickel. The minerals taenite (rich in nickel) and kamacite (deficient in nickel), together with plessite (compound) are the main components of these meteorites. They are called octahedrites because they are formed in a cubic octahedron pattern. This is called the Widmanstätten pattern (see description on page 20).
Contain about 5 – 6 % nickel and show only thin lines when treated with acid. These lines are called Neumann lines, and are caused by collision during formation of the meteorite. The nickel content is too low to form the Widmanstätten pattern.
If the nickel content exceeds 16 %, neither do these show any Widmanstätten pattern, and they are then an ataxite. Ataxites are also much more resistant to rust due to their high nickel content.
What is the Widmanstätten pattern?
This pattern was first discovered in 1804 by William Thompson in Naples. It was however the director of the royal porcelain company in Vienna, Alois von Widmanstätten, that had this pattern named after him when in 1908 he investigated an iron meteorite. Iron meteorites consist of nickel-iron, the minerals kamacite and taenite, and also some plessite. The compound of these minerals forms the cubic crystal form octahedron. Many iron meteorites are therefore called octahedrites. The amount of nickel in these minerals determines the breadth of this pattern, which is formed because cooling of this nickel-iron metal is 1˚C in the course of one million years! The pattern becomes apparent when the surface is treated with a mixture of alcohol and saltpetre acid.
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