The mineral olivine is a type of magnesium and iron silicate with the chemical formula (Mg, Fe)₂SiO₄. It belongs to a group of minerals called nesosilicates or orthosilicates. Olivine is a major part of Earth's upper mantle and is found deep underground, but it breaks down quickly when exposed to the surface. It is used as a gemstone called peridot (or chrysolite) and in industrial processes like metalworking.

Olivine has two main types based on the amount of magnesium and iron it contains: forsterite (Mg₂SiO₄) and fayalite (Fe₂SiO₄). The composition of olivine is often described using percentages of these two types, such as Fo70 Fa30. Forsterite has a very high melting point of about 1,900°C (3,450°F), while fayalite melts at a much lower temperature of around 1,200°C (2,190°F). Other properties of olivine also change gradually between these two types. Olivine contains only small amounts of elements other than oxygen, silicon, magnesium, and iron. Manganese and nickel are the most common additional elements found in olivine.

Olivine is the name of a group of minerals with a similar structure. This group includes tephroite (Mn₂SiO₄), monticellite (CaMgSiO₄), larnite (Ca₂SiO₄), and kirschsteinite (CaFeSiO₄).

Olivine's crystal structure is based on a type of repeating pattern called the orthorhombic P Bravais lattice. In this structure, each silicon-oxygen (SiO₄) unit is connected to metal ions, with each oxygen atom in the SiO₄ group bonded to three metal ions. Olivine has a structure similar to the mineral magnetite, but instead of using two trivalent and one divalent cation, it uses one quadrivalent cation and two divalent cations in the formula M₂MO₄.

Identification and paragenesis

Olivine is named for its usually olive-green color, which is thought to come from small amounts of nickel. However, it can sometimes turn reddish when iron reacts with oxygen.

Translucent olivine is sometimes used as a gemstone called peridot, named after the French word for olivine. It is also called chrysolite, from Greek words meaning "gold" and "stone," though this name is rarely used in English today. Some of the best-quality olivine used as a gem has come from mantle rocks on Zabargad Island in the Red Sea.

Olivine is found in both mafic and ultramafic igneous rocks, as well as in certain metamorphic rocks. Magnesium-rich olivine forms from magma that has a lot of magnesium and little silica. This magma cools to create mafic rocks like gabbro and basalt. Ultramafic rocks often contain large amounts of olivine, and those with more than 40% olivine are called peridotites. Dunite, a rock with over 90% olivine, may form when olivine crystals settle in magma or line magma channels. Olivine and its high-pressure forms make up more than half of Earth's upper mantle, and olivine is one of Earth's most common minerals by volume. Olivine can also form when magnesium-rich, low-silica sedimentary rocks like dolomite undergo metamorphism.

Iron-rich olivine, called fayalite, is much rarer. It appears in small amounts in some granites and rhyolites, and very iron-rich olivine can coexist with quartz and tridymite. In contrast, magnesium-rich olivine cannot form stable mixtures with silica minerals, as they would react to create orthopyroxene.

Magnesium-rich olivine remains stable at depths up to about 410 kilometers (250 miles). Because it is the most common mineral in Earth's upper mantle, olivine greatly influences how the mantle flows, which affects plate tectonics. Studies show that olivine under high pressure (about 360 km deep) can hold up to 8,900 parts per million of water by weight. This water reduces how hard olivine is to move, and because olivine is so common, it may hold more water than all Earth's oceans combined.

An olivine pine forest, a type of plant community, is found only in Norway. It grows on dry olivine ridges in the fjord areas of Sunnmøre and Nordfjord.

Examples of olivine include:
– Olivine grains eroded from lava on Papakolea Beach, Hawaii
– Light green olivine crystals in peridotite xenoliths from basalt in Arizona
– Olivine basalt from the Moon, collected by Apollo 15 in 1971
– Bright green olivine from Pakistan, showing a chisel-like shape and silky shine
– Olivine in lava from the Azores

Magnesium-rich olivine has also been found in meteorites, on the Moon, Mars, and in infant stars, as well as on asteroid 25143 Itokawa. Meteorites containing olivine include chondrites, which are fragments from the early solar system, and pallasites, which mix iron-nickel and olivine. Some rare asteroids, called A-type, are believed to have surfaces mostly covered in olivine.

The spectral signature of olivine has been observed in dust disks around young stars. Comet tails, which formed from dust around the early Sun, often show olivine's signature. In 2006, the Stardust spacecraft confirmed olivine in a comet sample. Magnesium-rich olivine similar to comets has also been detected in the planetesimal belt around the star Beta Pictoris.

