Forsterite

Forsterite is the magnesium-rich olivine mineral Mg₂SiO₄ and the magnesium endmember of the olivine solid-solution series.

Olivine: Olivine is a group of magnesium–iron silicate minerals with the general formula (Mg,Fe)₂SiO₄. Forsterite is the magnesium-rich endmember of this mineral group.
Olivine solid-solution series: The olivine solid-solution series spans compositions between magnesium-rich forsterite and iron-rich fayalite. Forsterite defines the magnesium end of this continuous compositional range.
Peridotite: Peridotite is a coarse-grained ultramafic rock composed mainly of olivine and pyroxene. Forsterite-rich olivine is a major constituent of many peridotites.
Dunite: Dunite is an ultramafic rock composed of at least 90 percent olivine. Its olivine can be strongly magnesium-rich and approach forsterite composition.
Fayalite: Fayalite is the iron-rich olivine endmember with the formula Fe₂SiO₄. Replacing magnesium with iron moves olivine composition toward this opposite endmember.
Cation substitution: Cation substitution is the replacement of one positively charged ion by another in a mineral structure. Iron can replace magnesium in forsterite’s metal sites, producing intermediate olivine compositions.
Earth's mantle: Earth’s mantle is the thick silicate-rock layer between the crust and the metallic core. Magnesium-rich olivine, including forsterite, is a major mantle mineral.
Pallasite: A pallasite is a stony-iron meteorite containing olivine crystals embedded in an iron–nickel matrix. Pallasitic olivine commonly includes magnesium-rich compositions near forsterite.
Silicate minerals: Silicate minerals are minerals built from silicon–oxygen tetrahedra, commonly combined with other elements. Forsterite belongs to the dominant mineral class in Earth’s crust and mantle.
Crystal structure: A crystal structure is the ordered, repeating arrangement of atoms or ions in a crystalline solid. The arrangement of magnesium, silicon, and oxygen determines forsterite’s physical properties.

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EnstatiteCompared with: Forsterite is another common magnesium-rich silicate, but belongs to olivine rather than pyroxene.