Silica << SIHL uh kuh >> is silicon dioxide, a compound of silicon and oxygen. Its chemical formula is SiO2. Silica is by far the most abundant substance in Earth’s crust and mantle, forming about three-fourths of the planet’s rocky outer layer. Silica forms the basic structure of an important group of rock-forming minerals called silicates. Silicates include feldspars, micas, pyroxenes, amphiboles, and olivine. Most glass, many ceramics, and some beach sand consist of nearly pure silica. Silica appears in four forms: (1) crystalline, (2) cryptocrystalline, (3) amorphous, and (4) hydrated. See Earth (The lithosphere).
Crystalline silica consists of large, visible crystals. It most commonly occurs as quartz. Other crystalline varieties include coesite, cristobalite, stishovite, and tridymite. Crystals too small to be seen without a microscope make up cryptocrystalline silica. It commonly appears in the quartz varieties chalcedony and chert.
Amorphous silica does not consist of crystals. It commonly forms the skeletons of microscopic marine organisms called radiolarians and diatoms and some species of sponges. The silica skeletons of radiolarians can accumulate to form the rock radiolarian chert, and diatom skeletons can form the rock diatomite, which is powdered to form diatomaceous earth.
Hydrous silica contains small amounts of water. The gem opal is made up of tiny spheres of hydrous silica. The spheres bend and reflect light, producing sparkling displays of color.
