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Encyclopedia Britannica - Main :: PYR-RAY |
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QUARTZ , a widely distributed mineral
Quartz is a mineral
drawn
paper and scouring-soap. It is also largely used in the manufacture of glass and porcelain, " silver sand " being a pure quartz sand.Quartz crystallizes in the trapezohedral
are 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 appearance. These striations (fig. 3) are horizontal
trapezohedral
of intersection with r; this serves to distinguish r and z, and thus, in the absence of x faces, to distinguish left- or right-handed crystals. Numerous other faces have been observed on crystals of quartz, but they are of rare occurrence. The basal plane, so common on calcite and many other rhombohedral minerals, is of the greatest rarity in quartz, and when present only appears as a small rough 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 character and can only be deciphered when the faces s and x are present, which is not often the 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 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 united in juxtaposition with their principal axis nearly at right angles (84 33'). A few magnificent specimens of rock-crystal twinned according to this law have been found at La Gardette in Isere, and in Japan they are somewhat abundant. The pyro-electric characters of quartz are closely connected with its 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 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 action of hydrofluoric 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 interest
Quartz has a hardness of 7 (being chosen as No. 7 on Mohs' scale), and it cannot be scratched with a 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 amethyst (q.v.), and is due to repeated inter-growths of right- and left-handed material. The mineral is a non-conductor of electricity ; it is unattacked by acids with the exception of hydrofluoric acid, and is only slightly dissolved by solutions of caustic alkalis. It is infusible before the gas blowpipe, but in the oxyhydrogen 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 " 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. " Sceptre quartz," in which a short thick crystal is mounted on the end of a long slender prism, indicates a change in the conditions of growth. Crystals with a helical twist are not uncommon. Enclosures of other minerals (rutile, chlorite, haematite, gothite, actinolite, 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 minute size and present in vast numbers. Usually these cavities contain a liquid (water, a saline solution, carbon dioxide or 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 matrix, but sometimes, especially when embedded in a soft matrix of clay, gypsum or 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 diameter and weighing half a ton. enantiomorphous, i.e. they are non-superposable, one being the 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 general appearance. Thus in the microcrystalline chalcedony (q.v.) the lustre is waxy, the fracture fibrous to even, and the external form botryoidal or stalactitic: flint and chert are compact and have a splintery fracture: jasper (q.r.) is a compact variety intermixed with much iron oxide. and clay and has a dull and even fracture. Further, these varieties may be of almost any colour, whereas transparent crystals have only a limited range of colour, being either colourless (rock-crystal), violet (amethyst), brown (smoky quartz) or yellow (citrine). Quartz occurs as a primary and essential constituent of igneous rocks of acidic composition such as granite, quartz- porphyry and rhyolite, being embedded in these either as irregularly shaped masses or as porphyritic crystals. In pegmatite (graphic granite) and granophyre it often forms a regular intergrowth with felspar. It is also a common constituent, as irregular grains, in many gneisses and crystalline schists, a quartz-schist being composed largely of quartz. By the weathering of silicates, silica passes into solution and quartz is deposited as a secondary product in the cavities of basic igneous rocks, and in fact in the crevices and along the joints of rocks of almost all kinds. Extensive veins of quartz are especially frequent in schistose rocks. Vein-quartz, often of economic importance as a matrix of gold, may, however, in some cases have been of igneous origin. In mineral veins and lodes crystallized quartz is usually the most abundant gangue
For particulars respecting the special
AVENTrRINE, BLOODSTONE
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