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Kyanite Mineral Facts:

Chemical Formula: Al₂SiO₅ 
Kyanite is a fairly common product of metamorphism in certain schists. The mineral is a basic metasilicate. The name kyanite is from the Greek word kuanos, suggesting a sky blue color noticed in many specimens.

Colors: blue or colorless, blue-green, rarely orange or black

Hardness: 5 parallel to length of crystals, 7 at right angles to this direction.

Density: 2.8 to 3

Cleavage: Perfect parallel to the pinacoids.

Crystallography: Triclinic - pinacoidal
Crystals Very few crystals are well developed. Their habit is columnar or tabular. More frequently the mineral occurs in long, flat, isolated blades, or in diverging flat plates. Some crystals are very complex.  Twinning is common according to several laws, most of which, however, yield twins in which the basal planes of the twinned individuals are parallel..

Luster:. Glassy, Vitreous to pearly luster (pearly on clevages).

Optics: (Refractive Index):  a = 1.7171, b=1.7222, y=1.7290.

Composition, Structure and Associated Minerals:
Kyanite is an accessory mineral in gneiss and mica-schist, often associated with garnet, staurolite, corundum, etc. Notable localities for its occurrence are St. Gothard, Switzerland; in the Tyrol; Litchfield, Connecticut; Chester and Delaware counties, Pennsylvania; Gaston, Rutherford and Yancey counties, North Carolina. Kyanite weathers to muscovite, talc  and pyrophyllite, and is itself an alteration product of the weathering of andalusite and corundum.

Identification and Diagnostics
Characterized by its bladed crystals, good cleavage, blue color and the fact that it is softer than a knife in the direction parallel to the length of the crystals but harder than a knife in the direction at right angles to this. Shape and color of crystals distinctive.

A fragment moistened with cobalt nitrate and ignited assumes a blue color (test for aluminium). It is insoluble in acids. It is distinguished from the few other minerals that it resembles by the great differences in hardness on its cleavage surfaces. At a high temperature (about 1350) it apparently changes to sillimanite.

Occurrence, Localities and Origins:
Kyanite occurs as large plates and small crystals in micaceous and other schists, and as an important constituent of some quartzites. At Horrsjoberg, in Wermland, Sweden, it forms a distinct layer of schist several meters thick. In a few places it is found in zones of contact metamorphism, but it is more frequently the result of dynamic regional metamorphism. Kyanite also occurs in gneiss, pegmatite, and quartz veins resulting from moderate to high-pressure regional metamorphism of principally pelitic rocks. Kyanite is one of the index minerals that are used to estimate the temperature, depth, and pressure at which a rock undergoes metamorphism.

Uses. Transparent kyanite is sometimes used as a semiprecious gemstone, which may display cat's eye chatoyancy, though this use is limited by its anisotropism and perfect cleavage. The cleavage makes it difficult to work as a gem, as the stone can break unexpectedly. Unusual gem color varieties include recently discovered orange kyanite from Tanzania. The orange color is due to inclusion of small amounts of manganese in the structure. Kyanite is primarily used industrially to enhance the refractory and strength characteristics of certain ceramic products, including porcelain plumbing fixtures and dishware. It is also used in electronics, electrical insulators and abrasives.

Localities. A fine blue material comes from Brazil, and a deep blue comes from Zaire. Crystals have been found at Greiner in the Tyrol; at
Mte. Campione in Switzerland; and at Graves Mt. in Lincoln Co., Ga.
The mineral also occurs in fine plates at Chesterfield, Mass. ; at Litchfield,
Conn.; a light blue material is found at Bakersville, N. C.; and on North Thompson River, B. C., Canada.

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