Stony meteorites

Meteorites made mostly of silicate minerals. They are by far the most common kind seen to fall.

Classification

Stony meteorites

Share of witnessed falls

about 94%

In our catalogue

44,694 meteorites, 1,005 seen to fall

Heaviest

Jilin, 4 t

Earliest recorded fall

Nogata, 860

Most recent fall

Chelyabinsk, 2013

Stony meteorites are made mainly of silicate minerals such as olivine, pyroxene and feldspar, often with a few per cent of iron-nickel metal and iron sulphide. About 94% of meteorites seen to fall are stones.

They split into two very different classes. Chondrites are primitive rocks that never melted: they are made of the dust and droplets that built the planets. Achondrites are igneous rocks from bodies that did melt, from small asteroids to the Moon and Mars.

Because they look much like Earth rocks and weather quickly, stony meteorites are under-represented among chance finds. Systematic searches in Antarctica and hot deserts recover them in their true proportions.

What they are made of

The minerals of stony meteorites are the same ones that build Earth's mantle and crust. Olivine and pyroxene, both silicates of magnesium and iron, dominate most of them; plagioclase feldspar is common in the melted kinds. Many also carry grains of iron-nickel metal and the iron sulphide troilite, which is why most stones attract a magnet and why their broken faces sparkle with tiny bright flecks.

Density is a useful clue. Most stony meteorites weigh between about 3.0 and 3.7 grams per cubic centimetre, heavier than most Earth rocks of the same size because of their metal, but far lighter than an iron meteorite at about 7.8.

Chondrites and achondrites

The great divide among stones is whether their parent body ever melted. Chondrites never did. They are sediments of the early Solar System: chondrules, refractory inclusions, metal and dust swept together and compacted, then gently heated or soaked in water. Achondrites did melt. They crystallised from magma, so they look like basalt, gabbro or peridotite, and they tell the story of how small worlds separated into crust, mantle and core.

Between the two sit the primitive achondrites, rocks that started to melt but never finished. Oxygen isotopes, which act like a postcode for each parent body, link some of them to particular chondrites and irons.

How many parent bodies

Comparing oxygen isotopes, chromium and titanium isotopes, and mineral chemistry, scientists count well over a dozen separate parent bodies among the stones alone, without even including the Moon and Mars. Isotopes of chromium, titanium and molybdenum split almost all meteorites into two great families: a 'non-carbonaceous' family that formed nearer the Sun, and a 'carbonaceous' family that formed farther out. The split was probably kept in place by the early growth of Jupiter, which held the two reservoirs apart for millions of years.

How to recognise a stony meteorite

A fresh stone has a thin black or dark brown fusion crust, less than a millimetre thick, formed as its surface melted in the atmosphere. Shallow thumbprint-like hollows called regmaglypts may cover it. Inside, a chondrite usually shows grey rock with small round chondrules and specks of shiny metal. Most respond to a strong magnet, though some achondrites barely do.

Weathering quickly turns metal to rust, so old finds are often brown and rusty right through. A streak test on unglazed tile is useful: most stony meteorites leave no streak or a faint grey one, while common look-alikes such as haematite leave a red-brown streak and magnetite a black one. Always check finds with a museum or university, and get permission before searching private land.

Classes and groups

Chondrites

Primitive stones that never melted, made of the first solids of the Solar System. The most common meteorites of all.

42,565 in the catalogue

Achondrites

Igneous stones from bodies that melted: asteroids, the Moon and Mars.

2,129 in the catalogue

Related groups

• Chondrites Primitive stones that never melted, made of the first solids of the Solar System. The most common meteorites of all.
• Achondrites Igneous stones from bodies that melted: asteroids, the Moon and Mars.
• Stony-iron meteorites Roughly equal parts metal and silicate. Rare, and among the most beautiful meteorites.
• Iron meteorites Almost pure iron-nickel metal, mostly from the cores of asteroids that melted and were later shattered.

Photos shared under Creative Commons licences (Wikimedia Commons).