Kyanite is a typically blue aluminosilicate mineral, found in aluminium-rich metamorphic pegmatites and sedimentary rock. It is the high pressure polymorph of andalusite and sillimanite, and the presence of kyanite in metamorphic rocks generally indicates metamorphism deep in the Earth's crust. Kyanite is also known as disthene or cyanite.

Kyanite is strongly anisotropic, in that its hardness varies depending on its crystallographic direction. In kyanite, this anisotropism can be considered an identifying characteristic, along with its characteristic blue color. Its name comes from the same origin as that of the color cyan, being derived from the Ancient Greek word κύανος. This is typically rendered into English as kyanos or kuanos and means "dark blue."

Kyanite is used as a raw material in the manufacture of ceramics and abrasives, and it is an important index mineral used by geologists to trace metamorphic zones.

Properties

thumb|left|Deep blue kyanite

thumb|left|Kyanite within quartz, [[Hunterian Museum, Glasgow]]

Kyanite is an aluminum silicate mineral, with the chemical formula . It is typically patchy blue in color, though it can range from pale to deep blue It typically forms sprays of bladed crystals, but is less commonly found as distinct euhedral (well-shaped) crystals, which are particularly prized by collectors.

Kyanite's elongated, columnar crystals are usually a good first indication of the mineral, as well as its color (when the specimen is blue). Associated minerals are useful as well, especially the presence of the polymorphs of staurolite, which occurs frequently with kyanite. However, the most useful characteristic in identifying kyanite is its anisotropism. If one suspects a specimen to be kyanite, verifying that it has two distinctly different hardness values on perpendicular axes is a key to identification; it has a hardness of 5.5 parallel to {001} and 7 parallel to {100}.

Occurrence

Kyanite occurs in biotite gneiss, mica schist, and hornfels, which are metamorphic rocks formed at high pressure during regional metamorphism of a protolith which is rich in aluminium (a pelitic protolith). Kyanite is also occasionally found in granite and pegmatites and associated quartz veins, and is infrequently found in eclogites. It occurs as detrital grains in sedimentary rocks, although it tends to weather rapidly. This makes the presence of kyanite in a metamorphic rock an indication of metamorphism at high pressure.

Kyanite is often used as an index mineral to define and trace a metamorphic zone that was subject to a particular degree of metamorphism at great depth in the crust. For example, G. M. Barrow defined kyanite zones and sillimanite zones in his pioneering work on the mineralogy of metamorphic rocks. Barrow was characterizing a region of Scotland that had experienced regional metamorphism at depth. By contrast, the metamorphic zones surrounding the Fanad pluton of Ireland, which formed by contact metamorphism at a shallower depth in the crust, include andalusite and sillimanite zones but no kyanite zone.

Kyanite is potentially stable at low temperature and pressure. However, under these conditions, the reactions that produce kyanite, such as:

:

never take place, and hydrous aluminosilicate minerals such as muscovite, pyrophyllite, or kaolinite are found instead of kyanite.

Bladed crystals of kyanite are very common, but individual euhedral crystals are prized by collectors. It is also found in pegmatites of the Appalachian Mountains and in Minas Gerais, Brazil. Splendid specimens are found at Pizzo Forno in Switzerland. Mullitized kyanite is used to manufacture refractory materials.

Kyanite has been used as a semiprecious gemstone, which may display cat's eye chatoyancy, though this effect is limited by its anisotropism and perfect cleavage. Color varieties include orange kyanite from Tanzania. The orange color is due to inclusions of small amounts of manganese (Mn<sup>3+</sup>) in the structure.

References

;Specific citations:

;General references:

  • Mineral Galleries
  • usgs.gov: Kyanite and related minerals (Mineral Commodity Summaries 2025)