World Library  
Flag as Inappropriate
Email this Article

CI chondrite

Article Id: WHEBN0027483741
Reproduction Date:

Title: CI chondrite  
Author: World Heritage Encyclopedia
Language: English
Subject: Meteorites, Meteorite find, Total known weight, Meteorite fall statistics, Refractory (planetary science)
Collection:
Publisher: World Heritage Encyclopedia
Publication
Date:
 

CI chondrite

CI chondrite
— Group —
Carbonaceous chondrites with a CI chondrite in the center (Tagish Lake - CI 2)
Type Chondrite
Structural classification ?
Class Carbonaceous chondrite
Subgroups
  • None?
Parent body Unknown
Composition ?
Total known specimens Six
TKW 17 kilograms (37 lb)
Alternative names CI chondrites, C1 chondrites, CI chondrite meteorites, C1 chondrite meteorites
CI chondrites, sometimes C1 chondrites, are a group of stony meteorites belonging to the carbonaceous chondrites. Amongst all the meteorites found so far their chemical composition resembles most closely the elemental distribution in the sun's photosphere.

Contents

  • Designation 1
  • History 2
  • Description 3
  • Mineralogy 4
  • Chemical composition 5
  • Physical parameters 6
  • Importance 7
  • Formation 8
  • See also 9
  • References 10

Designation

The abbreviation CI is derived from the C for carbonaceous and from the I for Ivuna, the type locality in Tanzania. The 1 in C1 stands for the type 1 meteorites in the classification scheme of Van Schmus-Wood. Type 1 meteorites normally have no recognizable chondrules.

History

There are very few Montauban. The meteorite had disintegrated into 20 pieces weighing a total 10 kilograms. In 1911 a meteorite was seen near Tonk (Rajasthan) in India. Only a few fragments were recuperated that weighed a mere 7.7 grams (0.27 oz). The meteorite of the type locality Ivuna in Tanzania fell in 1938 splitting into three pieces of altogether 705 grams (24.9 oz). This was followed in 1965 by a fery bright fall in Revelstoke, British Columbia, but only two tiny fragments of 1 gram (0.035 oz) were found. All in all roughly 17 kilograms of CI-chondrites exist so far.

During the Apollo 12 mission a meteorite was found 1969 on the moon, which was first thought to be a CI chondrite, but later turned out to be a closely related CM chondrite. In 2000 a fall occurred at Tagish Lake in the Yukon Territory. This meteorite is meanwhile included within the CI chondrites, although it contains chondrules. It has been designated CI 2.

Description

CI chondrites are very fragile and porous rocks, which easily disintegrate on their descent through the atmosphere. This explains why mainly small fragments have been discovered so far. A good example is the very bright Revelstoke fall which yielded only two tiny fragments weighing below one gram. CI chondrites are characterized by a black fusion crust which sometimes is difficult to distinguish from the very similar matrix. The opaque matrix is rich in carbonaceous material and contains black minerals like magnetite and pyrrhotite. At some places white, water-bearing carbonates and sulfates are incorporated.

The main characteristic of CI chondrites is the lack of recognizable chondrules (an exception being the sample from Tagish Lake). Yet small chondrule fragments and calcium-aluminium-rich inclusions (CAI's) do occur, but are quite rare.

Mineralogy

CI chondrite mineralogy is dominated by a fine-grained phyllosilicate matrix, hosting carbonates, sulfates, sulfides, and magnetite. CI-chondrites contain the following minerals:

All these ferromagnesian silicates are tiny, equidimensional, idiomorphic grains crystallized at high temperatures.

Water-bearing, clay-rich phyllosilicates like montmorillonite and serpentine-like minerals. Main constituents. As aqueous alteration minerals occur:

Carbonaceous minerals include:

The ferromagnesian minerals are isolated and show no signs of alteration.[1] As concerns the genesis of the montmorillonite and the serpentine-like minerals it is assumed that they were produced from magnesium-rich olivines and pyroxenes by aqueous alteration.[2]

Chemical composition

CI chondrites contain between 17and 22 weight % water. Their high porosity (of up to 30%) seems to be correlated to that fact. The water is not occurring freely, but is rather tied up in water-bearing silicates. Strong aqueous alteration at rather low temperatures (at 50 to 150 °C) [3] – a hallmark of CI chondrites – is indicated by the occurrence of minerals like epsomite, but also by carbonates and sulfates. Liquid water must have penetrated the parent body through cracks and fissures and then deposited the water-bearing phases.

Iron is present with 25 weight %, but mainly in oxidised form (magnetite). Iron sulfides like pyrrhotite, pentlandite, troilite and cubanite do occur, but elemental iron is absent.[4] The Mg/Si ratio of 1.07 is rather high.[5] Only CV chondrites are more strongly enriched in magnesium. The Ca/Si ratio of 0.057 is the lowest of all carbonaceous chondrites.[6] As regards the oxygen isotopes CI chondrites have the highest values in δ 17O and δ 18O amongst the carbonaceous chondrites. The ratio 17/18 compares with terrestrial values.

Physical parameters

Because of their high porosity CI chondrites have only a density of 2.2 g/cm3.

Importance

Amongst all the meteorites found up to this date CI chondrites possess the strongest similarity to the elemental distribution within the original solar nebula. For this reason they are also called primitive meteorites. Except for the volatile elements carbon, hydrogen, oxygen and nitrogen as well as the noble gases, which are deficient in the CI chondrites, the elemental ratios are nearly identical. Lithium is another exception, it is enriched in the meteorites (in the sun lithium is involved during nucleosynthesis and therefore diminished).

Because of this strong similarity it has become customary in petrology to normalize rock samples versus CI chondrites for a specific element, i. e. the ratio rock/chondrite is used to compare a sample with the original solar matter. Ratios > 1 indicate an enrichment, ratios < 1 a depletion of the sample. The normalization process is used mainly in spider diagrams for the rare earth elements.

CI chondrites also have a high carbon content. Besides anorganic carbon compounds like graphite, diamond and carbonates organic carbon compounds are represented. For instance amino acids have been detected. This is a very important fact in the ongoing search for the origin of life.

Formation

CI chondrites and the closely related CM chondrites are very rich in volatile substances, especially in water. It is assumed that they originally formed in the outer Comet Halley).

See also

References

  1. ^ Dodd, R. T.: Meteorites: A Petrologic-Chemical Synthesis. Cambridge University Press, New York 1981
  2. ^ Zolensky, M. E. & McSween, H.Y.: University of Arizona Press, Tucson 1988
  3. ^ Zolensky, M. E. & Thomas, K. L. (1995). GCA, 59, p. 4707 – 4712
  4. ^ Mason, B.: Meteorites. John Wiley and Son Inc., New York 1962
  5. ^ Von Michaelis, H., Ahrens, I. H. & Willis, J.P.: The compositions of stony meteorites – II. The analytical data and an assessment of their quality. In: Earth and Planetary Scientific Letters. 5, 1969
  6. ^ Van Schmus, W. R. & Hayes, J. M.: Chemical and petrographic correlations among carbonaceous chondrites. In: Geochimica Cosmochimica Acta. 38, 1974


This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 



Copyright © World Library Foundation. All rights reserved. eBooks from Hawaii eBook Library are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.