QUARTZ, a widely distributed mineral species, consisting of silicon dioxide, or silica (SiO2). It is the commonest of minerals, and is met with in a great variety of forms and with very diverse modes of occurrence. The various forms of silica have attracted attention from the earliest times, and the water-clear crystallized variety was known to the Greeks as κρύσταλλος (clear ice), being supposed by them to have been formed from water by the intense cold of the Alps; hence the name "crystal," or more commonly rock-crystal, applied to this variety. The name quartz is an old German word of uncertain origin: it was used by G. Agricola in 1529.

Quartz is a mineral which is put to many uses. Several of the varieties are cut into gems and ornaments, balance weights, pivot supports for delicate instruments, agate mortars, &c.; or used for engraving, for instance, cameos and the elaborately carved crystal vases of ancient and medieval times. Clear transparent rock-crystal is used for optical purposes and spectacle lenses. Fused quartz has recently been used for the construction of lenses and laboratory vessels, or it may be drawn out into the finest elastic fibres and used for suspending mirrors, &c., in physical apparatus. For striking fire, flint is used even to the present day. Buhrstone, a cellular variety of chalcedonic quartz from the Tertiary strata of the Paris basin, is largely used for millstones. Quartz is a valuable grinding and polishing material, and is used for making sandpaper 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-hemihedral class of the rhombohedral division of the hexagonal system. Crystals of this class possess neither planes nor centre of symmetry, but only axes of symmetry: perpendicular to the principal triad axis there are three uniterminal dyad axes of symmetry. Usually, however, this lower form of symmetry is not indicated by the faces developed on the crystals. The majority of crystals of quartz are bounded not only by the faces of a hexagonal prism m{211} and a hexagonal bipyramid, (fig. 1), though sometimes the prism is absent (fig. 2). Frequently the faces are of different sizes (fig. 3): mis-shapen crystals 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 in direction, being parallel to the edges of intersection between the prism and pyramid faces, and are due to the frequent oscillatory combination of these faces. The apparent hexagonal bipyramid is really a combination of two rhombohedra, the direct rhombohedron r{100} and the inverse rhombohedron z{221}. The faces of these two rhombohedra exhibit differences in surface characters, those of r being usually brighter in lustre than those of z; further, the former often predominates in size (figs. 4 and 5), and the latter may 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 s{412} situated on alternate corners at each end of the crystal, are called "rhomb" faces, because of their shape; if extended they would give a trigonal bipyramid. The "trapezohedral," or "plagihedral," faces x{412} belong to a trigonal trapezohedron. The two crystals shown in figs. 4 and 5 are enantionmorphous, 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 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 {521} (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 Isère, and in Japan they are somewhat abundant.

The pyro-electric characters of quartz are closely connected with its peculiar types of symmetry and especially with the three uniterminal dyad axes. A crystal become 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 figure, both natural and artificial (in the latter case produced by the action of hydrofluoric acid), on the faces of 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, since it is one of the two minerals (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 centres of symmetry. A ray of plane-polarized 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 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 blue (G) light 43°. Such a section when examined in the polariscope shows an interference figure with a coloured centre, there being no black cross inside the innermost 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 Airy's spirals in the polariscope. The indices of refraction for yellow (D) light are ω=1·5442 and ε=1·5533; the optic sign is therefore positive.

Quartz has a hardness of 7 (being chosen 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 intergrowths 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 oxyhydrgen flame fuses to a clear, colourless glass, which has a hardness of 5 and a specific gravity of 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, göthite, 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. 1). 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.

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 microcrystalline chalcedony (q.v.) the lustre is waxy, the fracture fibrous to even, and the external form botryoidal or stalactite: flint and chert are compact and have a splintery fracture: jasper (q.v.) 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 appears 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 feldspar. 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 case have been of igneous origin. In mineral veins and lodes crystallized quartz is usually the most abundant gangue material; the crystals are often arranged perpendicular to the walls of the lode, giving rise to a "comby" structure. In limestones of various kinds it occurs as nodules and bands of chert and flint, being in this case of organic origin. Quartz being a mineral very resistant to weathering agencies, it forms the bulk of sands and sandstones; and when the sand grains are cemented together by a later deposit of secondary quartz a rock known as quartzite results. Pseudomorphous quartz, i.e. quartz replacing other minerals, is of frequent occurrence, and as a petrifying material replacing organic remains it is often met with. As a deposit from hot springs, quartz is much less common than opal. Crystals of quartz may be readily prepared artificially by a number of methods; for example, by heating glass or gelatinous silica with water under pressure.

For particulars respecting the special characters, modes of occurrence and localities of the more important varieties of quartz, reference may be made to the following articles: AGATE, AMETHYST, AVENTURINE, BLOODSTONE, CAIRNGORM, CARNELIAN, CAT’S-EYE, CHALCEDONY, CHRYSOPRASE, FLINT, HELIOTROPE, JASPER, MOCHA-STONE, ONYX, ROCK-CRYSTAL, SARD, SARDONYX. For other forms of silica see OPAL and TRIDYMITE.  (L. J. S.)