Olivine
45159
224936422
2008-07-11T02:17:53Z
Fangjian
4807131
{{Infobox mineral
| name = Olivine
| category = [[Mineral]] Group
| boxwidth =
| boxbgcolor =
| image = Peridot2.jpg
| caption =
| formula = (Mg, Fe)<sub>2</sub>SiO<sub>4</sub>
| molweight =
| color = Yellow to yellow-green
| habit = Massive to granular
| system = [[Orthorhombic]]
| twinning =
| cleavage = Poor
| fracture = Conchoidal - brittle
| mohs = 6.5–7
| luster = Vitreous
| refractive = nα = 1.630–1.650 nβ = 1.650–1.670 nγ = 1.670–1.690
| opticalprop = Biaxial (+)
| birefringence = δ = 0.040
| pleochroism =
| streak = White
| gravity = 3.27–3.37
| melt =
| fusibility =
| diagnostic =
| solubility =
| diaphaneity = Transparent to translucent
| other =
| references = <ref>http://webmineral.com/data/Olivine.shtml Webmineral</ref><ref>http://www.mindat.org/min-2983.html Mindat</ref>
}}
The [[mineral]] '''olivine''' (when gem-quality also called '''[[peridot]]''') is a [[magnesium]] [[iron]] [[Silicate minerals|silicate]] with the formula ([[magnesium|Mg]],[[iron|Fe]])<sub>2</sub>[[silicon|Si]][[oxygen|O]]<sub>4</sub>. It is one of the most common minerals on Earth, and has also been identified in [[meteorite]]s and on the [[Moon]], [[Mars]], and comet [[Wild 2]].
The ratio of magnesium and iron varies between the two [[Endmember (mineralogy)|endmember]]s 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 fayalite (Fa) (e.g., Fo<sub>70</sub>Fa<sub>30</sub>). Forsterite has an unusually high melting temperature at atmospheric pressure, almost 1900°C, but the melting temperature of fayalite is much lower (about 1200°C). The 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 (the '''olivine group''') which includes [[tephroite]] ([[manganese|Mn]]<sub>2</sub>SiO<sub>4</sub>), [[monticellite]] ([[calcium|Ca]]MgSiO<sub>4</sub>), and [[kirschsteinite]] (CaFeSiO<sub>4</sub>).
==Identification and paragenesis==
[[Image:OlivineUSGOV.jpg|thumb|Olivine basalt]]
[[Image:Peridot in basalt.jpg|thumb|Peridotite xenoliths in basalt--olivines are light green crystals. Location: San Carlos Indian Reservation, Gila Co., Arizona, USA.]]
Olivine is usually 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 [[conchoidal fracture]] and is rather [[brittle]]. The [[Mohs scale of mineral hardness|hardness]] of olivine is 6.5–7, its [[relative density]] is 3.27–3.37, and it has a [[vitreous]] [[Lustre (mineralogy)|luster]]. It is transparent to translucent.
Transparent olivine is sometimes used as a [[gemstone]] called [[peridot]], the [[French language|French]] word for olivine. It is also called chrysolite, from the [[Greek language|Greek]] words for [[gold]] and stone. Some of the finest gem-quality olivine has been obtained from a body of [[Mantle (geology)|mantle]] rocks on [[Zabargad]] island in the [[Red Sea]].
Olivine/peridot occurs in both [[mafic]] and [[ultramafic]] [[igneous rock]]s and as a primary mineral in certain [[metamorphic rock]]s. 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 rock]]s with high magnesium and low silica content also produces Mg-rich olivine, or forsterite.
Fe-rich olivine is relatively much less common, but it occurs in [[igneous rock]]s in small amounts in rare [[granite]]s and [[rhyolite]]s, 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,Fe)<sub>2</sub>Si<sub>2</sub>O<sub>6</sub>).
Mg-rich olivine is stable to pressures equivalent to a depth of about 410 km 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 (e.g., 12 [[GPa]], the pressure at depths of 360 kilometers or so) can contain at least as much as about 8900 parts per million (weight) of water, and that such water contents drastically reduce the resistance of olivine to solid flow; moreover, because olivine is so abundant, more water may be dissolved in olivine of the mantle than contained in Earth’s oceans.<ref> Smyth, J. R., Frost, D. J., Nestola, F., Holl, C. M., Bromiley, G., ''Olivine hydration in the deep upper mantle: Effects of temperature and silica activity.'' Geophysical Research Letters, v. 33, L15301, doi:10.1029/2006GL026194, 2006</ref>
Mg-rich olivine has also been discovered in [[meteorite]]s, on Mars, and on Earth's [[moon]]. Such meteorites include [[chondrite]]s, collections of debris from the early solar system, and [[pallasite]]s, 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 (which formed from the dust disk around the young [[Sun]]) often have the spectral signature of olivine, and the presence of olivine has recently been verified in samples of a comet from the [[Stardust (spacecraft)#Sample analysis|Stardust spacecraft]]. <ref>[http://stardust.jpl.nasa.gov/news/status/060313.html Press Release 06-091]. Jet Propulsion Laboratory Stardust website, retrieved May 30, 2006. </ref>
==Crystal structure==
[[Image:Atomic structure of olivine 1.png|left|thumbnail|'''Figure 1:''' The atomic scale structure of olivine looking along the ''a'' 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 the [[orthorhombic]] system ([[space group]] P''bnm'') with isolated silicate tetrahedra, meaning that olivine is a [[Silicate minerals|nesosilicate]]. In an alternative view, the atomic structure can be described as a hexagonal, close-packed array of oxygen [[ion]]s 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 figure 1), two distinct metal sites (M1 and M2), and only one distinct silicon site. O1, O2, M2, and Si all lie on [[mirror plane]]s, 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 410 km olivine undergoes a [[phase transition]] to the [[silicate minerals|sorosilicate]], [[wadsleyite]] and, at about 520 km depth, wadsleyite transforms into [[ringwoodite]], which has the [[spinel]] structure. These phase transitions lead to a discontinuous increase in the density of the Earth's [[mantle (geology)|mantle]] that can be observed by [[seismic]] methods.
