Pure* iron end-member (Fe2SiO4) of the olivine solid solution series and an important mineral in meteorites. When iron (Fe) is completely substituted by magnesium, it yields the the pure Mg-olivine end-member, forsterite (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 FaX = Fo(100-X) where X represents the Fe (mol%) and (100-X) represents the Mg (mol%) in the olivine.

Geochemistry 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 chondrites, 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 chondrites and others. (personal response from Jeff Grossman, 10 July 2020)