Forsterite

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.

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Composition

Pure forsterite is composed of magnesium, oxygen and silicon. The chemical formula is Mg₂SiO₄. A forsterite is an endmember of the olivine solid solution series the composition can vary. The Mg²⁺ cation can be replace by Fe²⁺ or Ca²⁺ to form other minerals such as the fayalite (Fe₂SiO₄) endmember and monticellite (CaMgSiO₄) a calcium bearing phase found in contact metamorphosed dolomites.^[1]

Geologic occurrence

Forsterite olivine forms most of the mantle. The portion of forsterite varies in the mantle due to several factors such as depth, temperature and pressure.^[6] Forsterite olivine is a common mineral in mantle derived ultramafic magma. Although pure forsterite does not occur in igneous rocks, dunite often contains olivine with forsterite content of Fo₉₂ (92% forsterite - 8% fayalite) and peridotite contains olivine with Fo₈₈.^[7] Forsterite also occurs in dolomitic marble which results from the metamorphism of high magnesium limestones and dolostones.^[8]

Structure and formation

Forsterite is mainly composed of the anion SiO^2-₄ and the cation Mg²⁺.^[9] Silicon is the central atom in the SiO^2-₄ 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.  

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 and form fayalite instead of forsterite. Forsterite and fayalite can exist in mafic and ultramafic rocks to form olivine. The portion of forsterite and fayalite varies due to crystallization conditions.

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 temperature of the magma increased as the gases escape. As a result, the internal energy of the magma increased. Since the magma is not in a solid phase, there are free ions in the magma. The high internal energy of the magma oxidized the iron(II) ions in Stromboli. Iron became unavailable to form olivine. Magnesium formed olivine 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.^[12][13]

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. ^ http://www.mindat.org/min-1584.html
4. ^ http://webmineral.com/data/Forsterite.shtml
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