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SILICA
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,
diatoms, and sponges, and in the stems and other tissue
of higher plants.
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.
The chemical compound silicon dioxide, also known as
silica, is the oxide of silicon, chemical formula SiO2.
It is found in nature in several forms, including quartz
and opal. In fact, silica has 17 crystalline forms.
Also, many forms of life include silica structures,
including microorganisms such as diatoms, plants such
as horsetail, and animals such as hexactinellid sponges.
It is manufactured in several forms including glass
(in colorless high purity form called fused silica),
synthetic amorphous silica and silica gel (used e.g.
as desiccants in brand new clothes and leather goods).
Silica is a major ingredient of Portland cement. The
ceramic re-entry heat protection tiles mounted on the
bottom side of the Space Shuttles are made mostly of
silica, as are the firebricks used in steel processing.
The most common constituent of sand in inland continental
settings and non-tropical coastal settings, is silica
(silicon dioxide), usually in the form of quartz because
of the considerable hardness of this mineral. However,
the composition of sand varies according to local rock
sources and conditions. Inhaling crystalline silica
dust can lead to silicosis.
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