Two polymorphs of forsterite are known: wadsleyite (also orthorhombic) and ringwoodite (isometric). Both are mainly known from meteorites.
Mg2SiO4 + SiO2 → 2MgSiO3.
Forsterite also occurs in dolomitic marble which results from the metamorphism of high magnesium limestones and dolostones. 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.
Forsterite is mainly composed of the anion SiO2-4 and the cation Mg2+. 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.
This structure of forsterite can form a complete solid solution by replacing the magnesium with iron. 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. 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.
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