Spinel
A hard magnesium aluminium oxide that is one of the first minerals to form around the young Sun.
Key facts
- Formula
- MgAl2O4
- Family
- Oxides
- Crystal system
- Cubic
What it is
Spinel is an oxide of magnesium and aluminium. It gives its name to the spinel group, which includes chromite and magnetite, all sharing the same compact cubic structure. It is hard, about 8 on the Mohs scale, and resistant to change.
Where it is found
Spinel is common in calcium-aluminium-rich inclusions in carbonaceous chondrites, often as small octahedra enclosed in other minerals. Presolar spinel grains, made in other stars, have been found in primitive meteorites. On the Moon, a spinel-rich rock type was discovered from orbit by Chandrayaan-1.
How it is recognised
In CAIs it forms small, colourless to pink, eight-sided crystals with high relief in thin section. It is isotropic, dark under crossed polarisers.
Did you know?
The famous Black Prince's Ruby in the British Imperial State Crown is actually a red spinel.
Spinel in CAIs is often richer in the light oxygen isotope oxygen-16 than almost any other material, close to the Sun's value.