Chondrites
Chondrites are the most primitive, undifferentiated meteorites that have recorded and preserved the first moments of the solar system, through the first objects formed in the protosolar nebula they contain. They come from parent bodies formed later than those of the achondrites or not big enough to have been able to differentiate. Thus, they contain unique information about the origin of the solar system by the preservation of the first components. As such they are a precious source of information on the physico-chemical processes which took place and which led to our solar system such as we know it today.
Components
Refractory inclusions
Refractory inclusions are of two types: calcium aluminium rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs). CAIs are the oldest objects in the solar system, whose age estimated by the radiogenic datation is 4.567 Ga (billion years). It is the age dating of this inclusions that made it possible to date the age of the solar system. Most CAIs are composed of oxides and silicates of aluminum and calcium such as: melilite, diopside, anorthosite, spinel, hibonite and perovskite. They retain also isotopic anomalies originating from the birth of the solar system. Some CAIs would have formed by gas condensation at around 1000-1500°C. They have an irregular shape and a fine-grained texture. Others, on the other hand, have textures indicating crystallisation, with coarser grains. Refractory inclusions are more frequent in carbonaceous CV-type chondrites, although this component is quite rare, less than 0.1% in the majority of chondrites.
Chondrules
Chondrules are the main constituents of chondrites, which term was defined in 1864 by the German mineralogist Gustav Rose. Chondrules are micro-metric droplets (diameter ≺ 1 mm) of silicates partially or completely melted in a gas at high temperature (1500-1850°C) and cooled in a few minutes, a few hours at most. Indeed, their igneous textures prove to us that they were partially or totally melted before being incorporated into the parent body of chondrites and the presence of glass demonstrates that they cooled too quickly to fully crystallise. The possible mechanisms that led to the melting of these droplets are still subject to debate, as well as the precursors of those chondrules.
Chondrules have a wide range of chemical composition and textures. The estimated age of chondrules indicate that they would have formed only between 1 and 3 million years after CAIs, at a distance of about 5 Astronomical Unit (Earth is 1 AU from the Sun), in the median plane of the protosolar disk.
Matrix
The matrix of chondrites is composed of grains which fill the space between the components and which cement the whole. It is composed of a mixture of many minerals, such as olivine and pyroxene, preponderant in chondrites, oxides, sulphides, metal. It also contains either signs of exposure to hydrothermal alteration, by the presence of phyllosilicates, or signs of exposure to thermal alteration, by the presence of a recrystallised texture. The abundance of matrix is highly variable depending on the type of chondrite and is greater than 70% in the most primitive types of carbonaceous chondrites (cf. table below).
Chondrites represent more than 90% of objects in world collections. They are made up of an aggregate of several components: chondrules, refractory inclusions, metal and sulphide grains, caught in a matrix of fine grains. The matrix may also contain carbonaceous material and presolar grains.
Groups
Chondrites are separated into clans and groups according to 3 criteria: the overall chemical composition of the rock, the petrology and the texture and the oxygen isotopy. A distinction is thus made between ordinary chondrites (OCs), carbonaceous chondrites (CCs) and enstatite chondrites (ECs) who form the clans. Each clan is composed of chondrites sharing many textural and mineralogical characteristics, and presumably originating from the same region of the solar system.
Within these clans, chondrites are classified into 15 groups , again on petrographic and chemical criteria. For example, each group of chondrites has its own assembly of matrix components, chondrules, metal and refractory inclusions (cf. table below). Oxygen isotopes are also a powerful tool for distinguishing the different groups of chondrites (cf. graph below). The naming of carbonaceous chondrites groups is mostly related to the name of a typical meteorite of the group. For enstatite chondrites and ordinary chondrites, LL, L and H indicate the increase in the proportion of iron metal (L-low; H-high).
Chondrites have undergone various alterations, aqueous or thermal, which have modified their chemistry original. Thus, we assign them a type petrological (see table below), ranging from 1 to 6 depending on the intensity of the alteration suffered. Petrology type 3 corresponds to unweathered chondrites, while chondrites 2 and 1 suggest an increase in aqueous alteration, and types 4 to 6 an increase in thermal alteration, also called thermal metamorphism. Each group of chondrites also underwent preferential alteration: for example, enstatite or ordinary chondrites underwent thermal metamorphism, while carbonaceous chondrites underwent aqueous alteration. Thus, the carbonaceous chondrites are said to be hydrated, as they contain water, up to 10% of their mass.
Achondrites
Stony irons
Irons
