Peridot, a gemstone known for its vibrant lime-green to olive color, is the gem-quality variety of the common mineral olivine. Unlike most gemstones, which form within the Earth’s crust, peridot originates much deeper. Its formation is tied directly to the planet’s extreme geological forces, involving high pressure and intense heat, as well as processes that occurred far beyond Earth’s atmosphere.
The Basic Chemistry of Olivine
The mineral olivine, the base for peridot, is chemically classified as a magnesium iron silicate. Its general chemical formula is (Mg, Fe)2SiO4, indicating a solid solution where magnesium and iron substitute for one another. The characteristic green hue of peridot is caused by the presence of iron (Fe) within this silicate structure. Peridot is an idiochromatic gem, meaning its color is inherent to its fundamental chemical composition, not caused by trace impurities. The specific shade of green, ranging from yellowish-green to deep olive, depends on the precise ratio of iron to magnesium.
Deep Earth Formation and Volcanic Transport
Peridot’s formation begins deep within the Earth’s upper mantle, the layer lying just beneath the crust. At depths ranging from 20 to 55 miles, extremely high temperatures and intense pressure allow magnesium, iron, and silica to combine into dense olivine crystals. Peridot and diamond are two of the only gemstones that form exclusively in the mantle.
The crystals remain embedded in the mantle rock until violent geological events provide a pathway to the surface. Peridot is brought up almost exclusively by volcanic activity, particularly within rapidly ascending basaltic magma flows.
These crystals are often found encased in pieces of mantle rock, known as xenoliths, which are torn off the conduit walls during the magma’s fast journey upward. This rapid transport is important because it prevents the peridot crystals from chemically altering or dissolving as they move out of the high-pressure, high-temperature environment of the mantle. The gem material is then found as nodules in solidified lava flows or within these basaltic volcanic rocks.
Peridot from Space: Extraterrestrial Origins
The second source of peridot is entirely extraterrestrial, connecting the gem to the formation of the early solar system. Gem-quality peridot crystals have been discovered embedded within stony-iron meteorites called pallasites. These pallasite meteorites are thought to be remnants of shattered planetesimals destroyed by ancient collisions in the asteroid belt.
The peridot crystals in these meteorites formed at the boundary between the metallic core and the rocky mantle of their parent body. They are suspended within a matrix of iron and nickel metal, a composition different from their terrestrial counterparts. These crystals formed in space before falling to Earth, making them older than Earth’s continental crust. Although chemically similar to Earth-formed peridot, they often lack certain trace elements, which helps scientists differentiate their cosmic origin.