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The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of Nesosilicates. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickly on the surface. Olivine has many uses, such as the gemstone peridot (or chrysolite), as well as industrial applications like metalworking processes.
The ratio of magnesium to iron varies between the two endmembers of the solid solution series: forsterite (Mg-endmember: ) and fayalite (Fe-endmember: ). Compositions of olivine are commonly expressed as molar percentages of forsterite (Fo) and/or fayalite (Fa) ( e.g., Fo70Fa30, or just Fo70 with Fa30 implied). Forsterite's melting temperature is unusually high at atmospheric pressure, almost , while fayalite's is much lower – about . Melting temperature varies smoothly between the two endmembers, as do other properties. Olivine incorporates only minor amounts of elements other than oxygen (O), silicon (Si), magnesium (Mg) and iron (Fe). Manganese (Mn) and nickel (Ni) commonly are the additional elements present in highest concentrations.
Olivine gives its name to the group of minerals with a related structure (the olivine group) – which includes tephroite (manganese2SiO4), monticellite (calcium4), larnite (Ca2SiO4) and kirschsteinite (CaFeSiO4) (commonly also spelled kirschteinite).
Olivine's crystal structure incorporates aspects of the orthorhombic P Bravais lattice, which arise from each silica (SiO4) unit being joined by metal divalent cations with each oxygen in SiO4 bound to three metal ions. It has a spinel-like structure similar to magnetite but uses one quadrivalent and two divalent cations M22+ M4+O4 instead of two trivalent and one divalent cations.Ernst, W. G. Earth Materials. Englewood Cliffs, NJ: Prentice-Hall, 1969. p. 65
Translucent olivine is sometimes used as a gemstone called peridot ( péridot, the French language word for olivine). It is also called chrysolite (or chrysolithe, from the Greek language words for gold and stone), though this name is now rarely used in the English language. Some of the finest gem-quality olivine has been obtained from a body of mantle rocks on Zabargad Island in the Red Sea. St. John's Island peridot information and history at Mindat.org
Olivine occurs in both mafic and ultramafic and as a primary mineral in certain . Mg-rich olivine crystallizes from magma that is rich in magnesium and low in silica. That magma crystallizes to mafic rocks such as gabbro and basalt. Ultramafic rocks usually contain substantial olivine, and those with an olivine content of over 40% are described as . Dunite has an olivine content of over 90% and is likely a cumulate formed by olivine crystallizing and settling out of magma or a vein mineral lining magma conduits. (2024). 9780521880060, Cambridge University Press. ISBN 9780521880060 Olivine and high pressure structural variants constitute over 50% of the Earth's upper mantle, and olivine is one of the Earth's most common minerals by volume. The metamorphism of impure dolomite or other with high magnesium and low silica content also produces Mg-rich olivine, or forsterite.
Fe-rich olivine fayalite is relatively much less common, but it occurs in in small amounts in rare and , and extremely Fe-rich olivine can exist stably with quartz and tridymite. In contrast, Mg-rich olivine does not occur stably with silica minerals, as it would react with them to form orthopyroxene ().
Mg-rich olivine is stable to pressures equivalent to a depth of about within Earth. Because it is thought to be the most abundant mineral in Earth's mantle at shallower depths, the properties of olivine have a dominant influence upon the rheology of that part of Earth and hence upon the solid flow that drives plate tectonics. Experiments have documented that olivine at high pressures (12 GPa, the pressure at depths of about ) can contain at least as much as about 8900 parts per million (weight) of water, and that such water content drastically reduces the resistance of olivine to solid flow. Moreover, because olivine is so abundant, more water may be dissolved in olivine of the mantle than is contained in Earth's oceans.
Olivine pine forest (a plant community) is unique to Norway. It is rare and found on dry olivine ridges in the fjord districts of Sunnmøre and Nordfjord.
The spectral signature of olivine has been seen in the dust disks around young stars. The tails of comets (which formed from the dust disk around the young Sun) often have the spectral signature of olivine, and the presence of olivine was verified in samples of a comet from the Stardust spacecraft in 2006. Press Release 06-091 . Jet Propulsion Laboratory Stardust website, retrieved May 30, 2006. Comet-like (magnesium-rich) olivine has also been detected in the planetesimal belt around the star Beta Pictoris.
There are three distinct oxygen sites (marked O1, O2 and O3 in figure 1), two distinct metal sites (M1 and M2) and only one distinct silicon site. O1, O2, M2 and Si all lie on , while M1 exists on an inversion center. O3 lies in a general position.
The pressure at which these phase transitions occur depends on temperature and iron content. At , the pure magnesium end member, forsterite, transforms to wadsleyite at and to ringwoodite at pressures above . Increasing the iron content decreases the pressure of the phase transition and narrows the wadsleyite stability field. At about 0.8 mole fraction fayalite, olivine transforms directly to ringwoodite over the pressure range . Fayalite transforms to spinel at pressures below . Increasing the temperature increases the pressure of these phase transitions.
Because of its rapid weathering, olivine is rarely found in sedimentary rock.
Hans Strøm in 1766 described the olivine's typical red color on the surface and the blue color within. Strøm wrote that in Norddal district large quantities of olivine were broken from the bedrock and used as .Strøm, Hans: Physisk og Oeconomisk Beskrivelse over Fogderiet Søndmør beliggende i Bergen Stift i Norge. Published in Sorø, Denmark, 1766.
Kallskaret near Tafjord is a nature reserve with olivine.
The aluminium foundry industry uses olivine sand to cast objects in aluminium. Olivine sand requires less water than silica sands while still holding the mold together during handling and pouring of the metal. Less water means less gas (steam) to vent from the mold as metal is poured into the mold.
In Finland, olivine is marketed as an ideal rock for sauna stoves because of its comparatively high density and resistance to weathering under repeated heating and cooling.
Gem-quality olivine is used as a gemstone called peridot.
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