Mineral - Orthoclase
Orthoclase
Chemical Properties
Mineral Class
Silicates
Toxicity
none
Formula
K[AlSi₃O₈]
Unit Cell
a=8.56 b=12.96 c=7.30 β=116° Z=4
Physical Properties
Hardness
6 - 6.5
Streak
white
Density
Normal (2-3,5)
Cleavage
Perfect after {001}, distinct after {010}
Fracture
uneven
Tenacity
Brittle
Optical Properties
Color
whitebrowngrayredyellowcolorless
Birefringence
0.005
Luster
Vitreous,
Pearly
Transparency
translucent,
transparent,
opaque
Pleochroism
Geomineralogical Properties
Crystal System
monoclinic
Point Group
monoclinic-prismatic - 2/m
Space Group
C2/m (12)
Habit
blocky, coarse, lath-shaped, prismatic, tabular
Formation
(1) Primarily magmatic, formed by crystallization from silica-rich melts (plutonic/volcanic rocks) as well as in granitic pegmatites.
(2) Metamorphic, formed by the alteration of aluminum-rich sediments or granitoid protoliths under medium- to high-grade conditions (amphibolite to granulite facies).
Paragenesis
Twinning
Penetration twins after {001} – Karlsbader law, Contact twins after {001} – Manebach law, Contact twins after {021} – Baveno law
Rarity
Very common
Synonyms
Orthoclase
Composition
Elements
Mass
SiO₂
65.39
Al₂O₃
18.45
BaO
0.02
Na₂O
1.08
K₂O
14.76
Rb₂O
0.49
California (Dana 1895)