Kyanite and its related mineral “cousins,” sillimanite and andalusite, are called polymorphs. This means that they are three distinct minerals, but they all have the same chemical formula, Al2SiO5 (aluminum silicate). Because they are chemically the same, they can all be used in the same applications.
All three form in metamorphic rocks (rocks that are changed by intense heat and pressure), specifically in schists and gneisses that were formed out of sedimentary rocks with a high clay content. Studies have shown that each mineral forms under very specific temperature/pressure (T/P) conditions. Relative to one another, kyanite forms in a lower temperature/higher pressure environment; andalusite forms in a lower temperature/lower pressure environment, and sillimanite forms in a higher temperature/higher pressure environment.
Kyanite forms bladed crystals. It is generally blue, but can also be green or gray. It has a glassy luster. Kyanite has a unique physical feature in that it has two different hardnesses. When its hardness is measured across the crystal, it is 7; when it is measured down the length of the crystal, it is 5. All other minerals have a single hardness no matter where it is measured on the crystal.
Kyanite is the variant spelling of the original name of this mineral, cyanite. The name was derived from the Greek word kyanos meaning blue in reference to this mineral’s most common color. The name was given by Abraham Gottlob Werner in 1789.
Sillimanite was named in honor of Professor Benjamin Silliman (1779-1864) who was the first professor of mineralogy at Yale University (as well as professor of chemistry for a time). The name was given by G.T. Bowen in 1824.
Andalusite was named after Andalusia, a province in southern Spain, where this mineral is found. The name was given by Jean Claude Delametherie in 1798.
There are substantial deposits of kyanite in the United States. The most important deposits are in Idaho and the Appalachian Mountain region in Eastern United States. Gneisses in Southern California also have significant kyanite resources. Presently, however, it is not economical to mine these deposits. Should economic conditions change, these deposits may be worth mining. South Africa supplies most of the andalusite imported for industrial consumption in the United States. France and India also produce andalusite and kyanite, respectively.
Kyanite and its related minerals are used to make a variety of refractory materials. Refractory materials are those that are resistant to very high temperatures. As a result, more than half of the kyanite consumed is used in refractories for the production of steel. Kyanite is also used to produce refractories for nonferrous (non-iron-bearing) metals. Some is consumed to make refractories for glass and heat-resistant ceramics. Kyanite is also used to make spark plugs and is used for non-refractory applications.
Substitutes and Alternative Sources
For refractory purposes, high-alumina materials, fire clays, and a product called synthetic mullite (produced in the United States and elsewhere), can be used in place of kyanite and its related minerals. Synthetic mullite is made from bauxite (aluminum ore), clays, and silica (quartz) sand.
- Common Minerals and Their Uses, Mineral Information Institute.
- More than 170 Mineral Photographs, Mineral Information Institute.
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