Chromite
A hard, black iron-chromium oxide mineral found in most stony meteorites and remarkably resistant to weathering.
Chromite is an oxide of iron and chromium, FeCr₂O₄, belonging to the spinel family of minerals. It occurs as small, black, slightly glassy grains in ordinary chondrites, achondrites such as diogenites, and many other meteorite types. In equilibrated chondrites, the composition of chromite shifts with metamorphism, so it can help assess petrologic type.
Chromite's great talent is survival. It is hard, dense and chemically tough, so it can last for hundreds of millions of years after the rest of a meteorite has weathered away. This has made it the star of studies of fossil meteorites: in Ordovician limestone in southern Sweden, around 466 million years old, researchers have found many fossil meteorites that are now almost entirely replaced by other minerals except for their chromite grains. The chromite chemistry shows they were L chondrites, linked to the break-up of the L-chondrite parent asteroid. Extraterrestrial chromite grains extracted from sediments are now used to trace changes in the flux of meteorites through time.
In meteorites, chromite appears as tiny black, metallic-looking grains that are weakly magnetic and harder than steel. Identification needs a polished section and microscope or microprobe.
Related terms
Level: Enthusiast Teens and adults