Quartz

QUARTZ, a widely distributed mineral species, consisting of silicon dioxide, or silica, Si02. 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 (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 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, etc. ; or used for engraving, for instance, cameos and the elaborately carved crystal vases of ancient and mediaeval times. Clear transparent rock-crystal is used for optical purposes ; fused quartz is also used for the construction of lenses and laboratory vessels, and is drawn out into the finest elastic fibres and used for suspending mirrors, etc., in physical apparatus; for striking fire, flint is used even to the present day; and 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 mate rial, and is used for making sandpaper and scouring-soap, also in the manufacture of glass and porcelain, "silver sand" being a pure quartz sand.

Crystallography.

Quartz crystallizes in the trapezohedral class of the rhombohedral division of the hexagonal system. Crys tals 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 degree of symmetry is not indicated by the faces developed on the crystals. The majority of crystals of quartz are bounded only by the faces of a hexagonal prism m { 2111 and a hexagonal bipyramid (fig. I), though sometimes the prism is absent (fig. 2). Frequently the faces are of different sizes (fig. 3) : misshapen 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 inter section 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 ioo 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 predominate in size (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 situated on alternate corners at each end of the crystal, are called the "rhumb" faces, because of their shape ; if extended they would give a trigonal bipyramid. The "trapezohedral," or "plagihedral," faces x{415} belong to a trigonal trapezohedron. The two crystals shown in figs. 4 and 5 are 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 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 in are also small and of exceptional occurrence. 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.