Magnesiochromite
The magnesium-rich partner of chromite, found in some chondrules and primitive meteorites.
Key facts
- Formula
- MgCr2O4
- Family
- Oxides
- Crystal system
- Cubic
What it is
Magnesiochromite is a spinel of magnesium and chromium. It forms a continuous series with chromite, FeCr2O4, as magnesium and iron swap places. The balance between them depends on the temperature and on the rocks around the grain.
Where it is found
It occurs in some chondrules of unequilibrated chondrites, in ureilites and in some carbonaceous chondrites. Chromite-family grains, including some rich in magnesium, also survive as fossils in ancient sediments long after the rest of a meteorite has weathered away.
How it is recognised
It is black and opaque in hand specimen and grey in reflected light. Microprobe analysis shows whether magnesium or iron dominates.
Did you know?
Chromite grains are so tough that they survive for hundreds of millions of years in sediment after the rest of a meteorite has gone.
The colour of ruby and emerald also comes from chromium.