IRON METEORITES
Iron meteorites are composed almost entirely of nickel-iron with trace amounts of carbon, sulfur and silicates. They are believed to be from the core of a large asteroid (though some are formed by impact melting and metal segregation of surface rocks). Iron meteorites show varying textures depending upon their nickel content. Though they are quite common in our collections, iron meteorites make up only about 5% of all meteorite falls.
Hexahedrites: Lowest in nickel content (6% or less) these are composed almost entirely of the mineral kamacite. They show weak scratch-like lines on acid etching called “Neumann lines” which form by shock in a hexahedral structure.
Octahedrites: These have a nickel content of about 7% to 10%, and are composed of a mix of kamacite and taenite that crystallized in an octahedral structure. This is seen after acid etching as the beautiful “Widmanstatten” structure, which is unique to meteorites. They are broken down into fine, medium, coarse, etc, depending upon the width of the bands in the etch structure (higher nickel content = narrower bands).
Ataxites: These are the highest in nickel content (as high as 30% or more), and show no structure when etched. These are composed completely of the high Ni/Fe alloy taenite.
Stone meteorites are by far the most common, making up nearly 94% of all that fall. They are generally believed to be material from the crust (surface) and mantle of asteroids and (in a few rare cases), planets.
CHONDRITES: Chondrites make up 86% of all meteorite falls. They are composed of small round mineral blebs (called chondrules) composed of olivine and pyroxene packed together with varying amounts of metal (scattered as small flakes through out the stone). In many stones, the chondrules have been partly destroyed through impacts and heating. The number associated with a meteorite’s classification indicates how much the meteorite has been altered. The lower the number, the less altered the meteorite is from its pristine state. The higher numbers indicate that the meteorite has seen more metamorphism and generally has fewer visible chondrules.
ORDINARY
CHONDRITES
H group: These are generally higher in visible iron than other stone types, but have less iron locked up in the minerals. There is some evidence that these (or at least some of them) came from the asteroid 6 Hebe.
L group: Lower overall iron content, (though their olivine and pyroxenes have a higher iron content than H types) these generally have a lot less visible iron than H-types.
LL group: These have the lowest overall iron content of the “ordinary” chondrites. They often have very little visible iron (and generally don’t attract a magnet well). They also tend to have, on average, the largest chondrules.
OTHER
CHONDRITES
Enstatite chondrites: These rare stones are composed of enstatite (iron free pyroxene) as the silicate and, as all of their iron is in the form of free metal, have a very high visible iron content.
R chondrites: These are very rare and are a new addition to the meteorite classifications. They have the highest oxidation state (chemically) of all chondrites, showing virtually no visible metal and do not attract a magnet at all.
Carbonaceous chondrites: There are many types of this rare group of meteorites. They contain many unusual features such as organic compounds (including amino acids- the building blocks of life), interstellar materials and micro- diamonds that formed before and from outside of our solar system (the only such material we have). Due to their primitive nature and these features, these are the most studied of all meteorite types.
ACHONDRITES: These rare meteorites make up less than 1% of our collections (but nearly 8% of all falls). There are many different types. They do not contain chondrules, rarely show free iron, and often tend to resemble common Earth rocks, so they are generally not recognized as meteorites unless they were seen to fall or still retain obvious fusion crust. These are generally igneous rocks (lavas and mantle rocks) from asteroids (Vesta in the case of several types) and planets (some have been identified as being from Mars and a few Moon rocks have also been identified).
Stony-Iron meteorites are generally about 50% metal and 50% silicates. As a group, they are the rarest of the major meteorite types, making up only about 1.5% of falls.
Pallasites: Easily the most beautiful of all meteorites, being composed of olivine crystals in a nickel-iron matrix, these are believed to form at the core-mantle boundary of an asteroid, where iron from the core mixes with mantle olivine.
Mesosiderites: These are a varying mix of metal and silicates (pyroxene, plagioclase and some olivine). From the shattered appearance of the components of these meteorites, it has been suggested that they likely formed from the violent impact of an iron rich asteroid into a silicate rich asteroid.
It is not known for certain how these form. They are composed entirely of dry glass. Many theories have been suggested for their formation over the years (ranging from glass meteorites to melted soil or volcanics from the Moon). Current theory says that they are melted soil that was blasted high into the atmosphere (and even outside of it in some cases) by a large impact here on Earth. Many tektites show stretching, pitting and flow marks that apparently formed from their high speed flights through the atmosphere.