It is rare and found on dry olivine ridges in the fjord districts of Sunnmøre and Nordfjord. Olivine pine forest (a plant community) is unique to Norway. Moreover, because olivine is so abundant, more water may be dissolved in olivine of the mantle than is contained in Earth's oceans. Experiments have documented that olivine at high pressures ( e.g., 12 GPa, the pressure at depths of about 360 km (220 mi)) 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. 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. Mg-rich olivine is stable to pressures equivalent to a depth of about 410 km (250 mi) within Earth. In contrast, Mg-rich olivine does not occur stably with silica minerals, as it would react with them to form orthopyroxene ( (Mg,Fe) 2Si 2O 6). The metamorphism of impure dolomite or other sedimentary rocks with high magnesium and low silica content also produces Mg-rich olivine, or forsterite.įe-rich olivine fayalite is relatively much less common, but it occurs in igneous rocks in small amounts in rare granites and rhyolites, and extremely Fe-rich olivine can exist stably with quartz and tridymite. 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. 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. Ultramafic rocks usually contain substantial olivine, and those with an olivine content of over 40% are described as peridotites. That magma crystallizes to mafic rocks such as gabbro and basalt. Mg-rich olivine crystallizes from magma that is rich in magnesium and low in silica. Olivine occurs in both mafic and ultramafic igneous rocks and as a primary mineral in certain metamorphic rocks. Some of the finest gem-quality olivine has been obtained from a body of mantle rocks on Zabargad Island in the Red Sea. It is also called chrysolite (or chrysolithe, from the Greek words for gold and stone), though this name is now rarely used in the English language. Translucent olivine is sometimes used as a gemstone called peridot ( péridot, the French word for olivine). Olivine is named for its typically olive-green color, thought to be a result of traces of nickel, though it may alter to a reddish color from the oxidation of iron. It has a spinel-like structure similar to magnetite but uses one quadrivalent and two divalent cations M 2 2+ M 4+O 4 instead of two trivalent and one divalent cations.
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Olivine's crystal structure incorporates aspects of the orthorhombic P Bravais lattice, which arise from each silica (SiO 4) unit being joined by metal divalent cations with each oxygen in SiO 4 bound to three metal ions. Olivine gives its name to the group of minerals with a related structure (the olivine group) – which includes tephroite ( Mn 2SiO 4), monticellite ( CaMgSiO 4), larnite (Ca 2SiO 4) and kirschsteinite (CaFeSiO 4) (commonly also spelled kirschteinite ). Manganese (Mn) and nickel (Ni) commonly are the additional elements present in highest concentrations. Olivine incorporates only minor amounts of elements other than oxygen (O), silicon (Si), magnesium (Mg) and iron (Fe). Melting temperature varies smoothly between the two endmembers, as do other properties. Forsterite's melting temperature is unusually high at atmospheric pressure, almost 1,900 ☌ (3,450 ☏), while fayalite's is much lower – about 1,200 ☌ (2,190 ☏). Compositions of olivine are commonly expressed as molar percentages of forsterite (Fo) and fayalite (Fa) ( e.g., Fo 70Fa 30).
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The ratio of magnesium to iron varies between the two endmembers of the solid solution series: forsterite (Mg-endmember: MgĤ). Olivine also has many other historical uses, such as the gemstone peridot (or chrysolite), as well as industrial applications like metalworking processes. For this reason, olivine has been proposed as a good candidate for accelerated weathering to sequester carbon dioxide from the Earth's oceans and atmosphere, as part of climate change mitigation. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickly on the surface. It is a type of nesosilicate or orthosilicate. ɪ ˌ v iː n/) is a magnesium iron silicate with the chemical formula ( Mg 2+, Fe 2+) 2 Si O 4. For other uses, see Olivine (disambiguation).