Silicon Dioxide or Silica, is one of the most common chemical compounds. SiO2 crystals are found in nature in three polymorphic forms, the most common of which is quartz. Sand, agate, onyx, opal, amethyst and flint are silicon dioxide with traces of impurities.silica or silicon dioxide, chemical compound, SiO2. It is insoluble in water, slightly soluble in alkalies, and soluble in dilute hydrofluoric acid. Pure silica is colorless to white. It occurs in several forms and is widely and abundantly distributed throughout the earth, both in the pure state and in silicates, e.g., in quartz (agate, amethyst, chalcedony, flint, jasper, onyx, and rock crystal), opal, sand, sandstone, clay, granite, and many other rocks; in skeletal parts of various protists and animals, such as certain sarcodines (see Sarcodina), diatoms, and sponges, and in the stems and other tissue of higher plants (this is the type of silicon that is used as a dessicant). Silica has many important uses. It is used as a filler for paint and rubber; in making ordinary glass; in ceramics; in construction; and in the preparation of other substances, e.g., silicon carbide. Fused quartz is pure amorphous silica; it is used in special chemical and optical apparatus. Because it has a low thermal coefficient of expansion, it withstands sudden changes in temperature and can be used in parts that are subjected to wide ranges of heat and cold. Unlike ordinary glass, it does not absorb infrared and ultraviolet light.
