Fayalite (Fe₂SiO₄) is the iron rich end-member of the olivine solid-solution series. In common with all minerals in the olivine group, fayalite crystalises in the orthorhombic system (space group Pbnm) with cell paramiters a 4.82 Å, b 10.48 Å and c Å 6.09.

Iron rich olivine is a relitivly common constituent of acidic and alkaline igneous rocks such as volcanic obsidians, rhyolites, trachytes and phonolites and plutonic quartz syenites where it is assoceated with amphiboles. It also occours in medium-grade thermally metamorphosed iron-rich sediments.

References

• Deer, W. A., Howie, R. A., and Zussman, J. (1992). An introduction to the rock-forming minerals (2nd ed.). Harlow: Longman ISBN 0-582-30094-0