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 |
| 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 - 3.7 |
| Distinguishing Characteristics |
Color, cleavage, bladed crystals. |
| Chemical Composition |
Al2SiO5 |
| Crystal System |
Triclinic |
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.
Contributor: Hobart King
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| Green kyanite blades in quartzite from Avery County, North Carolina. Specimen is about four inches (ten centimeters) across. |
| 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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| 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. |
| A very common habit of kyanite is blue bladed crystals. Image by Aelwyn, creative commons license. |
| Green bladed kyanite is also common. Green kyanite blades in quartzite from Avery County, North Carolina. |
| The porcelain insulator on this spark plug was made with kyanite. © iStockphoto / Juergen Barry |
| Kyanite is used in the porcelain of sanitary fixtures. © iStockphoto / Carl Kelliher |
| Kyanite is used as a heat resistant binding medium in cutting tools and grinding wheels. © iStockphoto / Ron Sumners |
| Sometimes kyanite occurs as radiating masses of crystals such as this specimen from Petaca, New Mexico. Specimen is about 4 inches (ten centimeters) across. |
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