Chondrule
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A chondrule (from
Abundance and size
Different kinds of the stony, non-metallic meteorites called chondrites contain different fractions of chondrules (see table below). In general, carbonaceous chondrites contain the smallest percentage (by volume) of chondrules, including the CI chondrites which, paradoxically, do not contain any chondrules despite their designation as chondrites, whereas ordinary and enstatite chondrites contain the most. Because ordinary chondrites represent 80% of the meteorites that fall to earth, and because ordinary chondrites contain 60–80% chondrules, it follows that most of the meteoritic material that falls on earth (excluding dust) is made up of chondrules.
Chondrules can range in diameter from just a few micrometers to over 1 centimetre (0.39 in). Again, different kinds of
Chondrite group | abundance (vol%) | avg. diam. (mm) |
---|---|---|
CI | 0 | – |
CM | 20 | 0.3 |
CO | 50 | 0.15 |
CV | 45 | 1 |
CK | 45 | 1 |
CR | 50–60 | 0.7 |
CH | 70 | 0.02 |
CB | 20–40 | 10 (a subgroup), 0.2 (b subgroup) |
H | 60–80 | 0.3 |
L | 60–80 | 0.7 |
LL | 60–80 | 0.9 |
EH | 60–80 | 0.2 |
EL | 60–80 | 0.6 |
R | >40 | 0.4 |
K | 30 | 0.6 |
Mineralogy and petrology
Most chondrules are composed primarily of the
Chondrules display a wide variety of textures, which can be seen when the chondrule is sliced open and polished. Some show textural evidence for extremely rapid cooling from a molten or nearly completely molten state. Pyroxene-rich chondrules that contain extremely fine-grained, swirling masses of fibrous crystals only a few micrometers in size or smaller are called cryptocrystalline chondrules. When the pyroxene fibers are coarser, they may appear to radiate from a single nucleation site on the surface, forming a radial or excentroradial texture. Olivine-rich chondrules may contain parallel plates of that mineral, surrounded by a continuous shell of olivine and containing feldspathic glass between the plates; these are known as barred textures. Other observed textural features that are clearly the result of very rapid cooling are dendritic and hopper-shaped olivine grains, and chondrules that are composed entirely of glass.
More commonly, chondrules display what is known as a
The composition of olivine and pyroxene in chondrules varies widely, although the range is usually narrow within any single chondrule. Some chondrules contain very little iron oxide (FeO), resulting in olivine and pyroxene that are close to forsterite (Mg2SiO4) and enstatite (MgSiO3) in composition. These are commonly called Type I chondrules by scientists, and often contain large amounts of metallic Fe. Other chondrules formed under more oxidizing conditions and contain olivine and pyroxene with large amounts of FeO (e.g., olivine with the formula (Mg,Fe)
2SiO
4). Such chondrules are called Type II. Most chondrites contain both Type I and Type II chondrules mixed together, including those with both porphyritic and nonporphyritic textures, although there are exceptions to this.
Formation
Chondrules are believed to have formed by a rapid (flash) heating (within minutes or less) and melting of solid dust aggregates of approximately Solar composition under temperatures of about 1000 K. These temperatures are lower than those under which
Proposed heating mechanisms are:
- Impacts between molten planetesimals
- Meteorablation
- Hot inner nebula
- FU Orionis-type outburst of the early sun
- Energetic bipolar-shaped outflows
- Nebular lightning
- Magnetic flares
- Shock waves in the protoplanetary disk shocks[1]
- Supernova radiation and shock wave
Isotope studies indicate a nearby supernova explosion added fresh material to what became the Solar System. The Ningqiang carbonaceous chondrite contained sulfur-36 derived from chlorine-36. As chlorine-36 has a half-life of only 300,000 years, it could not have travelled far from its origin. The presence of iron-60 also indicates a nearby supernova.[3] Such proximity implies the radiation and shock wave would have been significant, although the degree of heating is not known.
In contrast, the fine grained matrix, in which the chondrules are embedded after their accretion into the chondrites parent body, is assumed to have been condensed directly from the solar nebula.
Types
There are a couple of different ways to organize different chondrules into textural types according to their appearance.
See also
- Glossary of meteoritics
- List of meteorite minerals
- Carbonaceous chondrites
- Chondrites
- Cosmochemistry
- Radiometric dating
References
- ^ PMID 23118187.
- ^ Weisberg et al. (2006) "Systematics and Evaluation of Meteorite Classification". In, Meteorites and the Early Solar System II, 19–52 (D.S. Lauretta and H.Y. McSween, Eds.), Univ. Arizona Press
- ^ G. Quitte et al. (2007). "Correlated iron 60, nickel 62 and zirconium 96 in refractory inclusions and the origin of the solar system", Astrophysical Journal (655): 678–84
Further reading
- Wlotzka F., Heide F. (1995). Meteorites: Messengers from Space, Springer Verlag, ISBN 0-387-58105-7
- Hewins R.H., ISBN 0-521-55288-5
- Oliver Botta, Jeffrey L. Bada (2002). "Extraterrestrial Organic Compounds in Meteorites", Surveys in Geophysics 23 (5): 411–467. .
- Vogel N. (2003). Chondrule formation and accretion processes in the early solar nebula – Clues from noble gases in different constituents of unequilibrated chondrites, Der Andere Verlag, Osnabrück, ISBN 3-89959-055-4
External links
- A Pictorial of Chondrules – Meteorites Australia (Meteorites.com.au)
- Chondrules and their Origins