What is "kyanite"

Cyanite \Cy"a*nite\ (-n?t), n. [See Cyanic.] (Min.) A mineral occuring in thin-bladed crystals and crystalline aggregates, of a sky-blue color. It is a silicate of aluminium. [Written also kyanite.]

n. a blue neosilicate mineral, Al₂SiO₅, found in metamorphic rocks

n. a gray or greenish-blue mineral consisting of aluminum silicate in crystalline form; occurs in metaphoric rock, used as a refractory [syn: cyanite]

Kyanite, whose name derives from the Greek word kuanos sometimes referred to as "kyanos", meaning deep blue, is a typically blue silicate mineral, commonly found in aluminium-rich metamorphic pegmatites and/or sedimentary rock. Kyanite in metamorphic rocks generally indicates pressures higher than four kilobars. Although potentially stable at lower pressure and low temperature, the activity of water is usually high enough under such conditions that it is replaced by hydrous aluminosilicates such as muscovite, pyrophyllite, or kaolinite. Kyanite is also known as disthene, rhaeticite and cyanite.

Kyanite is a member of the aluminosilicate series, which also includes the polymorph andalusite and the polymorph sillimanite. Kyanite is strongly anisotropic, in that its hardness varies depending on its crystallographic direction. In kyanite, this anisotropism can be considered an identifying characteristic.

At temperatures above 1100 °C kyanite decomposes into mullite and vitreous silica via the following reaction: 3(AlO·SiO) → 3AlO·2SiO + SiO. This transformation results in an expansion.