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 [greek: kyanos], 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. [illustration] 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 deg. 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 4(1/2). the name disthene, from [greek: dis], two, and [greek: sthenos], 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 deg. 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.) cyanogen (gr. [greek: kyanos], blue [greek: gennan], to produce), c2n2, in chemistry, a gas composed of carbon and nitrogen. the name was suggested by prussian blue, the earliest known compound of cyanogen. it was first isolated in 1815 by j. gay-lussac, who obtained it by heating mercury or silver cyanide; this discovery is of considerable historical importance, since it recorded the isolation of a "compound radical." it may also be prepared by heating ammonium oxalate; by passing induction sparks between carbon points in an atmosphere of nitrogen (see h. von wartenburg, _abs. j.c.s._, 1907, i. p. 299), or by the addition of a concentrated solution of potassium cyanide to one of copper sulphate, the mixed solutions being then heated. it also occurs in blast-furnace gases. when cyanogen is prepared by heating mercuric cyanide, a residue known as para-cyanogen, (cn)_x, is left; this is to be regarded as a polymer of cyanogen. it is a brownish amorphous solid, which is insoluble in water. cyanogen is a colourless gas, possessing a peculiar characteristic smell, and is very poisonous. it burns with a purple flame, forming carbon dioxide and nitrogen; and may be condensed (by cooling to -25 deg. c.) to a colourless liquid, and further to a solid, which melts at -34.4 deg. c. (m. faraday, _ann._, 1845, 56, p. 158). it dissolves readily in water and the aqueous solution decomposes on standing; a dark-brown flocculent precipitate of azulmic acid, c4h5n5o, separating whilst ammonium oxalate, urea and hydrocyanic acid are found in the solution. in many respects it resembles chlorine in its chemical behaviour, a circumstance noted by gay-lussac; it combines directly with hydrogen (at 500 deg. to 550 deg. c.) to form hydrocyanic acid, and with chlorine, bromine, iodine and sulphur, to form cyanogen chloride, &c.; it also combines directly with zinc, cadmium and iron to form cyanides of these metals. it combines with sulphuretted hydrogen, in the presence of water, to form the compound c2n2.h2s, and in the presence of alcohol, to form the compound c2n2.2h2s. concentrated hydrochloric acid converts it into oxamide. potash solution converts it into a mixture of potassium cyanide and cyanate. when heated with hydriodic acid (specific gravity 1.96) it forms amino-acetic acid, and with tin and hydrochloric acid it yields ethylene diamine. cyaxares (pers. _uvakhshatra_), king of media, reigned according to herodotus (i. 107) forty years, about 624-584 b.c. that he was the real founder of the median empire is proved by the fact that in darius's time a median usurper, fravartish, pretended to be his offspring (behistun inscr. 2. 43); but about his history we know very little. herodotus narrates (i. 103 ff.) that he renewed the war against the assyrians, in which his father phraortes had perished, but was, while he besieged nineveh, attacked by a great scythian army under madyas, son of protothyes, which had come from the northern shores of the black sea in pursuit of the cimmerians. after their victory over cyaxares, the scythians conquered and wasted the whole of western asia, and ruled twenty-eight years, till at last they were made drunk and slain by cyaxares at a banquet (cf. another story about cyaxares and a scythian host in herod, i. 73). as we possess scarcely any contemporary documents it is impossible to find out the real facts. but we know from the prophecies of jeremiah and zephaniah that syria and palestine were really invaded by northern barbarians in 626 b.c., and it is probable that this invasion was the principal cause of the downfall of the assyrian empire (see media and persia: _ancient history_). after the destruction of the scythians cyaxares regained the supremacy, renewed his attack on assyria, and in 606 b.c. destroyed nineveh and the other capitals of the empire (herod. i. 106; berossus _ap._ euseb. _chron._ i. 29, 37, confirmed by a stele of nabonidus found in babylon: scheil in _recueil de travaux_, xviii.; messerschmidt, "die inschrift der stele nabonaids," in _mitteilungen der vorderasiatischen gesellschaft_, i., 1896). according to berossus he was allied with nabopolassar of babylon, whose son nebuchadrezzar married amyitis, the daughter of the median king (who is wrongly called astyages). the countries north and east of the tigris and the northern part of mesopotamia with the city of harran (carrhae) became subject to the medes. armenia and cappadocia were likewise subdued; the attempt to advance farther into asia minor led to a war with alyattes of lydia. the decisive battle, in the sixth year, was interrupted by the famous solar eclipse on the 28th of may 585 predicted by thales. syennesis of cilicia and nebuchadrezzar (in herodotus named labynetus) of babylon interceded and effected a peace, by which the halys was fixed as frontier between the two empires, and alyattes's daughter married to cyaxares's son astyages (herod. i. 74). if herodotus's dates are correct, cyaxares died shortly afterwards. in a fragmentary letter from an assyrian governor to king sargon (about 715 b.c.) about rebellions of median chieftains, a dynast uvakshatar (i.e. cyaxares) is mentioned as attacking an assyrian fortress (kharkhar, in the chains of the zagros). possibly he was an ancestor of the median king. (ed. m.)