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QUARTZ
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QUARTZ , a widely distributed See also:mineral See also:species, consisting of See also:silicon dioxide, or See also:silica (SiO2). It is the commonest of minerals, and is met with in a See also:great variety of forms and with very diverse modes of occurrence. The various forms of silica have attracted See also:attention from the earliest times, and the See also:water-clear crystallized variety was known to the Greeks as KpbaTa? oc (clear See also:ice), being supposed by them to have been formed from water by the intense See also:cold of the See also:Alps; hence the name " crystal," or more commonly See also:rock-crystal, applied to this variety. The name quartz is an old See also:German word of uncertain origin: it was used by G. See also: It is also largely used in the manufacture of See also:glass and See also:porcelain, " See also:silver sand " being a pure quartz sand. Quartz crystallizes in the See also:trapezohedral-hemihedral class of the See also:rhombohedral See also:division of the hexagonal See also:system. Crystals of this class possess neither planes nor centre of symmetry, but only axes of symmetry: perpendicular to the See also:principal triad See also:axis there are three uniterminal dyad axes of symmetry. Usually, however, this See also:lower degree of symmetry is not indicated by the faces See also:developed on the crystals. The See also:majority of crystals of quartz are bounded only by the faces of a hexagonal See also:prism m12i11 and a hexagonal bipyramid (fig. I), though sometimes the prism is absent (fig. 2). Frequently the faces are of different sizes (fig. 3) : mis-shapen crystals are See also:common and sometimes very puzzling, but they can always be orientated by the aid of the very characteristic striations on the prism faces, which serve also to distinguish quartz from other minerals of similar See also:appearance. These striations (fig. 3) are See also:horizontal in direction, being parallel to the edges of intersection between the prism and See also:pyramid faces, and are due to the frequent oscillatory See also:combination of these faces. The apparent hexagonal bipyramid is really a combination of two rhombohedra, the See also:direct rhombohedron r{See also:loo} and the inverse rhombohedron z1224 The faces of these two rhombohedra exhibit See also:differences in See also:surface characters, those of r being usually brighter in lustre than those of z; further, the former often predominate in See also:size (See also:figs. 4 and 5), and the latter may sometimes be completely absent. When both the prism and the rhombohedron z are absent, the crystals resemble cubes in appearance, since the angles between the faces of the rhombohedron are 85° 46'. The additional faces s and x (figs. 4 and 5), which indicate the true degree of symmetry of quartz,are of comparatively rare occurrence except on crystals from certain localities. The six small faces s141z1 situated on alternate corners at each end of the crystal, are called the " rhomb " faces, because of their shape; if extended they would give a trigonal bipyramid. The " trapezohedral," or " plagihedral," faces x14121 belong to a trigonal trapezohedron. The two crystals shown in figs. 4 and 5 are of intersection with r; this serves to distinguish r and z, and thus, in the See also:absence of x faces, to distinguish See also:left- or right-handed crystals. Numerous other faces have been observed on crystals of quartz, but they are of rare occurrence. The basal See also:plane, so common on See also:calcite and many other rhombohedral minerals, is of the greatest rarity in quartz, and when present only appears as a small rough See also:face formed by the corrosion of the crystal. Faces of prisms other than m are also small and of exceptional occurrence. Twinned crystals of quartz are extremely common, but are complex in See also:character and can only be deciphered when the faces s and x are present, which is not often the See also:case. Usually they are interpenetration twins with the principal axis as twin-axis; the prism planes of the two individuals coincide, and the faces r and z also fall into the same plane. Such twins may therefore be mistaken for See also:simple crystals unless they are attentively studied; but the twinning is often made evident by the presence of irregularly bounded areas of the duller z faces coinciding with the brighter r faces. In a rarer type of twinning, in which the twin-plane is 15211 (a plane truncating the edge between r and z), the two individuals are See also:united in juxtaposition with their principal axis nearly at right angles (84° 33'). A few magnificent specimens of rock-crystal twinned according to this See also:law have been found at La Gardette in See also:Isere, and in See also:Japan they are somewhat abundant. The pyro-electric characters of quartz are closely connected with its See also:peculiar type of symmetry and especially with the three uniterminal dyad axes. A crystal becomes positively and negatively electrified in alternate prism edges when its temperature changes. A similar See also:distribution of electric charges is produced when a crystal is subjected to pressure; quartz being thus also piezo-electric. Etched figures, both natural and artificial (in the latter case produced by the See also:action of hydrofluoric See also:acid), on the faces of the crystals are in accordance with the symmetry, and may serve to distinguish left- and right-handed crystals. In its optical characters, quartz is also of See also:interest, since it is one of the