Concept

Forsterite

Summary
Forsterite (Mg2SiO4; commonly abbreviated as Fo; also known as white olivine) 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 (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). 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 (also orthorhombic) and ringwoodite (isometric, Cubic crystal system). Both are mainly known from meteorites. Peridot is the gemstone variety of forsterite olivine. Pure forsterite is composed of magnesium, oxygen and silicon. The chemical formula is Mg2SiO4. Forsterite, fayalite (Fe2SiO4) and tephroite (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 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. Forsterite-rich olivine is the most abundant mineral in the mantle above a depth of about ; pyroxenes are also important minerals in this upper part of the mantle. Although pure forsterite does not occur in igneous rocks, dunite often contains olivine with forsterite contents at least as Mg-rich as (92% forsterite – 8% fayalite); common peridotite contains olivine typically at least as Mg-rich as . Due to its high melting point, olivine crystals are the first minerals to precipitate from a magmatic melt in a cumulate process, often with orthopyroxenes. Forsterite-rich olivine is a common crystallization product of mantle-derived magma.
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