Chromite
A black iron–chromium oxide found in almost every stony meteorite, and the survivor that reveals fossil meteorites.
Key facts
- Formula
- FeCr2O4
- Family
- Oxides
- Crystal system
- Cubic
What it is
Chromite is a spinel-group oxide of iron and chromium, often with magnesium, aluminium and titanium. It is hard, dense and very resistant to weathering and chemical attack.
Where it is found
It makes up about half a per cent of ordinary chondrites, as small grains and occasional chromite-rich chondrules, and occurs in pallasites, mesosiderites, iron meteorites and most achondrites. In equilibrated chondrites its composition changes with the class, which helps classify them.
In limestone about 466 million years old at Kinnekulle in Sweden, geologists found more than a hundred fossil meteorites in which every mineral had been replaced except chromite. The chromite grains still carried the chemistry of L chondrites, the debris of a huge collision in the asteroid belt.
How it is recognised
It forms black, opaque, slightly magnetic grains with a dark brown streak. In polished section it is grey and moderately reflective. A microprobe gives its chromium and titanium contents.
Did you know?
Chromite grains from L chondrites are about a hundred times more common in some 466-million-year-old rocks than in younger ones.
Chromite is the world's main ore of chromium.