Three kinds of meteorite

Meteorites come in three big families: stony, iron and stony-iron. Each one tells a different story about the asteroid it came from.

Stony meteorites

Stony meteorites are made mostly of minerals, the same kinds of natural crystals that make up rocks on Earth. Most stony meteorites also contain small flecks of shiny iron-nickel metal.

Stony meteorites are by far the most common kind. Of all the meteorites that people have seen fall, about 94 out of every 100 are stony. Even so, they are often hard to spot on the ground, because they can look a lot like ordinary Earth rocks once they have weathered.

Many stony meteorites contain tiny round beads called chondrules, usually a millimetre or so across. These beads formed as melted droplets floating in space before the planets were built. Stony meteorites with chondrules are called chondrites. You will meet them again later in the course.

Iron meteorites

Iron meteorites are made almost entirely of metal: mostly iron, mixed with some nickel. They are very heavy for their size, about two and a half times heavier than a normal Earth rock of the same size.

Iron meteorites come from the centres, or cores, of asteroids that once got so hot that they melted. The heavy metal sank to the middle, just as it did inside Earth. Later, crashes broke those asteroids apart and exposed their metal hearts.

About 4 or 5 in every 100 meteorite falls are irons. But because irons are tough and easy to notice, many of the biggest meteorites ever found are irons. The largest of all, Hoba in Namibia, is an iron weighing about 60 tonnes.

Stony-iron meteorites

Stony-iron meteorites are a mixture of roughly equal amounts of metal and stone. They are the rarest of the three families, only about 1 in every 100 falls.

The most beautiful are called pallasites. When a pallasite is cut and polished, it shows golden-green crystals of a mineral called olivine set inside shiny metal, a bit like stained glass. Scientists think many pallasites formed where metal and rock met deep inside an asteroid.

The other main kind, called mesosiderites, look like a jumble of broken rock and metal stuck together. They probably formed when asteroids smashed into each other.

So, to recap: stony meteorites are mostly rock, irons are mostly metal, and stony-irons are a bit of both. Each family gives scientists a different window into the insides of asteroids.

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