two minerals (See also:cinnabar being the other) which are circularly polarizing. This phenomenon is connected with the symmetry of the crystals, and is also shown by the crystals of certain other substances in which there are neither planes nor centre of symmetry. A See also:ray of plane-polarized See also:light traversing a right-handed crystal of quartz in the direction of the triad axis has its plane of polarization rotated to the right, while a left-handed crystal rotates it to the left. A See also:section 1 mm. thick, cut perpendicular to the principal axis of a quartz crystal, rotates the plane of yellow (D) light through 22°, and of See also:blue (G) light through 43°. Such a section when examined in the polariscope shows an interference figure with a coloured centre, there being no See also:black See also:cross inside the innermost See also:ring (this is not shown in very thin sections). Superimposed sections of right- and left-handed quartz, as may sometimes be present in sections of twinned crystals, exhibit See also:Airy's spirals in the polariscope. The indices of See also:refraction of quartz for yellow (D) light are w =1.5442 and =1.5533; the optic sign is therefore See also:positive. Quartz has a hardness of 7 (being chosen as No. 7 on See also:Mohs' See also:scale), and it cannot be scratched with a See also:knife; its specific gravity is 2.65. There is no distinct cleavage; though an imperfect cleavage may sometimes be developed parallel to the faces of the rhombohedron r by plunging a heated crystal into cold water. The glassy conchoidal fracture is a characteristic feature of the crystallized mineral. A peculiar rippled or " thumb-marked " fracture is sometimes to be seen, especially in See also:amethyst (q.v.), and is due to repeated inter-growths of right- and left-handed material. The mineral is a non-conductor of See also:electricity ; it is unattacked by acids with the exception of hydrofluoric acid, and is only slightly dissolved by solutions of See also:caustic alkalis. It is infusible before the See also:gas See also:blowpipe, but in the oxyhydrogen See also:flame fuses to a clear colourless glass, which has a hardness of 5 and specific gravity 2.2. Many peculiarities of the growth of crystals are well illustrated by the mineral quartz. Thus in " See also:ghost quartz," in which one crystal is seen inside another, the stages of growth are marked out by thin layers of enclosed material. In " capped quartz " these layers are thicker, and the successive shells of the crystal may be easily separated. " See also:Sceptre quartz," in which a See also:short thick crystal is mounted on the end of a See also:long slender prism, indicates a See also:change in the conditions of growth. Crystals with a helical twist are not uncommon. Enclosures of other minerals (See also:rutile, See also:chlorite, See also:haematite, See also:gothite, actinolite, See also:asbestos and many others) are extremely frequent in crystals of quartz. Cavities, either rounded or with the same shape (" negative crystals ") as the surrounding crystal, are also common; they are often of See also:minute size and present in vast See also:numbers. Usually these cavities contain a liquid (water, a saline See also:solution, See also:carbon dioxide or See also:petroleum) and a movable bubble of gas. The presence of these enclosed impurities impairs the transparency of crystals. Crystals of quartz are usually attached at one end to their rocky See also:matrix, but sometimes, especially when embedded in a soft matrix of See also:clay, See also:gypsum or See also:salt, they may be bounded on all sides by crystal faces (fig. I). In size they vary between wide limits, from minute sparkling points encrusting rock surfaces and often so thickly clustered together as to produce a drusy effect, to large single crystals measuring a yard in length and See also:diameter and weighing See also:half a ton. enantiomorphous, i.e. they are non-superposable, one being the See also:mirror reflection of the other: they are left-handed and right-handed crystals respectively. The faces s are striated parallel to their edge The characters as given above apply more particularly to crystals .of quartz, but in the various massive and compact varieties the material may be quite different in See also:general appearance. Thus in the microcrystalline See also:chalcedony (q.v.) the lustre is waxy, the fracture fibrous to even, and the See also:external See also:form botryoidal or stalactitic: flint and chert are compact and have a splintery fracture: See also:jasper (q.r.) is a compact variety intermixed with much See also:iron See also:oxide. and clay and has a dull and even fracture. Further, these varieties may be of almost any See also:colour, whereas transparent crystals have only a limited range of colour, being either colourless (rock-crystal), See also:violet (amethyst), See also: As a deposit from hot springs, quartz is much less common than See also:opal. Crystals of quartz may be readily prepared artificially by a number of methods; for example, by See also:heating glass or gelatinous silica with water under pressure.
For particulars respecting the See also:special characters, modes of occurrence and localities of the more important varieties of quartz, reference may be made to the following articles: AGATE, AMETHYST,
AVENTrRINE, See also:BLOODSTONE, See also:CAIRNGORM, See also:CARNELIAN, See also:CAT'S-See also:EYE, CHALCEDONY, CIIRYSOPRASE, FLINT,See also:HELIOTROPE, JASPER, MOCHA-See also: Additional information and CommentsThere are no comments yet for this article.
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