Calcium-aluminium-rich inclusion (CAI)
CAIs are pale, irregular inclusions rich in calcium and aluminium that are the oldest dated solids formed in the Solar System.
A calcium-aluminium-rich inclusion, or CAI, is a small mineral assemblage made of minerals that form at very high temperatures, such as spinel, melilite, perovskite, hibonite and calcium-rich pyroxene. Calculations show these are among the first minerals expected to condense from hot gas of solar composition, so CAIs are thought to have formed very close to the young Sun.
Their importance is hard to overstate. Lead–lead dating gives CAIs an age of about 4,567 million years, and that date is used to mark the start of the Solar System. They also once contained radioactive aluminium-26, now long decayed, whose traces let scientists build a timeline of the first few million years. Their oxygen isotopes differ markedly from those of Earth rocks, adding to the evidence that they formed in a distinctive region of the early disc.
The classic example is the Allende CV3 chondrite, which fell in Mexico in 1969 and supplied tonnes of material rich in CAIs. On a cut slice of a CV chondrite, CAIs appear as white, cream or pinkish patches, often irregular or fluffy in outline, from under a millimetre up to a centimetre or more across, standing out clearly against the dark grey matrix.
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