A calcium–aluminium-rich inclusion or Ca–Al-rich inclusion (CAI) is a submillimeter- to centimeter-sized light-colored calcium- and aluminium-rich inclusion found in carbonaceous chondrite meteorites. They are probably the oldest substances in the Solar System, which formed approximately 4567.30 ± 0.16 Ma.
CAIs consist of minerals that are among the first solids condensed from the cooling protoplanetary disk. They are thought to have formed as fine-grained condensates from a high temperature (>1300 K) gas that existed in the protoplanetary disk at early stages of Solar System formations. Some of them were probably remelted later resulting in distinct coarser textures. The most common and characteristic minerals in CAIs include anorthite, melilite, perovskite, aluminous spinel, hibonite, calcic pyroxene, and forsterite-rich olivine.
Using lead (Pb–Pb) isotopic data determined on CAIs, an age of 4567.30 ± 0.16 million years has been calculated, which can be interpreted as the beginning of the formation of the planetary system. However, due to possible disturbances of the lead isotopic system within the CAIs, this age is possibly only a lower limit of the true age. Radiometric dating with Pb–Pb, Al–Mg and Cr–Mn chronometers shows that the CAIs formed up to 3 million years before the chondrules appeared, although some chondrules formed simultaneously with CAIs.
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