A chondrite (, ) is a stony (non-metallic) meteorite that has not been modified by either melting or differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar System accreted to form primitive asteroids. Some such bodies that are captured in the planet's gravity well become the most common type of meteorite by arriving on a trajectory toward the planet's surface. Estimates for their contribution to the total meteorite population vary between 85.7% and 86.2%.

Their study provides important clues for understanding the origin and age of the Solar System, the synthesis of organic compounds, the origin of life and the presence of water on Earth. One of their characteristics is the presence of chondrules (from the Ancient Greek χόνδρος chondros, grain), which are round grains formed in space as molten or partially molten droplets of distinct minerals. Chondrules typically constitute between 20% and 80% of a chondrite by volume.

Chondrites can be distinguished from iron meteorites by their low iron and nickel content. Non-metallic meteorites that lack chondrules are achondrites, which are believed to have formed more recently than chondrites. There are currently over 27,000 chondrites in the world's collections. The largest individual stone ever recovered, weighing 1770 kg, was part of the Jilin meteorite shower of 1976. Chondrite falls range from single stones to extraordinary showers consisting of thousands of individual stones. An instance of the latter occurred in the Holbrook fall of 1912, in which an estimated 14,000 stones were grounded in northern Arizona.

Origin and history

Chondrites were formed by the accretion of particles of dust and grit present in the primitive Solar System which gave rise to asteroids over 4.54 billion years ago. These asteroid parent bodies of chondrites are (or were) small to medium-sized asteroids that were never part of any body large enough to undergo melting and planetary differentiation. Dating using <sup>206</sup>Pb/<sup>204</sup>Pb gives an estimated age of 4,566.6 ± 1.0 Ma, matching ages for other chronometers. Another indication of their age is the fact that the abundance of non-volatile elements in chondrites is similar to that found in the atmosphere of the Sun and other stars in the Milky Way galaxy.

thumb|200x200px|Thin slice of ordinary chondrite NWA 17172 observed under polarized light/analyzed.

Although chondritic asteroids never became hot enough to melt based upon internal temperatures, many of them reached high enough temperatures that they experienced significant thermal metamorphism in their interiors. The source of the heat was most likely energy coming from the decay of short-lived radioisotopes (half-lives less than a few million years) that were present in the newly formed Solar System, especially <sup>26</sup>Al and <sup>60</sup>Fe, although heating may have been caused by impacts onto the asteroids as well. Many chondritic asteroids also contained significant amounts of water, possibly due to the accretion of ice along with rocky material.

As a result, many chondrites contain hydrous minerals, such as clays, that formed when the water interacted with the rock on the asteroid in a process known as aqueous alteration. In addition, all chondritic asteroids were affected by impact and shock processes due to collisions with other asteroids. These events caused a variety of effects, ranging from simple compaction to brecciation, veining, localized melting, and formation of high-pressure minerals. The net result of these secondary thermal, aqueous, and shock processes is that only a few known chondrites preserve in pristine form the original dust, chondrules, and inclusions from which they formed.

Characteristics

Prominent among the components present in chondrites are the enigmatic chondrules, millimetre-sized spherical objects that originated as freely floating, molten or partially molten droplets in space; most chondrules are rich in the silicate minerals olivine and pyroxene.

Chondrites also contain refractory inclusions (including Ca–Al inclusions), which are among the oldest objects to form in the Solar System, particles rich in metallic Fe-Ni and sulfides, and isolated grains of silicate minerals. The remainder of chondrites consists of fine-grained (micrometre-sized or smaller) dust, which may either be present as the matrix of the rock or may form rims or mantles around individual chondrules and refractory inclusions. Embedded in this dust are presolar grains, which predate the formation of the Solar System and originated elsewhere in the galaxy. The chondrules have distinct texture, composition and mineralogy, and their origin continues to be the object of some debate. The scientific community generally accepts that these spheres were formed by the action of a shock wave that passed through the Solar System, although there is little agreement as to the cause of this shock wave.

