Cyanite

Cyanite, a native aluminium silicate, Al2SiO5, 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 characters. P. Groth writes the formula as a metasilicate (AlO)2SiO3. The name cyanite was given by A. G. Werner in 1789, from κύανος, blue, in allusion to the characteristic colour of the mineral; the form kyanite is also in common use, and the name disthène, proposed by R. J. Haüy in 1801, is used by French writers.

Distinctly developed crystals with terminal planes are rare, the mineral being commonly found as lamellar cleavage masses or long blade-shaped crystals embedded in crystalline rocks. The colour is usually a pale sky-blue, but may be white, greenish or yellowish; it 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. The accompanying figure represents a crystal twinned on the plane m (100). A negative biaxial optic figure is seen in convergent polarized light through the cleavage plane m, the axial plane being inclined at about 30° to the edge between m and t. 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, whilst in a direction parallel to the edge between m and t it is 4½. The name disthène, from δίς, two, and σθένος, strong, has reference to these differences in hardness.

Analyses of cyanite often show the presence of a small amount (usually less than 1%) of ferric oxide and sometimes traces of copper, and to these constituents the blue or green colour 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 colour are abundant. In the gneiss of the Pfitscher Tal near Sterzing in Tirol a white variety known as rhaetizite 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 colour (deep cornflower-blue) the mineral has a limited application as a gem-stone; it is usually cut en cabochon.

(L. J. S.)

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