Microcline



Optical properties - Microcline
 
Crystal System: Triclinic
Color in PPL: Colorless
Pleochroism: N/A
Habit/Shape: Can be albite.
Relief: Low negative relief
Cleavage/Fracture Habit: Has perfect cleavage parallel to {001} and good cleavage on {010}. Cleavages intersect at 90°41'. It can be difficult to see cleavage in thin section due to microcline's low relief.
pericline twinning. This combination leads to a grid pattern, hence microcline displays gridiron twinning. Can also display carlsbad twinning, simple twins, or lack twinning altogether. Lamellae in microcline are discontinuous and "pinch and swell."
Birefringence: Up to first order white (roughly 0.007)
Extinction Habit/Angle: Inclined extinction to cleavage
Length Slow/Fast: N/A
Optic Sign: Biaxial negative
2Vx: 65-88°
Alteration: Commonly alters to sericite or clay.
Distinguishing Characteristics: plagioclase because the lamellae in plagioclase are continuous and do not "pinch and swell."

Microcline (KAlSi3O8) is an important igneous rock forming triclinic microcline.

Microcline may be chemically the same as twinning which forms a grating-like structure that is unmistakable.   Perthite is either microcline or orthoclase with thin lamellae of exsolved albite.

Amazon stone, or amazonite, is a beautiful green variety of microcline. It is not found anywhere in the Amazon basin, however, Spanish explorers who named it apparently confused it with another green mineral from that region.

A sodic alkali feldspar named anorthoclase also occurs; it is a crystalline solid solution of KAlSi3O8 and NaAlSi3O8, the sodium-aluminium silicate being in larger proportion.

See also

References

  • Alkali feldspars U. Texas
  • Mindat
 
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