Crystal structure

Minerals in the olivine group form crystals in the orthorhombic system (space group Pbnm) with silicate tetrahedra that are separate from one another, making olivine a type of nesosilicate. The structure consists of oxygen ions arranged in a hexagonal, close-packed pattern. In this arrangement, half of the octahedral positions are filled with magnesium or iron ions, and one-eighth of the tetrahedral positions are filled with silicon ions.

There are three different oxygen positions (labeled O1, O2, and O3 in the figure), two different metal positions (M1 and M2), and one distinct silicon position. O1, O2, M2, and Si are located on mirror planes, while M1 is positioned at an inversion center. O3 is in a general position not aligned with any symmetry plane.

High-pressure polymorphs

At very high temperatures and pressures deep inside the Earth, the mineral olivine can no longer stay in its usual structure. Below about 410 km (250 mi), olivine changes into a different form called wadsleyite in a process that releases heat. At around 520 km (320 mi) depth, wadsleyite changes again into ringwoodite, which has a structure similar to spinel. At about 660 km (410 mi) depth, ringwoodite breaks down into two new minerals: silicate perovskite ((Mg,Fe)SiO₃) and ferropericlase ((Mg,Fe)O) in a process that absorbs heat. These changes cause a sudden increase in the density of the Earth's mantle, which scientists can detect using seismic waves. These changes also affect how the mantle moves, as heat-releasing changes help movement across the boundary, while heat-absorbing changes slow it down.

The pressure at which these changes happen depends on temperature and the amount of iron in the minerals. At 800 °C (1,070 K; 1,470 °F), the pure magnesium form of olivine, called forsterite, changes into wadsleyite at 11.8 gigapascals (116,000 atm) and into ringwoodite at pressures above 14 GPa (138,000 atm). Adding more iron lowers the pressure needed for these changes and reduces the range where wadsleyite is stable. When olivine contains about 0.8 mole fraction of fayalite, it changes directly into ringwoodite between 10.0 and 11.5 GPa (99,000–113,000 atm). Fayalite changes into a mineral called Fe₂SiO₄ spinel at pressures below 5 GPa (49,000 atm). Higher temperatures increase the pressure required for these changes to occur.

Weathering

Olivine is one of the less stable common minerals on Earth's surface, as shown by the Goldich dissolution series. It changes into iddingsite (a mix of clay minerals, iron oxides, and ferrihydrite) quickly when water is present. Speeding up the weathering process of olivine, such as by spreading finely ground olivine on beaches, has been suggested as a low-cost method to remove carbon dioxide from the atmosphere. Finding iddingsite on Mars would indicate that liquid water existed there in the past and could help scientists determine when the last liquid water was present on the planet. Due to its fast weathering, olivine is seldom found in sedimentary rock.

Mining

Norway is the primary source of olivine in Europe, especially in an area from Åheim to Tafjord and from Hornindal to Flemsøy in the Sunnmøre district. Olivine is also found in Stad Municipality. Approximately 50% of the world’s olivine used for industrial purposes is produced in Norway. In Svarthammaren, located in Norddal Municipality (now Fjord Municipality), olivine was mined from about 1920 to 1979, with daily production reaching up to 600 metric tons. Olivine was also collected during construction of hydroelectric power stations in Tafjord. An open-pit mine at Robbervika in Norddal Municipality has operated since 1984. The red color of olivine is reflected in local names such as Raudbergvik (Red Rock Bay) and Raudnakken (Red Ridge).

In 1766, Hans Strøm described the typical red color of olivine on its surface and the blue color inside. He noted that large amounts of olivine were broken from bedrock in the Norddal district and used as sharpening stones.

Kallskaret near Tafjord is a nature reserve that contains olivine.

Applications

Olivine is used instead of dolomite in steel production.

The industry that makes aluminum objects uses olivine sand to shape metal. Olivine sand needs less water than silica sand, but it still keeps the mold strong during handling and pouring of the metal. Using less water means less steam is released from the mold when the metal is poured.

In Finland, olivine is sold as a good choice for sauna stoves because it is dense and stays strong when heated and cooled many times.

Olivine that is high quality is used as a gemstone called peridot.

Experimental uses

Scientists are studying a method to remove carbon dioxide from the air by reacting it with crushed olivine. The reaction produces silicon dioxide, magnesium carbonate, and iron oxides. A public benefit organization called Project Vesta is testing this method on beaches, where waves help break the olivine into smaller pieces, increasing its surface area for better reactions.

Another use of olivine is in creating cement that does not release carbon dioxide into the air or may even remove some carbon dioxide from the air.