The ratio of magnesium to iron varies between the twoendmembers of thesolid solution series:forsterite (Mg-endmember:Mg 2SiO 4) andfayalite (Fe-endmember:Fe 2SiO 4). Compositions of olivine are commonly expressed asmolar 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 1,900 °C (3,450 °F), while fayalite's is much lower – about 1,200 °C (2,190 °F). Melting temperature varies smoothly between the two endmembers, as do other properties. Olivine incorporates only minor amounts of elements other thanoxygen (O),silicon (Si),magnesium (Mg) andiron (Fe).Manganese (Mn) andnickel (Ni) commonly are the additional elements present in highest concentrations.
Olivine gives its name to the group of minerals with a related structure (theolivine group) – which includestephroite (Mn2SiO4),monticellite (CaMgSiO4),larnite (Ca2SiO4) andkirschsteinite (CaFeSiO4) (commonly also spelled kirschteinite[10]).
Olivine's crystal structure incorporates aspects of theorthorhombic PBravais 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 aspinel-like structure similar to magnetite but uses one quadrivalent and two divalent cations M22+ M4+O4 instead of two trivalent and one divalent cations.[11]
Olivine is named for its typically olive-green color,thought to be a result of traces ofnickel,[citation needed] though it may alter to a reddish color from the oxidation of iron.[citation needed]
Translucent olivine is sometimes used as agemstone calledperidot (péridot, theFrench word for olivine). It is also called chrysolite (orchrysolithe, from theGreek words forgold 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 ofmantle rocks onZabargad Island in theRed Sea.[12][13]
Olivine occurs in bothmafic andultramaficigneous rocks and as a primary mineral in certainmetamorphic rocks. Mg-rich olivine crystallizes frommagma that is rich in magnesium and low insilica. That magma crystallizes to mafic rocks such asgabbro andbasalt.[14] Ultramafic rocks usually contain substantial olivine, and those with an olivine content of over 40% are described asperidotites.Dunite has an olivine content of over 90% and is likely acumulate formed by olivine crystallizing and settling out of magma or avein mineral lining magma conduits.[15] 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.[16] Themetamorphism of impuredolomite or othersedimentary rocks with high magnesium and low silica content also produces Mg-rich olivine, orforsterite.
Fe-rich olivinefayalite is relatively much less common, but it occurs inigneous rocks in small amounts in raregranites andrhyolites, and extremely Fe-rich olivine can exist stably withquartz andtridymite. In contrast, Mg-rich olivine does not occur stably withsilica minerals, as it would react with them to formorthopyroxene ((Mg,Fe)2Si2O6).
Mg-rich olivine is stable to pressures equivalent to a depth of about 410 km (250 mi) 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 therheology of that part of Earth and hence upon the solid flow that drivesplate tectonics. Experiments have documented that olivine at high pressures (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. Moreover, because olivine is so abundant, more water may be dissolved in olivine of the mantle than is contained in Earth's oceans.[17]
Olivine pine forest (aplant community) is unique to Norway. It is rare and found on dry olivine ridges in the fjord districts of Sunnmøre and Nordfjord.[18]
Thespectral 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 youngSun) often have the spectral signature of olivine, and the presence of olivine was verified in samples of a comet from theStardust spacecraft in 2006.[26] Comet-like (magnesium-rich) olivine has also been detected in theplanetesimal belt around the starBeta Pictoris.[27]
The atomic scale structure of olivine looking along thea axis. Oxygen is shown in red, silicon in pink, and magnesium/iron in blue. A projection of the unit cell is shown by the black rectangle.
Minerals in the olivine group crystallize in theorthorhombic system (space group Pbnm) with isolated silicate tetrahedra, meaning that olivine is anesosilicate. The structure can be described as a hexagonal, close-packed array of oxygenions 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 (marked O1, O2 and O3 in the figure), two distinct metal sites (M1 and M2) and only one distinct silicon site. O1, O2, M2 and Si all lie onmirror planes, while M1 exists on an inversion center. O3 lies in a general position.
