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Illustration by Timothy Arbon
HAVE YOU FOUND A SPACE ROCK?
AN INTRODUCTORY GUIDE TO METEORITE IDENTIFICATION
The third in a series of articles by Geoffrey Notkin, Aerolite Meteorites
Meteorwrong: Slag—sometimes called cinder or runoff—is a by-product of metal smelting and usually consists of a conglomerate of metal oxides. Slag is one of the substances most commonly mistaken for meteorites, as it appears burned and melted on the surface and often sticks to a magnet due to its high iron content. It is used in road and railroad building, as ballast, and even in the manufacture of fertilizer. In other words, it is all over the place. Take special note of the vesicles—small holes and cavities created by escaping gases. Vesicles are not found in meteorites, so an experienced eye will immediately identify this as a meteor-wrong. The scale cube pictured is 1 cm. Photo by Geoffrey Notkin, copyright Aerolite Meteorites. Click to enlarge.
How Rare Are Meteorites?
One of my happy tasks as a meteorite hunter is running a web site that
specializes in my favorite subject. We receive hundreds of thousands
of visitors each year, and I try to maintain a fair balance on the site
between education, photographs and reports about our expeditions, and
commercial sales of meteorites. One of the most frequently visited
sections of the site is a detailed guide to meteorite identification.
As a result of that guide we receive, almost daily, inquiries by letter
and email from hopeful individuals who think they may have found a rock
from outer space.
Meteorites are among the rarest materials that exist on our planet-far
less common than gold, diamonds, or even emeralds. So, the chances of
discovering a new example are slim-even for those of us who make their
living hunting for, and studying, meteorites. I do spend a significant
amount of time each year assisting people who think they may have
found the real thing, but the odds are against it. Out of the many
hundreds of suspected space rocks sent to us for testing, far less
than one percent turn out to be genuine visitors from outer space.
Stone meteorite with fusion crust: This 307.1-gram stone meteorite fell as part of a shower on October 16, 2006 in Mauretania. It is an ordinary chondrite (H5) and an excellent example of a complete fusion crusted stone. This specimen was picked up immediately after the fall. Note the very fresh, rich black fusion crust which is reminiscent of a charcoal briquette. Fusion crust is thin and fragile and will weather away over time, so a recently fallen stone will exhibit a dark black crust with no weathering or rust stains. Photo by Geoffrey Notkin, copyright Aerolite Meteorites. Click to enlarge.
What Are Meteor-Wrongs?
A specimen that is thought to be a
meteorite, but turns out instead to be a common earth rock is
affectionately and humorously dubbed a meteor-wrong. The surface of
our planet is rich in terrestrial iron oxides such as magnetite and
hematite (many of which will stick to a magnet), dark black rocks
such as basalt, and many different types of man-made metallic by-products
such as runoff (slag) from old smelters, and castoff iron implements that
have corroded over time. All of these materials are frequently mistaken
for meteorites. Identification of a genuine meteorite takes a practiced
eye, but there are a number of simple tests that can help hopeful rock
hounds determine if they have stumbled across a rare space rock, or
just a common earthbound stone.
Iron meteorite with flow lines: This close-up image of the main mass of the Bruno iron meteorite (found near Bruno, Saskatchewan, 1931) shows a delicate and intricate pattern of flow lines, created as the surface of the meteorite literally melted and flowed. Flow lines may be found on the surface of irons, stones, and stony-irons but, like fusion crust, they are fragile and may disappear over time, due to the processes of terrestrial erosion. Actual size of area pictured is approximately 10 cm across. Photo by Geoffrey Notkin, copyright Aerolite Meteorites. Click to enlarge.
Visual Identification of Meteor-Wrongs
Meteorites tend to look different from the ordinary terrestrial rocks around them.
They do not contain the common earth mineral quartz, and in general do not contain
vesicles. When gas escapes from cooling molten material, it creates small pinprick holes or
cavities in a rock's surface. The volcanic rock pumice, often used in skin care
for the removal of callouses, contains vesicles which is one of the reasons it is
very light in weight. If a suspected meteorite looks like a sponge, with lots of
tiny holes, it is probably volcanic rock or slag of earthly origin.
Iron meteorite - Campo del Cielo: This beautiful 654.9-gram Campo del Cielo iron meteorite was found in Chaco Province, Argentina. It is one of the world's oldest-known meteorites and was first discovered by the Spanish in 1576. This example displays excellent regmaglypts (thumbprints), as well as a rare natural hole. This specimen is also oriented. Its leading edge (pictured) is dome-shaped and heavily thumbprinted. The trailing edge is smooth and slightly concave. This specimen measures 114 by 78 mm. Photo by Geoffrey Notkin, copyright Aerolite Meteorites. Click to enlarge.
The Magnet Test
Meteorites are divided into three basic groups: irons, stones, and stony-irons.
Practically all meteorites contain a significant amount of extraterrestrial
iron and nickel, so the first step in identifying a possible meteorite is the
magnet test. Iron and stony-iron meteorites are rich in iron, and will stick to
a powerful magnet so strongly that it can be difficult to separate them! Stone
meteorites also, for the most part, have a high iron content and a good magnet
will happily adhere to them. Many earth rocks will also attract a magnet, so
this is not a definitive test, but it's a good step in the right direction.
Lunar and Martian meteorites, and most achondrites (stone meteorites without
chondrules) contain little or no iron and even a powerful magnet will generally
have no effect on them. However, these meteorite types are so extremely rare that,
as a general rule, we discount specimens that will not adhere to a magnet.
