Anorthoclase

Anorthoclase is named from Greek "an" - not, plus "orthos" - right, plus "klan" - to break, alluding to the fact that the cleavages are not at right angles.

Anorthoclase forms opaque to translucent , rarely transparent, colorless, also white, pale creamy yellow, occasionally greenish or pink, red crystals.
General Information
Chemical Formula
(Na,K)AlSi
 
3
O
 
8
Michael O’Donoghue, Gems, Sixth Edition (2006)
Photos of natural/un-cut material from mindat.org
Physical Properties of Anorthoclase
Mohs Hardness6 to 6.5
Michael O’Donoghue, Gems, Sixth Edition (2006)
Specific Gravity2.56 to 2.62
Michael O’Donoghue, Gems, Sixth Edition (2006)
Cleavage QualityPerfect,Good
Michael O’Donoghue, Gems, Sixth Edition (2006) Perfect to good
PartingPrismatic
Michael O’Donoghue, Gems, Sixth Edition (2006)
FractureUneven,Conchoidal
Michael O’Donoghue, Gems, Sixth Edition (2006)
Optical Properties of Anorthoclase
Refractive Index1.518 to 1.534
Michael O’Donoghue, Gems, Sixth Edition (2006) Alpha 1.518–1.527; beta 1.522–1.532; gamma 1.522–1.534
Optical CharacterBiaxial/-
Michael O’Donoghue, Gems, Sixth Edition (2006)
Birefringence0.006 to 0.008
Michael O’Donoghue, Gems, Sixth Edition (2006)
PleochroismWeak
Michael O’Donoghue, Gems, Sixth Edition (2006)
DispersionWeak
Michael O’Donoghue, Gems, Sixth Edition (2006)
Colour
Colour (General)Colorless, also white, pale creamy yellow, occasionally greenish or pink, red.
Michael O’Donoghue, Gems, Sixth Edition (2006)
TransparencyTranslucent,Opaque
Michael O’Donoghue, Gems, Sixth Edition (2006) Rarely transparent
LustreVitreous,Pearly
Michael O’Donoghue, Gems, Sixth Edition (2006) May be pearly on cleavages
Crystallography of Anorthoclase
Crystal SystemMonoclinic
Michael O’Donoghue, Gems, Sixth Edition (2006)
Geological Environment
Where found:As phenocrysts in high-temperature volcanic rocks such as trachytes, phonolites and andesites and in tuffs.
Michael O’Donoghue, Gems, Sixth Edition (2006)
Further Information
Mineral information:Anorthoclase information at mindat.org
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