Editing Forsterite

{{Short description|Magnesium end-member of olivine, a nesosilicate mineral}}
{{Infobox mineral
| name        = Forsterite
| category    = [[Nesosilicates]]
| boxwidth    = 
| boxbgcolor  = 
| image       = Forsterite on Sanidine - Ochtendung, Eifel, Germany.jpg
| imagesize   = 260px
| caption     = Forsterite (big tabular and colorless) on [[sanidine]] (little colorless crystals)<br />with [[hematite]] (reddish)
| formula     = [[Magnesium]] [[silicate]] ([[Magnesium|Mg]]<sub>2</sub>[[Silicon|Si]][[Oxygen|O]]<sub>4</sub>)
| IMAsymbol   = Fo<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}</ref>
| molweight   = {{Chem molar mass|Mg=2|Si=1|O=4}}
| strunz      = 9.AC.05
| system      = [[Orthorhombic]]
| class       = Dipyramidal (mmm) <br/>[[H-M Symbol]]: (2/m 2/m 2/m)
| symmetry    = ''Pbnm''
| unit cell   = a = 4.7540 [[Ångström|Å]], b = 10.1971&nbsp;Å <br/>c = 5.9806&nbsp;Å; Z&nbsp;=&nbsp;4
| color       = Colorless, green, yellow, yellow green, white
| habit       = Dipyramidal prisms often tabular, commonly granular or compact massive
| twinning    = On {100}, {011} and {012}
| cleavage    = Perfect on {010} imperfect on {100}
| fracture    = [[Conchoidal]]
| mohs        = 7
| luster      = Vitreous
| refractive  = n<sub>α</sub> = 1.636 – 1.730 n<sub>β</sub> = 1.650 – 1.739 n<sub>γ</sub> = 1.669 – 1.772
| opticalprop = Biaxial (+)
| birefringence = δ = 0.033 – 0.042
| pleochroism = 
| 2V          = 82° 
| streak      = White
| gravity     = 3.21 – 3.33
| density     = 
| melt        = 1890 °C<ref name=Klein>{{cite book |last1=Klein |first1=Cornelis |first2=Cornelius Jr. |last2=Hurlbut |year=1985 |title=Manual of Mineralogy |edition=20th |publisher=Wiley |pages=[https://archive.org/details/manualofmineralo00klei/page/373 373–375] |isbn=978-0-471-80580-9 |url-access=registration |url=https://archive.org/details/manualofmineralo00klei/page/373 }}</ref>
| fusibility  = 
| diagnostic  = 
| solubility  = 
| diaphaneity = Transparent to translucent
| other       = 
| references  = <ref name=Handbook>http://rruff.geo.arizona.edu/doclib/hom/forsterite.pdf Handbook of Mineralogy</ref><ref name=Mindat>http://www.mindat.org/min-1584.html Mindat.org: Forsterite mineral information and data</ref><ref name=Webmin>http://webmineral.com/data/Forsterite.shtml Webmineral: Forsterite Mineral Data</ref>
}}
'''Forsterite''' (Mg<sub>2</sub>SiO<sub>4</sub>; commonly abbreviated as '''Fo'''; also known as white olivine) is the magnesium-rich [[Endmember|end-member]] of the [[olivine]] [[solid solution]] series. It is [[Isomorphism (crystallography)|isomorphous]] with the iron-rich end-member, [[fayalite]]. 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&nbsp;nm) and ''c'' 5.98 Å (0.598&nbsp;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>{{cite journal | doi = 10.1126/science.1109602 | last1 = Lauretta | first1 = Ds. | last2 = Keller | first2 = L.P. | last3 = Messenger | first3 = S. | year = 2005 | title = Supernova olivine from cometary dust | journal = Science | volume = 309 | issue = 5735| pages = 737–741 | pmid = 15994379 |bibcode = 2005Sci...309..737M | s2cid = 23245986 | doi-access = free }}</ref> In 2011 it was observed as tiny crystals in the dusty clouds of gas around a forming star.<ref>[http://www.physorg.com/news/2011-05-spitzer-crystal-outer-clouds-infant.html Spitzer sees crystal 'rain' in outer clouds of infant star], Whitney Clavin and Trent Perrotto, Physorg.com,  May 27, 2011 . Accessed May 2011</ref>

Two [[Polymorphism (materials science)|polymorphs]] of forsterite are known: [[wadsleyite]] (also [[orthorhombic]]) and [[ringwoodite]] (isometric, [[cubic crystal system]]). Both are mainly known from meteorites.

