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New England
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Carbonaceous Chondrites
Murchison, Australia,
carbonaceous chondrite, CM2
Carbonaceous (C)
chondrites are some of the most complex of
all meteorites. They are rare, primitive and contain organic compounds. Most
importantly they contain water-bearing minerals which is evidence of water
moving slowly through their interiors not long after formation.
C chondrites are further divided
according to chemical and mineralogical differences into the sub-classes CI,
CM, CV, CO, CK and CR. These sub-classes are named for the type specimen of
each group, Ivuna, Mighei, Vigarano, Ornans, Karoonda and Renazzo
respectively.
The fine-textured, small
chondrule attributes of the carbonaceous chondrite Isna, Egypt are displayed in
this image
Like the ordinary chondrites,
C chondrites also have petrologic grade
designations. In addition to grades 3 to 6, indicating increasing heat
metamorphism of chondrules, grades 2 and
1 indicate increasing metamorphism of the meteorite by water. Carbonaceous
chondrites are the only meteorites known with petrologic grades 1 and
2.
Two carbonaceous chondrites,
Allende and Murchison, are of particular interest to scientists and, curiously,
both fell in 1969, but on opposite sides of the world.
Fusion-crusted fragment,
Orgueil, France, Carbonaceous chondrite, CI1
Allende, Mexico (14K GIF) is classed as CV3
and represents some of the oldest known matter. The meteorite formed 4.56
billion years ago and contains interstellar grains within calcium/aluminum rich
inclusions (CAIs). Interstellar grains are
remnants of a prior star that lived out its life and exploded before the
formation of our Sun. It is possible that this explosion was the trigger for
the formation of our solar system. 
Section of the Allende,
Mexico CV3 meteorite.
Murchison, Australia, image at
the top of the page, is a CM2 chondrite and believed to be of cometary origin
because of its high water content, 12%. To date 92 amino acids (the building
blocks of protein) have been found in Murchison. Only nineteen of them are
found on Earth.
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