Forsterite

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Forsterite

Forsterite (peridot) crystal captured in space by NASA's Stardust spacecraft.
General
Category Silicate mineral
Chemical formula Magnesium silicate (Mg2SiO4)
Identification
Color Colorless, green, yellow, yellow green, white
Crystal habit Dipyramidal prisms often tabular, commonly granular or compact massive
Crystal system Orthorhombic
Twinning On {100}, {011} and {012}
Cleavage Perfect on {010} imperfect on {100}
Fracture Conchoidal
Mohs Scale hardness 7
Luster Vitreous
Refractive index nα = 1.636 - 1.730 nβ = 1.650 - 1.739 nγ = 1.669 - 1.772
Optical Properties Biaxial (+)
Birefringence δ = 0.033 - 0.042
Streak White
Specific gravity 3.21 - 3.33
Melting point 1890 °C[1]
Diaphaneity Transparent to translucent
References [2][3][4]

Forsterite (Mg2SiO4) 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 Å (0.475 nm), b 10.20 Å (1.020 nm) and c 5.98 Å (0.598 nm).[1]

Forsterite is associated with igneous and metamorphic rocks and has also been found in meteorites. In 2005 it was also found in cometary dust returned by the Stardust probe.[5]

Peridot is the gemstone variety of forsterite olivine.

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

Mg2SiO4 + SiO2 → 2MgSiO3.

Contents

[edit] Composition

Pure forsterite is composed of magnesium, oxygen and silicon. The chemical formula is Mg2SiO4. Forsterite and fayalite (Fe2SiO4) 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 (CaMgSiO4), 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 metamorphosed dolomites.[1]

[edit] Geologic occurrence

Forsterite-rich olivine is the most abundant mineral in the mantle above a depth of about 400 km; pyroxenes are also important minerals in this upper part of the mantle.[6] Although pure forsterite does not occur in igneous rocks, dunite often contains olivine with forsterite contents at least as Mg-rich as Fo92 (92% forsterite - 8% fayalite); common peridotite contains olivine typically at least as Mg-rich as Fo88.[7] 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 limestones and dolostones.[8] Nearly pure forsterite occurs in some metamorphosed serpentinites. 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 formations.

[edit] Structure and formation

Forsterite is mainly composed of the anion SiO2-4 and the cation Mg2+.[9] Silicon is the central atom in the SiO2-4 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.

FIG.1: Forsterite Structure ( See Klein 23rd FIG 18.4 Pg 439)
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.[10] Iron can form two different cations which are Fe2+ and Fe3+. The iron(II) ion has the same charge as magnesium ion and it has a very similar ionic radius to magnesium. Consequently, Fe2+ 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.[11] 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.

[edit] 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.[12][13]

[edit] References

  1. ^ a b c Klein, Cornelis and Cornelius Hurlbut Jr., 1985, Manual of Mineralogy, 20th ed., Wiley, p 373-375 ISBN 0-471-80580-7
  2. ^ http://rruff.geo.arizona.edu/doclib/hom/forsterite.pdf
  3. ^ Forsterite: Forsterite mineral information and data
  4. ^ Forsterite Mineral Data
  5. ^ Lauretta, Ds, Keller, LP, & Messenger, S. (2005). Supernova Olivine from Cometary Dust. Science, 309, 737-741
  6. ^ Kushiro, I. The System Forsterite- Diopside- Silica with and without Water at High Pressure, American Journal of Science, 267, 269-294
  7. ^ 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
  8. ^ 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
  9. ^ Iishi, K. (1978). Lattice Dynamics of Forsterite. American Mineralogist, 63, 1198 - 1208
  10. ^ Keppla, O. J, & Wood, B. J. Thermochemistry of forsterite- fayalite olivine solutions. Geochimica et Cosmochimica Acta, 45, 529- 534
  11. ^ 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
  12. ^ 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
  13. ^ http://minrec.org/labels.asp?colid=726 Mineralogical Record, Biographical Archive
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