Kyanite

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. 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 and can also be gray or white or, infrequently, light green. It typically forms sprays of bladed crystals, but is less commonly found as distinct euhedral (well-shaped) crystals, which are particularly prized by collectors. It has a perfect {100} cleavage plane, parallel to the long axis of the crystal, and a second good cleavage plane {010} that is at an angle of 79 degrees to the {100} cleavage plane. Kyanite also shows a parting on {001} at an angle of about 85 degrees to the long axis of the crystal. Cleavage surfaces typically display a pearly luster. The crystals are slightly flexible. 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.