The oxide of the element silicon, with the chemical formula SiO2. Pure (or nearly pure silica) can occur in several mineral forms: quartz, tridymite, and cristobalite, depending on temperature and pressure. Silica combined with other metal oxide comprise a large part of many rock-forming minerals, and hence silica is a major gangue component or many ores. Hence, silica is a major component of many smelting slags, although occasionally some copper smelting slags can have very low silica content as there is a low melting point region in calcium-ferrite system.