Microcline
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| Optical properties - Microcline | |
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| 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 (KAlSi3O8) is an important igneous rock forming tectosilicate mineral. It is a potassium-rich alkali feldspar, usually containing minor amounts of sodium, and is common in granite and pegmatites, and is often secondary after orthoclase. It is very stable at lower temperatures. Microcline may be clear, white, pale-yellow, brick-red, or green and is generally characterized by cross-hatch twinning.
Microcline is chemically the same as orthoclase, but belongs to the triclinic crystal system, the prism angle being 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 all physical properties and can be distinguished only by optical examination; under a polarizing microscope microcline exhibits a minute multiple twinning which results from a grating-like structure that is unmistakable. It is probable that much orthoclase is actually microcline.
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 soda microcline named anorthoclase is known, which is an isomorphous mixture of KAlSi3O8 and NaAlSi3O8, the sodium-aluminium silicate being in larger proportion.
