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Kyanite


A mineral used to make porcelain, as an abrasive and occasionally as a gemstone



Kyanite



Kyanite is a mineral found mainly in metamorphic rocks. It most often forms from the high pressure alteration of clay minerals during the metamorphism of sedimentary rocks. It is found in the schists and gneisses of regionally metamorphosed areas and less often in quartzite or eclogite.

Kyanite's typical habit is a bladed crystal although it sometimes occurs as radiating masses of crystals. Kyanite is often associated with other metamorphic minerals such as garnet, staurolite and corundum.


Kyanite's Unusual Hardness



Kyanite specimens have a variable hardness. The long crystals have a Mohs hardness of about 4.5 to 5 if tested parallel to the length of a crystal and a hardness of 6.5 to 7 if tested across the short dimension of a crystal. The mineral was once commonly called "disthene" which means "two strengths".


Polymorphs of Al2SiO5



Three minerals have a chemical composition of Al2SiO5 These are kyanite, andalusite, and sillimanite. Kyanite is the high pressure polymorph, sillimanite forms at high temperature and andalusite is the low pressure polymorph.


Physical Properties of Kyanite

Chemical Classification silicate
Color Blue, white, gray, green, colorless
Streak White, colorless.
Luster Vitreous, pearly
Diaphaneity transparent to translucent
Cleavage perfect in two directions, faces sometimes striated
Mohs Hardness Kyanite often occurs in long, bladed crystals. These have a hardness of 4.5 to 5 along the length of the crystals and 6.5 to 7 across the width of the crystals.
Specific Gravity 3.5 to 3.7
Diagnostic Properties color, cleavage, bladed crystals
Chemical Composition Al2SiO5
Crystal System triclinic
Uses ceramics, gemstones


Many Industrial Uses of Kyanite



Kyanite is used to manufacture a wide range of products. An important use is in the manufacture of refractory products such as the bricks, mortars and kiln furniture used in high temperature furnaces. For foundries, the molds that are used for casting high temperature metals are often made with kyanite.

Kyanite is also in products used in the automotive and railroad industries where heat resistance is important. Mullite, a form of calcined kyanite, is used to make brake shoes and clutch facings.


Use in High Refractory Strength Porcelain



Kyanite has properties that make it exceptionally well suited for the manufacture of a high refractory-strength porcelain - a porcelain that holds its strength at very high temperatures. A familiar use of this type of porcelain is the white porcelain insulator on a spark plug.

Kyanite is also used in some of the more common forms of porcelain, such as those used to make dentures, sinks and bathroom fixtures.


Use in Abrasive Products



Kyanite's heat resistance and hardness makes it an excellent material for use in the manufacture of grinding wheels and cutting wheels. It is not used as the primary abrasive, instead it is used as part of the binding agent that holds the abrasive particles together in the shape of a wheel.


Expansion of Kyanite When Heated



Kyanite, unlike most other minerals, can expand significantly when heated. Depending upon particle size, temperatures and heating conditions, kyanite can expand to up to twice its original volume when heated. This expansion is predictable. In the manufacture of certain refractory products, specific amounts of kyanite are added to the raw material (which shrinks during heating) to maintain volume in the finished product.


Kyanite Use as a Gemstone



Occasionally, clear and colorful kyanite is encountered. This material is highly prized by those who cut gemstones. It can be cut en cabochon, faceted, or used to produce beads and pendants. Kyanite is considered to be an "exotic" gemstone because it is not often used in jewelry. Photos of Kyanite Gemstones.


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green kyanite crystal
Kyanite is often cut "en cabochon" or as a faceted gemstone. Shown above are kyanite cabochons ranging in color from clear, to blue to green and black.




blue kyanite crystal
A very common habit of kyanite is blue bladed crystals. Image by Aelwyn, creative commons license.


green kyanite crystal
Green bladed kyanite is also common. Green kyanite blades in quartzite from Avery County, North Carolina.


kyanite spark plug
The porcelain insulator on this spark plug was made with kyanite. © iStockphoto / Juergen Barry


kyanite spark plug
Kyanite is used in the porcelain of sanitary fixtures. © iStockphoto / Carl Kelliher


kyanite spark plug
Kyanite is used as a heat resistant binding medium in cutting tools and grinding wheels. © iStockphoto / Ron Sumners


Radiating kyanite
Sometimes kyanite occurs as radiating masses of crystals such as this specimen from Petaca, New Mexico. Specimen is about 4 inches (ten centimeters) across.


green kyanite crystal
Green kyanite blades in quartzite from Avery County, North Carolina. Specimen is about four inches (ten centimeters) across.


green kyanite crystal
Green bladed kyanite (same specimen as above) - looking down the long axis of the blades. Specimen is about 4 inches (10 centimeters) across.


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