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8.18 Types of Meteorites

Thousands of rocks are mistakenly identified as meteorites every year. Many people assume that every odd-looking rock they find is a meteorite. The truth is, meteorites are not only the rarest type of rock, but they often appear quite normal. There are some ways you can recognize a meteorite just by looking at it, but in general, a trained scientist needs to test a piece of the rock in a laboratory to determine if it really has an extraterrestrial origin. Of course, if you actually witness a rock fall from the sky, it's most likely a meteorite. This is called a meteorite "fall" as opposed to a meteorite "find" (which is when a meteorite is discovered after it's already on the ground.) 

The most obvious characteristic of a freshly fallen meteorite is its fusion crust. This is a shiny, dark brown or black layer on the outside of a meteorite. Because of their relative orbital motions, interplanetary bodies collide with the Earth at very high speeds, usually 11 to 60 kilometers per second (24,000 to 134,000 mph). At these speeds, the impacting material is heated by friction with the air, and the outer layer of a meteorite will melt. Blobs of liquid rock ooze off as it travels through the atmosphere, leaving characteristic thumbprint-shaped hollows. Dust grains to pea-sized pieces of material burn up before striking the ground, but larger pieces may not break apart and burn up completely — some fragment (or fragments!) may reach the ground. Since meteorites pass through the atmosphere too quickly for their interiors to be strongly heated, stories of meteorites remaining red-hot for hours after falling are untrue. Instead, the melted outer layer cools quickly,forming a thin, glassy fusion crust. Meteorites that have been lying on the Earth's surface for an extended time often lack a fusion crust, because the thin, fragile layer erodes away easily.

Most meteorites belong to the group of stony meteorites, and the majority of these are chondrites. Chondrites are the most primitive, ancient rocks in the Solar System. They are named after the tiny spheres of rock they contain, called chondrules. If you look very closely at these meteorites, you will see the millimeter-sized round balls sticking out of the surrounding rock. These chondrules were originally free-floating droplets of molten rock in the solar nebula. They cooled and solidified quickly, and were later incorporated into larger bodies that became asteroids. These chondrites have survived since the early Solar System, meaning the asteroids they were originally a part of (their parent bodies) never completely melted or differentiated into layers. Because the metal was never separated out into a core, bits of metal are mixed throughout some chondrites. Some other types of chondrites contain high amounts of dark carbonaceous material. Some of these rocks contain organic material (carbon based molecules), as well as evidence that they were exposed to liquid water in the past. Liquid water could have been in the form of impacting craters.

Some stony meteorites are not chondrites. They are pieces of crustal rock from a parent body that melted and differentiated into separate layers. Some of these are from large asteroids with enough internal heat to drive volcanism, and some are from other planetary surfaces. When geologists studied the lunar samples returned to Earth in the 1960s, they were able to recognize a dozen mysterious rocks that had been found lying on Antarctic ice flows — rocks that did not match any known terrestrial geology. They were Moon rocks, too! This was proof that material could be blasted off the Moon's surface by a meteoric impact, drift through space, and then fall to Earth. Martian rocks can reach our planet by the same scenario. Since we realized this, we've found about fifty Martian meteorites. At first glance, these meteorites are difficult to differentiate from terrestrial rocks, because they appear so similar. They're a result of the same geological processes, so they have most of the same minerals and textures as Earth rocks. 

Iron meteorites are rare, but they are much more easily recognized. They are very dense alloys of almost pure iron and a small amount of nickel. When the metal solidified, it formed crystals. These crystals are visible in a distinctive Widmanstätten pattern when an iron meteorite's polished surface is etched with a weak solution of acid. These meteorites are also samples from a differentiated body — in this case, the metallic core. At some point after the parent body separated into a core, mantle, and crust, it experienced a collision big enough to disrupt it. Some fragments that were originally in the core were thrown into Earth-crossing orbits, to eventually land on the surface of our planet.

The last, and least common, type of meteorite is a combination of stone and iron. The stony part of these meteorites usually consists of a mineral called olivine, which is the same mineral that makes up the Earth's mantle. These rocks are also samples of a differentiated asteroid, presumably from the boundary between the metallic core and the olivine mantle.

Not all meteors can be clearly placed in one category or another. Brecciated meteorites can be mixtures of any of the above types of meteorites. They're formed when fragments of rock, often radically different types, are jumbled together and welded by pressure and heat into one rock. Such fragmentation and welding is caused when two asteroids collide, further proof of the violent history of many asteroids. Because two different types of asteroids may be jumbled together during the collision, the brecciated meteorites can contain several kinds of rock.

Park Forest Meteorite (large individual with dark fusion crust) - the only rock from space to impact in a modern urban area is the Park Forest Meteorite. Click here for original source URL.