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Uncommonly Common Kyanite

Most people, even geologists, don’t think much about minerals like kyanite.  Kyanite is not useful as a gemstone, doesn’t usually display a strikingly brilliant color, and it is composed of some very common elements: oxygen, silicon and alumina – not much romance there.  But minerals likekyanite, and the kyanite cousins, andalusite, and sillimanite, are very important industrial minerals that are the critical raw materials in the manufacture another naturally occurring, but very uncommon mineral: Mullite.

Kyanite gives its name to the Kyanite group of minerals that are composed of the most common elements on the earth crust: oxygen, silicon, and alumina.  The three most prominent members of the group, kyanite, andalusite, and sillimanite all have the same chemical formula, but display very different crystal forms. Kyanite is the most common species in the group that includes the fluorine-flush Topaz, and the boron-bearing Dumortierite as rarer members.

The minerals in the Kyanite group are probably best characterized as high-alumina minerals typically containing 59% to 63% alumina (compared with about 52% for bauxite, the ore of aluminum).  The name kyanite is likely derived from the Greek word for dark blue, kyanos, but several references list an alternative name for the mineral cyanite, related to another Greek word meaning blue.   Kyanite is usually a bluish-gray, bladed mineral but can be white, gray, green, yellow or even pink.  The hardness ranges from 4 to 7, a peculiarity due to its crystal structure, and varies with the crystal face.  Its habit is long, single bladed single crystals, which can be disseminated in gneisses or schists, or in visually striking bladed masses.

Actually kyanite, for all its common elemental character, is not really common at all.  The minerals of the kyanite group are restricted to a very narrow high-temperature and high-pressure environment of formation.  These minerals occur in highly regionally or contact metamorphosed terranes.  These areas are places where high-alumina sedimentary rocks, such as shales and slates, or clay-rich sands or sandstones, have been metamorphosed to gneiss or schist.  These schists commonly contain quartz, feldspar along with kyanite, and kyanite cousins, and other minerals known for having high alumina contents such as sericite, or garnet.  As mentioned earlier, the cousins kyanite, andalusite and sillimanite have the same chemical composition, but differing temperatures and pressures of metamorphism result in differing crystal forms.

The kyanite group minerals are used primarily to produce synthetic mullite, which actually is a naturally occurring, but very rare high-alumina raw material.  It is used for the manufacture of refractory materials.  A refractory material is one that can retain its physical shape and chemical characteristics in very high temperature environments.  Refractory materials are used as material in foundry brick, moulds in steel or glass manufactuire, brick in high temperature kilns, and in ceramic materials such as in spark plugs.

Kyanite is especially desirable because it has the lowest temperature requirements for the formation of mullite.  In the US, kyanite is presently produced commercially from highly metamorphosed rocks in Virginia and Georgia.  Although no longer mined in California, commercial deposits were formerly mined in the Chocolate Mountains of Imperial County and in the White Mountains of Mono County.   So, a mineral that is composed of some very common elements, it is nevertheless, uncommonly important.