Forsterite
925638
212270889
2008-05-14T03:11:38Z
Omphacite
1590117
parts rewritten, minor content changes
{{Infobox mineral
| name = Forsterite
| category = [[Silicate mineral]]
| boxwidth =
| boxbgcolor =
| image = Forsterite_Stardust.jpg
| imagesize =
| caption = Forsterite ([[peridot]]) crystal captured in space by [[NASA]]'s [[Stardust (spacecraft)|Stardust]] spacecraft.
| formula = [[Magnesium]] [[silicate]] ([[Magnesium|Mg]]<sub>2</sub>[[Silicon|Si]][[Oxygen|O]]<sub>4</sub>)
| molweight =
| color = Colorless, green, yellow, yellow green, white
| habit = Dipyramidal prisms often tabular, commonly granular or compact massive
| system = [[Orthorhombic]]
| twinning = On {100}, {011} and {012}
| cleavage = Perfect on {010} imperfect on {100}
| fracture = Conchoidal
| mohs = 7
| luster = Vitreous
| refractive = nα = 1.636 - 1.730 nβ = 1.650 - 1.739 nγ = 1.669 - 1.772
| opticalprop = Biaxial (+)
| birefringence = δ = 0.033 - 0.042
| pleochroism =
| streak = White
| gravity = 3.21 - 3.33
| density =
| melt = 1890 °C<ref name=Klein>Klein, Cornelis and Cornelius Hurlbut Jr., 1985, ''Manual of Mineralogy'', 20th ed., Wiley, p 373-375 ISBN 0-471-80580-7 </ref>
| fusibility =
| diagnostic =
| solubility =
| diaphaneity = Transparent to translucent
| other =
| references = <ref>http://rruff.geo.arizona.edu/doclib/hom/forsterite.pdf</ref><ref>[http://www.mindat.org/min-1584.html Forsterite: Forsterite mineral information and data<!-- Bot generated title -->]</ref><ref>[http://webmineral.com/data/Forsterite.shtml Forsterite Mineral Data<!-- Bot generated title -->]</ref>
}}
'''Forsterite''' (Mg<sub>2</sub>SiO<sub>4</sub>) is the magnesium rich end-member of the [[olivine]] [[solid solution]] series. Forsterite crystallizes in the [[orthorhombic]] system ([[space group]] ''Pbnm'') with cell parameters ''a'' 4.75 [[Ångström|Å]] (0.475 [[Nanometre|nm]]), ''b'' 10.20 Å (1.020 nm) and ''c'' 5.98 Å (0.598 nm).<ref name=Klein/>
Forsterite is associated with [[Igneous rock|igneous]] and [[metamorphic rock]]s and has also been found in [[meteorite]]s. In 2005 it was also found in [[comet]]ary dust returned by the [[Stardust probe]].<ref>Lauretta, Ds, Keller, LP, & Messenger, S. (2005). Supernova Olivine from Cometary Dust. ''Science'', 309, 737-741</ref>
[[Peridot]] is the [[gemstone]] variety of forsterite olivine.
Forsterite reacts with [[quartz]] to form the [[orthopyroxene]] mineral [[enstatite]] in the following reaction:
Mg<sub>2</sub>SiO<sub>4</sub> + SiO<sub>2</sub> → 2MgSiO<sub>3</sub>.
==Composition==
Pure forsterite is composed of magnesium, oxygen and silicon. The chemical formula is Mg<sub>2</sub>SiO<sub>4</sub>. Forsterite and fayalite (Fe<sub>2</sub>SiO<sub>4</sub>) are the endmembers of the olivine solid solution series; other elements such as Mn, Ni, and Ca substitute for Fe and Mg in olivine, but only in minor proportions in natural occurrences. Other minerals such as [[monticellite]] (CaMgSiO<sub>4</sub>), an uncommon calcium-rich mineral, share the olivine structure, but solid solution between olivine and these other minerals is limited. Monticellite is found in [[Contact metamorphism|contact metamorphosed]] [[dolomite]]s.<ref name=Klein/>
==Geologic occurrence==
Forsterite-rich olivine is the most abundant mineral in the [[Mantle (geology)|mantle]] above a depth of about 400 km; pyroxenes are also important minerals in this upper part of the mantle.<ref>Kushiro, I. ''The System Forsterite- Diopside- Silica with and without Water at High Pressure'', American Journal of Science, 267, 269-294</ref> Although pure forsterite does not occur in [[igneous rock]]s, [[dunite]] often contains olivine with forsterite contents at least as Mg-rich as Fo<sub>92</sub> (92% forsterite - 8% fayalite); common [[peridotite]] contains olivine typically at least as Mg-rich as Fo<sub>88</sub>.<ref>Deer, W. A., Howie, R. A., and Zussman, J. (1992). ''An introduction to the rock-forming minerals'' (2nd ed.). Harlow: Longman ISBN 0-582-30094-0</ref> Forsterite-rich olivine is a common crystallization product of mantle-derived [[magma]]. Olivine in mafic and ultramafic rocks typically is rich in the forsterite endmember.
