Chondrites and their Components

What are Chondrites?Chondrites are meteorites that provide the best% clues to the origin of the solar system. They are the oldest known rocks - their components formed during the birth of the solar system ca. 4,567 Ma - and their abundances of nonvolatile elements are close to those in the solar photosphere. Chondrites are broadly ultramafic in composition, consisting largely of iron, magnesium, silicon, and oxygen. The most abundant constituents of chondrites are chondrules, which are igneous particles that crystallized rapidly in minutes to hours. They are composed largely of olivine and pyroxene, commonly contain metallic Fe,Ni and are 0.01-10 mm in size. Some chondrules are rounded as they were once entirely molten but many are irregular in shape because they were only partly melted or because they accreted other particles as they solidified. Chondrites themselves were never molten. The definition of a chondrite has expanded recently with the discovery in Antarctica and the Sahara Desert of extraordinary meteorites with chondrules 10-100 μm in size, and chondrites so rich in metallic Fe,Ni that they were initially classified as iron meteorites with silicate inclusions. Thus, in meteoritics, as in other fields of planetary science, new discoveries sometimes require definitions to be modified.Chondrites are so diverse in their mineralogical and textural characteristics that it is not possible to describe a typical chondrite. We show one with diversely textured chondrules including prominent, aesthetically pleasing, rounded chondrules (Figure 1(a)), and another with more uniformly textured chondrules (Figure 1(b)). Owing to the high abundance of rounded or droplet chondrules in the abundant, so-called "ordinary" chondrites ( Figure 1(a)), studies of the origin of chondrules have commonly been based on these chondrites. (7K)Figure 1. Maps showing magnesium concentrations in two chondrites: (a) PCA91082, a CR2 carbonaceous chondrite, and (b) Tieschitz, an H/L3.6 ordinary chondrite. In CR chondrites, as in most carbonaceous chondrites, nearly all chondrules have porphyritic textures and are composed largely of forsterite (white grains), enstatite (gray), and metallic Fe,Ni (black). The subscripts show type I chondrules, which are common, and type II, which are FeO-rich and rare in this chondrite. Tieschitz, like other ordinary chondrites, is composed of all kinds of chondrules with diverse FeO concentrations. Key to chondrule types: BO, barred olivine; C, cryptocrystalline, PO, porphyritic olivine; POP, porphyritic olivine-pyroxene; PP, porphyritic pyroxene; RP, radial pyroxene. These maps were made with an electron microprobe from Mg Kα X-rays. Chondrites contain diverse proportions of three other components: refractory inclusions (0.01-10 vol.%), metallic Fe,Ni (<0.1-70%), and matrix material (1-80%). Refractory inclusions are tens of micrometers to centimeters in dimensions, lack volatile elements, and are the products of high-temperature processes including condensation, evaporation, and melting. Two types are recognized: calcium- and aluminum-rich inclusions or CAIs, and amoeboid olivine aggregates. CAIs are composed of minerals such as spinel, melilite, hibonite, perovskite and Al-Ti-diopside, which are absent in other chondritic components (see Chapter 1.08). Amoeboid olivine aggregates consist of fine-grained olivine, Fe,Ni metal, and a refractory component largely composed of aluminum-diopside, anorthite, spinel and rare melilite. Grains of metallic Fe,Ni occur inside and outside the chondrules as grains up to a millimeter in size and, like the chondrules and refractory inclusions, formed at high temperatures. Matrix material is volatile-rich, and fine-grained (5-10 μm) and forms rims on other components and fills the interstices between them. Chondrite matrices have diverse mineralogies: most are disequilibrium mixtures of hydrated and anhydrous silicates, oxides, metallic Fe,Ni, sulfides, and organic material and contain rare presolar grains.