Olivine


The mineral olivine is a magnesium iron silicate with the formula 2. Thus, it is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickly on the surface.
The ratio of magnesium to iron varies between the two endmembers of the solid solution series: forsterite and fayalite. Compositions of olivine are commonly expressed as molar percentages of forsterite and fayalite . 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, silicon, magnesium and iron. Manganese and nickel commonly are the additional elements present in highest concentrations.
Olivine gives its name to the group of minerals with a related structure —which includes tephroite, monticellite, larnite and kirschsteinite .
Olivine's crystal structure incorporates aspects of the orthorhombic P Bravais lattice, which arise from each silica 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.
Olivine gemstones are called peridot and chrysolite.
Olivine rock is usually harder than surrounding rock and stands out as distinct ridges in the terrain. These ridges are often dry with little soil. Drought-resistant Scots pine is one of the few trees that thrive on olivine rock. Olivine pine forest is unique to Norway. It is rare and found on dry olivine ridges in the fjord districts of Sunnmøre and Nordfjord. Olivine rock is hard and base-rich.

Identification and paragenesis

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.
Translucent olivine is sometimes used as a gemstone called peridot. It is also called chrysolite. Some of the finest gem-quality olivine has been obtained from a body of mantle rocks on Zabargad Island in the Red Sea.
Olivine occurs in both mafic and ultramafic igneous rocks and as a primary mineral in certain metamorphic rocks. 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 such as peridotite and dunite can be residues left after extraction of magmas, and typically they are more enriched in olivine after extraction of partial melts. 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 sedimentary rocks 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 igneous rocks in small amounts in rare granites and rhyolites, 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 can contain at least as much as about 8900 parts per million 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.
– feldspar, pyroxenes, olivine revealed.

Extraterrestrial occurrences

Mg-rich olivine has also been discovered in meteorites, on the Moon and Mars, falling into infant stars, as well as on asteroid 25143 Itokawa. Such meteorites include chondrites, collections of debris from the early Solar System; and pallasites, mixes of iron-nickel and olivine.
The spectral signature of olivine has been seen in the dust disks around young stars. The tails of comets 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. Comet-like olivine has also been detected in the planetesimal belt around the star Beta Pictoris.

Crystal structure

Minerals in the olivine group crystallize in the orthorhombic system with isolated silicate tetrahedra, meaning that olivine is a nesosilicate. In an alternative view, the atomic structure can be described as a hexagonal, close-packed array of oxygen ions with half of the octahedral sites occupied with magnesium or iron ions and one-eighth of the tetrahedral sites occupied by silicon ions.
There are three distinct oxygen sites, two distinct metal sites and only one distinct silicon site. O1, O2, M2 and Si all lie on mirror planes, while M1 exists on an inversion center. O3 lies in a general position.

High-pressure polymorphs

At the high temperatures and pressures found at depth within the Earth the olivine structure is no longer stable. Below depths of about olivine undergoes an exothermic phase transition to the sorosilicate, wadsleyite and, at about depth, wadsleyite transforms exothermically into ringwoodite, which has the spinel structure. At a depth of about, ringwoodite decomposes into silicate perovskite and ferropericlase in an endothermic reaction. These phase transitions lead to a discontinuous increase in the density of the Earth's mantle that can be observed by seismic methods. They are also thought to influence the dynamics of mantle convection in that the exothermic transitions reinforce flow across the phase boundary, whereas the endothermic reaction hampers it.
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.

Weathering

Olivine is one of the weaker common minerals on the surface according to the Goldich dissolution series. It alters into iddingsite readily in the presence of water. Artificially increasing the weathering rate of olivine, e.g. by dispersing fine-grained olivine on beaches, has been proposed as a cheap way to sequester CO2. The presence of iddingsite on Mars would suggest that liquid water once existed there, and might enable scientists to determine when there was last liquid water on the planet.

Mining

Norway

Norway is the main source of olivine in Europe, particularly in an area stretching from Åheim to Tafjord, and from Hornindal to Flemsøy in the Sunnmøre district. There is also olivine in Eid municipality. About 50% of the world's olivine for industrial use is produced in Norway. At Svarthammaren in Norddal olivine was mined from around 1920 to 1979, with a daily output up to 600 metric tons. Olivine was also obtained from the construction site of the hydro power stations in Tafjord. At Robbervika in Norddal municipality an open-pit mine has been in operation since 1984. The characteristic red color is reflected in several local names with "red" such as Raudbergvik or Raudnakken.
, Hurtigruten ship passing.
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 sharpening stones.
Kallskaret near Tafjord is a nature reserve with olivine.

Uses

A worldwide search is on for cheap processes to sequester CO2 by mineral reactions, called enhanced weathering. Removal by reactions with olivine is an attractive option, because it is widely available and reacts easily with the CO2 from the atmosphere. When olivine is crushed, it weathers completely within a few years, depending on the grain size. All the CO2 that is produced by burning one liter of oil can be sequestered by less than one liter of olivine. The reaction is exothermic but slow. To recover the heat produced by the reaction to produce electricity, a large volume of olivine must be thermally well-isolated. The end-products of the reaction are silicon dioxide, magnesium carbonate, and small amounts of iron oxide.
Olivine is used as a substitute for dolomite in steel works. Olivine is also used to tap blast furnaces in the steel industry, acting as a plug, removed in each steel run.
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 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.