Meteorite or “Meteor-wrong”?

Posted: December 16, 2011 - 13:00 , by ROM

ROM Earth Scientists receive dozens of requests each year to identify possible meteorites. This is especially the case when there is a spectacular fireball similar to the one which recently streaked across southern Ontario on December 12 of this year (the video was captured by astronomers at the University of Western Ontario). Do you think you have found a space rock?

Meteorites have an other-worldly appearance because, well, they are from other worlds. As a result, freshly fallen meteorites often stand out from the run-of-the-mill Earth-rocks you encounter. They typically have a fusion crust: a blackened, charred-looking exterior. This crust is the result of the meteor making it’s fiery passage through our atmosphere when the exterior of the stone is white-hot. During this period tremendous amounts of molten material are ripped from the surface which also produces a gouged, dimpled surface. Such a surface is often referred to as being ‘thumb printed’ since it resembles a piece of clay in which someone has repeatedly pressed their thumb, creating divots. The Greek term is regmaglypt but thumb printed is easier to say!

Many types of meteorites are also magnetic. Unfortunately there are any number of objects – both natural and manmade – that can mimic some or all of these features which we often refer to as ‘meteor-wrongs’. Over the next few installments we will discuss and show examples of some of the more typical meteor-wrongs that we often see. If you’d like to see one of the best selections of ‘meteor-rights’ on public display then be sure to stop by the Teck Suite of Galleries: Earth’s Treasures on your next visit to the ROM. Our meteorite display is second to none.

The number one, all-time, King of meteor-wrongs is slag. Slag is an industrial by-product usually left over from the refining of iron or other metals. Probably 90% of what we see as potential meteorites is some form of slag. It’s easy to understand why. It is very striking in appearance and usually appears out of place wherever you see it. It also turns up in some of the most unlikely places (I routinely find small pieces in my garden!) making people think that it must have fallen from the sky.

Typically slag has a black, brown or brownish-red, glassy-looking surface often with patches of grey or white. Not uncommonly it also contains prominent blebs of metal. Whether there is visible metal present or not slag is also quite often very magnetic. The combination of these features can convince people that they have found a meteorite since meteorites often do have a blackened exterior and can be slightly to very magnetic. However slag is also typically magnetic to some degree due to metal left over from the smelting process. So, a blackened exterior and magnetism are definitely not enough to go on.

One feature that slag almost always has, which meteorites typically do not, is bubbles. These were created when the molten slag trapped gases released in the refining process. As the slag cooled some of the gas was trapped, leaving void spaces behind. Such voids are exceptionally rare in real meteorites – only a handful are known out of the more than 40,000 meteorites in world collections. So, bubbles are not usually a good sign.

As mentioned earlier slag has a glassy appearance because it is in fact a glass. It generally cools fairly quickly so the components it is made of have little or no time to crystallize. As a result it breaks in much the same way as bottle glass does, with the classic scallop-shaped surface also known as a conchoidal fracture. The word conchoidal comes from the Greek, and refers to the curved shape of a mussel shell. Most meteorites on the other hand break more like terrestrial rocks.

One last tell-tale feature of slag is its internal appearance. When a meteor streaks through the sky it is only melting on the surface. Its interior is generally not affected. As a result, on a broken surface a meteorite will have the darkened, fusion-crusted exterior but have an internal appearance similar to many types of Earth-rocks, or in the case of iron-nickel meteorites be a very obvious silvery-colour. Slag generally has the same texture and appearance throughout and typically also shows bubbles in the interior.

These three images demonstrate many of these features.

A closer look at a black  rock with bubbles.

charred-looking surface, yes, but bubbles...not a good sign.

A closer look at a black rock with a curved break and shiny surface.

A glassy, ‘conchoidal’ fracture and a similar appearance internally and externally (and more bubbles!)

An closer look at a true meteorite

Meteor-right! This is a piece of a rare meteorite called a diogenite. You can see the typical, charred fusion crust on the outside while the interior has a distinctly different appearance.

We encourage people to be aware of their surroundings while they are out and about, this is how discoveries are made. But bear in mind that finding a real meteorite is a very rare experience and that most of us in Southern Ontario live in areas that have seen a lot of industrial activity over the past century or more. I still don’t know why slag turns up in my garden!

If you still think you have a meteorite, bring your specimen to an ROM ID Clinic and one of our experts will take a closer look.