Kyanite

Kyanite
Kyanite crystals.jpg
General
Category Mineral
Chemical formula Al2SiO5
(aluminum silicate)
Identification
Color Blue; also green, white, grey, black
Crystal habit Columnar; fibrous
Crystal system Triclinic
Cleavage [100] Perfect, [010] Imperfect
Fracture Brittle
Mohs Scale hardness 4.5-5 parallel to one axis
6.5-7 perpendicular to that axis
Luster Vitreous; Pearly
Refractive index 1.71 - 1.75
Pleochroism Trichroic, colorless to pale blue to blue
Streak White
Specific gravity 3.56 - 3.67
Solubility None
Diaphaneity Transparent to translucent

Kyanite is a member of the aluminosilicate subgroup of silicate minerals. It is typically blue, but its color may vary. It is used in porcelain plumbing fixtures, dinnerware, electrical insulators, and abrasives. It has also been used as a gemstone, and it is a collector's mineral.

Contents

Etymology and alternative names

The name kyanite derives from the Greek word kyanos, meaning blue. This mineral is also known by several alternative names, including disthene, munkrudite, and cyanite. White-gray kyanite is also called rhaeticite.

Occurrence

Kyanite is commonly found in aluminum-rich metamorphic pegmatites and sedimentary rocks. It is a diagnostic mineral of the Blueschist Facies of metamorphic rocks.

Characteristics

Kyanite has two polymorphs: andalusite and sillimanite. Kyanite and its polymorphs have the same chemical formula, Al2SiO5, but they differ in crystalline structure.

An interesting property of kyanite is that it undergoes irreversible expansion when fired at high temperature. In addition, this mineral is strongly anisotropic, in that its hardness varies depending on its crystallographic direction.

Identifying traits

Kyanite's elongated, columnar crystals are usually a good first indication of the mineral, as well as its color (when the specimen is blue). Associated minerals are useful as well, especially the presence of the polymorphs or staurolite, which occur frequently with kyanite. However, the most useful characteristic in identifying kyanite is its anisotropism. If one suspects a specimen to be kyanite, verifying that it has two distinctly different values of hardness on perpendicular axes is a key to identification.

Associated minerals

Kyanite is usually found in association with its polymorphs and other silicate minerals. They include:

  • andalusite, Al2SiO5
  • sillimanite, Al2SiO5
  • quartz, SiO2
  • staurolite, Fe2Al9Si4O22(OH)2
  • micas, AB2-3(X, Si)4O10(O,F,OH)2
  • garnets, A3B2(SiO4)3

Uses of kyanite

In Buckingham County, Virginia kyanite from the mine at Mt. Willis is transported by train on the Buckingham Branch Railroad.

Kyanite is used primarily in refractory and ceramic products, including porcelain plumbing fixtures and dinnerware. It is also used in electrical insulators and abrasives. Kyanite has also been used as a gemstone, though this use is limited by its anisotropism and perfect cleavage. Finally, as with most minerals, kyanite is a collector's mineral.

See also

References

  • Deer, W.A., R.A. Howie, and J. Zussman. 1996. An Introduction to the Rock-Forming Minerals. 2nd ed. Upper Saddle River, NJ: Prentice Hall. ISBN 0582300940
  • Farndon, John. 2006. The Practical Encyclopedia of Rocks & Minerals: How to Find, Identify, Collect and Maintain the World's best Specimens, with over 1000 Photographs and Artworks. London: Lorenz Books. ISBN 0754815412
  • Klein, Cornelis, and Barbara Dutrow. 2007. Manual of Mineral Science. 23rd ed. New York: John Wiley. ISBN 978-0471721574
  • Pellant, Chris. 2002. Rocks and Minerals. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 0789491060
  • Schumann, Walter. 2006. Gemstones of the World. 3rd ed. New York: Sterling. ISBN 1402740166
  • Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. 2001. Rocks, Gems and Minerals. Rev. ed. New York: St. Martin's Press. ISBN 1582381321

External links

All links retrieved June 16, 2018.

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