Fayalite n. : A black, greenish, or brownish mineral of the chrysolite group. It is a silicate of iron.

Example:
A rock is 5.5% by mass Fayalite (Fe2SiO4), 7.4% by mass forsterite (Mg2SiO4), ----please see details ----?

Taxonomy
Fayalite, quartz, free encyclopedia, oxygen, Å, reaction, end-member, solid-solution, solution series, minerals, olivine group, pressure, fractures, search, Greenish yellow, yellow-brown, pale yellow, thin section.

Defination from the web:
fayalite crystallizes in the orthorhombic system (space group Pbnm) with cell parameters a 4.82 ... Fayalite is stable with quartz at low pressures, whereas more magnesian olivine is not, because of the reaction olivine + quartz = orthopyroxene. ...