200px|thumb|Chondrules in the chondrite classification Grassland. A millimeter scale is shown.

200px|thumb|Chondrules in the Carbonaceous Chondrite NWA 13887

thumb|200x200px|Chondrules in a thin section of ordinary chondrite observed under polarized light/analyzed.

A chondrule (from Ancient Greek χόνδρος chondros, grain) is a round grain found in a chondrite. Chondrules form as molten or partially molten droplets in space before being accreted to their parent asteroids. Because chondrites represent one of the oldest solid materials within the Solar System

!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 feldspathic 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 dendritic 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 () and enstatite () 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 ). 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 CAIs are thought to have formed. However, the environmental setting, the energy source for the heating, and the precursor material are not known. The solar nebula or a protoplanetary environment are possible places of formation.

Proposed heating mechanisms are:

  • Impacts between molten planetesimals
  • Meteor ablation
  • 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 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

Chondrules may be organized into textural types according to such characteristics as their appearance, color, texture, and crystalline structure.

{| class="wikitable" border="1"

! Name !! Abbreviation !! Picture

|-

| Porphyritic olivine || PO

|-

| Porphyritic pyroxene || PP

|-

| Porphyritic olivine-pyroxene || POP

|-

| Radial pyroxene || RP

|-

| Barred olivine || BO

|-

| Cryptocrystalline || C

|-

| Granular olivine-pyroxene || GOP

|-

| Glassy chondrules || – || frameless|200px

|}

See also

  • Glossary of meteoritics
  • List of meteorite minerals
  • Carbonaceous chondrites
  • Chondrites
  • Cosmochemistry
  • Radiometric dating

References

Further reading

  • Wlotzka F., Heide F. (1995). Meteorites: Messengers from Space, Springer Verlag,
  • Hewins R.H., Jones R.H., and Scott E.R.D. eds. (1996). Chondrules and the Protoplanetary Disk, Cambridge University Press, UK,
  • 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,
  • A Pictorial of Chondrules – Meteorites Australia (Meteorites.com.au)
  • Chondrules and their Origins