Iron meteorites and their hidden patterns

Iron meteorites are pieces of the metal cores of shattered asteroids. Cut one open and etch it, and many reveal a beautiful crystal pattern found nowhere else in nature.

Hearts of asteroids

Most iron meteorites formed in the cores of differentiated asteroids. When an asteroid melted, its dense metal sank to the centre. The core then cooled and crystallised extremely slowly, insulated by the rocky mantle around it. Later, violent collisions broke many of these asteroids apart, freeing pieces of the core.

Iron meteorites are made mainly of iron, usually with about 5 to 20 per cent nickel, though a few contain much more, plus small amounts of cobalt, phosphorus, sulphur and carbon. They often contain nodules of troilite, an iron sulphide, and graphite.

Iron meteorites are important because we can never sample Earth's own core directly. Studying them helps scientists understand how planetary cores form and behave. Some irons, however, probably did not form in cores at all but in pools of melted metal produced by impacts.

The Widmanstätten pattern

When many iron meteorites are cut, polished and lightly etched with a weak acid by an expert, a criss-cross pattern of bands appears. This is the Widmanstätten pattern, named after Count Alois von Beckh Widmanstätten, who described it in 1808. An Englishman, William Thomson, had noticed the same effect a few years earlier.

The pattern is made of two iron-nickel minerals. Kamacite is low in nickel and forms broad bands. Taenite is richer in nickel and forms thin borders around the kamacite. Areas where the two are finely intergrown are called plessite.

The pattern grows only when hot metal cools by a few degrees to thousands of degrees per million years. It cannot be made in a laboratory or furnace, because no human process can cool metal that slowly.

Three main structures

Iron meteorites are sorted into three main structural classes based on their nickel content and appearance.

Octahedrites are the most common. They contain roughly 6 to 13 per cent nickel and show a Widmanstätten pattern, with bands that run parallel to the faces of an octahedron, an eight-sided shape. They are described as coarse, medium or fine depending on the width of the kamacite bands. Gibeon from Namibia is a well-known fine octahedrite.

Hexahedrites contain less nickel, about 4.5 to 6.5 per cent, and are made almost entirely of kamacite. They show no Widmanstätten pattern but often display fine parallel lines called Neumann lines, produced by shock.

Ataxites contain the most nickel, usually more than about 16 per cent, and show no visible structure. The giant Hoba meteorite is an ataxite.

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