Identifying a possible meteorite

Most suspected meteorites turn out to be Earth rocks. This lesson sets out a careful, non-destructive approach, and explains what only a laboratory can confirm.

Start with the outside

Begin by examining the whole rock carefully, ideally with a hand lens. Look for these features.

Fusion crust: a thin, dark, often matt or slightly glossy coating, usually less than a millimetre thick. On some achondrites it can be shiny black. On weathered finds it may be brown or partly missing.

Shape: irregular but rounded edges, possibly with regmaglypts. Sharp, angular edges are normal on fragments broken during the fall, but these should show a fresher interior.

No vesicles: holes left by gas bubbles are characteristic of volcanic rocks and slag, not meteorites. A very small number of meteorite types contain vesicles, but they are exceptions.

No layering: meteorites do not show bands or layers like sedimentary rocks, nor visible crystals of quartz or mica.

Simple tests

Magnetism: most meteorites attract a strong magnet because of their metal. Remember that lunar meteorites, many achondrites and some carbonaceous chondrites react weakly or not at all, and that magnetite-rich Earth rocks and slag are also magnetic.

Density: weigh the rock, then find its volume, for example by displacing water while the rock is sealed in a thin plastic bag so it does not get wet. Divide mass by volume. Ordinary chondrites are usually about 3.2 to 3.7 grams per cubic centimetre, irons about 7.5 to 8, while many common crustal rocks are about 2.6 to 2.8.

Streak: rubbing the rock on the unglazed back of a ceramic tile leaves little or no streak for most stony meteorites. A strong red-brown streak suggests hematite; a black streak suggests magnetite.

None of these tests proves a meteorite; together they rule many rocks out.

What the laboratory does

Confirmation requires expert analysis, and the most valuable tests require only a small sample.

If metal is present, a meteorite will usually show nickel in it, typically at least about 5 per cent, along with cobalt. Natural iron metal on Earth is very rare, and man-made iron usually contains little nickel. Testing for nickel involves chemicals and should be done by specialists.

A laboratory will usually prepare a thin section or polished mount and examine it under a microscope and electron microprobe to measure mineral compositions. For many stony meteorites, oxygen isotope analysis confirms an extraterrestrial origin and helps identify the group.

Museums and university geology departments often offer identification services. Contact them first, send clear photographs with a scale, and follow their advice rather than cutting or grinding the stone yourself.

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