Plagioclase series

Chemical Formula : (Na,Ca)Al(Al,Si)Si₂O₈
Hardness : 6
Cleavage : Three, two good, one poor
Luster : Vitreous, to pearly.
Color : Whtie or gray as end members, interneal series members black, white, reddish brown.
Streak : White
Density : 2.62-2.76br> Diagnostic Features and Trivia : The plagioclase series runs from albite NaAlSi₃O₈ (0-10% anorthite), oligoclase (10-30% anorthite), andesine (30-50% anorthite), labradorite (50-70% anorthite), bytownite (30-10% anorthite), and anorthite CaAl₂Si₂O₈ (90-100% anorthite). As a whole, plagioclase sommonly has twinning striations which help in identification. Bytownite and labradorite commonly have schiller.
Crystal System and Habit : Triclinic; typically tabular parallel to {010}.
Twinning : Nearly universal polysynthetic twinning according to one or both albite and pericline laws.
Polymorphs : None
Index of Refraction : alhpa= 1.528-1.575; beta=1.533-1.584; gamma=1.539-1.589. Increasing with increasing anorthite.