| Microcline | |
|---|---|
 | |
| General | |
| Category | Tectosilicateminerals,feldspargroup,alkali series |
| Formula | KAlSi3O8 |
| IMA symbol | Mcc[1] |
| Crystal system | Triclinic |
| Space group | P1 (no. 2) |
| Identification | |
| Color | White, grey, greyish yellow, yellowish, tan, salmon-pink, bluish green, green. |
| Crystal habit | Can be anhedral or euhedral. Grains are commonly elongate with a tabular appearance. May contain lamellae which formed from exsolved albite. |
| Twinning | Typically displaysalbite twinning andpericline twinning. This combination leads to a grid pattern, hence microcline displaysgridiron twinning. Can also displaycarlsbad twinning, simple twins, or lack twinning altogether. Lamellae in microcline are discontinuous and "pinch and swell". |
| Cleavage | 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. |
| Fracture | Uneven |
| Tenacity | Brittle |
| Mohs scale hardness | 6–6.5 |
| Luster | Vitreous |
| Streak | White |
| Diaphaneity | Transparent, translucent |
| Specific gravity | 2.5–2.6 |
| Optical properties | Biaxial negative |
| Refractive index | nα = 1.514 – 1.529 nβ = 1.518 – 1.533 nγ = 1.521 – 1.539 |
| Birefringence | Up to first order white (roughly 0.007) |
| Pleochroism | N/A |
| 2V angle | 65–88° |
| Extinction | Inclined extinction to cleavage |
| Diagnostic features | Gridiron twinning distinguishes microcline from other feldspars. Distinguishable from plagioclase because the lamellae in plagioclase are continuous and do not "pinch and swell." |
| Alters to | Commonly alters to sericite or clay. |
| Relief | Low negative relief |
| Optical sign | Biaxial negative |
| Color inPPL | Colorless |
Microcline (KAlSi3O8) is an importantigneous rock-formingtectosilicatemineral. It is apotassium-richalkali feldspar. Microcline typically contains minor amounts ofsodium. It is common ingranite andpegmatites. Microcline forms during slow cooling oforthoclase; it is more stable at lower temperatures than orthoclase.Sanidine is apolymorph of alkali feldspar stable at yet higher temperature. Microcline may be clear, white, pale-yellow, brick-red, or green; it is generally characterized by cross-hatch twinning that forms as a result of the transformation ofmonoclinic orthoclase intotriclinic microcline.
The chemical compound name is potassium aluminium silicate, and it is known asE number referenceE555.
Microcline may be chemically the same asmonoclinic orthoclase, but because it belongs to thetriclinic crystal system, the prism angle is slightly less than right angles; hence the name "microcline" from theGreek "small slope". It is a fully orderedtriclinic modification ofpotassiumfeldspar and isdimorphous withorthoclase. Microcline is identical to orthoclase in many physical properties, and can be distinguished by x-ray or optical examination. When viewed under apolarizing microscope, microcline exhibits a minute multipletwinning which forms a grating-like structure that is unmistakable.
Perthite is either microcline or orthoclase with thin lamellae of exsolved albite.
Amazon stone, oramazonite, is a green variety of microcline. It is not found anywhere in theAmazon Basin, however. TheSpanish explorers who named it apparently confused it with another green mineral from that region.
The largest documented single crystals of microcline were found in Devil's Hole Beryl Mine,Colorado, US and measured ~50 × 36 × 14 m. This could be one of the largest crystals of any material found so far.[2]
Microcline is exceptionally active ice-nucleating agent in the atmosphere.[3] Recently it has been possible to understand how water binds to the microcline surface.[4]
The chemical compound name is potassium aluminium silicate, and it is known asE number referenceE555. It was the subject in 2018 of a Call for technical and toxicological data from theEFSA.[5]
In 2008, it (along with other Aluminum compounds) was the subject of a Scientific Opinion of the Panel on Food Additives, Flavourings, Processing Aids and Food Contact Materials from the EFSA.[6]
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