Five silica categories are covered under silica: industrial sand and gravel, high-purity quartz, industrial quartz crystal, special silica stone, and tripoli. Industrial sand and gravel consists of high-silica sands and gravels used in a wide range of industrial applications such as hydraulic fracturing and glassmaking.
High-purity quartz contains less than 100 parts per million of total impurities, equivalent to at least 99.99% silica. Ultra-high-purity material may contain 10 parts per million or less of impurities and is used in demanding applications, including fused quartz crucibles for semiconductor manufacturing. For some end uses, synthetic high-purity silica can be produced from silicon tetrachloride, a byproduct of polysilicon production.
Industrial cultured quartz crystal is used mainly for its piezoelectric and optical properties. It is produced from mined natural quartz feedstock, known as lascas, or from rejected cultured quartz. U.S. mining of natural quartz crystal ended in 1997 because of competition from lower-cost imports.
Silica stone products, commonly made from novaculite, quartzite, and other microcrystalline quartz, are used in abrasive tools and related applications. Tripoli is a very fine-grained crystalline silica containing about 98% to 99% silica and is used in various industrial products. Both are generally mined by open pit methods and processed for their intended uses.
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