An article published in 2005 proposed that the gravitational instability of the gaseous disk that formed Jupiter generated a shock wave with a velocity of more than 10&nbsp;km/s, which resulted in the formation of the chondrules.

Chondrite classification

Chondrites are divided into about 15 distinct groups (see Meteorites classification) on the basis of their mineralogy, bulk chemical composition, and oxygen isotope compositions (see below). The various chondrite groups likely originated on separate asteroids or groups of related asteroids. Each chondrite group has a distinctive mixture of chondrules, refractory inclusions, matrix (dust), and other components and a characteristic grain size. Other ways of classifying chondrites include weathering and shock.

Chondrites can also be categorized according to their petrologic type, which is the degree to which they were thermally metamorphosed or aqueously altered (they are assigned a number between 1 and 7). The chondrules in a chondrite that is assigned a "3" have not been altered. Larger numbers indicate an increase in thermal metamorphosis up to a maximum of 7, where the chondrules have been destroyed. Numbers lower than 3 are given to chondrites whose chondrules have been changed by the presence of water, down to 1, where the chondrules have been obliterated by this alteration.

A synthesis of the various classification schemes is provided in the table below.

{| class="wikitable"

|-----

! Type

! Subtype

! Distinguishing features/Chondrule character

! Letter designation

|-----

| rowspan="5" style="background:#FFFF00" | Enstatite chondrites

| rowspan="5" |

| style="background:#FFFFB3" | Abundant || E3, EH3, EL3

|-----

| style="background:#FFFFB3" | Distinct || E4, EH4, EL4

|-----

| style="background:#FFFFB3" | Less distinct || E5, EH5, EL5

|-----

| style="background:#FFFFB3" | Indistinct || E6, EH6, EL6

|-----

| style="background:#FFFFB3" | Melted || E7, EH7, EL7

|-----

| rowspan="15" style="background:#FFFF00" | Ordinary chondrites

| rowspan="5" style="background:#FFDDB3" | H

| style="background:#FFFFB3" | Abundant || H3–H3,9

|-----

| style="background:#FFFFB3" | Distinct || H4

|-----

| style="background:#FFFFB3" | Less distinct || H5

|-----

| style="background:#FFFFB3" | Indistinct || H6

|-----

| style="background:#FFFFB3" | Melted || H7

|-----

| rowspan="5" style="background:#FFDDB3" | L

| style="background:#FFFFB3" | Abundant || L3–L3,9

|-----

| style="background:#FFFFB3" | Distinct || L4

|-----

| style="background:#FFFFB3" | Less distinct || L5

|-----

| style="background:#FFFFB3" | Indistinct || L6

|-----

| style="background:#FFFFB3" | Melted || L7

|-----

| rowspan="5" style="background:#FFDDB3" | LL

| style="background:#FFFFB3" | Abundant || LL3–LL3,9

|-----

| style="background:#FFFFB3" | Distinct || LL4

|-----

| style="background:#FFFFB3" | Less distinct || LL5

|-----

| style="background:#FFFFB3" | Indistinct || LL6

|-----

| style="background:#FFFFB3" | Melted || LL7

|-----

| rowspan="10" style="background:#FFFF00" | Carbonaceous chondrites

| style="background:#FFDDB3" |Ivuna || Phyllosilicates, Magnetite || CI

|-----

| style="background:#FFDDB3" | Mighei || Phyllosilicates, Olivine || CM1–CM2

|-----

| style="background:#FFDDB3" | Vigarano|| Olivines rich in Fe, Ca minerals and Al || CV2–CV3.3

|-----

| style="background:#FFDDB3" | Renazzo|| Phyllosilicates, Olivine, Pyroxene, metals || CR

|-----

| style="background:#FFDDB3" | Ornans || Olivine, Pyroxene, metals, Ca minerals and Al || CO3–CO3.7

