Fayalite

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.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 is 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.
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General Information:
 Chemical Formula:
Fe++2SiO4
 Composition:
(Molecular Weight = 203.78 gm)
Iron 54.81 % Fe
Silicon 13.78 % Si
Oxygen 31.41 % O 
 Empirical Formula:
Fe2+2(SiO4)
 Environment:
Ultra mafic silica-poor igneous rocks.
 Locality:
Lipari Islands, Sicily.
Physical Properties:
 Cleavage:
[010] Indistinct
 Color:
brown, black, or black.
 Density:
4.39
 Diaphaniety:
Transparent to Translucent
 Habits:
Massive - Granular - Common texture observed in granite and other igneous rock., Granular - Generally occurs as anhedral to subhedral crystals in matrix.,
 Hardness:
6.5 - Pyrite
 Luminescence:
None.
 Luster:
Vitreous (Glassy)
 Streak:
white
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B: (Kimia) fayalit
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