Fayalite (Fe2SiO4) is the iron-rich end-member of the olivine solid-solution series. In common with all minerals in the olivine group, fayalite crystallizes in the orthorhombic system (space group Pbnm) with cell parameters a 4.82 Å, b 10.48 Å and c Å 6.09.
Fayalite is stable with quartz at low pressures, whereas more magnesian olivine is not, because of the reaction olivine + quartz = orthopyroxene. Iron stabilizes the olivine + quartz pair. The pressure and compositional dependence of the reaction can be used to calculate constraints on pressures at which assemblages of olivine + quartz formed.
Fayalite can also react with oxygen to produce magnetite + quartz: the three minerals together make up the "FMQ" oxygen buffer. The reaction is used to control the fugacity of oxygen in laboratory experiments. It can also be used to calculate the fugacity of oxygen recorded by mineral assemblages in metamorphic and igneous processes.
The name fayalite is derived from Faial (Fayal) Island in the Azores where it was first described in 1840.
* Forsterite, (Mg2SiO4), the magnesium-rich end-member of the olivine solid-solution series.
1. ^ a b http://rruff.geo.arizona.edu/doclib/hom/fayalite.pdf Handbook of Mineralogy
* 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