Description
Silicon dioxide, or silica, is the best-known oxide of silicon and has the molecular formula SiO2. The pure substance occurs as a white crystalline powder, which is virtually insoluble in water. When combined with water, silicon dioxide forms silicic acid (although this process is extremely slow). Silicon dioxide occurs naturally in various forms, both crystalline and non-crystalline (amorphous). Silicon dioxide is the most common silicon compound and also the main constituent of the Earth's crust, either in its free form as quartz or as part of silicates such as feldspar and clay minerals.
