Olivine
The green magnesium–iron silicate that is the most common mineral in stony meteorites and the gem crystals of pallasites.
Key facts
- Formula
- (Mg,Fe)2SiO4
- Family
- Silicates
- Crystal system
- Orthorhombic
What it is
Olivine is a silicate in which single groups of one silicon and four oxygen atoms are held together by magnesium and iron. The two metals swap freely, so olivine is really a series running from pure magnesium forsterite to pure iron fayalite. Meteoriticists describe where a grain sits on that scale by its fayalite number, written Fa: Fa18 means 18% of the metal sites hold iron.
Olivine crystallises early from hot melts and is stable at high temperatures, so it is one of the first minerals to form in chondrules and in the mantles of planets.
Where it is found
It makes up roughly 35 to 50% of most ordinary chondrites and is the main mineral of many chondrules. Pallasites contain large olivine crystals set in iron-nickel metal. It is also abundant in ureilites, brachinites, chassignites and some shergottites, and in carbonaceous chondrites it occurs in chondrules and amoeboid olivine aggregates. Olivine grains were among the dust caught from comet Wild 2 and the asteroid Itokawa.
How it is recognised
In hand specimens olivine is glassy and olive to yellow-green, turning brown or orange-red when weathered. Under the polarising microscope it shows bright interference colours and no cleavage. In classification its exact iron content, measured with an electron microprobe, sorts ordinary chondrites into H (about Fa16 to Fa20), L (Fa23 to Fa26) and LL (Fa27 to Fa32).
Did you know?
Gem olivine is called peridot, and peridot cut from pallasites is among the rarest gemstones in the world.
Olivine weathers quickly, which is why many desert meteorites look rusty brown inside.