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)



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