Corundum
Corundum is aluminium oxide, the first mineral predicted to condense from hot solar gas, and some meteoritic grains are presolar.
Corundum is aluminium oxide, Al2O3. On Earth, coloured varieties are the gemstones ruby and sapphire. It is very hard, ranking 9 on the Mohs scale.
Calculations show that corundum should be the first major mineral to condense as a gas with the Sun's composition cools, at roughly 1,400 to 1,500 °C at the low pressures thought to apply in the early disc. Tiny corundum grains do occur in some calcium-aluminium-rich inclusions.
The most exciting meteoritic corundum is presolar. Some micrometre-sized grains extracted from primitive meteorites by dissolving the surrounding rock in acid have oxygen isotope ratios utterly unlike anything in the Solar System. They formed in the outflows of red giant stars and other stars that lived and died before the Sun, then drifted into the cloud from which our Solar System was born. Each grain is a sample of a different star.
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