SILICON While the element silicon does not occur free in nature, its compounds make up approximately 88% of the earth's crust.
Silicon is found in the form of silicon dioxide, either as quartz or as silicates, generally in combination with alumina (clay, shale, feldspar, etc.). Industrially useful and commercially important, silicious resources are consequently generally available in all countries. Of primary importance are hydrous aluminium sili cates, broadly termed clays, varying in purity of chemical com position and in physical properties from kaolin, which approaches the pure aluminium silicate, kaolinite, to the shales which contain enough kaolinite to render them workable in ceramic processes. The uses which clays find in industry may be summarized as fol lows: kaolins (white burning clays of high purity) for making paper, porcelain and chinaware, electrical porcelain, wall tile and rubber; fire clays (high fusing clays generally with some free silica and 2 to 4% of iron, lime, and alkalies) for fire clay brick and other metallurgical refractories, glass-house refractories, boiler furnaces, incinerators, and in general for use to withstand high temperatures, sewer pipes, chimney tiles, clay and graphite cruci bles and in the manufacture of aluminium sulphate; common clays and shales (miscellaneous rocks which become plastic on grinding in water) for common brick, sewer pipes, building tiles, portland cement, electrical conduit tiles, chemical stoneware.
Silica (silicon dioxide) occurs chiefly as quartz, in the form of sand, sandstone, quartzite rock, vein quartz and rock crystal and also as infusorial earth. Quartz sand and crushed quartzite rocks
find uses in abrasives, glass, glazes and enamels, silica refracto ries, electric furnace products (silicon carbide and many metal silicon alloys), porcelains, sand lime brick and other ceramic products. Vein quartz or rock crystal is necessary for the produc tion of transparent fused silica, but the translucent material may be made from quartz sand. Large crystals find use in radio trans mission. Gannister, a form of quartzite, is employed in man ufacturing silica refractories. Infusorial earth, or diatomaceous earth, is employed for thermal insulation and for filtering and clarifying in chemical manufacture, and "fuller's earth," which is properly a "clay," is similarly used for bleaching and clarifying oils and fats. Among the useful silicate minerals is feldspar, of pri mary importance in porcelain, whiteware and other ceramic prod ucts, glazes, enamels and glass. The minerals sillimanite, andalu site and kyanite are important raw materials for electrical insula tors and also for greatly improved metallurgical refractories. Asbestos, a hydrous magnesium silicate (chrysotile) is woven into fire-resisting fabrics and for brake linings, and the shorter fibres are used in shingles and roofing sheets, coverings for the insulation of steam pipes and for parts of electrical equipment. Talc and mica are also important. (E. W. T.)