METEORITE BASICS: STONES

Most stone meteorites are composed largely of minerals which are common on the earth, such as olivine, pyroxene and feldspar. All three of these are silicates, the same minerals commonly found in volcanic lavas on earth. Based on their texture, stone meteorites are divided into two types, the chondrites and the achondrites.

The chondrites are so-named because they contain small BB-like, glassy spheres, called chondrules. These chondrules, unique little bodies never found in earthly rocks, appear to have formed very early as the Solar System, with its Sun and planets, was taking shape. For reasons not well known, these molten droplets formed within the "dust cloud" that was to be our future Solar System. These chondrite meteorites are often referred to as "primitive" because these relics of earliest times have orbited unchanged in space for 4.5 billion years before coming to earth.

The much rarer achondrites are so-named because they do not contain chondrules. These achondrites have experienced major changes, unlike the unchanged and "primitive" chondrites discussed above. In most cases, melting of the entire rock occurred at some previous time so that any chondrules which were present then disappear. The same chemicals are still present but now the BB-like chondrules are replaced by a texture composed of a mosaic of interlocking crystals. These new textures are similar to igneous rocks, formed on earth, because they crystallized from a liquid state. So where are achondrites formed? The answer is in large asteroids and planets. Many asteroids grew large enough and had sufficient radioactive fuel so that wholesale melting occurred even though they didn't reach the size of planets. The largest asteroid known, by the way, is about 600 miles in diameter. To recap, imagine an asteroid composed of chondrules and "primordial dust" which grew large enough to melt, thereby forming a crystallized achondrite on cooling. In addition to the asteroids, our known Mars and Lunar meteorites are all achondrites because they originated on large bodies where volcanic activity occurred, similar to that on earth.

[left] Leedey, a stone meteorite which fell in Dewey County, Oklahoma, in 1943