CARBONACEOUS CHONDRITES
Carbonaceous chondrites are dark gray to
blackish-colored chondrite meteorites with a relatively carbon-rich
matrix. The first large sampling of carbonaceous chondrites available to
meteoriticists came with the 1969 fall of the Allende Meteorite.
Before Allende, carbonaceous chondrite material was exceedingly rare.
Allende has become the most heavily studied and the most famous carbonaceous
chondrite.
Allende impacted on Earth at 1:05 AM on 8 February
1969. Its known strewn field trends southwest-to-northeast in the
vicinity of the town of Allende in southeastern Chihuahua State, northern
Mexico.
Carbonaceous chondrite (1.4 cm across) - Allende Meteorite, small broken
individual. The blackish material is the original fusion crust. The
fusion crust represents the outer portions of the original rock fragment that
partially melted as the Allende fireball passed through Earth's
atmosphere. The lighter gray areas show the internal appearance of the
rock (= where the fusion crust is broken away).
Carbonaceous chondrite - large individual (just under 12 kg) of the Allende
Meteorite with fusion crust. FMNH Me 2681 (Field Museum of Natural
History meteorite collection, Chicago, Illinois, USA).
(More info. on the
Allende Meteorite)
Carbonaceous chondrite (above & below; field of view 1.65 cm across) -
cut slice of the Allende Meteorite. The slice shows the internal
structure & composition of the Allende carbonaceous condrite. Allende
has small, spherical to subspherical structures called chondrules (all
chondrites have these). Allende also contains whitish, irregularly-shaped
patches called CAIs ("calcium-aluminum inclusions"), composed of
high-temperature Ca-Al-Ti silicates & oxides. The blackish, fine-grained,
carbon-rich matrix consists of Fe-olivine & poorly graphitized
carbon. A few tiny specks of metallic iron-nickel alloy also occur.
Allende rocks represent the near-oldest meteoritic
material known. The olivine chondrules in Allende rocks date to 4.560
billion years. The CAIs in Allende rocks date to 4.568 billion years.
Carbonaceous chondrite - large individual of the Murchison Meteorite with
fusion crust. FMNH Me 2640 (Field Museum of Natural History meteorite
collection, Chicago, Illinois, USA).
Another famous carbonaceous chondrite is the Murchison
Meteorite. It landed on Earth in 1969, the same year as
Allende. Murchison impacted in the late morning on 28 September 1969 in
Victoria, Australia. Samples were quickly recovered to minimize
contamination with Earth materials. Murchison is important for containing
extraterrestrial amino acids. Amino acids are the "building blocks"
of life on Earth. This discovery demonstrated that these organic
molecules may have originally appeared on Earth not by chemical synthesis in
the early oceans (although that is still a strong possibility), but may have
arrived from outer space.
(More info. on the
Murchison Meteorite)