|-----

| style="background:#FFDDB3" | Karoonda || Olivine, Ca minerals and Al || CK

|-----

| style="background:#FFDDB3" | Bencubbin || Pyroxene, metals || CB

|-----

| style="background:#FFDDB3" | Loongana || Chondrules and CAIs, metals || CL

|-----

| style="background:#FFDDB3" | High Iron || Pyroxene, metals, Olivine || CH

|-----

| style="background:#FFDDB3" | Tagish Lake || Phyllosilicates, Magnetite, Ca-Mg-Fe carbonates || TAG

|-----

| style="background:#FFFF00" | Kakangari-type || &nbsp; || &nbsp; || K

|-----

| style="background:#FFFF00" | Rumurutiites || &nbsp; ||Olivine, Pyroxenes, Plagioclase, Sulfides || R

|}

Enstatite chondrites

thumb|225px|The Saint Sauveur [[enstatite chondrite (EH5)]]

Enstatite chondrites (also known as E-type chondrites) are a rare form of meteorite thought to comprise only about 2% of the chondrites that fall to Earth. Only about 200 E-Type chondrites are currently known.

Ordinary chondrites

Ordinary chondrites are by far the most common type of meteorite to fall to Earth: about 80% of all meteorites and over 90% of chondrites are ordinary chondrites.), and smaller chondrules than L and LL chondrites. They are formed of bronzite, olivine, pyroxene, plagioclase, metals and sulfides and ~42% of ordinary chondrite falls belong to this group (see Meteorite fall statistics).

  • L chondrites have low total iron contents (including 7–11% Fe–Ni metal by mass). ~46% of ordinary chondrite falls belong to this group, which makes them the most common type of meteorite to fall on Earth.
  • LL chondrites have low total iron and low metal contents (3–5% Fe–Ni metal by mass of which 2% is metallic Fe and they also contain bronzite, oligoclase and olivine). They are characterized by the presence of carbon compounds, including amino acids. They are thought to have been formed the farthest from the sun of any of the chondrites as they have the highest proportion of volatile compounds. They are characterized by large amounts of dusty matrix and oxygen isotope compositions similar to carbonaceous chondrites, highly reduced mineral compositions and high metal abundances (6% to 10% by volume) that are most like enstatite chondrites, and concentrations of refractory lithophile elements that are most like ordinary chondrites.

Many of their other characteristics are similar to the O, E and C chondrites.

Rumuruti chondrites

Rumuruti (R) type chondrites are a very rare group, with only one documented fall out of almost 900 documented chondrite falls. They have a number of properties in common with ordinary chondrites, including similar types of chondrules, few refractory inclusions, similar chemical composition for most elements, and the fact that <sup>17</sup>O/<sup>16</sup>O ratios are anomalously high compared to Earth rocks. However, there are significant differences between R chondrites and ordinary chondrites: R chondrites have much more dusty matrix material (about 50% of the rock); they are much more oxidized, containing little metallic Fe–Ni; and their enrichments in <sup>17</sup>O are higher than those of ordinary chondrites. Nearly all the metal they contain is oxidized or in the form of sulfides. They contain fewer chondrules than the E chondrites and appear to come from an asteroid's regolith.

Composition

Because chondrites accumulated from material that formed very early in the history of the Solar System, and because chondritic asteroids did not melt, they have very primitive compositions. "Primitive," in this sense, means that the abundances of most chemical elements do not differ greatly from those that are measured by spectroscopic methods in the photosphere of the sun, which in turn should be well-representative of the entire Solar System (note: to make such a comparison between a gaseous object like the sun and a rock like a chondrite, scientists choose one rock-forming element, such as silicon (Si), to use as a reference point, and then compare ratios. Thus, the atomic ratio of Mg/Si measured in the sun (1.07) is identical to that measured in CI chondrites).

