Meteorites: Stones from the Sky

There are three basic origins of meteorites. This leads to a classification of meteorites into three types: stony, stony-iron, and iron. Meteoriticists recognize many more types of meteorites, and have reconstructed a marvelously detailed history of the solar system from their subtle differences.

The most easily recognized meteorites are the iron meteorites. Since even a casual examination shows that they are not ordinary rocks, they tend to be very common in collections. However, they are rare in space. They are heavy and, except for a thin crust (made by the melting of the surface by the passage through the atmosphere), they look and feel like metal. Chemically, they are mostly iron with a few percent nickel and a little cobalt. When sawed in half and polished, they display a geometrical pattern called a Widmanstatten pattern (see figure at the left which shows an iron found in Henbury, Australia). This pattern is the result of the meteorite cooling very slowly under very high pressure. Iron meteorites were once the cores of larger, differentiated bodies, most likely asteroids. Because of differentiation, these large asteroids developed an iron core and a stony outer mantle. Between the core and the mantle was a stony-iron region, more iron toward the core, more stony toward the mantle. Collisions in the asteroid belt break up asteroids, sending particles into the inner solar system. Occasionally one of the bits runs into the Earth and falls as a meteorite.

The iron meteorites we are handling in class are samples of the cores of worlds formed out beyond the orbit of Mars.

The most common meteorites that fall to the Earth are called stony meteorites. Many are from the outer parts of an asteroid that was destroyed by collision, but some are pieces of material that was never part of a much larger body. Meteorites that come from such a small, undifferentiated body are called primitive meteorites. The stony meteorites vary in appearance: some are light, some dark, some are coarse grained, some fine grained. Chemically they are also diverse; however, they all have a telltale composition that tells us that they are definitely not from Earth. Their diversity and the fact that they tend to look like ordinary rocks to the untrained eye means that stony meteorites are difficult to recognize in the field. Unless someone sees them fall, they usually go unnoticed. Therefore, although stony meteorites are the most common type out in space, they are rarer than irons in collections on Earth. The stony meteorite shown in the figure at right is from Silverton, Texas.

Pieces of material from the boundary zone between the cores and mantles of the now-destroyed asteroids are also found. These very rare meteorites are called stony-iron meteorites. They tend to look like irons with pieces of stone in them, or stones with pieces of iron.

This stony-iron meteorite was found in Kansas.	This stony-iron meteorite was found in Dalgaranga, Western Australia.

An especially important type of meteorite is the carbonaceous chondrite. These are stony meteorites of a very special kind, usually black or dark gray in color. They are rich in the element carbon (thus their black color) and they contain small spherical droplet-like inclusions called chondules. They are among the most unchanged (primitive) objects in the solar system, having survived literally untouched for 4.6 billion years. It has been recently learned that some chondrules were formed outside of our solar system and thus were around long before our solar system was even formed. So, not only are carbonaceous chondrite meteorites an important probe into the early history of our solar system, but they may supply us with materials from beyond our solar system. Although carbonaceous chondrites are fairly abundant among meteorites that fall to the Earth, they look enough like Earth rocks that they are rare in collections. They also weather very easily and do not survive long on the surface of the Earth.

This is a section of the most famous carbonaceous chondrite, the Orgueil, found in France. It is one of the most studied meteorites; some amino acids (the "building blocks of life") are found in this meteorite. Notice the extremely black color.	This is a section of a carbonaceous chondrite found in Allende, Mexico. The small white specks are chondrules. Most of the chondrules are roughly spherical in shape.

The tables in the lab have a number of meteorites of these different types. Examine them carefully with the idea that afterwards you will be identifying meteorites for which the types are not going to be given. Pay particular attention to the densities, colors, fusion crust, chondrules, and metallic materials. On this page, write down the characteristics of the different types of meteorites that will allow you to identify similar ones later.

Meteorite Type	Location of Find	Specific Characteristics
		Density	Colors	Chondrules?	Metallic?	Other
Iron	Canyon Diablo
	Odessa
	Henbury
	Guadalupe
Stony	Wellman
	Dalgety Downs
	Millbillillie
	Cocklebiddy
Stony-Iron	Dalgaranga
	Brenham
Carbonaceous Chondrite	Allende (1)
	Allende (2)

 

Astronomy 150	Name ______________________________

Meteorite Quiz

Now that you have had the opportunity to look over the various types of meteorites, you should be able to apply your newly found knowledge to a possible real-life situation.

A good friend of yours has learned that you are taking Astronomy 150 and excitedly invited you over to take a look at his new meteorite collection. The collection has cost him one-month's salary, but he thinks it is worth it. He lays out the six meteorites on the table in front of you and asks you to identify what he has just purchased. Write down what type of meteorite you think each one is and why. What characteristics of each rock lead you to classify it as you did? Unfortunately, you have very bad news for your friend; you suspect that one or more of his purchases are not meteorites at all, but are terrestrial rocks. If you think the sample is not a meteorite, then just write down "rock" as its type.

Unknown	Meteorite Type	Reason for this Identification
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