Is it a meteorite?

March 17, 2015

People sometimes bring unusual rocks to us at Stardome to see if they may have found a meteorite. Sadly, they almost never are. How can we tell? Short of having the chemistry of the rock properly analysed you usually can’t be 100% sure but there are certain tell-tail signs that your rock is probably not a meteorite.

Does it stick to a magnet? Meteorites typically have high iron content and will attract a magnet. Rocks that don’t do this are probably not meteorites (non-magnetic meteorite exist but are rare). If you rock does attract a magnet it still may not be a meteorite; some terrestrial rocks do this too.

Can it easily scratch a glass bottle? If it can then it is probably NOT a meteorite. Meteorites typically don’t contain any minerals harder than glass and thus cannot scratch it. Many terrestrial rocks, however, contain quartz which absolutely will scratch glass. Not all do though, many terrestrial rocks are also softer than glass and can’t scratch it.

Does it have a streak? Find a piece of unglazed porcelain and rub a clean piece of the rock across it. Do you see a red or black mark? Then it is NOT a meteorite. Meteorites should leave no colour at all. If you get a red mark, your rock is probably hematite. If you get a black mark, it is probably magnetite. These are both naturally occurring iron-ores which are very commonly confused for meteorites. Most of the alleged meteorites we get are one of these (usually hematite). Note that many terrestrial rocks also won’t leave a streak. Lack of streak does not mean that you have a meteorite.

Does it have little air bubbles in it? Then it is almost certainly NOT meteorite. Air bubbles are commonly found in volcanic rocks on Earth. They are formed by expanding gases trapped in the rock as it solidifies. They are almost unheard of in meteorites.

Finally, does one face of the rock appear oddly smooth compared to the rest, with a texture somewhat like leather? As meteors pass through the atmosphere, the leading edge becomes very hot and begins of melt. Most of the melt will slough off the meteor as it travels through the atmosphere. However, once the meteor as slowed down enough, it will cool and the last of the melt will harden into a glassy layer 1-2 mm thick. This is known as fusion crust and it is a unique feature of meteorites. You will not find true fusion crust on terrestrial rocks although some rocks may have features that are similar at first glance. Freshly fallen meteorites will always have fusion crust, however, if the meteorite as been sitting on Earth for a while, it may have eroded away.

Does your rock attract a magnet? Does it have no streak? Does it fail to scratch glass? Does it lack air bubbles? Does it have fusion crust? If you rock passes all these tests then you might have a meteorite!

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