If you were out prospecting on the side of a hill, and stumbled over a meteorite, would you recognize it? Probably not. Although the earth’s crust is sprinkled with these rare prizes, few of them ever are ever recovered. Like many other rare stones, they have a very drab exterior and the average person on a field trip would not give them a second glance. While it is seldom possible to identify a meteorite by looking at its drab exterior, there are certain characteristics which if understood, will at least give the finder a clue.
Meteorites are of two principal varieties, stony and nickel-iron. A stony meteorite consists mainly of rock material, but in most cases has numerous small grains of nickel-iron distributed all through it. The nickel-iron meteorites are almost solid nickel-iron or steel. These latter are about three times as heavy as ordinary rocks of the same size. The stony meteorites are about one and a half times as heavy as ordinary rocks of the same size. However, there are several kinds of terrestrial rocks or minerals which are quite as heavy, or even heavier, than the stony meteorites. Some of these are magnetite, hematite, limonite, pyrite, chalcopyrite, galena, and certain varieties of basaltic lava. These should not be confused with meteorites.
Another feature of all meteorites is the peculiar crust, covering their surfaces and known as the fusion crust, which has been formed by melting at the surface as they plunge through the atmosphere. When a large shooting-star blazes across the sky, leaving in its wake a train of fire, this train consists of a stream of sparks, which are molten bits or detached fragments of the meteorite, stripped off by the resisting air. This powerful resistance finally slows down the meteorite to a point where it ceases to burn. At this moment the mass is naturally covered with a layer of this molten or fused material which solidifies and becomes the hard black crust which one sees on freshly fallen meteorites.
Stony meteorites contain lumps of material which are harder to melt than the remainder of the mass. These metallic grains and other hard lumps give the stone an uneven surface. This un-eveness shows under the fusion crust, making the meteorite appear much as a lump of concrete would look if it were given a coat of paint. Someone has likened the surface of a brown stony meteorite to the crust of cracked wheat bread. It is a very good comparison. This peculiar crust is one of the best marks by which to recognize meteorites.
There is a third group which is intermediate between the stony and the metallic; these are known as stony-iron or iron-stony meteorites. They consist of about equal portions of stony and metallic material. In these the nickel-iron may be arranged in a network of irregular bands or it may be in the form of more or less disconnected masses embedded in a stony matrix. These metallic constituents may be in the form of very small grains or in larger lumps.
Meteorites are dark in color. If lately fallen to the earth they are usually covered with a black crust, regardless of the color of their interior. In the majority of cases the interior is gray and cement-like in appearance, but they are sometimes dark inside as well as outside.
After meteorites have lain on or in the soil for a few years the black crust is changed to a rusty brown color by the formation of iron rust (oxides of iron). Since meteorites do not fall frequently in any locality, it naturally follows that the majority of those to be found are of this brown color and not black as described above.
Meteorites vary greatly in shape. In fact, they are usually characterized by two peculiarities which help in their identification. First, they seldom have sharp corners or edges. This is because the extreme heat of friction while they are coming through the atmosphere flames off any sharp points or edges, leaving them dull and rounded, just as when a broken piece of ice is passed through a flame. Second, they are irregular in form and they seem always to present the appearance at first sight of an individual, isolated and complete in itself. Upon closer examination, however, they usually show evidence of having been broken from a larger mass and subsequently fused over. Occasionally a meteorite is definitely cone-shaped. This is due to its having traveled in a straight forward position without turning over. The majority of meteorites, however, travel in a tumbling fashion and do not have a chance to be carved into a symmetrical form.
In many meteorites there is another characteristic which is quite useful for identification. These are referred to the peculiar pittings or “thumb marks” which indent the surface of many. These pits are caused by the unequal melting of the substance of the meteorite, in combination with the violent air blast which drives off the molten material as fast as it is formed. The pits are shallow, with rounded edges. They are of various shapes and sizes but often appear like marks made by pressing the finger tips into plastic clay. After one becomes familiar with these markings, they may attract attention to specimens which would otherwise escape notice. It should be mentioned, however, that pittings which are very similar are often produced by running water, with its load of sand and gravel, and by a sand-blasting action of the wind in desert regions. A careful inspection will reveal that the fusion crust is absent from such specimens.
For the great majority of stony meteorites, chondrules are an important identification mark. These are rounded bodies of various sizes which differ from sand grains and from all other rock structures to some extent. They resemble oolitic structures more closely than any other feature of terrestrial rocks. Generally, chondrules are of many different sizes in the same stone. They may be so small as to require a lens to see them, or they may be several millimeters in diameter. A common size is about like a pin head. The color is usually white or gray but sometimes brown to black.
In some meteorites, these chondrules are loosely embedded in a fragmental matrix so that on a broken surface they stand out conspicuously. In others, they break with the matrix, in which case it is often necessary to polish a small surface in order to see them. A 10-power hand lens is very useful in looking for small chondrules. While the great majority of known stony meteorites are chondritic there are several varieties which are not. Therefore, one cannot depend on this feature alone.