Kyanite

Kyanite
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.

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, AB_2-3(X, Si)₄O₁₀(O,F,OH)₂
• garnets, A₃B₂(SiO₄)₃

Uses of kyanite

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

• Garnet
• Mica
• Quartz
• Silicate

References

ISBN links support NWE through referral fees

• 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.

• Kyanite Mineral Data Webmineral.com.
• Kyanite Mindat.org.