Meteorites are the rarest types of rock found on Earth. Thousands of rocks are sent to laboratories each year by individuals thinking that they have found a meteorite. At best only one or two of these thousands is actually a rock from space. This article is designed to show the basic characteristics of meteorites. It will provide some simple at home tests that can guide the reader in making a better determination of whether or not a rock is a meteorite or one of many terrestrial rocks that are often referred to “meteor wrongs.”
There are dozens of precise classifications of meteorites but most meteorites fall into one of three larger families. These are the Stone, Iron, and Stony-iron meteorite families. The names describe them well. The Stone family contains meteorites that are composed of rock. The Iron meteorites are composed of nearly pure nickel-iron metal. The stony-iron meteorites are a mixture of approximately equal parts of stone and metal.
All meteorites fall through the atmosphere at such high velocity that material burns off their exterior. This melting leaves on the meteorites that reach the ground a glassy outer coating called a fusion crust. The fusion crust is usually a dark gray to charcoal black in color. It can be shiny or a dull velvety texture. The fusion crust can have lines and flow marks from the movement of the molten rock across the surface. Many meteorites even fresh ones are found with broken and chipped off fusion crust. Meteorites found after a long time may have little remaining fusion crust.
Meteorites are not round balls, they are irregularly shaped masses. They are often angular or blocky and will usually have rounded off corners. Some meteorites will be marked on their surface with pits that are commonly called thumbprints. Scientists call these marks regmaglyphs, they are formed by the hot air currents eroding the rock as it plunges through the atmosphere. Round rusted iron masses are actually found quite often and confused for meteorites. Cannonballs and mill balls are often the actual source of these "meteor-wrongs". When heavily rusted some of these look much like iron meteorites and a chemical test is required to determine if they are man made. The iron in meteorites is always alloyed with nickel. If there is no nickel than it is not an iron meteorite.
Slag from industrial processes like iron and steel manufacturing can have metallic particles in it. Slag is one of the materials often confused for meteorites. Slag however, will usually be porous or even bubbly and this is a clue that it is not a space rock. Meteorites are not porous or bubbly. Meteorites are solid rocks, yes they may have pits on their surface but inside they are dense. Volcanic rocks: basalts and lavas, are porous and for some reason are often thought to be meteorites.
When meteorites are fresh they are often black and their fusion crusts may show flow lines and details that can help in identification. After a long time on the ground meteorites change color and fusion crusts wear away and details disappear. The iron in meteorites just like iron in tools rusts from being out in the weather. As the iron metal of meteorites rusts it stains the inside rock matrix and the outside surface. Starting as spots of red or orange on the black fusion crust as time goes on the whole stone will become rusty brown colored. The fusion crust will often still be visible but no longer black.
Meteorites are generally heavier than earth rocks of the same size. This is because of the metallic iron that most meteorites contain. Many meteorite hunters carry a diamond file with them to grind a small spot off rocks to see if there are any iron grains inside. With only a few rare exceptions metallic iron metal is not found in earth rocks. During eons of erosion and weathering the iron in earth rocks is now in mineral form and not metallic. The metal found in meteorites will be shiny and look like the chrome. It will not be just a shiny gray sheen that is often seen on some rocks. Iron metal grains in a rock that also looks like a meteorite are good indicators. However, there are more tests and there are things to watch out for.
The vast majority of meteorites contain some metallic iron, while others are nearly solid iron. This makes most meteorites responsive to a magnet. Many stone meteorites will hold a strong magnet placed on them even if they are of a low iron meteorite type. This magnet test is one of the first simple tests that can be done at home or in the field to test rocks. Rare earth magnets are inexpensive and available from many sources and are by far the best for this test. But, there are earth rocks that also will hold a magnet. To make matters worse they are quite common. The two most often misidentified rocks are those that contain the minerals hematite and magnetite. These sometimes also look a little like meteorites. Other simple tests can rule these rocks out.
It is a common practice in geology to rub a mineral or rock across a streak test plate and observe the color of the mineral powder line that is made. This is very useful for examining suspect space rocks as well. Meteorites, even old weathered ones will give a brown streak when rubbed on a test plate. The two most commonly misidentified rocks as mentioned earlier are magnetite and hematite. The streak color of hematite is a dark red. In fact the coloring for rouge used in cosmetics was derived from crushed hematite. The streak color of magnetite is black. So even though the stone is responsive to a magnet the mineral's streak and lack of actual metal will reveal it is terrestrial. You do not need to obtain an actual testing plate to make the streak. The unglazed back side of a white ceramic tile will work fine for the test and can be obtained for a few cents at a home improvement store.
The largest group of space rocks is the chondrite type of stone meteorite. These meteorites contain nickel-iron metal in the form of tiny grains scattered throughout the matrix of the rock. If the stone is fresh and has a nice fusion crust it may not be desirable or necessary to cut it or grind off a spot to look for metal grains especially if the stone responds to a magnet. The appearance of the fusion crust may be enough evidence of it cosmic origin. But, if the stone is an old desert rock that is brown but meteorite looking then grinding away a spot with a diamond file is a place to start. Then using a hand lens to look for metal grains is a good field test to determine for sure if it is a meteorite. Earth rocks do not have iron grains in them. Some chondrites will have little metal while others will have much more. But the presence of metal grains is a good indicator that a stone maybe a meteorite. There are man-made rock-like materials such as slags that may have bits of metal but they will not have real solid rock at the same time. The iron metal grains of chondrite meteorites are normally irregular in shape and only rarely are round or oval as in impact-melt meteorites. Because the grains are made of iron which rusts it is common for them to be altered or somewhat absent in old weathered stone meteorites. Usually even very old chondrite stones will still reveal some metal grains upon careful inspection of a cut surface or ground off spot. If you think it is a meteorite and see no metal grains on the small spot you grind with a diamond file it is good to bring the stone home and do more tests on a larger spot or to cut the end of the stone. Especially, if the stone produced a brown powder when ground and holds a magnet. A small number of stone meteorites produce a gray powder when you grind them and it is wise to bring these suspects home as well for further testing if they have other meteorite characteristics.
Nothing is more important in learning to identify meteorites than seeing many images of real meteorites. Over thousands of years of weathering on Earth meteorites change so much it can become hard to recognize them. Fresh meteorites have a lot more clues to help in their recognition. We have many images on our site that will aid in meteorite identification starting with our Stone Meteorites which are divided between the Chondrites, Achondrites, and Carbonaceous Meteorites. Then we have our Iron Meteorites and Stony-Iron Meteorites for more images that can help familiarize the reader with how true meteorites look. Real meteorites are found every year by hikers and hunters and others who spend time out of the city. But, the vast majority of rocks thought to be meteorites are in fact earth rocks of many common types. We do not accept samples of rocks for examination. However, you can find a list of laboratories that do take samples here. Learning the characteristics of meteorites is a challenging task, but it can be accomplished with a little study.
Author: James Tobin [Google]
Copyright © 1996–2015 The Meteorite Exchange, Inc. All Rights Reserved.
The Meteorite Exchange, 4216 Pacific Coast Hwy, #10465, Torrance CA 90505-924415
Users online: 47 anonymous customer(s)