Meteorite Recognition

With examples of Meteorites
and "Meteorwrongs"



All kinds of people think they have found a meteorite, and eventually they, a friend or relative bring it to a museum or university for authentication, often after the treasured specimen has sat on a mantelpiece or beside the barn for a year or a decade! This month let's examine a few examples of the real thing...

Stone and iron [97 kb] Kitchener chondrite [10 kb]

1. Left: On the left is an ordinary chondrite, displaying a black fusion crust and a pale interior with traces of terrestrial rust-staining around mm-size metallic flecks of nickel-iron alloy. On the right is a small piece of iron meteorite showing the dimpling typical of many irons.


2. Right: The Kitchener chondrite, a small (200-g) 1998 fall in southern Ontario. Note the large chondrule near the centre, visible in outline beneath the intact fusion crust.



Toronto iron 1 [88 kb] Toronto iron 2 [86 kb]
3. Left: One view of the Toronto iron meteorite, a "find" recognized in 1997 and, for lack of precise information on its recovery, named for the city in which its meteoritic origin was finally verified.

4. Right: A second view of Toronto, a 2.7-kg (6-pound) iron.






"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.



Meteorites have caught the attention of the public in the past decade, in part because of well-publicized episodes such as the 1992 Peekskill fall in upstate New York, which struck a car as it landed, and bright "fireball" meteors which resulted in recovery of freshly fallen meteorite "showers" such as St-Robert (Quebec, 1994) and Tagish Lake (northern British Columbia, 2000). There is also the question of scarcity and value: trade prices of most meteorites have risen sharply in the 1990s, despite the fact that most aesthetically-beautiful specimens are priced beyond the means of humble collectors. An alternative reason for the increased interest resides in the exotic origins of these "space rocks", distant in space and time from our own perspective, on the one hospitable body in the Solar System.

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.
As employed for railway use, slag fragments are typically 3-8 cm in size, and few are larger than 12 cm in maximum dimension. Study these photographs of typical slag samples:

Slag [22 kb]

Slag [18 kb]


Left: a large sample, 15x10x3 cm, courtesy of John Rucklidge (University of Toronto).

Right: sample 1896, smelter slag from Sudbury, Ontario.






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

References

GRIBBIN,JR and GRIBBIN,M (1996) Fire on Earth: Doomsday, Dinosaurs, and Humankind. St. Martin's Press, New York, 264pp.

HUTCHISON,R and GRAHAM,A (1994) Meteorites. Natural History Museum, London, 2nd impression, 61pp.

McSWEEN,HY (1999) Meteorites and their Parent Bodies. Cambridge University Press, 2nd edition, 310pp.

NORTON,OR (1994) Rocks from Space: Meteorites and Meteorite Hunters. Mountain Press Publishing Co., 449pp.

NORTON,OR and CHITWOOD,LA (2008) Field Guide to Meteors and Meteorites. Springer-Verlag London Limited, 287pp.

ZANDA,B and ROTARU,M (editors) (2001) Meteorites, their Impact on Science and History. Cambridge University Press, 128pp.


Graham Wilson, for September 2001, last amended 21 November 2010.

Interested? Please see THIS (updated 2-page review, as 494 kb pdf file)

Visit the Turnstone Meteorite Index

or the broader "Rock of the Month" Archives!

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