Olivine and peridot are two names that are sometimes used interchangeably, but they actually refer to slightly different variations of the same mineral. Both olivine and peridot belong to the silicate mineral group known as olivine. So what exactly is the difference between olivine and peridot?
Olivine is the broader term that refers to the entire mineral group of iron-magnesium silicates. The chemical formula for olivine is (Mg,Fe)2SiO4, which means it contains varying amounts of magnesium and iron along with silica. Olivine is an abundant mineral found in Earth's mantle and is a common rock-forming mineral in mafic and ultramafic igneous rocks.
Peridot refers more specifically to the transparent gem-quality olivine that is used in jewelry. Peridot has a characteristic bright green color that makes it distinctive as a gemstone. Its color results from its chemical composition containing mostly magnesium along with some iron. Peridot's chemical formula is Mg2SiO4, magnesium silicate.
While olivine is composed of a solid solution series between two end-member minerals – forsterite (Mg2SiO4) and fayalite (Fe2SiO4) – peridot is confined to olivine crystals with higher levels of forsterite. Specifically, peridot refers to olivine crystals containing at least 20% iron by composition. Anything with less than 20% iron is simply called forsterite rather than peridot.
So:
- Olivine is the broad term for the mineral group of iron-magnesium silicates. It includes all compositions along the forsterite-fayalite solid solution series.
- Peridot is the name given specifically to transparent gem-quality olivine with a composition containing at least 20% iron. This gives it the bright green color that makes it valuable as a gemstone.
- Chemically, peridot is magnesium silicate (Mg2SiO4), while olivine can contain varying amounts of iron and magnesium.
- Peridot is a variety of olivine optimized for gemstone use. Not all olivine can be classified as peridot, only the high-clarity transparent crystals.