Meteorite analysis laboratory: A partial view of the impressive Ion Beams for Analysis of Materials (IBeAM) facility at Arizona State University in Tempe. This remarkable device allows specialists to study the composition of suspected meteorites (and other materials) in great detail. A small specimen is placed in a chamber and then bombarded by accelerated ions. The results appear on an adjacent computer screen in seconds. The author gratefully acknowledges the generous assistance of the ASU IBeAM Facility during the preparation of this article. Photo by Geoffrey Notkin, copyright Aerolite Meteorites. Click to enlarge.
Weight and Density
Iron is heavy and most meteorites feel much heavier in the hand than an ordinary
earth rock should. A softball-sized iron meteorite will likely weigh five or
six pounds, making it seem unnaturally dense. Imagine holding a steel ball
bearing as big as a grapefruit and you'll get the idea.
|More About Meteorite Identification|
|If you would like to learn more about meteorite identification, and discover
how to perform some other simple tests at home, please visit The Aerolite
Guide to Meteorite Identification. Meteorites are very valuable both to the
scientific community and to enthusiastic collectors. So, if you think one
landed in your backyard, be sure to get it checked out!
Visual Identification: Fusion Crust
When a meteoroid (a potential meteorite) streaks through our atmosphere,
tremendous heat is generated by atmospheric pressure. The surface of the rock
melts and the air around it incandesces. As a result of this brief but intense
heating, the surface burns and forms a thin, dark rind called fusion crust.
Meteorites literally began to burn up in our atmosphere, so they tend to appear
darker than the terrestrial rocks around them. Desert varnish forms on the surface
of some earth rocks, particularly in arid areas, and can easily be mistaken for
fusion crust by an untrained eye. True fusion crust does not occur on earth
rocks. It is delicate and will weather away over time, but a freshly fallen
meteorite will exhibit a rich black crust, much like a charcoal briquette.
Chondrite meteorite: A prepared end section of the ordinary chondrite Northwest Africa 869 (L4-6, found Tindouf, Algeria, 2000) displays a wealth of colorful grain-like chondrules and multiple tiny flakes of extraterrestrial nickel-iron. The specimen pictured weighs 38.3 grams and measures 60 by 33 mm. Chondrites are the most abundant meteorite group and take their name from the ancient chondrules they contain.
Photo by Geoffrey Notkin, copyright Aerolite Meteorites. Click to enlarge.
Visual Identification: Regmaglypts
Regmaglypts, popularly known as thumbprints, are oval depressions-often about the
size of a peanut-found on the surface of many meteorites. These indentations look
much like the marks a sculptor might make with his fingers on a wet lump of clay,
hence their name. Regmaglypts are created as the meteorite's outer layer melts
during flight and they are another feature unique to meteorites.
Visual Identification: Flow Lines
As our typical meteorite burns through the atmosphere, its surface may melt
and flow in tiny rivulets known as flow lines. These patterns formed by flow
lines can be minute, often thinner than a strand of human hair, and they are
one of the most unique and intriguing surface characteristics of meteorites.
Chondrules and Metal Flakes
Stone meteorites known as chondrites are the most abundant meteorite type. They are
composed largely of chondrules, which are miniscule, grain-like spheroids, often of
differing colors. Chondrules are believed to have formed in the solar disk before the
planets in our solar system and are not present in earth
rocks. Chondrites are also typically rich in metal flakes of iron-nickel, and shiny
blobs of this extraterrestrial alloy are often visible on their surfaces, though you
may need a hand lens to see them. A simple test involves removing a small corner of a
suspected stone meteorite with a file or bench grinder and examining the exposed face
with a loupe. If the interior displays metal flakes and small, round, colorful
inclusions, it may well be a stone meteorite. Please see the accompanying photographs
for illustrations of these and other features.
Lab Testing of Meteorites: Nickel
Nickel is rare on earth but almost always present in meteorites.
If a suspected meteorite passes the magnet test and looks promising
following a visual inspection we may elect to conduct a test for nickel.
Assay labs can perform an analysis of the nickel content for a few dollars,
but it is necessary to cut off a modest sample in order to perform such a
test. Some labs and universities with meteoritics departments can perform
more sophisticated tests without damaging a specimen. I recently had the
pleasure of visiting the Ion Beams for Analysis of Materials (IBeAM)
facility at Arizona State University in Tempe. ASU curates the world's
largest university-based meteorite collection and they also utilize some
of the most hi-tech meteorite identification equipment available today.
The IBeAM uses accelerated ions to determine, with great accuracy, the
composition of samples. In simple terms, that means we can discover the
chemical makeup of a specimen without cutting it up on a diamond saw.
The results appear on a computer screen within a few seconds, and a
compositional analysis showing somewhere between three and ten percent
nickel will almost certainly indicate an authentic meteorite.
About the Author
|Photograph by |
Leigh Anne DelRay
Geoffrey Notkin is a meteorite hunter, science writer, photographer, and musician. He was born in New York City, raised in London, England, and now makes his home in the Sonoran Desert in Arizona. A frequent contributor to science and art magazines, his work has appeared in Reader's Digest, The Village Voice, Wired, Meteorite, Seed, Sky & Telescope, Rock & Gem, Lapidary Journal, Geotimes, New York Press, and numerous other national and international publications. He works regularly in television and has made documentaries for The Discovery Channel, BBC, PBS, History Channel, National Geographic, A&E, and the Travel Channel.
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