Fayalite

Fayalite is named after its discovery locality, on Fayal Island, Azores; although believed to occur in the local lavas. Fayalite is iron-bearing olivine mineral.

Fayalite is very rare in nature, but common in man-made (metallurgy) iron slags.
General Information
A variety or type of:Peridot
Chemical Formula
Fe
 
2
SiO
 
4
Michael O’Donoghue, Gems, Sixth Edition (2006)
Photos of natural/un-cut material from mindat.org
Physical Properties of Fayalite
Mohs Hardness6.5 to 7
Anthony et al, Handbook of mineralogy (2001)
Specific Gravity4.392
Anthony et al, Handbook of mineralogy (2001)
TenacityBrittle
Anthony et al, Handbook of mineralogy (2001)
Cleavage QualityImperfect
Anthony et al, Handbook of mineralogy (2001)
FractureConchoidal
Anthony et al, Handbook of mineralogy (2001)
Optical Properties of Fayalite
Refractive Index1.827 to 1.879
Anthony et al, Handbook of mineralogy (2001)
Optical CharacterBiaxial/-
Anthony et al, Handbook of mineralogy (2001)
PleochroismFaint: pale yellow - yellow-orange, reddish brown.
Anthony et al, Handbook of mineralogy (2001)
DispersionWeak
Anthony et al, Handbook of mineralogy (2001)
Colour
Colour (General)Greenish yellow, yellow-brown, brown; pale yellow to amber in thin section
Anthony et al, Handbook of mineralogy (2001)
TransparencyTransparent
Anthony et al, Handbook of mineralogy (2001)
LustreVitreous,Resinous
Anthony et al, Handbook of mineralogy (2001) Vitreous to resinous on fractures
Crystallography of Fayalite
Crystal SystemOrthorhombic
Anthony et al, Handbook of mineralogy (2001)
HabitCrystals are thick to thin, tabular, typically with wedge-shaped terminations, producing eight-sided cross sections. Commonly granular, compact, or massive.
Anthony et al, Handbook of mineralogy (2001)
Geological Environment
Where found:In ultramafic volcanic and plutonic rocks, less commonly in felsic plutonic rocks; rarely in granite pegmatite; in lithophysae in obsidian. In metamorphosed iron-rich sediments and
impure carbonate rocks.
Anthony et al, Handbook of mineralogy (2001)
Further Information
Mineral information:Fayalite information at mindat.org
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