Orthoclase



Orthoclase
Optical properties
Crystal System: Monoclinic
Color in PPL: Colorless, commonly with a dusty or cloudy appearance; white to pink in hand-sample
Pleochroism: N/A
Habit/Shape: Can be euhedral. Grains are commonly elongate with a tabular appearance.
Relief: Low negative relief
Cleavage/Fracture Habit: Has perfect cleavage on {001} and good cleavage on {010}. Cleavages intersect at 90°. It can be difficult to see cleavage in thin section due to orthoclase's low relief.
Twinning: Typically displays carlsbad twinning. Baveno and manebach twins have also been reported in orthoclase.
Birefringence: Up to first order white (roughly 0.007)
Extinction Habit/Angle: Parallel extinction to cleavage
Length Slow/Fast: N/A
Optic Sign: Biaxial negative
2Vx: 40-70°
Alteration: Commonly alters to sericite or clay.
Distinguishing Characteristics: Distinguishable from microcline by a lack in gridiron twinning. Distinguishable from sanidine by a larger 2Vx.

Orthoclase (endmember formula KAlSi3O8) is an important 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 pegmatite masses.

Typical orthoclase is a perthite.

The higher-temperature microcline.

albite.

Together with the other potassium porcelain, and as a constituent of scouring powder.

It is the state gem of Florida.  

See also

References

  • Alkali feldspars U. Texas
  • Mindat
 
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