Chondrite

OCCURRENCE

Chondrites are the most abundant meteorite class, making up about 91-92% of the approximately 20,000 Meteorite s classified. The largest was the Jilin Meteorite , a H Chondrite .

ORIGIN

The parent bodies of chondrites are (or were) small to medium sized Asteroid s that were never part of any body large enough to undergo melting and Planetary Differentiation . These bodies Accreted right at the beginning of the Solar System's history, about 4.5 billion years ago, and were largely unchanged since.

COMPOSITION

Chondrites consist of the Chondrules (up to 80%) embedded in a fine rock Matrix . This is typically composed of small mineral or metal grains, chondrule fragments, as well as a variety of minerals formed due to fluid activity on the parent bodies. Ca-Al Inclusions are also a common component embedded in the matrix along with the chondrules. Some interstellar mineral grains which formed around other stars in the solar neighbourhood prior to accretion are also present. Some chondrites have been Breccia ted by impacts. The chondrules are not always visibly apparent due to metamorphic or water-based alteration.

The metal grains are composed mainly of Iron and Nickel . A common way to check whether a candidate rock is a meteorite is to test for the presence of this Nickel .

The average chemical composition resembles the original undifferentiated Solar Nebula from 4.5 billion years ago, excepting volatile elements such as Hydrogen or Helium . There are, however, some differences in the chemical abundances, presumably due to two factors: The different conditions existing at accretion time at various distances from the Sun, and also due to later impact mixing and physical processes occurring on the parent asteroids.

The classes based on chemical composition are:

Carbonaceous (C) Chondrite s (about 3.5% of the total)

Ordinary (O) Chondrite s, about 95% of the total, divided into the:

--- H Chondrite s (44%)

--- L Chondrite s (38%)

--- LL Chondrite s (13%)

E (Enstatite) Chondrites (just above 1%)

R (Rumuruti) Chondrites (rare)

K (Kakangari) Chondrites (rare)

F (Forsterite) Chondrites (rare)

PETROLOGIC TYPES

Apart from chemical differences which determine a chondrite's class (above), a Petrological type with labels 1-7 is assigned, based on the amount of physical alteration after accretion into the parent body:

Types 1-2 have been significantly altered by the action of liquid water, and the Chondrule s may be less distinct. For type 2, the water did not reach more than 20° C , while the more strongly altered type 1 were exposed to water at up to 50°C.

Type 3 is the baseline type which has not been significantly altered from the primordial state. Great numbers of pristine Chondrule s are readily apparent. These are the meteorites most closely resembling the original Solar Nebula material.

Types 4-6 have been increasingly altered by thermal Metamorphism , presumably due to burial deep within the parent body, which was heated by Radioactive Decay in the first few million years after Accretion . The chondrules become less distinct with increasing petrologic type.

Type 7 have been so strongly altered by metamorphism, that the chondrules are not visible, although the rock has retained its chondritic composition. These are a transition type towards the Primitive Achondrite s.

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Chondrite