The pressure at which these phase transitions occur depends on temperature and iron content (Deer et al. 1992). At 800°C the pure magnesium end member, forsterite, transforms to wadsleyite at 11.8 [[gigapascal]]s (118 kbar) and to ringwoodite at pressures above 14 GPa (140 kbar). 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 10–11.5 GPa (100–115 kbar). Fayalite transforms to Fe<sub>2</sub>SiO<sub>4</sub> spinel at pressures below 5 GPa (50 kbar). Increasing the temperature increases the pressure of these phase transitions.
==Historical and mythical uses==
The [[Septuagint]] names ''chrysolithos'' as a stone on the [[Hoshen]] in the verse [[Exodus]] 28:20; the [[masoretic text]] has the word ''tarshish'', which has uncertain meaning, in the same place. According to the [[New International Version]] and [[Bahya ben Asher|Rebbenu Bachya]], the word ''tarshish'' refers to chrysolite (olivine) and Rebbenu Bachya claims it was the stone representing the tribe of [[Asher]]. However, Chrysolite took its modern meaning much more recently, and in Greek times just meant ''golden stone'' (''chryso-lithos''), and could refer not only to yellowish olivine, but also to [[Topaz]], [[Amber]], yellow [[Jasper]], yellow [[Serpentine]], or even [[lapis lazuli]] which has golden flecks within its mainly blue surface and fits with the [[targum]] descriptions of the ''tarshish'' stone as being sea-colored. ''Tarshish'' probably refers to [[Tarshish]], a place, though this doesn't identify the stone much more. In the Biblical account, there is a stone, on an earlier row, that scholars think was translucent and yellow, so scholars think that ''chrysolithos''/''tarshish'' here is unlikely to refer to olivine, because that would place two translucent stones next to each other, and be quite jarring; instead scholars favour yellow Jasper or Serpentine. There is a wide range of views among traditional sources about which tribe the stone refers to.
==Uses==
A worldwide search is on for cheap processes to sequester CO<sub>2</sub> by mineral reactions. Removal by reactions with olivine is an attractive option, because it is widely available and reacts easily with the (acid) CO<sub>2</sub> from the atmosphere. When olivine is crushed, it weathers completely within a few years, depending on the grain size. All the CO<sub>2</sub> that is produced by burning 1 liter of oil can be sequestered by less than 1 liter of olivine. The reaction is exothermic but slow. In order to recover the heat produced by the reaction to produce electricity, a large volume of olivine must be thermally well isolated. Then it can produce power, while at the same time removing CO<sub>2</sub>. The end-products of the reaction are silicon dioxide, magnesium carbonate and small amounts of iron oxide.<ref>http://www.netl.doe.gov/publications/proceedings/01/carbon_seq/6c1.pdf</ref><ref>http://www.springerlink.com/index/78528604337V3773.pdf</ref><ref>http://www.rockcollector.co.uk/infocus/olivine.asp</ref>
==See also==
* [[List of minerals]]
* [[Bowen's reaction series]]
==References==
{{Commonscat|Olivine}}
<references/>
*{{cite book
| last = Klein
| first = Cornelis
| authorlink =
| coauthors = and C. S. Hurlburt
| title = Manual of Mineralogy (21rst ed.)
| publisher = John Wiley & Sons
| date = 1985
| location = New York
| pages = 681 pp
| url =
| doi =
| id = ISBN 0-471-80580-7}}
*{{cite book
| last = Deer
| first = W. A.
| authorlink =
| coauthors = R. A. Howie, and J. Zussman
| title = An Introduction to the Rock-Forming Minerals (2nd ed.)
| publisher = Longman
| date = 1992
| location = London
| pages = 696 pp
| url =
| doi =
| id = ISBN 0-582-30094-0}}
[[Category:Magnesium minerals]]
[[Category:Iron minerals]]
[[Category:Nesosilicates]]
[[bn:অলিভিন]]
[[bs:Olivin]]
[[ca:Grup de l'Olivina]]
[[cs:Olivín]]
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[[he:אוליבין]]
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