Allende meteorite

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Allende fragment
Type Chondrite
Class Carbonaceous chondrite
Group CV3
Composition 23.85% total iron
Shock stage S1
Country Mexico
Region Pueblito de Allende, Allende, Chihuahua
Coordinates 26°58′N 105°19′W﻿ / ﻿26.967°N 105.317°W﻿ / 26.967; -105.317Coordinates: 26°58′N 105°19′W﻿ / ﻿26.967°N 105.317°W﻿ / 26.967; -105.317
Observed fall Yes
Fall date 01:05 local time (07:05 GMT) on 1969 February 8
TKW 2 metric tons
Strewn field Yes
Allende meteorite.jpg
Chondrules of Allende

The Allende meteorite is the largest carbonaceous chondrite ever found on Earth. The fireball was witnessed at 01:05 on February 8, 1969, falling over the Mexican state of Chihuahua.[1] After breaking up in the atmosphere, an extensive search for pieces was conducted and it is often described as "the best-studied meteorite in history". The Allende meteorite is notable for possessing abundant, large calcium-aluminium-rich inclusions, which are among the oldest objects formed in the Solar System.



The original stone is believed to have been approximately the size of an automobile traveling towards the Earth at more than 10 miles per second. The fall occurred in the early morning hours of February 8, 1969. At 01:05 a huge, brilliant fireball approached from the southwest and lit the sky and ground for hundreds of miles. It exploded and broke up to produce thousands of fusion crusted individuals. This is typical of falls of large stones through the atmosphere and is due to the sudden braking effect of air resistance. The fall took place in northern Mexico, near the village of Pueblito de Allende in the state of Chihuahua. Allende stones became one of the most widely distributed meteorites and provided a large amount of material to study, far more than all of the previously known carbonaceous chondrite falls combined.


Stones were scattered over a huge area – one of the largest meteorite strewnfields known. This strewnfield measures approximately 8 by 50 kilometers. The region is desert, mostly flat, with sparse to moderate low vegetation. Hundreds of meteorites were collected shortly after the fall. Approximately 2 or 3 tonnes of specimens were collected over a period of more than 25 years. Some sources guess that an even larger amount was recovered (estimates as high as 5 tonnes can be found), but there is no way to make an accurate estimate.[a] Even today, over 40 years later, specimens are still occasionally found. Fusion crusted individual Allende specimens ranged from 1 gram (0.035 oz) to 110 kilograms (240 lb).


Path of the fireball and the area in northern Mexico where the meteorite pieces landed (the strewnfield).[3][4]




Allende meteorite - image by Matteo Chinellato; cube = 1 cm
Allende meteorite slice ASU

The meteorite was formed from nebular dust and gas during the early formation of the solar system. It is a "stone" meteorite, as opposed to an "iron," or "stony iron," the other two general classes of meteorite. Most Allende stones are covered, in part or in whole, by a black, shiny crust created as the stone descended at great speed through the atmosphere as it was falling towards the earth from space. This causes the exterior of the stone to become very hot, melting it, and forming a glassy "fusion crust."

When an Allende stone is sawed into two pieces and the surface is polished, the structure in the interior can be examined. This reveals a dark matrix embedded throughout with mm-sized, lighter-colored chondrules, tiny stony spherules found only in meteorites and not in earth rock (thus it is a chondritic meteorite). Also seen are white inclusions, up to several cm in size, ranging in shape from spherical to highly irregular or "amoeboidal." These are known as calcium-aluminum-rich inclusions or "CAIs", so named because they are dominantly composed of calcium- and aluminum-rich silicate and oxide minerals. Like many chondrites, Allende is a breccia, and contains many dark-colored clasts or "dark inclusions" which have a chondritic structure that is distinct from the rest of the meteorite. Unlike many other chondrites, Allende is almost completely lacking in Fe-Ni metal.




Subsequent research[edit]

Close examination of the chondrules in 1971, by a team from Case Western Reserve University, revealed tiny black markings, up to 10 trillion per square centimeter, which were absent from the matrix and interpreted as evidence of radiation damage. Similar structures have turned up in lunar basalts but not in their terrestrial equivalent which would have been screened from cosmic radiation by the Earth's atmosphere and geomagnetic field. Thus it appears that the irradiation of the chondrules happened after they had solidified but before the cold accretion of matter that took place during the early stages of formation of the solar system, when the parent meteorite came together.[7]

The discovery at California Institute of Technology in 1977 of new forms of the elements calcium, barium and neodymium in the meteorite was believed to show that those elements came from some source outside the early clouds of gas and dust that formed the solar system. This supports the theory that shockwaves from a supernova - the explosion of an aging star - may have triggered the formation of, or contributed to the formation of our solar system. As further evidence, the Caltech group said the meteorite contained Aluminum 26, a rare form of aluminum. This acts as a "clock" on the meteorite, dating the explosion of the supernova to within less than 2 million years before the solar system was formed.[8] Subsequent studies have found isotopic ratios of krypton, xenon, nitrogen and some other elements whose forms are also unknown in our solar system. The conclusion, from many studies with similar findings, is that there were a lot of substances in the presolar disc that were introduced as fine "dust" from nearby stars, including novas, supernovas, and red giants. These specks persist to this day in meteorites like Allende, and are known as presolar grains.


a. ^ The number of specimens and total weight can never be known with certainty. Clarke et al. (1971) reported that "At least two tons of meteoritic stones have been recovered." Hundreds more have been found since it was published.[9]

See also[edit]


  1. ^ Marmet. "The Allende Meteorite (Mexico)". Marmet-Meteorites. 
  2. ^ "Learn About the Allende Carbonaceous Chondrite Meteorite". The Meteorite Market. 
  3. ^ Norton, O. Richard (1998). Rocks From Space. Mountain Press Publishing Company. ISBN 0-87842-373-7. 
  4. ^ Wasson, J.T. (2006). "Learn About the Allende Carbonaceous Chondrite Meteorite". Encyclopedia of Meterorites. Pierre-Marie Pele. Archived from the original on 2008-02-29. Retrieved 2008-02-06. 
  5. ^ Amelin, Yuri; Alexander Krot (July/August 2007). "Pb isotopic age of the Allende chondrules". Meteoritics & Planetary Science (University of Arizona) 42 (7/8): 1321–1335. Bibcode:2007M&PS...42.1321A. doi:10.1111/j.1945-5100.2007.tb00577.x. 
  6. ^ Bryner, Jeanna. "1969 Fireball Meteorite Reveals New Ancient Mineral". Live Science. Retrieved June 26, 2012. 
  7. ^ Darling, David. "Allende meteorite". The Internet Encyclopedia of Science. The Worlds of David Darling. Archived from the original on 24 January 2008. Retrieved 2008-02-06. 
  8. ^ "Meteorite Gives Clue To Solar System". Associated Press (Indiana Evening Gazette). 1977-11-12. p. 8. 
  9. ^ Clarke, Roy S., Jr.; Eugene Jarosewich, Brian Mason, Joseph Nelen, Manuel Gomez, Jack R. Hyde. (1971-02-17). "Allende, Mexico, Meteorite Shower". Smithsonian Contributions to the Earth Sciences; 5. Smithsonian Institution. hdl:10088/809. 

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