Microcline (KAlSi₃O₈) is an important igneous rock-forming tectosilicate

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

. It is a

potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...

-rich alkali feldspar. Microcline typically contains minor amounts of

sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...

. It is common in

granite Granite ( ) is a coarse-grained (phanerite, phaneritic) intrusive rock, intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly coo ...

and pegmatites. Microcline forms during slow cooling of orthoclase; it is more stable at lower temperatures than orthoclase. Sanidine is a polymorph 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 of

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three Vector (geometric), vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in t ...

orthoclase into triclinic microcline. The chemical compound name is potassium aluminium silicate, and it is known as

E number E numbers, short for Europe numbers, are codes for substances used as food additives, including those found naturally in many foods, such as vitamin C, for use within the European Union (EU) and European Free Trade Association (EFTA). Commonly ...

reference E555.

Geology

Microcline may be chemically the same as

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

feldspar Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagiocl ...

and is dimorphous with orthoclase. Microcline is identical to orthoclase in many physical properties, and can be distinguished by x-ray or optical examination. When 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 Amazonite, also known as amazonstone, is a green tectosilicate mineral, a variety of the potassium feldspar called microcline. Its chemical formula is KAlSi3O8, which is Polymorphism (materials science), polymorphic to orthoclase. Its name is ta ...

, is a green variety of microcline. It is not found anywhere in the

Amazon Basin The Amazon basin is the part of South America drained by the Amazon River and its tributary, tributaries. The Amazon drainage basin covers an area of about , or about 35.5 percent of the South American continent. It is located in the countries ...

, however. The Spanish 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 Colorado is a U.S. state, state in the Western United States. It is one of the Mountain states, sharing the Four Corners region with Arizona, New Mexico, and Utah. It is also bordered by Wyoming to the north, Nebraska to the northeast, Kansas ...

, US and measured ~50 × 36 × 14 m. This could be one of the largest crystals of any material found so far. Microcline is exceptionally active ice-nucleating agent in the atmosphere. Recently it has been possible to understand how water binds to the microcline surface.

As food additive

The chemical compound name is potassium aluminium silicate, and it is known as

reference E555. It was the subject in 2018 of a Call for technical and toxicological data from the EFSA. 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.

References

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Alkali feldspars U. Texas
Potassium minerals Aluminium minerals Tectosilicates Triclinic minerals Feldspar Luminescent minerals E-number additives Minerals in space group 2

Geology

As food additive

See also

References