Quartz

I

Introduction

Quartz, most common of all minerals, composed of silicon dioxide, or silica, SiO₂. Distributed all over the world as a constituent of rocks and in the form of pure deposits, it is an essential constituent of igneous rocks such as granite, rhyolite, and pegamite, which contain an excess of silica. In metamorphic rocks, it is a major constituent of the various forms of gneiss and schist; the metamorphic rock quartzite is composed almost entirely of quartz. Quartz forms veins and nodules in sedimentary rock, principally limestone. Sandstone, a sedimentary rock, is composed mainly of quartz.

Many widespread veins of quartz deposited in rock fissures form the matrix for many valuable minerals. Precious metals, such as gold, are found in sufficient quantity in quartz veins to warrant the mining of quartz to recover the precious mineral. Quartz is also the primary constituent of sand.

II

Properties

Quartz crystallizes in the rhombohedral system. The size of the crystals varies from specimens weighing a tonne to minute particles that sparkle in rock surfaces. Quartz is also common in massive forms, which contain particles ranging in size from coarse-grained to cryptocrystalline (grains invisible to the naked eye but observable under a microscope). The mineral has a hardness of 7 and relative density of 2.65. The lustre in some specimens is vitreous; in others it is greasy or splendent (shining glossily). Some specimens are transparent; others are translucent. In the pure form, the mineral is colourless, but it is commonly coloured by impurities.

Quartz crystals exhibit a property called the piezoelectric effect, that is, they produce an electric voltage when subjected to pressure along certain directions of the crystal. Because of this property, quartz crystal has important applications in the electronics industry for controlling the frequency of radio waves. It also has the optical property of rotating the plane of polarized light and is used in polarizing microscopes.

Quartz crystals undergo structural transformations when heated. Ordinary, or low, quartz, when heated to 573° C (1063.4° F), is converted into high quartz, which has a different crystal structure and different physical properties. When cooled, however, high quartz reverts to low quartz. Between 870° and 1470° C (1598° and 2678° F), quartz exists in the form called tridymite, and above 1470° C (2678° F), the stable form is known as cristobalite. At about 1710° C (3078° F), the mineral melts.

III

Varieties

The coarsely crystalline varieties of quartz are, in general, transparent and lustrous. Rock crystal, a colourless form of quartz, usually occurs in distinct crystals. Rose quartz is coarsely crystalline but without distinct crystal form and is coloured rose red or pink, the colour often fading on exposure to light. Smoky quartz, or cairngorm stone, occurs in crystals ranging from smoky yellow to dark brown. Amethyst, a semi-precious variety of quartz, is coloured purple or violet.

Many other minerals form inclusions in crystalline varieties of quartz. Rutilated quartz contains fine needles of rutile that penetrate crystals of colourless quartz. Aventurine is a variety of quartz containing brilliant scales of haematite or mica. Liquids and gases also occur as inclusions in quartz. Milky quartz owes its milky-white colour to the presence of numerous minute liquid or gaseous inclusions.

The cryptocrystalline varieties of quartz are often divided into two general classes, fibrous and granular. The fibrous varieties, which include agate, cornelian, heliotrope, onyx, and chrysoprase, are all forms of chalcedony. The granular varieties include chert, flint, jasper, and prase.

IV

Uses

The different forms of chalcedony and many of the crystalline varieties of quartz are used as gemstones and other ornamental materials. Pure rock crystal is used in optical and electronic equipment. In the form of sand, quartz is used extensively in the manufacture of glass and silica brick, and is also used in cement and mortar. Ground quartz is used as an abrasive in stonecutting, sandblasting, and glass grinding. Powdered quartz is used in making porcelain, scouring soaps, sandpaper, and wood fillers. Large amounts of quartz are used as a flux in smelting operations. Almost all natural high-grade quartz crystal, which is an important raw material in the electronics industry, is imported from Brazil—the only country with large deposits of the mineral in commercial quantities. Quartz crystals can also be made synthetically.