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The meteorites in our collections have a wide range of compositions and histories, but traditionally they have been placed into three broad classes. First are the irons , composed of nearly pure metallic nickel-iron. Second are the stones , the term used for any silicate or rocky meteorite. Third are the rarer stony-irons , made (as the name implies) of mixtures of stone and metallic iron ( [link] ).
Of these three types, the irons and stony-irons are the most obviously extraterrestrial because of their metallic content. Pure iron almost never occurs naturally on Earth; it is generally found here as an oxide (chemically combined with oxygen) or other mineral ore. Therefore, if you ever come across a chunk of metallic iron, it is sure to be either man-made or a meteorite.
The stones are much more common than the irons but more difficult to recognize. Often laboratory analysis is required to demonstrate that a particular sample is really of extraterrestrial origin, especially if it has lain on the ground for some time and been subject to weathering. The most scientifically valuable stones are those collected immediately after they fall, or the Antarctic samples preserved in a nearly pristine state by ice.
[link] summarizes the frequencies of occurrence of the different classes of meteorites among the fall, find, and Antarctic categories.
| Frequency of Occurrence of Meteorite Classes | |||
|---|---|---|---|
| Class | Falls (%) | Finds (%) | Antarctic (%) |
| Primitive stones | 88 | 51 | 85 |
| Differentiated stones | 8 | 2 | 12 |
| Irons | 3 | 42 | 2 |
| Stony-irons | 1 | 5 | 1 |
It was not until the ages of meteorites were measured and their compositions analyzed in detail that scientists appreciated their true significance. The meteorites include the oldest and most primitive materials available for direct study in the laboratory. The ages of stony meteorites can be determined from the careful measurement of radioactive isotopes and their decay products. Almost all meteorites have radioactive ages between 4.50 and 4.56 billion years, as old as any ages we have measured in the solar system. The few younger exceptions are igneous rocks that have been ejected from cratering events on the Moon or Mars (and have made their way to Earth).
The average age for the most primitive meteorites, calculated using the most accurate values now available for radioactive half-lives, is 4.56 billion years, with an uncertainly of less than 0.01 billion years. This value (which we round off to 4.5 billion years in this book) is taken to represent the age of the solar system —the time since the first solids condensed and began to form into larger bodies.
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