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Chondrites are more or less undifferentiated, primordial matter that has remained nearly unchanged for the last 4.5 billion years. These stony meteorites formed nearly simultaneously with the central star of our system, the Sun. It is thought that small droplets of olivine and pyroxene condensed and crystallized from the hot primordial solar nebula in form of small spheres that we nowadays call chondrules. This process of solidification and crystallization is not completely understood, and different scientists suggest different theories for chondrule formation. However, they all agree that those chondrules accreted with other material that condensed from the solar nebula forming a matrix, and of course, the larger parent bodies of those primitive meteorites; i.e. smaller and larger asteroids of chondritic nature.
In their chemical composition, chondrites resemble the Sun, depleted of the most volatile elements like hydrogen and helium. However, the distribution of elements has not been uniform in the original solar nebula - elemental composition varied as did the conditions under which the chondritic parent bodies formed. Different asteroids formed in various regions of the primordial solar nebula under different conditions. Those parent bodies were further subjected to different thermal and chemical processes as well as to impacts with other asteroids resulting in a variety of chondrites, which have been categorized into several clans, groups, and subgroups by modern meteoritics and cosmochemistry.
Petrologic Types of Chondrites
Chondrites are not only differentiated into clans and groups reflecting chemical and isotopic relationships to each other, common parent bodies, or regions of formation in the primordial solar nebula. The chondrites of each clan and group are further subdivided according to petrologic viewpoints and are classified into petrologic types.
Each type is designated with a number from 1 to 7 whereas type 3 builds the base line and describes a type of chondrite that has suffered little or any alteration by neither water nor any thermal metamorphism. The petrologic types mirror the degree of chemical equilibrium within the minerals of a chondrite. Petrologic types 1 to 3 represent highly unequilibrated chondrites due to a lack of thermal metamorphism while the types 4 to 7 are increasingly equilibrated due to extended thermal processes. For this reason researchers sometimes talk of unequilibrated / equilibrated chondrites when they are referring to certain petrologic types.
Unequilibrated chondrites of petrologic types 2 and 1 have been subjected to an increasing degree of aqueous alteration with the result that type 1 chondrites don't show any chondrules - they are virtually absent even though the meteorite is of chondritic composition and certainly contained chondrules in its early history. Type 2 chondrites exhibit only a sparse distribution of more or less aqueously altered chondrules. Both types are represented only by members of the carbonaceous chondrite clan, and we don't find them in other chondrite groups.
Petrologic types 3 to 7 have been exposed to increasing thermal metamorphism that is reflected in an increasing alteration of the chondrules. Type 3 exhibits abundant, unaltered and distinct chondrules while the chondrules of petrologic types 4 to 6 are increasingly indistinct due to thermal metamorphism and (non-igneous, solid-solution) recrystallization. In chondrites of petrologic type 7 we can witness the end of this process since chondrules are completely absent even though the meteorite has retained its chemical composition revealing its chondritic nature. Those type 7 chondrites can actually be regarded as true transitional specimens that form a link between chondrites and primitive achondrites.