"Rock of the Month # 3, posted September 2001" --- four images, a "special" presentation to mark the annual meeting of MIAC, the Meteorites & Impacts Advisory Committee to the Canadian Space Agency. Photographs by Karyn Gorra, Department of Geology, University of Toronto.

Although no one set of features will define all meteorites, these examples illustrate ordinary chondrites and irons, the broad families of meteorites which a lucky finder is most likely to encounter. In brief, some of the most useful pointers are as follows:

• 1. A thin black fusion crust, like a thin glassy glaze, covers many meteorites, and is most prominent on many of the stony types, where it forms a sharp contrast to the pale interior typical of common chondrites (photo 1 - left side: photo 2). The fusion crust may show fine lineations indicative of flow of liquid glass along the face of the meteorite tumbling in the atmosphere, rather like rain running down a car's windshield.
• 2. The interior of the most common stony meteorites often contains small (1 mm or less) flecks of silvery metal and bronzey sulphide, often stained by rust on weathered samples.
• 3. Broken interiors of many of the most abundant class of meteorites, the chondrites, contain pale rounded structures which are mostly less than 3 mm across, but exceptionally larger than 5 mm. These "chondrules" (photo 2) are spherical aggregates of silicate minerals, thought to have formed in the solar nebula shortly before the origin of the Earth and the other planets (meteorites are old!).
• 4. Iron meteorites are intensely magnetic, while most but not all stony meteorites are somewhat magnetic due to the disseminated blebs of metal. Iron meteorites are very dense, and may weigh three times as much as similar-sized pebbles of common Earth rocks, such as limestone or granite.
• 5. The surfaces of many iron meteorites are variably indented (photo 1 - right side: photos 3, 4) with rounded depressions. These rather resemble the marks of a thumb or finger in putty.
• 6. It is worth noting the absence of other features, such as abundant bubbles, often concentrated along particular planes, and coarse, ropy flow textures, more like a lava flow than the delicate features noted in (1) above. Both these textural features are typical of blast-furnace slag, which often has a reddish surface tinge, and is commonly found in abundance as ballast along railway lines.

Anyone who thinks that they may have found a meteorite is encouraged to take their find to a museum or university department with expertise in geology, astronomy or related branches natural history, for verification. Although the proportion of "found" samples that turn out to be meteorites is usually small, less than one in a hundred, the "treasure" is sometimes notable for other reasons, which probably made the sample sufficiently unusual to attract the finder's eye and awaken a sense of curiosity. As a caveat, novice finders of potential meteorites should study the cautionary tale of Randy Korotev, of Washington University in St. Louis.

Here are five modern books which offer distinct, informative introductions to meteorites and related topics, including tektites and impact events.

References