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Kyanite |
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Kyanite is a metamorphic mineral that most often forms from the high pressure alteration of clay-rich 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 HardnessKyanite specimens have a variable hardness. The long crystals have a 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 Al2SiO5Three 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. Many Industrial Uses of KyaniteKyanite 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 PorcelainKyanite 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 ProductsKyanite'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 HeatedKyanite, 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 GemstoneOccasionally, 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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Kyanite - Mineral Properties and Uses |
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Uses |
Kyanite is most often used where heat resistance is important. It is used to make high refractory strength porcelain, refractory bricks and mortars, kiln furniture, brake shoes, clutch facings, grinding wheels, cutting disks. It is also used in standard porcelain of dentures, sinks and bathroom fixtures. When kyanite is highly transparent and of attractive color it can be cut into cabochon and faceted gemstones. |
Color |
blue, white, gray, green, colorless |
Streak |
white, colorless |
Luster |
vitreous, pearly |
Diaphaneity |
transparent to translucent |
Cleavage |
perfect in two directions, faces sometimes striated |
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 |
color, cleavage, bladed crystals |
Crystal System |
triclinic |
Chemical Classification |
Silicate - Nesosilicate |
Chemical Composition |
Al2SiO5 (kyanite is polymorphic with sillimanite and andalusite) |
Kyanite is often cut as a gemstone. Shown above are kyanite cabochons ranging in color from clear, to blue to green and black. Photos of kyanite gemstones.
Blue bladed kyanite from Minas Gerais, Brazil. Specimen is approximately two inches (5 centimeters) across.
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.
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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Mineral photographs that appear on this page with a Geology.com copyright mark are property of Geology.com and may not be used beyond our website. If you like these photos and would like to share them with others please link to this page or email its URL. The photos were taken and composed by Ann Bryant. |