plagioclase

Plagioclase is a very important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series (from the Greek "oblique fracture", in reference to its two cleavage angles). The series ranges from albite to anorthite endmembers (with respective compositions NaAlSi3O8 to CaAl2Si2O8), where sodium and calcium atoms can substitute for each other in the mineral's crystal lattice structure.
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Noun
1. any of a rock-forming series of triclinic feldspars
(synonym) oligoclase
(hypernym) feldspar, felspar
(hyponym) albite, white feldspar
(substance-holonym) basalt

n. plagioclase, mineral of the feldspar group

(n.)
A general term used of any triclinic feldspar. See the Note under Feldspar.
  

General Information:
 Chemical Formula:
(Na,Ca)(Si,Al)4O8
 Composition:
(Molecular Weight = 270.77 gm)
Sodium 4.25 % Na
Calcium 7.40 % Ca
Aluminum 9.96 % Al
Silicon 31.12 % Si
Oxygen 47.27 % O 
 Empirical Formula:
Na0.5Ca0.5Si3AlO8
 Environment:
Igneous and metamorphic rocks.
 IMA Status:
Not IMA Approved
 Locality:
Common world wide occurrences.
 Name Origin:
From the Greek plagios - "oblique" and klao - "I cleave" in allusion to the obtuse cleavage angles of the good cleavages.
Physical Properties:
 Cleavage:
[001] Good, [010] Good
 Color:
white, gray, bluish white, reddish white, or greenish white.
 Density:
2.61 - 2.76, Average = 2.68
 Diaphaniety:
Transparent to translucent
 Habits:
Massive - Granular - Common texture observed in granite and other igneous rock., ,
 Hardness:
6-6.5 - Orthoclase-Pyrite
 Luminescence:
Non-fluorescent.
 Luster:
Vitreous (Glassy)
 Streak:
white
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