|The pallasites are a class of stony–iron meteorite.|
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.
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.
Below you will find descriptions of seven different tests you can do to determine if the rock in question is a meteorite.
Most meteorites contain at least some metal. Do you see the metal shining on a broken surface? If so, you might have a meteorite.
Density - Those meteorites that do have a lot of metal tend to be very dense compared to regular rocks. Do you have something very dense such that it could be a meteorite? But remember that not all meteorites are dense.
Magnetic Properties - A lot of meteorites contain shiny iron-nickel metal grains or consist largely of iron-nickel metal. The iron in the metal attracts a magnet. Is a magnet attracted to the surface of your sample? If so, you might have a meteorite. But remember that a lot of normal rocks on the Earth are also magnetic. So, just because something is magnetic, it doesn't mean that it is a meteorite.
Some primitive meteorites have little round pieces of stony material in them. These little round pieces are called chondrules. Some sedimentary and volcanic rocks can have spherical particles that look somewhat like chondrules. Does your sample contain chondrules? If it does, you might have a meteorite.
5. Fusion crust
When a meteorite is falling through the atmosphere, it begins to heat up because of the extreme compression of the atmosphere. The meteor gets so hot that the outer surface begins to melt, which produces a thin black/brown coating on the surface of the rock called a fusion crust. Iron meteorites may show evidence of melted metal on their surface, but this is less common. Fusion crusts are present on freshly fallen meteorites, but the crusts are fragile and can weather away from samples that fell a long time ago. Small patches of fusion crust can sometimes remain in hollows of the sample. Does your sample have a fusion crust? If so, you have a meteorite.
6. Regmaglypt texture/thumbprints
When the surface of the meteorite begins to melt during entry into the atmosphere, some areas of the meteorites are eroded by the melting more than others, almost like someone is taking little scoops of material out. This leaves a bunch of small dents in the surface of the rock, making it look like someone put thumbprints into clay. The surface of most meteorite samples have these thumbprints called “regmaglypts,” which can vary in size from less than a centimeter up to as much as 10 centimeters. Does your sample have Regmaglypt texture/thumbprints? If so, you have a meteorite.
Most meteorites won’t leave a streak, but the surfaces of some meteorites might leave a reddish streak if they have been oxidized (rusted). If you drag your sample across this “streak plate,” and it leaves a red/orange line, then the sample is probably a common mineral on the Earth called hematite. If the sample is magnetic and leaves a black or gray streak, then it might be the common terrestrial iron-oxide mineral called magnetite. Does your sample cause a streak on a “streak plate?” If not, you may have a meteorite.
The above post is reprinted from Materials provided by NASA’s Dawn Mission.