Olivine
Template:Short description Template:Other uses Template:Infobox mineral
The mineral olivine (Template:IPAc-en) is a magnesium iron silicate with the chemical formula Template:Chem2. It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle,<ref name=DCH>Template:Cite book</ref> 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: Template:Chem) and fayalite (Fe-endmember: Template:Chem). 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 Template:Convert, while fayalite's is much lower – about Template:Convert. 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 (Mn2SiO4), monticellite (CaMgSiO4), larnite (Ca2SiO4) and kirschsteinite (CaFeSiO4) (commonly also spelled kirschteiniteTemplate:Sfn).
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.<ref>Ernst, W. G. Earth Materials. Englewood Cliffs, NJ: Prentice-Hall, 1969. p. 65</ref>
Identification and paragenesis
[edit]Olivine is named for its typically olive-green color, Template:Cn span though it may alter to a reddish color from the oxidation of iron.
Translucent olivine is sometimes used as a gemstone called peridot (péridot, the French word for olivine). 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. Some of the finest gem-quality olivine has been obtained from a body of mantle rocks on Zabargad Island in the Red Sea.<ref>St. John's Island peridot information and history at Mindat.org</ref><ref>Template:Cite journal</ref>
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.Template:Sfn Ultramafic rocks usually contain substantial olivine, and those with an olivine content of over 40% are described as peridotites. 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.<ref>Template:Cite book</ref> 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.<ref>Template:Cite journal</ref> 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 (Template:Chem2).
Mg-rich olivine is stable to pressures equivalent to a depth of about Template:Convert 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 Template:Convert) 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.<ref>Template:Cite journal</ref>
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.<ref>Template:Cite journal</ref>
-
Light green olivine crystals in peridotite xenoliths in basalt from Arizona
-
Bright green olivine from Pakistan, showing chisel termination and silky luster
-
Olivine in lava from the Azores
Extraterrestrial occurrences
[edit]
Mg-rich olivine has also been discovered in meteorites,<ref>Fukang and other Pallasites Template:Webarchive. Farlang.com (2008-04-30). Retrieved on 2012-06-16.</ref> on the Moon<ref name="MareBasalt1">Template:Cite web</ref> and Mars,<ref>Pretty Green Mineral.... Template:WebarchiveMission Update 2006... Template:Webarchive UMD Deep Impact Website, University of Maryland Ball Aerospace & Technology Corp. retrieved June 1, 2010</ref><ref>Hoefen, T.M., et al. 2003. "Discovery of Olivine in the Nili Fossae Region of Mars". Science 302, 627–30. "Template:Cite journal"</ref> falling into infant stars,<ref>Spitzer Sees Crystal Rain... Template:Webarchive NASA Website</ref> as well as on asteroid 25143 Itokawa.<ref>Japan says Hayabusa brought back asteroid grains... Template:Webarchive retrieved November 18, 2010</ref> Such meteorites include chondrites, collections of debris from the early Solar System; and pallasites, mixes of iron-nickel and olivine. The rare A-type asteroids are suspected to have a surface dominated by olivine.<ref>Template:Cite journal</ref>
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 spacecraftTemplate:Broken anchor in 2006.<ref>Press Release 06-091 Template:Webarchive. Jet Propulsion Laboratory Stardust website, retrieved May 30, 2006.</ref> Comet-like (magnesium-rich) olivine has also been detected in the planetesimal belt around the star Beta Pictoris.<ref name="deVries2012">Template:Cite journal</ref>
Crystal structure
[edit]
Minerals in the olivine group crystallize in the orthorhombic system (space group Pbnm) with isolated silicate tetrahedra, meaning that olivine is a nesosilicate. The 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 (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 on mirror planes, while M1 exists on an inversion center. O3 lies in a general position.
High-pressure polymorphs
[edit]At the high temperatures and pressures found at depth within the Earth the olivine structure is no longer stable. Below depths of about Template:Convert olivine undergoes an exothermic phase transition to the sorosilicate, wadsleyite and, at about Template:Convert depth, wadsleyite transforms exothermically into ringwoodite, which has the spinel structure. At a depth of about Template:Convert, ringwoodite decomposes into silicate perovskite (Template:Chem2) and ferropericlase (Template:Chem2) 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.<ref>Template:Cite journal</ref>
The pressure at which these phase transitions occur depends on temperature and iron content.<ref>Template:Cite book</ref> At Template:Convert, the pure magnesium end member, forsterite, transforms to wadsleyite at Template:Convert and to ringwoodite at pressures above Template:Convert. 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 Template:Convert. Fayalite transforms to Template:Chem spinel at pressures below Template:Convert. Increasing the temperature increases the pressure of these phase transitions.
Weathering
[edit]
Olivine is one of the less stable common minerals on the surface according to the Goldich dissolution series. It alters into iddingsite (a combination of clay minerals, iron oxides and ferrihydrite) readily in the presence of water.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite web</ref><ref>Template:Cite web</ref> 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.<ref>Template:Cite journal</ref>
Because of its rapid weathering, olivine is rarely found in sedimentary rock.<ref>Template:Cite journal</ref>
Mining
[edit]Norway
[edit]
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 Stad Municipality. About 50% of the world's olivine for industrial use is produced in Norway. At Svarthammaren in Norddal Municipality (now Fjord Municipality), olivine was mined from around 1920 to 1979, with a daily output up to Template:Convert. 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 (Red rock bay) or Raudnakken (Red ridge).<ref>Furseth, Astor (1987): Norddal i 150 år. Valldal: Norddal kommune.</ref><ref>Geological Survey of Norway. Kart over mineralressurser Template:Webarchive. Accessed 9.12.2012.</ref><ref>Template:Cite web</ref><ref>Gjelsvik, T. (1951). Oversikt over bergartene i Sunnmøre og tilgrensende deler av Nordfjord Template:Webarchive. Norge geologiske undersøkelser, report 179.</ref>
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.<ref>Strøm, Hans: Physisk og Oeconomisk Beskrivelse over Fogderiet Søndmør beliggende i Bergen Stift i Norge. Published in Sorø, Denmark, 1766.</ref>
Kallskaret near Tafjord is a nature reserve with olivine.<ref>Template:Cite web</ref>
Applications
[edit]Olivine is used as a substitute for dolomite in steel works.<ref>Mineralressurser i Norge ; Mineralstatistikk og bergverksberetning 2006. Trondheim: Bergvesenet med bergmesteren for Svalbard. 2007.</ref>
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.<ref>Template:Cite book</ref>
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.<ref>Template:Cite web</ref>
Gem-quality olivine is used as a gemstone called peridot.
Experimental uses
[edit]Removal of atmospheric CO2 via reaction with crushed olivine has been considered. The end-products of the very slow reaction are silicon dioxide, magnesium carbonate, and iron oxides.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> 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.<ref>Template:Cite web</ref>
See also
[edit]References
[edit]External links
[edit]- Pretty Green Mineral – Pretty Dry Mars? by Linda M.V. Martel, Planetary Science Research Discoveries, Hawai'i Institute of Geophysics and Planetology
- Olivine Page Farlang library: Historic sources + modern articles on Olivine and Peridot
- Geological information and several microscopic images University of North Dakota