[[Peridot]] is the [[gemstone]] variety of forsterite olivine.

==Composition==
[[File:Forsterite orange - Ochtendung, Eifel, Germany.jpg|thumb|left|Orange forsterite with a portion of tephroite]]
Pure forsterite is composed of magnesium, oxygen and silicon. The chemical formula is Mg<sub>2</sub>SiO<sub>4</sub>. Forsterite, [[fayalite]] (Fe<sub>2</sub>SiO<sub>4</sub>) and [[tephroite]] (Mn<sub>2</sub>SiO<sub>4</sub>) are the end-members of the olivine solid solution series; other elements such as 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]] dolomites.<ref name=Klein/>

==Geologic occurrence==
Forsterite-rich olivine is the most abundant mineral in the [[Mantle (geology)|mantle]] above a depth of about {{convert|400|km|abbr=on}}; [[pyroxene]]s are also important minerals in this upper part of the mantle.<ref>{{cite journal | last1 = Kushiro | first1 = I. | title = The system forsterite – diopside – silica with and without water at high pressure | url =http://earth.geology.yale.edu/~ajs/1969/ajs_267A_11.pdf/269.pdf | journal = American Journal of Science | volume = 267 | pages = 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 {{chem2|Fo92}} (92% forsterite – 8% fayalite); common [[peridotite]] contains olivine typically at least as Mg-rich as {{chem2|Fo88}}.<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>  Due to its high melting point, olivine crystals are the first minerals to precipitate from a magmatic melt in a [[cumulate rock|cumulate]] process, often with orthopyroxenes. Forsterite-rich olivine is a common crystallization product of mantle-derived [[magma]]. Olivine in [[mafic]] and [[ultramafic]] rocks typically is rich in the forsterite end-member.

Forsterite also occurs in dolomitic [[marble]] which results from the metamorphism of high magnesium [[limestone]]s and [[Dolomite (rock)|dolomite]]s.<ref>{{cite journal | last1 = Tormmsdof | first1 = V. | year = 1966 | title = Progressive metamorphose kieseliger karbonatgesteine in den Zentralalpen zwischen Bernina und Simplon | journal = Schweizerische Mineralogische und Petrographische Mitteilungen | volume = 46 | pages = 431–460 }}</ref>
Nearly pure forsterite occurs in some [[Metamorphic rock|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, formation, and physical properties==

Forsterite is mainly composed of the anion SiO<sub>4</sub><sup>4−</sup> and the cation Mg<sup>2+</sup> in a molar ratio 1:2.<ref>{{cite journal | last1 = Iishi | first1 = K. | year = 1978 | title = Lattice dynamics of forsterite | url =http://www.minsocam.org/ammin/AM63/AM63_1198.pdf | journal = American Mineralogist | volume = 63 | issue =11–12 | pages = 1198–1208 }}</ref> Silicon is the central atom in the SiO<sub>4</sub><sup>4−</sup> anion. Each oxygen atom is bonded to the silicon by a single covalent bond. The four oxygen atoms have a partial negative charge because of the covalent bond with silicon. Therefore, oxygen atoms 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 anions. M1 and M2 are slightly different. M2 site is larger and more regular than M1 as shown in Fig.&nbsp;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. 
[[File:Atomic structure of olivine 1.png|thumb|left|Fig.&nbsp;1: The atomic scale structure of forsterite looking along the ''a'' axis. Oxygen is shown in red, silicon in pink, and Mg in blue. A projection of the unit cell is shown by the black rectangle.]]

This structure of forsterite can form a complete [[solid solution]] by replacing the magnesium with iron.<ref>{{cite journal | title=Thermochemistry of forsterite – fayalite olivine solutions | journal=Geochimica et Cosmochimica Acta | volume=45| issue=4 | pages= 529–534 |doi = 10.1016/0016-7037(81)90185-X | first1=B. J. |last1=Wood | first2=O. J.|last2= Kleppa|bibcode = 1981GeCoA..45..529W | year=1981 }}</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>{{cite journal | doi = 10.1093/petrology/egl012 | last1 = Wilson | first1 = M. | last2 = Condliffe | first2 = E. | last3 = Cortes | first3 = J.A | last4 = Francalanci | first4 = L. | year = 2006 | title = The occurrence of forsterite and highly oxidizing conditions in basaltic lavas from Stromboli volcano, Italy | journal = Journal of Petrology | volume = 47 | issue = 7| pages = 1345–1373 | bibcode = 2006JPet...47.1345C | doi-access = free }}</ref> As the iron(II) ions [[oxidize]] and become iron(III) ions, iron(III) ions cannot form olivine because of their 3+ 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 Fe-rich olivine (fayalite). Hence, the crystallizing olivine was Mg-rich, and igneous rocks rich in forsterite were formed.

