a mineral. The term “quartz” is applied to two crystalline modifications of silicon dioxide, SiO2: hexagonal quartz (or α-quartz), which is stable at a pressure of 1 atm (or 100 kilonewtons per square meter) at temperatures ranging from 573° to 870°C, and trigonal (β-quartz), which is stable at temperatures below 573°C. The β-quartz is most abundant in nature, and it crystallizes in the trigonal-trapezohedral class of the trigonal system.
The crystalline structure of the skeletal forms of quartz consists of silica tetrahedrons placed helically (with a right or left twist) in relation to the main axis of the crystal. Depending on this placement, a distinction is made between right-handed and left-handed structural morphological crystal forms. These forms are externally differentiated by the symmetry of the placement of certain faces (for example, the trapezohedron). The absence of planes and a center of symmetry in quartz crystals results in the piezoelectric and pyroelectric properties of quartz. Most often, quartz crystals have an elongated prismatic appearance with the predominant development of the faces of the hexagonal prism and two rhombohedrons (the crystal head). More rarely, the crystals assume the appearance of a pseudohexagonal dipyramid. Externally, regular quartz crystals are usually complexly twinned, most frequently forming twinned sections according to the Brazil law or Dauphiné law. Dauphiné twins arise not only with the growth of the crystals but also as a consequence of internal structural rearrangements. These rearrangements, in turn, are the result of mechanical deformations and thermal α— β inversions accompanied by compression.
The color of quartz crystals, grains, and aggregates varies widely. Colorless, milky-white, or gray varieties of quartz are most common. Transparent or semitransparent attractively colored crystals have been given individual names. For example, the colorless or transparent ones are called rock crystals, purple crystals are known as amethyst, smoky colored crystals are known as smoky quartz, black crystals are called morion, and golden yellow crystals are called citrine. Variety in color usually results from structural defects with replacement of Si4+ by Fe3+ or Al3+ by the simultaneous inclusion of Na1+, Li1+, or (OH)1- in the matrix. Multicolored quartz is also encountered, caused by microinclusions of foreign minerals. For example, green prase has inclusions of actinolite or chlorite microcrystals, and golden sparkling aventurine has inclusions of mica or hematite. The cryptocrystalline varieties of quartz—agate and chalcedony—consist of minute fibrous formations.
Quartz is optically uniaxial and positive. The indexes of refraction (for daylight, λ = 589.3) are ne = 1.553 and n0= 1.544. Quartz is transparent for ultraviolet and partially infrared rays. In passing a plane-polarized light beam in the direction of the optical axis, left-handed quartz crystals rotate the polarization plane to the left, and right-handed crystals to the right. In the visible part of the spectrum, the angle of rotation (for the thickness of a 1-mm quartz plate) varies from 32.7° (for λ 486 nm) to 13.9° (λ 728 nm). The dielectric permittivity (єij), the piezoelectric modulus (djj), and the coefficients of elasticity (Sij) are, respectively (at room temperature), є11 = 4.58 and є33 - 4.70,d11 = -6.76 X 10-8 and d14 =2.56 X 10-8, and S11 = 1.279, S12 = -0.159,S13 = -0.110, S14 = -0.446, S33= 0.956,and S44 = 1.978. The coefficients of linear expansion are 13.4 X 10-6 perpendicular to the threefold axis and 8 X 10-6 parallel to it. The heat of inversion for β—α quartz equals 2.5 kilocalories per mole (10.45 kilojoules per mole). Quartz has a hardness of 7 on the mineralogical scale and a density of 2,650 kg/m3. It melts at a temperature of 1710°C and upon cooling forms quartz glass. Fused quartz is a good insulator; the resistance of a cube with a 1-cm edge at 18°C equals 5 X 1018 Ω/cm, and the coefficient of linear expansion is 0.57 X 10-6 cm/°C.
An economically feasible method has been developed for growing monocrystals of synthetic quartz. The crystal is obtained from aqueous solutions of SiO2 at high pressures and temperatures (hydrothermal synthesis). The synthetic quartz crystals possess such valuable technical properties as stable piezoelectric properties, radiation stability, and high optical uniformity.
Natural quartz is a common mineral and an essential constituent of many rocks and of mineral deposits of the most diverse origin. Industrially, the most important quartz materials are quartz sand, quartzite, and monocrystalline quartz. The last is rarely encountered and is highly valued. In the USSR the most important quartz crystal deposits are located in the Urals, the Ukrainian SSR (Volyn’), the Pamir, and the Aldan River basin. Abroad, quartz deposits are found in Brazil and the Malagasy Republic. Quartz sand is an important raw material of the ceramic and glass industries. Quartz monocrystals are used in radio engineering (piezoelectric frequency stabilizers, filters, resonators, piezoplates in ultrasound units). Quartz is also used in optical instruments (prisms for spectrographs and monochromators, lenses for ultraviolet optics). Fused quartz is used in the manufacture of special chemical vessels. Quartz is also used for obtaining pure silicon. Transparent colored varieties of quartz are considered as semiprecious stones and are widely used in jewelry-making.
V. P. BUTUZOV