Olivine is an abundant silicate mineral that can be found in many different rock types around the world. Here is an overview of the primary locations and geologic settings where olivine occurs naturally.
The Mantle
Olivine is one of the major constituents of Earth's mantle, the layer found between the crust and the core. The composition of the mantle is estimated to be around 55% olivine overall. Olivine makes up over 60% of the upper mantle approximately 400 kilometers deep. Below that depth, the percentage of olivine decreases but remains a major component all the way to the core-mantle boundary 2900 kilometers deep.
Basalts
Olivine is a common phenocryst found in mafic igneous rocks such as basalt. Phenocrysts are early-forming crystals that became part of the lava as it erupted and cooled. Because olivine is one of the early crystallizing minerals in mafic magmas, it is found as phenocrysts in many basalts. Basalts are common along mid-ocean ridges and on ocean islands, so anywhere these rocks occur, olivine can be found if the lava cooled quickly enough to preserve phenocrysts.
Kimberlites
Kimberlite is an ultramafic igneous rock that originates deep within the mantle. Kimberlites are less common than basalts, but are economically important since they can transport diamonds to the surface from over 150 kilometers deep. Olivine is typically found in kimberlites as they erupt rapidly from great depths where olivine is stable. The presence of olivine phenocrysts helps identify kimberlites.
Peridotite
Peridotite is an ultramafic plutonic rock that makes up large portions of the upper mantle. It is composed predominantly of olivine with lesser amounts of other minerals like pyroxenes. Sometimes tectonic forces bring pieces of mantle peridotite to the surface in rock bodies called ophiolites. The olivine in peridotite alters easily, so fresh, unaltered olivine is not as common. But in some ophiolites, like in Oman, fresh olivine crystals up to several centimeters wide can be found.
Green Sand Beaches
The famous green sand beaches of Hawaii get their color from olivine grains eroded out of basalt lava flows. The famous Papakōlea Beach on the Big Island has sand composed of 90% olivine crystals. This olivine sand is also found on many other Hawaiian beaches with high concentrations of it occurring in bays near recent olivine-rich lava flows. Though less common, green olivine sand beaches can also occur in other volcanic regions like Iceland and New Zealand.
So, olivine is most abundant deep underground in Earth's mantle, but various geologic processes like volcanism and tectonics allow this common mineral to be found at the surface in specific rock types and locations around the globe. Its distinctive green color makes olivine easy to identify and unique among the minerals that form the rocks of our planet.
