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CYANITE

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Originally appearing in Volume V07, Page 680 of the 1911 Encyclopedia Britannica.
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CYANITE , a native See also:

aluminium silicate, Al2SiO5, crystallizing in the anorthic See also:system. It has the same percentage chemical See also:composition as See also:andalusite and See also:sillimanite, but differs from these in its crystallographic and See also:physical characters. P. See also:Groth writes the See also:formula as a metasilicate (AlO)2SiO3• The name cyanite was given by A. G. See also:Werner in 1789, from. ebavos, See also:blue, in allusion, to the characteristic See also:colour of the See also:mineral; the See also:form kyanite is also in See also:common use, and the name disthene,. proposed by R. J. See also:Hauy in 1801, is used by See also:French writers: , Distinctly See also: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 See also:pale See also:sky-blue, but may be See also:white, greenish or yellowish; it varies in intensity in different bands, so that the crystals usually See also:present a more or less striped See also:appearance. There is a perfect cleavage parallel to the broad See also:face m (See also:loo), and a less perfect one parallel to t (or)): the basal See also:plane p (oo1), oblique to the See also:prism See also:zone, is a gliding plane on which secondary twinning is produced by pressure, giving rise to characteristic See also: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 See also: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 See also:

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 See also:Sis, two, and oOivos, strong, has reference to these See also:differences in hardness. Analyses of cyanite often show the presence of a small amount (usually less than 1%) of ferric See also:oxide and sometimes traces of See also:copper, and to these constituents the blue or See also:green colour of the mineral is doubtless due. The mineral is infusible before the See also: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-See also:line rocks—gneiss, schist, See also:granulite and eclogite—and is often associated with See also:garnet and See also:staurolite. A typical occurrence is in the white, See also:fine-scaled paragonite-schist of See also:Monte Campione, near St Gotthard in See also:Switzerland, where See also:long transparent crystals of a fine blue colour are abundant. In the See also:gneiss of the Pfitscher Tal near Sterzing in See also:Tirol a white variety known as rhaetizite is found. It occurs at several places in See also:Scotland, for instance, at Botriphnie in See also:Banffshire, with See also:muscovite in a See also:quartz-vein. Fine specimens are found in See also:mica-schist at See also:Chesterfield in See also:CYAXARES See also:Massachusetts, and at several other: localities in the See also:United States. It is found in the See also:gold-washings of the See also:southern Urals and in the See also:diamond-washings of See also:Brazil. As See also:minute crystal fragments it is met with in many sands and sandstones.

When of sufficient transparency and See also:

depth of colour (deep cornflower-blue) the mineral has a limited application as a See also:gem-See also:stone; it is usually cut en cabochon. (L. J.

End of Article: CYANITE

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