page_title: Forsterite
section_title: Structure, formation, and physical properties
hierarchical_section_title: Forsterite / Structure, formation, and physical properties
context_page_description: Forsterite is the magnesium-rich end-member of the olivine solid solution series. It is isomorphous with the iron-rich end-member, fayalite. Forsterite crystallizes in the orthorhombic system with cell parameters a 4.75 Å, b 10.20 Å and c 5.98 Å.
  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. In 2011 it was observed as tiny crystals in the dusty clouds of gas around a forming star.
  Two polymorphs of forsterite are known: wadsleyite and ringwoodite. Both are mainly known from meteorites.
  Peridot is the gemstone variety of forsterite olivine.
context_section_description: Forsterite is mainly composed of the anion SiO₄⁴⁻ and the cation Mg²⁺ in a molar ratio 1:2. 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 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 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.
  At high pressure, forsterite undergoes a phase transition into wadsleyite; under the conditions prevailing in the Earth's upper mantle, this transformation would occur at pressures of ca. 14–15 GPa. 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 and quartz react to form forsterite, calcite and carbon dioxide:
  Forsterite reacts with quartz to form the orthopyroxene mineral enstatite in the following reaction:
caption: Molar volume vs. pressure at room temperature