Chondrites are meteorites that have chemical compositions similar to that of the Sun. Since the Sun makes up about 99 percent of the mass of the solar system, it is assumed that the composition of the Sun represents the average composition for the entire solar system, and that this average composition resembles the original composition of the solar system when it was formed. Therefore since chondrites and the Sun have similar compositions, chondrites are thought to have very primitive compositions that are close to the average starting material that formed the solar system.
Chondritic meteorites contain small round nodules called chondrules that consist of a mixture of crystals and glass. Most interpretations for the origin of chondrules suggest that they represent small droplets of liquid that condensed during the earliest stages of the formation of the solar system, before they were incorporated into the meteorites. Chondrules that have been dated yield isotopic ages of 4.568 billion years, the time that the solar system began to condense from the solar nebula. Thus, chondrules represent small remnants of the earliest solar system material. Other chondritic meteorites, such as the famous Allende meteorite, that have been dated also give ages of 4.566 billion years, similar to that obtained from the chondrites.
Many experiments have been done on chon-drules to determine their exact components and the conditions in which they formed since they are interpreted to have formed during the early stages of the formation of the solar system. Knowing the conditions of their formation yields information about the conditions in the early solar system and solar nebula. Most experiments show that the chondrules formed at temperatures of at least 2,700°F (1,500°C) and that the chondrules cooled rapidly. Some chondrules contain unusual minerals. one group of these contains a suite of very high-temperature minerals and is called calcium-aluminum inclusions (CAIs), typically exhibiting textures like concentric skins of an onion. Experiments on the temperature of formation of these CAIs indicate that they formed at temperatures of at least 3,100°F (1,700°C) and underwent slow cooling. From these extraordinarily high temperatures scientists infer that these CAIs represent the oldest parts of the oldest fragments of the early solar system.
Most chondritic meteorites consist of mixtures of chondrules and the minerals olivine and pyroxene, and show little evidence of being heated or metamorphosed since they formed. Some, however, show textures like partial melting that indicate they were heated to temperatures of up to 1,800°F (1,000°C) after they formed. still others are cut by veins that have minerals with water in their structures such as carbonates, sulfates, and magnetite. Thus water existed in the asteroid belt in the early solar system. The range in the amount of heating of chondrites likely reflects that they were incorporated into a larger body, with the higher temperature heating happening deeper inside this now-destroyed asteroid or protoplanetary body.
Isotopic dating techniques have shown that most chondritic meteorites cooled within 60 million years of the time of the formation of the solar system. some time after that, impacts in the asteroid belt between the orbits of Mars and Jupiter broke the larger proto-planets or asteroid parent bodies into smaller pieces now preserved as the asteroid belt. some chondrites are composed of strongly fragmented rock called breccia, produced by these early collisions in the asteroid belt. Calculations of the pressures needed to produce these breccias indicate that the pressures reached 75 giga Pascals, or the equivalent of 750,000 times the atmospheric pressure on Earth.
Chondritic meteorites are divided into a number of classes with similar compositions and textures, thought to represent formation in similar parts of the solar system. These in turn are divided into groups thought to represent fragments of the same parent body.
The main classes of chondrites include the ordinary chondrites, carbonaceous chondrites, and enstatite chondrites. Some classifications add further subdivisions based on the type of alteration or metamorphism, such as alteration by water, or meta-morphism by late heating. Ordinary chondrites are thought to have formed in parent bodies that were 100-120 miles (165-200 km) in diameter. Carbonaceous chondrites contain organic material such as hydrocarbons in rings and chains, and amino acids. Even though these organic molecules can serve as building blocks of life, no life has been found on any meteorites unless they were contaminated on Earth. Enstatite chondrites contain small sulfide minerals that indicate very rapid cooling, suggesting that the original material came from deep in a larger body that was broken apart by strong impacts, and the deep material cooled quickly in space after the violent collisions. The enstatite chondrites typically have impact breccias in their structures.
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