At the high temperatures and pressures found at depth within the Earth the olivine structure is no longer stable. Below depths of about 410 km (250 mi) olivine undergoes an exothermicphase transition to thesorosilicate,wadsleyite and, at about 520 km (320 mi) depth, wadsleyite transforms exothermically intoringwoodite, which has thespinel structure. At a depth of about 660 km (410 mi), ringwoodite decomposes intosilicate perovskite ((Mg,Fe)SiO3) andferropericlase ((Mg,Fe)O) in an endothermic reaction. These phase transitions lead to a discontinuous increase in the density of the Earth'smantle that can be observed byseismic methods. They are also thought to influence the dynamics ofmantle convection in that the exothermic transitions reinforce flow across the phase boundary, whereas the endothermic reaction hampers it.[28]
The pressure at which these phase transitions occur depends on temperature and iron content.[29] At 800 °C (1,070 K; 1,470 °F), the pure magnesium end member, forsterite, transforms to wadsleyite at 11.8gigapascals (116,000 atm) and to ringwoodite at pressures above 14 GPa (138,000 atm). Increasing the iron content decreases the pressure of the phase transition and narrows thewadsleyite stability field. At about 0.8mole fraction fayalite, olivine transforms directly to ringwoodite over the pressure range 10.0 to 11.5 GPa (99,000–113,000 atm). Fayalite transforms toFe 2SiO 4 spinel at pressures below 5 GPa (49,000 atm). Increasing the temperature increases the pressure of these phase transitions.
Olivine is one of the less stable common minerals on the surface according to theGoldich dissolution series. It alters intoiddingsite (a combination of clay minerals, iron oxides andferrihydrite) readily in the presence of water.[30] 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.[31][32] 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.[33]
Because of its rapid weathering, olivine is rarely found insedimentary rock.[34]
Norway is the main source of olivine in Europe, particularly in an area stretching fromÅheim toTafjord, and fromHornindal to Flemsøy in theSunnmøre district. There is also olivine inStad Municipality. About 50% of the world's olivine for industrial use is produced in Norway. At Svarthammaren inNorddal Municipality (nowFjord Municipality), olivine was mined from around 1920 to 1979, with a daily output up to 600metric tons (590long tons; 660short 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 asRaudbergvik (Red rock bay) orRaudnakken (Red ridge).[35][36][37][38]
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 assharpening stones.[39]
Kallskaret near Tafjord is a nature reserve with olivine.[40]
Olivine is used as a substitute fordolomite in steel works.[41]
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.[42]
In Finland, olivine is marketed as an ideal rock forsauna stoves because of its comparatively high density and resistance to weathering under repeated heating and cooling.[43]
Removal of atmospheric CO2 via reaction with crushed olivine has been considered. The end-products of the very slow reaction aresilicon dioxide,magnesium carbonate, and iron oxides.[44][45] A public benefit corporation,Project Vesta, is investigating this approach on beaches which increase the agitation and surface area of crushed olivine through wave action.[46]
Another experimental use for olivine is in making carbon-neutral or carbon-negative cement.[47]
^"Olivine". Science.smith.edu. Archived fromthe original on 2014-01-20. Retrieved2013-11-14.G = 3.22 to 4.39. Specific gravity increases and hardness decreases with increasing Fe.
^Philpotts, Anthony R.; Ague, Jay J. (2009).Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. pp. 44, 138, 142, 385.ISBN9780521880060.
^Meyer, C. (2003)."Mare Basalt Volcanism"(PDF).NASA Lunar Petrographic Educational Thin Section Set.NASA.Archived(PDF) from the original on 21 December 2016. Retrieved23 October 2016.
^De Vries, B. L.; Acke, B.; Blommaert, J. A. D. L.; Waelkens, C.; Waters, L. B. F. M.; Vandenbussche, B.; Min, M.; Olofsson, G.; Dominik, C.; Decin, L.; Barlow, M. J.; Brandeker, A.; Di Francesco, J.; Glauser, A. M.; Greaves, J.; Harvey, P. M.; Holland, W. S.; Ivison, R. J.; Liseau, R.; Pantin, E. E.; Pilbratt, G. L.; Royer, P.; Sibthorpe, B. (2012). "Comet-like mineralogy of olivine crystals in an extrasolar proto-Kuiper belt".Nature.490 (7418):74–76.arXiv:1211.2626.Bibcode:2012Natur.490...74D.doi:10.1038/nature11469.PMID23038467.S2CID205230613.
