Peridot is a distinctive gemstone, known for its vibrant green hues ranging from yellowish-green to deep olive. Its unique coloration contributes to its appeal, making it a sought-after gem. Its unusual origins hint at a remarkable journey from deep within the Earth.
Understanding Peridot
Peridot is the gem-quality variety of the mineral olivine, a magnesium iron silicate ((Mg,Fe)2SiO4). Iron within its crystal structure gives it an olive-green color, with intensity varying by iron percentage. Unlike most gemstones that form in the Earth’s crust, peridot originates much deeper, specifically in the planet’s upper mantle. This deep-earth formation distinguishes peridot.
Formation in the Earth’s Mantle
Peridot forms under extreme heat and immense pressure within the Earth’s upper mantle, at depths of approximately 20 to 90 miles (32 to 145 kilometers). Within this environment, peridot crystallizes from magma rich in both magnesium and iron.
Olivine is one of the first minerals to crystallize from cooling magma due to its high formation temperature, ranging from approximately 1100 to 1351 degrees Celsius. The dense, hot environment of the upper mantle provides the necessary ingredients and conditions for these magnesium and iron silicates to solidify into olivine crystals, which are then recognized as gem-quality peridot.
Ascent to the Surface
Once formed deep within the mantle, peridot crystals reach the Earth’s surface primarily through intense volcanic activity. During volcanic eruptions, magma originating from the mantle carries these peridot crystals upward.
The peridot crystals are often transported as xenoliths, which are fragments of foreign rock embedded within molten rock. As these basaltic lavas cool and solidify near or on the surface, the peridot crystals become trapped within the newly formed volcanic rock. Tectonic forces, such as the collision of Earth’s plates, also contribute to bringing mantle rocks containing peridot closer to the surface.
Extraterrestrial Peridot
Beyond its terrestrial origins, peridot also has an extraterrestrial presence. This gemstone is found within pallasites, a rare type of stony-iron meteorite. These meteorites are characterized by their distinctive composition, featuring olivine (peridot) crystals suspended within an iron-nickel metallic matrix.
Pallasite meteorites are believed to be remnants of asteroids that formed very early in the solar system’s history. The peridot crystals found in these meteorites can be billions of years old, predating the Earth itself. While pallasitic peridot is considerably rarer and typically yields smaller gem-quality crystals than its terrestrial counterpart, its cosmic origin offers valuable insights into the formation processes of our solar system.