Abstract: 4500-SiO2 A. Introduction
1. Occurrence and Significance
Silicon does not occur free in nature, but rather as free silica (SiO2) in coarsely crystalline (quartz, rock crystal, amethyst, etc.) and microcrystalline (e.g., flint, chert, jasper) varieties of quartz, the major component of sand and sandstone. Silicon is found in combination with other elements in silicates, represented by feldspar, hornblende, mica, asbestos, and other clay minerals. Silicates also occur in rocks, such as granite, basalt, and shale. Silicon therefore is usually reported as silica (SiO2) when rocks, sediments, soils, and water are analyzed. The average abundance of silica in different rock types is 7% to 80%, in typical soils 50% to 80%, and in surface and groundwater 14 mg/L.