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Kyanite



Kyanite

General
CategoryMineral
Chemical formulaAl2SiO5
(aluminium silicate)
Identification
ColorBlue; also green, white, grey, black
Crystal habitColumnar; fibrous
Crystal systemTriclinic
Cleavage[100] Perfect, [010] Imperfect
FractureBrittle
Mohs Scale hardness4.5-5 parallel to one axis
6.5-7 perpendicular to that axis
LusterVitreous; Pearly
Refractive index1.71 - 1.75
PleochroismTrichroic, colorless to pale blue to blue
StreakWhite
Specific gravity3.56 - 3.67
SolubilityNone
DiaphaneityTransparent to translucent

 

Kyanite, whose name derives from the Greek word kyanos, meaning blue, is a typically blue silicate mineral, commonly found in aluminium-rich metamorphic pegmatites and/or sedimentary rock. Kyanite is a diagnostic mineral of the Blueschist Facies of metamorphic rocks.

Kyanite is a member of the aluminosilicate series, which includes the polymorph andalusite and the polymorph sillimanite. Kyanite is strongly anisotropic, in that its hardness varies depending on its crystallographic direction. While this is a feature of almost all minerals, in kyanite this anisotropism can be considered an identifying characteristic.

Additional recommended knowledge

Contents

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. An interesting property of kyanite is that it undergoes an irreversible expansion when fired at high temperature.[citation needed] Kyanite has also been used as a gemstone, though this use is limited by its anisotropism and perfect cleavage. Kyanite is one of the index minerals that are used to estimate the temperature, depth, and pressure at which a rock undergoes metamorphism. Finally, as with most minerals, kyanite is a collector's mineral.

Associated minerals

Kyanite is usually found in association with its polymorphs, as well as other silicate minerals. These include:

Alternative names

Kyanite has several alternative names, including disthene, munkrudite and cyanite. White-grey kyanite is also called rhaeticite.

Notes for identification

  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 hardnesses on perpendicular axes is a key to identification.

References

  • WebMineral Listing
  • MinDat Listing
  • Mineral Galleries
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Kyanite". A list of authors is available in Wikipedia.
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