Chondrule

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Most meteorites that meteorite falls on Earth are chondrites, which are characterized by the presence of round grains called chondrules (from Greek chondros, grain). Chondrules formed as molten or partially molten droplets in space before being accretion to their parent asteroids. Because chondrites represent the oldest solid material within our solar system and are believed to be the building blocks of the planetary system, it follows that an understanding of the formation of chondrules is important to understand the initial development of the planetary system.

Abundance and size Different kinds of 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 chondrite 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 (exclusive of dust) is made up of chondrules.

Chondrules can range in diameter from just a few micrometers to over 1 cm. Again, different kinds of chondrites contain different ranges of chondrule sizes: they are smallest in CH, CM, and CO chondrites (see Meteorites classification), moderately large in CR, CV, L, LL, and R chondrites, and largest in some CB chondrites (see table). Other chondrite groups are intermediate between these.

{|class="wikitable" align=center|+ Table 1: Chondrule sizes and abundancesWeisberg 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!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 silicate minerals olivine and pyroxene, surrounded by feldspar material that may either be glassy or crystalline. Small amounts of other minerals are often present, including Fe sulfide (troilite), metallic Fe-Ni, oxides such as chromite, and phosphates such as merrillite. Less common types of chondrules may be dominantly composed of feldspathic material (again either glassy or crystalline), silica, or metallic Fe-Ni and sulfides.

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 dendrite (crystal) and hopper-shaped olivine grains, and chondrules that are composed entirely of glass.

More commonly, chondrules display what is known as a porphyritic texture. In these, grains of olivine and/or pyroxene are equidimensional and sometimes euhedral. They are named on the basis of the dominant mineral, i.e. porphyritic olivine (PO), porphyritic pyroxene (PP), and porphyritic olivine-pyroxene (POP). It seems likely that these chondrules cooled more slowly than those with radial or barred textures, however they still may have solidified in a matter of hours.

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 redox conditions and contain olivine and pyroxene with large amounts of FeO (e.g., olivine with the formula (Mg,Fe)2SiO4). 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 formed by a rapid heating (within minutes or less) of solid precursor material to temperatures between 1500°C and 1900°C and subsequent melting. This is followed by a cooling within one to several hours (Wood, 1999). However, the environmental setting, the energy source for the heating, and the precursor material are not known. The solar nebula or a Protoplanetary disc are possible places of formation.

Proposed energy sources are:

Impact melting
Meteor ablation
Hot inner nebula
FU Orionis outburst of the early sun
Energetic bipolar-shaped outflows
Nebular lightning
Magnetic flares
Accretion shocks
Nebular shocks
Supernova radiation and shockwave

Isotope studies indicate a nearby supernova explosion added fresh material to what became our 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. Such proximity implies the radiation and shockwave 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.

References {{cite conference | author=Wood, J.A.| title=Origin of the Solar System| booktitle=The New Solar System,Beatty, Petersen, and Chaikin, eds.| year=1990| pages=16-17 |publisher=Sky Publishing-->

Further reading

Wlotzka F., Heide F. (1995) Meteorites: Messengers from Space, Springer Verlag, ISBN 0-387-58105-7
Hewins R.H., Jones R.H., and Scott E.R.D. eds. (1996) Chondrules and the Protoplanetary Disk, Cambridge University Press, UK, ISBN 0-521-55288-5
Oliver Botta, Jeffrey L. Bada Extraterrestrial Organic Compounds in Meteorites, Surveys in Geophysics 23 (5): 411-467, 2002
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