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.