Although all chondrite compositions can be considered primitive, there is variation among the different groups, as discussed above. CI chondrites seem to be nearly identical in composition to the sun for all but the gas-forming elements (e.g., hydrogen (H), carbon (C), nitrogen (N), and noble gases: helium (He), neon (Ne), argon (Ar) etc.). Other chondrite groups deviate from the solar composition (i.e., they are fractionated) in highly systematic ways:

  • At some point during the formation of many chondrites, particles of metal became partially separated from particles of silicate minerals. As a result, chondrites coming from asteroids that did not accrete with their full complement of metal (e.g., L, LL, and EL chondrites) are depleted in all siderophile elements, whereas those that accreted too much metal (e.g., CH, CB, and EH chondrites) are enriched in these elements compared to the sun.
  • In a similar manner, although the exact process is not very well understood, highly refractory elements like Ca and Al became separated from less refractory elements like Mg and Si, and were not uniformly sampled by each asteroid. The parent bodies of many groups of carbonaceous chondrites contain over-sampled grains rich in refractory elements, whereas those of ordinary and enstatite chondrites were deficient in them.
  • No chondrites except the CI group formed with a full, solar complement of volatile elements. In general, the level of depletion corresponds to the degree of volatility, where the most volatile elements are most depleted.

Petrologic types

A chondrite's group is determined by its primary chemical, mineralogical, and isotopic characteristics (above). The degree to which it has been affected by the secondary processes of thermal metamorphism and aqueous alteration on the parent asteroid is indicated by its petrologic type, which appears as a number following the group name (e.g., an LL5 chondrite belongs to the LL group and has a petrologic type of 5). The current scheme for describing petrologic types was devised by Van Schmus and Wood in 1967.

It is thought possible that a proportion of the water present on the Earth comes from the impact of comets and carbonaceous chondrites with the Earth's surface.

Origin of life

thumb|upright|[[Amino acid general structure]]

Carbonaceous chondrites contain more than 600 organic compounds that were synthesized in distinct places and at distinct times. These organic compounds include: hydrocarbons, carboxylic acids, alcohols, ketones, aldehydes, amines, amides, sulfonic acids, phosphonic acids, amino acids, nitrogenous bases, etc. These compounds can be divided into three main groups: a fraction that is not soluble in chloroform or methanol, chloroform soluble hydrocarbons and a fraction that is soluble in methanol (which includes the amino acids).

The first fraction appears to originate from interstellar space and the compounds belonging to the other fractions derive from a minor planet. It has been proposed that the amino acids were synthesized close to the surface of a minor planet by the radiolysis (dissociation of molecules caused by radiation) of hydrocarbons and ammonium carbonate in the presence of liquid water. In addition, the hydrocarbons could have formed deep within a planetoid by a process similar to the Fischer–Tropsch process. These conditions could be analogous to the events that caused the origin of life on Earth.

thumb|The [[Murchison meteorite is on display at the Smithsonian's NMNH.]]

The Murchison meteorite has been thoroughly studied; it fell in Australia close to the town that bears its name on 28 September 1969. It is a CM2 and it contains common amino acids such as glycine, alanine and glutamic acid as well as other less common ones such as isovaline and pseudo-leucine.

Two meteorites that were collected in Antarctica in 1992 and 1995 were found to be abundant in amino acids, which are present at concentrations of 180 and 249 ppm (carbonaceous chondrites normally contain concentrations of 15 ppm or less). This could indicate that organic material is more abundant in the Solar System than was previously believed, and it reinforces the idea that the organic compounds present in the primordial soup could have had an extraterrestrial origin.

See also

  • Meteorite
  • Meteorite classification
  • Achondrite
  • Carbonaceous chondrite
  • Chondrule
  • Glossary of meteoritics
  • Iron meteorite
  • Stony-iron meteorite
  • Asteroid
  • Solar System

Notes

References

  • Natural History Museum, meteorite catalogue
  • Meteorite articles, including discussions of chondrites in Planetary Science Research Discoveries
  • The British and Irish Meteorite Society
  • Chondrite images from Meteorites Australia