Orthoclase

Orthoclase (endmember formula KAlSi3O8) is an important tectosilicate mineral, which forms igneous rock. It is also known as alkali feldspar and is common in granite and related rocks. Orthoclase is named based on the Greek for "straight fracture," because its two cleavages are at right angles to each other. Orthoclase crystallizes in the monoclinic crystal system. It has a hardness of 6, a specific gravity of 2.56-2.58, and a vitreous to pearly luster. It can be colored white, gray, yellow, pink, or red; rarely green. Twinned crystals are quite common. Orthoclase is a common constituent of most granites and other felsic igneous rocks and is often found in huge crystals and masses in pegmatite masses.
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Noun
1. a white or colored monoclinic feldspar
(hypernym) feldspar, felspar

n. orthoclase, mineral of the feldspar group consisting of a silicate of potassium and aluminum

(n.)
Common or potash feldspar crystallizing in the monoclinic system and having two cleavages at right angles to each other. See Feldspar.
  

General Information:
 Chemical Formula:
KAlSi3O8
 Composition:
(Molecular Weight = 278.33 gm)
Potassium 14.05 % K 
Aluminum 9.69 % Al
Silicon 30.27 % Si
Oxygen 45.99 % O 
 Empirical Formula:
KAlSi3O8
 Environment:
Intrusive and extrusive igneous, and metamorphic rocks.
 IMA Status:
Approved IMA 1960
 Locality:
Common world wide occurrences.
 Name Origin:
From the Greek orthos - "right" and kalo -" I cleave" in allusion to the mineral's right angle of good cleavage.
Physical Properties:
 Cleavage:
[001] Perfect, [010] Good
 Color:
colorless, greenish, grayish yellow, white, or pink.
 Density:
2.56
 Diaphaniety:
Transparent to translucent
 Habits:
Prismatic - Crystals Shaped like Slender Prisms (e.g. tourmaline)., Massive - Granular - Common texture observed in granite and other igneous rock., Blocky - Crystal shape tends to be equant (e.g. feldspars).
 Hardness:
6 - Orthoclase
 Luminescence:
None.
 Luster:
Vitreous (Glassy)
 Streak:
white
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