Meteorites: What is a Meteorite?

Most people are familiar with the term "shooting star," but few know its importance. Actually, it is not a star shooting across the sky, but a small piece of solid matter called a meteoroid colliding with the atmosphere. As the meteoroid enters the Earth's atmosphere, the friction created by its incoming velocity causes its surface to heat up, and the brilliant flash of light records the passage of a meteor. Should the object survive this firey plunge through the atmosphere and hit the ground, it then becomes a meteorite. On very rare occasions when an extremely bright meteor is observed, it is referred to as a fireball. It is from these fireballs that most meteorites of recoverable size originate.

The arrival of a meteorite is a totally unpredictable event. Meteor showers are regular annual occurences, but have never produced a recorded meteorite fall. When a large fireball is observed, recovery of specimens is almost solely dependent upon the accounts of chance observers who just happened to see the event. Even rarer is the observed impact of a meteorite on someone's roof or in a backyard. Meteorites recovered in this manner are termed falls, indicating that the specimen was observed while falling. The majority of meteorites are recorded as finds, those specimens which were not observed to fall. Finds are generally reported by people who happen to pick up a strange looking rock and later have it identified as an actual meteorite.

Classification of Meteorites

Meteorites are classified into three main groups because of their particular mineral compositions: irons, stony-irons, and stones. Mineralogically, meteorites consist of varying amounts of nickel-iron alloys, silicates, sulfides, and several other minor phases. Classification is then made on the basis of the ratio of metal to silicate present in the various compositions. No two meteorites are completely alike, and specific compositional and structural features give a particular meteorite its unique identity.

Iron meteorites are characterized by the presence of two nickel-iron alloy metals: kamacite and taenite. These, combined with minor amounts of non-metallic phases and sulfide minerals, form the three basic subdivisions of irons. Depending upon the percentage of nickel to iron, these subdivisions are classified as:

hexahedrites (4-6% Ni)
octahedrites (6-12% Ni)
ataxites (12+% Ni)

Octahedrites, which are the most common type of iron meteorite, exhibit a unique structural feature called the Widmanstatten pattern when etched with a weak acid. This unique crystal pattern is the result of the combination of the two nickel-iron minerals kamacite and taenite being present in approximately equal amounts.

Stony-irons consist of almost equal amounts of nickel-iron alloy and silicate minerals. Although all stony-irons may not be genetically related or have similar composition, they are combined into one group and divided into two subgroups for convenient classification. The Pallasite group is characterized by olivine crystals surrounded by a nickel-iron structure which forms a continuous enclosing network around the silicate portion. Mesosiderites, on the other hand, consist mainly of plagioclase and pyroxene silicates in the form of heterogeneous aggregates intermixed with the metal alloy. No distinct separation between the metal and silicate phases is readily apparent as it is with the Pallasites.

Stony meteorites are the most abundant of the three meteorite groups and come closest to resembling earth rocks in their appearance and composition. The major portion of these meteorites consists of the silicate minerals olivine, pyroxene, and plagioclase feldspars. Metallic nickel-iron occurs in varying percentages and is accompanied by an iron-sulfide mineral. Aside from being the most abundant meteorite type, stony meteorites have the greatest variety in composition, color, and structure. One particular structural feature called chondrules divides the group into two main subgroups:

Chrondrites, those with chondrules
Achondrites, those without chondrules

Many scientists believe that these small, rounded, nearly spherical chondrules may represent the most primitive material in the solar system.

What do Meteorites Look Like?

As compared to Earth rocks, meteorites have several features which can be used to establish their extraterrestrial origin. The surface of a meteorite is generally very smooth and featureless, but often has shallow depressions and deep cavities resembling clearly visible thumbprints in wet clay. Meteorites which have fallen recently may have a black "ash-like" crust on their surface. This provides evidence of their flaming entry through the atmosphere. However, this crust weathers to a rusty brown color after several years of exposure on the Earth's surface and will eventually disappear altogether.

The size of a meteorite will vary from microscopic to a very large mass several feet in diameter. Most recovered meteorites measure between 2 inches and 2 feet in diameter. The largest meteorite ever discovered is still in the ground in South Africa because it was too large to move. The shape of a meteorite is seldom round; they are usually very irregular in appearance and come in a variety of different shapes and sizes. Unusual weight is one of their more characteristic features. Iron meteorites are generally 3.5 times as heavy as ordinary Earth rocks, while stony meteorites are about 1.5 times as heavy.

Stony meteorites are not as obvious as the irons because they often resemble a common rock called basalt. They also contain metal as do the iron meteorites but in much smaller amounts which appear as small fragments within the surrounding rocky material. Very often this metal can be seen where a chip of the meteorite has been broken off. Because of their metal content most, but not all, meteorites will be strongly attracted to a magnet. This can be a good clue to the meteorite's true identity.

Several Earth materials are easily mistaken for meteorites. Furnace and smelter slag often look like meteorites, but because of their generally light weight and porous texture they can easily be distinguished from meteorites.

Iron ore minerals, such as hematite and magnetite, can also be confused with meteorites, but a simple laboratory test for nickel will determine their true nature. Earth rocks have either very small or very large amounts of nickel, but in meteorites the nickel content falls within a very specific range.

Why Study Meteorites?

Prior to the Apollo moon landings, meteorites supplied the only extraterrestrial materials available for study. In their basic characteristics, meteorites represent materials which formed in a region of the solar system far removed from Earth. They represent travelers from both space and time, and much can be learned from them about the conditions that existed when the solar system was formed. Meteorite studies have also provided spacecraft designers an economical means to observe the effects of high speed aerodynamics without the costly expense of experimental re-entry vehicles.

When scientists study a meteorite they look for clues to its origin by studying its mineral composition. Through the use of special microscopes and high magnification we can learn about the conditions in which the meteorite formed. A chemist can examine a meteorite's chemical composition and determine from where in the solar system it came, how long it has been in space, and its age since formation.