Orthoclase

Orthoclase
Property	Value	Comments
Formula	KAlSi₃O₈
Crystal System	Monoclinic	2/m
Crystal Habit	Fine-grained, tabular
Cleavage	perfect cleavage on {001}, good cleavage on {010}	Difficult to see in thin section due to very low relief. Look inside the crystal to find the cleavage. Right angle cleavage is characteristic.
Color/Pleochroism	Colorless in thin section No pleochroism	Can be cloudy or pale brown due to inclusions.
Optic Sign	Biaxial (-)
2V	60-65°	May have 2V as large as 85°, but usually is less than 70°
Optic Orientation	Z=b X^a = 5^o
Refractive Indices alpha = beta = gamma =	1.520 1.525 1.527
Max Birefringence	.005-.007	1st order grey & white
Elongation	Commonly elongated parallel to c or a
Extinction	rougly Parallel	0-12 degrees
Dispersion	r > v
Distinguishing Features	Intermediate 2V, twinning: carlsbad twinning is the most common but pericline and albite twinning have also been observed, very LOW relief. Orthoclase is often confused with quartz - try looking for cloudy patches on individual crystals in plain polarized light which indicate weathering of the crystal. Also take notice of the right angle cleavage.
Occurrence	Found in granite, granodiorite, syenite, and other felsic rocks. Also found in arkoses and certain contact and regional metamorphic rocks. Usually found with quartz and muscovite.
Editors	Kristen Clark (02), Lila Rose (03), Kriten Boyer (AC08), and Sarah Motti (10)

	This whitish orthoclase crystal exhibits low birefringince color under crossed polars
	Orthoclase crystal showing extinction patterns.
	Notice the cleavage of 90 degrees in the center of the crystal.
	Orthoclase crystal at extinction--notice inclusions and undulatory extinction under cross polarized light.
	Orthoclase sample JB-10 under cross polarized light. This view shows various forms of twinning including carlsbad, pericline, and albite. Notice the change in the view of the twins as the stage is rotated counter clockwise. This is a characteristic view of orthoclase under cross polarized light.
WWW Images	U.C.L.A. Petrographic Workshop
	Union College Igneous Rocks in Thin Section