Chondrites

As a group, chondrites make up approximately 86 % of all stone meteorites. Chondrite meteorites as so named because they contain small spherical crystals of minerals such as olivine and pyroxene. These crystals are called "chondrules." Some chondrules have undergone little, if any, chemical and physical change since the birth of the solar system and some have lost their distinctiveness through impact and heating. The distinctiveness of the chondrules vary and a numbering system (petrographic grades 1 through 6) has been devised to indicate the degree to which chondrules are distinct; the higher the number, the less distinct the chondrules. Chondrite meteorites also contain varying amounts of metal, which can be seen as small flakes when the meteorite is cut and sliced. Note also that a few rare type of stone meteorites are classified as chondrites even though they contain no chondrules. This is why classification of meteorites by chemical analysis is important.

Norton (1998) states that "with the exception of the lightest gases, hydrogen and helium, chondrites have an elemental composition very close to the sun's. It is as though pieces of the Sun, minus the light elements, had condensed into sold matter to form chondrites."

There are also subclasses of chondrites:

H group (also called olivine-bronzite chondrites). These meteorites contain a relatively high degree of iron (25 to 31%) both in mineral form and metal flake.

L Group (also called olivine-hypersthene chondrites). These meteorites contain less total iron (20 to 25%) and less visible iron than the H group.

LL Group (also called amphoterites). There is very little free iron visible as well as little iron in the minerals. Indeed, LL chondrites contain between 19 to 22% total iron and about 3% metal. As well, the LL group tend to be more composed of fragmented rocks than the other groups, a process call "brecciation."

Enstatite Chondrites. (also called E chondrites).  Enstatite Chondrites are composed the silicate enstatite (iron free pyroxene). They represent less than 2% of the stone meteorites. It is theorized that the E chondrites formed in an oxygen-depleted environment, possibly near Mercury's orbit. The E chondrites are subclassified into H and L subtypes depending on iron content.

Carbonaceous Chondrites. The carbonaceous chondrites are very rare and extremely interesting. These meteorites contain organic compounds, amino acids, interstellar material (material from exploded stars outside of our solar system). They tend to have a dark matrix and well defined chondrules. Carbonaeous chondreites are comprised of about 2 percent carbon by weight.  Carbonaceous chondrites make up only 2 to 3 percent of meteorite falls. Visually, they generally resemble charcoal briquettes and structurally, they are very fragile meteorites. Unfortunatly, they tend to weather rapidly when exposed to climatic conditions on the earth's surface.

Achondrites

As a group, achondrites make up approximately 7% of fallen stone meteorites. They are extremely rare. They generally do not contain chondrules nor do they show much metal flake (typically, none at all). Achondrite meteorites are thought to be lavas or impact breccias from the surface of asteroids, although a subgroup, the SNCs, are thought to originate from the planet Mars!

Octahedrites. Octahedrite irons contain about 7 to 10% nickel. When sliced and etched with acid, they display bands call "widmanstatten" bands. A further subdivision of octahedrites is made based upon the width of the bands: fine, medium, coarse, and coarsest. The bands narrows with increasing nickel content.

Hexahedrites. Hexahedrite iron meteorites contain a relatively low amount of nickel (approximately 6% or less) and when etched with acid, show very thin lines called "Neumann lines" which have a hexahedral structure.

Ataxites.  Ataxite irons are the highest in nickel content (approximately 16% or more) and show no structure when etched with acid.

Pallasites. Pallasite are composed of olivine crystals set in a nickel-iron matrix. They are believed to form at the core-mantle boundary of a large asteroid. When cut and polished, they are among the most beautiful of meteorites.

Mesosiderites. Mesosiderites are a mixture of metal grains, pyroxene, olivine, and plagioclas. It is believed that mesosiderites are formed when two asteroids, one metal-rich and the other silicate-rich, collide in a violent impact.

References

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