Fayalite

Pure* iron end-member (Fe₂SiO₄) of the olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion solid solutionCompositional variation resulting from the substitution of one ion or ionic compound for another ion or ionic compound in an isostructural material. This results in a mineral structure with specific atomic sites occupied by two or more ions or ionic groups in variable proportions. Solid solutions can be complete (with series and an important mineralInorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous in meteorites. When iron (Fe) is completely substituted by magnesium, it yields the the pure Mg-olivine end-member, forsteritePure* magnesium end-member (Mg2SiO4) of the olivine solid solution series and an important mineral in meteorites. When magnesium (Mg) is completely substituted by iron, it yields the the pure Fe-olivine end member, fayalite (Fe2SiO4). The various Fe and Mg substitutions between these two end-members are described based on their forsteritic (Fo) (Mg₂SiO₄). The various Fe and Mg substitutions between these two end-members are described based on their forsteritic (Fo) or fayalitic (Fa) content. Since the iron content (mole percentage) plus the magnesium content (mole percentage) in the olivine must equal 100%, then Fa_X = Fo_(100-X) where X represents the Fe (mol%) and (100-X) represents the Mg (mol%) in the olivine.

GeochemistryStudy of the chemical composition of Earth and other planets, chemical processes and reactions that govern the composition of rocks and soils, and the cycles of matter and energy that transport Earth's chemical components in time and space. example:
Fa₆₅ can also be written as Fayalite (mol%): 65
In this example, the iron in the olivines comprises 65 mol% of the total and the remaining 35 mol% is magnesium or Fo₃₅.

Terrestrial olivine composition is usually expressed as Fo because igneous olivine on Earth trends towards being more iron rich (forsteritic). Meteoritic olivine composition, especially unequilibrated chondritesChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such, is usually expressed as Fa since olivines trend towards being more magnesium rich (fayalitic).

* Note: There are references in the literature to “fayalite” being used to describe olivine with composition Fa slightly less 100 mol%. Therefore, depending on the author, the term “fayalite” can be used to indicate olivine that contains a very small amount of Mg. This occurs in CV chondritesMeteorite class named after the Vigarano meteorite that fell in Italy in 1910. They have abundant large, well-defined rimless (?) chondrules of magnesium-rich olivine (~0.7 mm diameter; 40-65 vol. %), often surrounded by iron sulfide. They also contain 7-20 vol. % CAIs. The often dark-gray matrix is dominated by Fe-rich and others. (personal response from Jeff Grossman, 10 July 2020)