Cyanite
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Cyanite ia a native aluminium silicate mineral, AlSiOi, crystallizing in the anorthic system. It has the same percentage chemical composition as andalusite and sillimanite, but differs from these in its crystallographic and physical characteristics. The formula is sometimes written as a metasilicate, (AlO)1SiO3. The name cyanite was given by A. G. Werner in 1789 in allusion to the characteristic blue color of the mineral. The name kyanite is also in common use.
Distinctly developed crystals with L-terminal planes are rare, the mineral being commonly found as lamellar cleavage masses or as lone blade-shaped crystals embedded in crystalline rocks.
The color is usually a pale sky-blue, but may be white, greenish or yellowish. The color varies in intensity in different bands, so that the crystals usually present a more-or-less striped appearance. There is a perfect cleavage parallel to the broad face m (100), and a less perfect one parallel to t (010); the basal plane, p (001), oblique to the prism zone, is a gliding plane on which secondary twinning is produced by pressure, giving rise to characteristic horizontal striations on the cleavage face m. A remarkable feature of cyanite is the great difference in hardness on different faces of the same crystal and in different directions on the same face; on the face m in a direction parallel to the edge between m and p the hardness is 7, while in a direction parallel to the edge between m and I it is 4 1/2.
Analyses of cyanite often show the presence of a small amount (usually less than 1%) of ferric oxide and sometimes traces of copper. It is to these constituents that the blue or green color of the mineral is doubtless due. The mineral is infusible before the blowpipe, and is not decomposed by acids. At a high temperature, about 1350 C, it becomes transformed into sillimanite, changing in specific gravity from 3.6 to 3.2.
Cyanite is a characteristic mineral of the metamorphic crystalline rocks gneiss, schist, granulite and eclogite and is often associated with garnet and staurolite. A typical occurrence is in the white, fine-scaled paragonite-schist of Monte Campione near St. Gotthard in Switzerland, where long, transparent crystals of a fine blue color are abundant. In the gneiss of the Pfitscher Tal near Sterzing in Tirol a white variety known as ihaetizite is found. It occurs at several places in Scotland, for instance, at Botriphnie in Banffshire, with muscovite in a quartz-vein. Fine specimens are found in mica-schist at Chesterfield in Massachusetts, and at several other localities in the United States. It is found in the gold-washings of the southern Urals and in the diamond-washings of Brazil. As minute crystal fragments it is met with in many sands and sandstones.
When of sufficient transparency and depth of color (deep cornflower-blue), the mineral has a limited application as a gem-stone; it is usually cut en cabochon.