American Heritage
Medical Dictionary

silicon dioxide n.
Silica.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
silicon dioxide  
See silica.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Britannica
Encyclopedia

silicon dioxide

compound of the two most abundant elements in the Earth's crust, silicon and oxygen, SiO2. The mass of the Earth's crust is 59 percent silica, the main constituent of more than 95 percent of the known rocks. Silica has three main crystalline varieties: quartz (by far the most abundant), tridymite, and cristobalite. Other varieties include coesite, keatite, and lechatelierite. Silica sand is used in buildings and roads in the form of portland cement, concrete, and mortar, as well as sandstone. Silica also is used in grinding and polishing glass and stone; in foundry molds; in the manufacture of glass, ceramics, silicon carbide, ferrosilicon, and silicones; as a refractory material; and as gemstones.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
Until now, that has been done using silicon dioxide-in other words, quartz.
It's called aerogel, and it's an extremely lightweight form of silicon dioxide.
The oxygen stolen from the metals turns the silicon into silicon dioxide, or
  quartz.
For decades, the gate dielectric has been made of an insulating material called
  silicon dioxide.
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