Calcium-Aluminum-rich Inclusions (or CAI's) are the light-colored inclusions common in carbonaceous chondrites. Like chondrules, CAI's are composed of refractory minerals, but there are several significant differences. As their name indicates, they are rich in calcium and aluminum minerals, and they are also quite poor in iron. They appear to be approximately 2 million years older than chondrules. Their shapes are less regular, while common chondrules are more uniformly spherical. Some scientists believe that the different composition and age of CAI's is strong evidence that the formation of the solar system was triggered by a nearby supernova, which would have provided the CAI's or (more likely) the gasses and dust from which they condensed. Alternatively, they may have been captured (via friction) from another passing sun as it formed (with a different composition) in the dense stellar nursery of our Sun's birth.
The minerals identified in CAI's are many, including anorthite (CaAl2Si2O8), hibonite ((Ca,Ce)(Al,Ti,Mg)12O19), melilite (Ca2Al2SiO7), perovskite (CaTiO3), pyroxenes such as hedenbergite (CaFeSi2O6) and diopside (CaMgSi2O6), spinel (MgAl2O4), and additional forsterite-rich olivine ((Mg, Fe)2SiO4, but very little iron is present).
Much of the variety of primordial minerals described in The Evolution of Minerals is due to the variety of species found in CAI's.
It is clear that the CAI's have been subjected to more high-temperature events than the associated chondrules. Whether that is the result of a different formation mechanism or merely due to a greater age is unknown.
CAI's are typically found in millimeter to centimeter size grains, but the largest are several centimeters across.
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