| Optical properties - Microcline | |
|---|---|
| Crystal System: | Triclinic |
| Color in PPL: | Colorless |
| Pleochroism: | N/A |
| Habit/Shape: | Can be anhedral or euhedral. Grains are commonly elongate with a tabular appearance. May contain lamellae which formed from exsolved 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. |
| Twinning: | Typically displays albite twinning and 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: | Gridiron twinning distinguishes microcline from other feldspars. Distinguishable from plagioclase because the lamellae in plagioclase are continuous and do not "pinch and swell." |
Microcline may be chemically the same as monoclinic orthoclase, but because it belongs to the triclinic crystal system, the prism angle is slightly less than right angles; hence the name "microcline" from the Greek "small slope." It is a fully ordered triclinic modification of potassium feldspar and is dimorphous with orthoclase. Microcline is identical to orthoclase in many physical properties; it can be distinguished by x-ray or optical examination; viewed under a polarizing microscope, microcline exhibits a minute multiple 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
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Last updated on Friday July 11, 2008 at 17:10:38 PDT (GMT -0700)
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