| Forsterite | |
|---|---|
 | |
| General | |
| Category | Nesosilicates |
| Formula | Magnesiumsilicate (Mg2SiO4) |
| IMA symbol | Fo[1] |
| Strunz classification | 9.AC.05 |
| Crystal system | Orthorhombic |
| Crystal class | Dipyramidal (mmm) H-M Symbol: (2/m 2/m 2/m) |
| Space group | Pbnm |
| Unit cell | a = 4.7540Å, b = 10.1971 Å c = 5.9806 Å; Z = 4 |
| Identification | |
| Formula mass | 140.691 g·mol−1 |
| Color | Colorless, green, yellow, yellow green, white |
| Crystal 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 scale hardness | 7 |
| Luster | Vitreous |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 3.21 – 3.33 |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 1.636 – 1.730 nβ = 1.650 – 1.739 nγ = 1.669 – 1.772 |
| Birefringence | δ = 0.033 – 0.042 |
| 2V angle | 82° |
| Melting point | 1890 °C[2] |
| References | [3][4][5] |
Forsterite (Mg2SiO4; commonly abbreviated asFo; also known as white olivine) is the magnesium-richend-member of theolivinesolid solution series. It isisomorphous with the iron-rich end-member,fayalite. Forsterite crystallizes in theorthorhombic system (space groupPbnm) with cell parametersa 4.75Å (0.475nm),b 10.20 Å (1.020 nm) andc 5.98 Å (0.598 nm).[2]
Forsterite is associated withigneous andmetamorphic rocks and has also been found inmeteorites. In 2005 it was also found incometary dust returned by theStardust probe.[6] In 2011 it was observed as tiny crystals in the dusty clouds of gas around a forming star.[7]
Twopolymorphs of forsterite are known:wadsleyite (alsoorthorhombic) andringwoodite (isometric,cubic crystal system). Both are mainly known from meteorites.
Peridot is thegemstone variety of forsterite olivine.
Pure forsterite is composed of magnesium, oxygen and silicon. The chemical formula is Mg2SiO4. Forsterite,fayalite (Fe2SiO4) andtephroite (Mn2SiO4) 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 asmonticellite (CaMgSiO4), an uncommon calcium-rich mineral, share the olivine structure, but solid solution between olivine and these other minerals is limited. Monticellite is found incontact metamorphosed dolomites.[2]
Forsterite-rich olivine is the most abundant mineral in themantle above a depth of about 400 km (250 mi);pyroxenes are also important minerals in this upper part of the mantle.[8] Although pure forsterite does not occur inigneous rocks,dunite often contains olivine with forsterite contents at least as Mg-rich asFo92 (92% forsterite – 8% fayalite); commonperidotite contains olivine typically at least as Mg-rich asFo88.[9] Due to its high melting point, olivine crystals are the first minerals to precipitate from a magmatic melt in acumulate process, often with orthopyroxenes. Forsterite-rich olivine is a common crystallization product of mantle-derivedmagma. Olivine inmafic andultramafic rocks typically is rich in the forsterite end-member.
Forsterite also occurs in dolomiticmarble which results from the metamorphism of high magnesiumlimestones anddolomites.[10]Nearly pure forsterite occurs in somemetamorphosedserpentinites. Fayalite-rich olivine is much less common. Nearly pure fayalite is a minor constituent in somegranite-like rocks, and it is a major constituent of some metamorphicbanded iron formations.
Forsterite is mainly composed of the anion SiO44− and the cation Mg2+ in a molar ratio 1:2.[11] Silicon is the central atom in the SiO44− 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. 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 completesolid solution by replacing the magnesium with iron.[12] 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.[13] As the iron(II) ionsoxidize 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 theStrombolivolcano 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.
At high pressure, forsterite undergoes aphase transition into wadsleyite; under the conditions prevailing in the Earth'supper mantle, this transformation would occur at pressures of ca. 14–15 GPa.[14] In high-pressure experiments, the transformation may be delayed so that forsterite can remain metastable at pressures up to almost 50 GPa (see fig.).
The progressive metamorphism between dolomite andquartz react to form forsterite,calcite andcarbon dioxide:[15]
Forsterite reacts with quartz to form theorthopyroxene mineralenstatite in the following reaction:
Forsterite was first described in 1824 for an occurrence atMount Somma,Vesuvius,Italy. It was named byArmand Lévy in 1824 after the English naturalist and mineral collectorAdolarius Jacob Forster.[16][17]
Forsterite is being currently studied as a potentialbiomaterial forimplants owing to its superior mechanical properties.[18]