[[File:Forsterite-pV.svg|thumb|right|Molar volume vs. pressure at room temperature]]
At high pressure, forsterite undergoes a [[phase transition]] into wadsleyite; under the conditions prevailing in the Earth's [[upper mantle (Earth)|upper mantle]], this transformation would occur at pressures of ca. 14–15&nbsp;GPa.<ref name="Presnall">D. C. Presnall (1995): Phase diagrams of Earth-forming minerals. In: Mineral Physics & Crystallography — A Handbook of Physical Constants, ed. by T. J. Ahrens, AGU Reference Shelf vol. 2, American Geophysical Union, Washington, D.C., pp. 248–268</ref> In high-pressure experiments, the transformation may be delayed so that forsterite can remain metastable at pressures up to almost 50&nbsp;GPa (see fig.).

The progressive metamorphism between dolomite and [[quartz]] react to form forsterite, [[calcite]] and [[carbon dioxide]]:<ref>{{Cite book |title=Rock-Forming Minerals: Orthosilicates, Volume 1A |last= Deer |first= William A. |date= Dec 1, 1982 |publisher= Geological Society of London|page= 264}}</ref>

<chem> 2CaMg(CO3)2 + SiO2 -> Mg2SiO4 + 2CaCO3 + 2CO2</chem>

Forsterite reacts with quartz to form the [[orthopyroxene]] mineral [[enstatite]] in the following reaction:

<chem>Mg2SiO4 + SiO2 -> 2MgSiO3</chem>

==Discovery and name==
[[File:Peridot olivine on basalt.JPG|thumb|Forsterite var. [[peridot]] with minor pyroxene (brown) on vesicular basalt. Collected near [[Peridot, Arizona]].]]
Forsterite was first described in 1824 for an occurrence at [[Mount Somma]], [[Vesuvius]], [[Italy]]. It was named by [[Armand Lévy (mineralogist)|Armand Lévy]] in 1824 after the English naturalist and mineral collector [[Adolarius Jacob Forster]].<ref>{{cite journal | doi = 10.1180/minmag.1972.038.297.02 | last1 = Frondel | first1 = C. | year = 1972 | title = Jacob Forster (1739–1806) and his connections with forsterite and palladium | url = http://www.minersoc.org/pages/Archive-MM/Volume_38/38-297-545.pdf | journal = Mineralogical Magazine | volume = 38 | issue = 297 | pages = 545–550 | bibcode = 1972MinM...38..545F | citeseerx = 10.1.1.605.3767 | s2cid = 93223692 | access-date = 2007-12-09 | archive-date = 2009-03-27 | archive-url = https://web.archive.org/web/20090327135924/http://www.minersoc.org/pages/Archive-MM/Volume_38/38-297-545.pdf | url-status = dead }}</ref><ref>http://minrec.org/labels.asp?colid=726 {{Webarchive|url=https://web.archive.org/web/20160303171456/http://minrec.org/labels.asp?colid=726 |date=2016-03-03 }} Mineralogical Record, Biographical Archive.</ref>

==Applications==
Forsterite is being currently studied as a potential [[biomaterial]] for [[Implant (medicine)|implants]] owing to its superior mechanical properties.<ref>{{cite journal | doi = 10.1016/j.jmbbm.2013.05.008 | last1 = Ramesh | first1 = S. | last2 = Yaghoubi | first2 = A. | last3 = Lee | first3 = K.Y.S. | last4 = Chin | first4 = K.M.C. | last5 = Purbolaksono | first5 = J. | last6 = Hamdi | first6 = M. | last7 = Hassan | first7 = M.A. | year = 2013 | title = Nanocrystalline forsterite for biomedical applications: Synthesis, microstructure and mechanical properties | journal = Journal of the Mechanical Behavior of Biomedical Materials | volume = 25 | pages = 63–69 | pmid=23726923}}</ref>

==References==
{{reflist}}

{{commons category|Forsterite}}

[[Category:Magnesium minerals]]
[[Category:Nesosilicates]]
[[Category:Orthorhombic minerals]]
[[Category:Minerals in space group 62]]