Everything about Fayalite totally explained
Fayalite (Fe
2SiO
4) 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.
Iron rich olivine is a relatively common constituent of acidic and alkaline
igneous rocks such as volcanic
obsidians,
rhyolites,
trachytes and
phonolites and
plutonic quartz
syenites where it's associated with
amphiboles. It also occurs in medium-grade thermally
metamorphosed iron-rich sediments.
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.
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