Do you think you have found a meteorite?
Many people have been to the desert, or river, or perhaps even their own backyard and found a rock that is unusual. Some of those rocks are thought to be meteorites. But how do we know? A good place to start is with helpful information and meteorite identification tests you can do to help answer your question “Is it a meteorite?”.
Meteorites come in three different large classes, Stone (Stony), Iron, and Stony-Iron meteorites. But, to be honest there are a lot more actual types within the three classes so it can be complicated to make a refined classification. But for our purpose here let’s work with the three main classes.
This is by far the largest of the three main types. These meteorites look like a rock since they are made of mostly mineral material similar to many rocks originating here on Earth. But, true meteorites are often much heavier for their size than an Earth rock. So ‘Heavy for Size’ is the first thing to examine in your suspect rock.
If the rock is broken it will be solid inside. It will not be porous like lava rocks are. It may have small round structures like tiny balls showing on the broken surface. These are called chondrules and many stone meteorites (the chondrites) will have them. But it will not have holes inside if it is a meteorite. Most stone meteorites will not have shiny crystals in them. They will not be layered or banded with different strips of mineral types. So ‘Solid’ and ‘Not Crystalline’ and ‘Not layered’ are phrases to remember.
Stone meteorites often have grains of nickel-iron in them. Metallic iron in rocks from the Earth is very rare. The moist atmosphere of the Earth has turned almost all native iron to some other chemical form of iron long ago. So, if you grind off a small spot on your suspect rock and find bright shiny metal spots this is another good indication that you might have a meteorite of the stone type. One thing to think about here is what I mean by shiny metal spots. These will not be a metallic luster or shininess. The metal spots in meteorites will be actual metal; they will look the way the chrome on a car looks. So having ‘Metal Grains’ is the next thing on our list of characteristics.
Because meteorites often have iron metal in them they will respond when a strong magnet is brought near them. If a magnet will stick to your rock or a magnet will pull the rock when it is hung from a string that may mean there is iron metal in the suspect rock. You will want to grind a little spot as discussed above and see if there is metal. But many rocks on the Earth that are not meteorites contain iron in chemical forms that are magnet responsive. Magnetite is one of the most common and is often found in rocks. It will stick to magnets. But, it will not have metal grains when ground and the powder produced by the grinding will be black. The powder produced by grinding most meteorites will be brown. Fresh meteorites may not make brown powder but older stone meteorites usually will. So ‘Responds to a Magnet’ and makes a ‘Brown Streak or Powder’ is the next characteristic.
Meteorites pass through the atmosphere of the Earth initially at the speed they had in space. This speed is thousands of miles per hour. They interact with the thin air high above the ground and are melted on the outside surface. This melted coating is called ‘Fusion Crust’. It is often black if the meteorite is freshly fallen, but will turn more and more brown as time passes laying on the ground. The iron grains and minerals will rust and weather making the meteorite browner. So ‘Black or Brown on the Outside’ is the feature we are now discussing but it maybe the first thing to look for in the rocks.
If your rock had some of the characteristics but not all it is still possible that it is a type of stone meteorite. Some stone meteorite do not have much metal or any metal so there is none to see when you grind them and they will not respond to a magnet very much. You will have only their external appearance to go by. If you think it is fusion crusted and looks like it has flight marking from traveling through the atmosphere while melting on the outside, send it to be examined.
Much more rare to find are meteorites made almost entirely of nickel-iron. These meteorites will be black or brown on the outside. They will be very heavy and a magnet will stick strongly to them since they are metal. If you file or grind on them they will show metal like any piece of iron from your garage or a junkyard that is rusted.
Iron meteorite and meteorites in general can be almost any shape. They will have smoothed off rounded corners, but do not have to be spherical in shape. Iron meteorite and other type may be marked on the outside surface by depressions. These are commonly call thumbprints. I use this name instead of the scientific term “regmaglyphs” since thumbprints describes them pretty well. Imagine pushing your thumb many times into potter’s clay. This is the kind of marks meteorites have sometimes on their exterior surface. Remember they are not porous. They will not have holes, but they may have these depressions on the surface.
Iron meteorites can easily be confused with rusted pieces of man made iron and steel. Old mill balls and cannon balls become good imitators of meteorites after many years of rusting. The same is true for airplane parts and car parts, motorcycle parts all are found in the desert and other isolated areas now. Testing for the presence of nickel is often required to determine for sure if a chunk of iron is meteoritic. All iron meteorites have nickel along with the iron. This test is complicated enough to make it something that probably is not for the first time rock finder. But send the rock to a testing facility for analysis. Or if you can remove a piece send that instead. A piece the size of a walnut will be plenty for a determination to be made. If it is a meteorite that much should also satisfy the requirement for official classification and inclusion in the Catalog of Meteorites. The finder is required to submit a finder’s report to the Meteoritical Society whose online address can be found below. Twenty grams of a meteorite or 20% of a small meteorite is donated to fulfill the requirement. This amount will be placed in a permanent collection for scientists to have access to after the analysis is complete.
These are the last main type of meteorite. As the name indicates they have characteristics of both the previous types. They are made of a mixture of nickel-iron and stone. It will be possible to file or grind and find abundant metal. But it will often be possible to find spots on the surface where a regular file is useless since the material is mineral. The mineral portion can be dark silicate minerals or it can be yellowish green or yellowish brown olivine crystals.
Stony-iron meteorites will be heavy since they contain a lot of iron. They will often be very rusted for the same reason. And the olivine crystals may have discolored on the outside or fallen out from weathering and rusting processes. Furnace slag is sometimes confused for stony-iron meteorite material, since it often has a residue of iron along with the melted rock component. But furnace slag is often porous and meteorites are not.
Author: James Tobin [Google]