kyanite - black kyanite in a spray of fine crystals, from São José de Safira, Minas Gerais, Brazil - hand specimen

kyanite Al₂SiO₅

Kyanite forms when aluminum-rich rocks are metamorphosed, and is frequently found in schists that formed at medium to high pressures and lower temperatures in areas of regional metamorphism. At higher temperatures, kyanite converts to sillimanite.

Occasionally the metamorphism produces a pegmatite with large crystals that form when a fluid-rich magma allows rapid movement of atoms. These large crystals of kyanite and the surrounding quartz are the product of that process. This kyanite was mined in the vicinity of the town of São José de Safira, Minas Gerais State, southeastern Brazil, a famous locality for this material. It is a polymorph of sillimanite and andalusite, sharing composition but differing in crystal structure.

The geologic setting at São José de Safira is a schist belt 30 km wide and 100 km long that is interpreted as an accretionary wedge in a basin that closed during the assembly of West Gondwanaland.

Kyanite was named by Abraham Gottlieb Werner in 1789 from kyanos, the Greek word for blue. The mineral was called "cyanite" by mineralogists using the French spelling, during most the 19th and early 20th centuries, though it can also be black (as these), white, and various shades of yellow, green and pink. It is mined at a small number of sites as an industrial mineral, used in the production of refractory materials, the ceramics designed to withstand very high temperatures in excess of 1,000˚F. Refractories are essential in the manufacture of petrochemicals and refining of gasoline.

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