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. ebavos, blue, in allusion, to the characteristic colour of the mineral; the form kyanite is also in common use, and the name disthene,. proposed by R . J . Hauy 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 lon' 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 (loo), and a less perfect one parallel to t (or)): the basal plane p (oo1), 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 (too) . A negative biaxial optic figure is seen, in convergent polarized light through the cleavage plane m, the axial plane being inclined at about 300 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 4i . The name disthene, from Sis, two, and oOivos, 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 crystal-line 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 CYAXARES 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 .