Spinel
Spinel is a magnesium-aluminium oxide that forms tiny hard crystals in CAIs and some impact rocks.
Spinel is an oxide of magnesium and aluminium, MgAl2O4, and the name of a wider family of oxides with the same crystal structure, which includes chromite and magnetite. Pure spinel can be red, pink, blue or colourless and is prized as a gemstone.
In meteorites, spinel is a common ingredient of calcium-aluminium-rich inclusions, where tiny crystals sit enclosed in melilite, pyroxene or anorthite. It is resistant to heat and to chemical change, so it preserves original isotopic signals from the early Solar System. Some spinel grains in primitive meteorites are presolar, formed around other stars before the Sun existed.
Spinel group minerals record oxygen levels and cooling histories, and their crystal structure is shared by several high-pressure forms of olivine, such as ringwoodite, created in shock veins. In the lab, spinel is identified by its chemistry and its cubic, often octahedral, crystal shape.
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