Forsterite also occurs in dolomitic [[marble]] which results from the metamorphism of high magnesium [[limestone]]s and [[dolostone]]s.<ref>Tormmsdof, V. (1966). Progressive Metamorphose kieseliger Karbonatgesteine in den Zentralalpen zwischen Bernina und Simplon. Schweizerische Mineralogische und Petrographische Mitteilungen = Bulletin Suisse de Mineralogie et Petrographie, 46, 431-460</ref>
Nearly pure forsterite occurs in some [[metamorphic|metamorphosed]] [[serpentinite]]s. Fayalite-rich olivine is much less common. Nearly pure fayalite is a minor constituent in some [[granite]]-like rocks, and it is a major constituent of some [[metamorphic]] [[banded iron formation]]s.
==Structure and formation==
Forsterite is mainly composed of the anion SiO<sup>2-</sup><sub>4</sub> and the cation Mg<sup>2+</sup>.<ref>Iishi, K. (1978). Lattice Dynamics of Forsterite. American Mineralogist, 63, 1198 - 1208</ref> Silicon is the central atom in the SiO<sup>2-</sup><sub>4</sub> anion. Each oxygen atom is bonded to the silicon by a single covalent bond. The four oxygens have a partial negative charge because of the covalent bond with silicon. Therefore, oxygens need to stay far from each other in order to reduce the repulsive force between them. The best geometry to reduce the repulsion is a tetrahedral shape. The cations occupy two different octahedral sites which are M1 and M2 and form ionic bonds with the silicate molecules. M1 and M2 are slightly different. M2 site is larger and more regular than M1 as shown in fig. 1. The packing in forsterite structure is dense. The space group of this structure is Pbnm and the point group is 2/m 2/m 2/m which is an orthorhombic crystal structure.
[[Image:Structure.jpg|thumb|right|FIG.1: Forsterite Structure ( See Klein 23rd FIG 18.4 Pg 439)]]
This structure of forsterite can form a complete [[solid solution]] by replacing the magnesium with iron.<ref>Keppla, O. J, & Wood, B. J. Thermochemistry of forsterite- fayalite olivine solutions. Geochimica et Cosmochimica Acta, 45, 529- 534</ref> Iron can form two different cations which are Fe<sup>2+</sup> and Fe<sup>3+</sup>. The iron(II) ion has the same charge as magnesium ion and it has a very similar ionic radius to magnesium. Consequently, Fe<sup>2+</sup> can replace the magnesium ion in the olivine structure.
One of the important factors that can increase the portion of forsterite in the olivine solid solution is the ratio of iron(II) ions to iron(III) ions in the magma.<ref>Wilson, M, Condliffe, E, Cortes, J. A, & Francalanci, L. (2006). The Occurrence of Forsterite and Highly oxidizing conditions in Basaltic Lavas from Stromboli Volcano, Italy. Journal of Petrology, 47, 1345- 1373</ref> As the iron(II) ions [[oxidize]] and become iron(III) ions, iron(III) ions can not form olivine because of their positive three charge. The occurrence of forsterite due to the oxidation of iron was observed in the [[Stromboli]] [[volcano]] in Italy. As the volcano fractured, gases and volatiles escaped from the magma chamber. The crystallization temperature of the magma increased as the gases escaped. Because iron(II) ions were oxidized in the Stromboli magma, little iron(II) was available to form olivine. Hence, the crystallizing olivine was Mg-rich, and igneous rocks rich in forsterite were formed.
==Discovery and name==
Forsterite was first described in 1824 for an occurrence at Mte. Somma, Vesuvius, Italy. It was named by [[Armand Lévy]] in 1824 for the English naturalist and mineral collector [[Jacob Forster]].<ref>Frondel, C. (1972). Jacob Forster (1739- 1806) and his connections with forsterite and palladium. ''Mineralogy Magazine'', 38, 545- 550 Available in PDF form at: http://www.minersoc.org/pages/Archive-MM/Volume_38/38-297-545.pdf </ref><ref>http://minrec.org/labels.asp?colid=726 Mineralogical Record, Biographical Archive</ref>
==References==
<references/>
{{commonscat|Forsterite}}
[[Category:Magnesium minerals]]
[[Category:Nesosilicates]]
[[ca:Forsterita]]
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[[fr:Forstérite]]
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[[lt:Forsteritas]]
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