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Microcline

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Microcline is one of the most common feldspar minerals. It can be colorless, white, cream to pale yellow, salmon pink to red, or bright green to blue-green. Microcline forms short prismatic or tabular crystals that are often of considerable size: single crystals can weigh several tons and reach yards in length. Crystals are often multiply twinned, with two sets of fine lines at right angles to each other. This gives a “plaid” effect that is unique to microcline among the feldspars. Microcline can also be massive. The mineral occurs in feldspar-rich rocks, such as granite, syenite, and granodiorite. It is found in granite pegmatites and in metamorphic rocks, such as gneisses and schists.

Polymorphism & Series: Dimorphous with orthoclase.

Mineral Group: Feldspar (alkali) group; (Si,Al) is completely ordered in low microcline.

Chemical Properties of Microcline

Chemical Classification Silicate
Chemical Composition K(AlSi3O8)

Physical Properties of Microcline

Color White, grey, greyish yellow, yellowish, tan, salmon-pink, bluish green, green.
Streak White
Luster Vitreous
Cleavage Perfect on [001], good on [010]
Diaphaneity Transparent, Translucent
Mohs Hardness 6 – 6½ on Mohs scale
Specific Gravity 2.54 – 2.57
Crystal System Triclinic
Tenacity Brittle
Parting on {100}{110}{110}{201}
Fracture Irregular/Uneven
Density 2.54 – 2.57 g/cm3 (Measured)    2.56 g/cm3 (Calculated)

Optical Properties of Microcline

Microcline grain in centre showing its distinctive cross-hatched twinning.
Type Anisotropic
Optical Extinction Inclined extinction to cleavage
Twinning Carlsbad, Baveno, Manebach, polysynthetic on albite and pericline laws.
Optic Sign Biaxial (-)
Birefringence δ = 0.007 – 0.010
Relief Low

Occurrence

Common in plutonic felsic rocks, as granites, granite pegmatites, syenites; in metamorphic rocks of the greenschist and amphibolite facies; in hydrothermal veins. A detrital component in sedimentary rocks and as authigenic overgrowths.

Uses Areas

  • The most important place of use is the production of porcelain.
  • Microcline is used industrially in the production of glass and ceramic products.
  • It is used as ornamental lapidary material with Amazonite in green color.
  • Sometimes feldspar is also used in the manufacture of glass.

Association

Quartz, sodic plagioclase, muscovite, biotite, hornblende.

Distribution

A widespread mineral. Notable occurrences include:

  • at FredriksvÄarn, Arendal, and Larvik, Norway.
  • In the Ilmen Mountains, Ural Mountains, and on the Kola Peninsula, Russia.
  • At St. Gotthard, Ticino, Switzerland.
  • On Mt. Greiner, Zillertal, Tirol, Austria.
  • At Baveno, Piedmont, Italy.
  • In the USA, at Amelia, Amelia Co., Virginia; Haddam, Middlesex Co., Connecticut; and Magnet Cove, Hot Spring Co., Arkansas.
  • In Colorado, in the Pikes Peak area, El Paso Co., Crystal Peak, Teller Co., with large crystals from the Devil’s Hole beryl mine, Fremont Co.; in the Black Hills, Pennington and Custer Cos., South Dakota.
  • At Bancroft, Ontario, Canada.
  • From Klein Spitzkopje, Namibia.
  • In Brazil, from Minas Gerais, at Fazenda do Bananal, Salinas, Urucum, and Capelinha.
  • At Ambositra, Madagascar.
  • From Kimpusan, Yamanshi Prefecture, and Tanakamiyama, Otsu, Shiga Prefecture, Japan.
  • At Broken Hill, New South Wales, Australia.

References

  • Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
  • Handbookofmineralogy.org. (2019). Handbook of Mineralogy. [online] Available at: http://www.handbookofmineralogy.org [Accessed 4 Mar. 2019].
  • Mindat.org. (2019). Microcline: Mineral information, data and localities.. [online] Available at: https://www.mindat.org
Cite this article as: Geology Science. (2019). Microcline. [online] Available at: http://geologyscience.com/minerals/microcline/ [5th December 2019 ]
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