Silicon

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Silicon is a grey, metallically lustrous, metalloid element with atomic number 14 and atomic weight 28.086. Silicon is not naturally present in its reduced elemental form as it undergoes spontaneous oxidation in air. Therefore elemental Silicon is not found free in nature, but occurs as oxide and as silicates. Silicon crystallises in the cubic diamond structure. There is also an amorphous allotropic modification of Silicon described in the literature. However, due to lack of data, this allothrophic form of Silicon is not dealt with or covered by this dossier, Crystalline Silicon is the element (substance) being covered by this dossier. Silicon is commercially available in the form of lumps, ingots, wafers, briquettes, flakes, granules and powders of different size ranges. The purity of commercial silicon ranges from 95 to 100 wt%. The main impurities include iron, calcium and aluminium. A thin layer of amorphous silicon oxide is formed on the surface of silicon exposed to air, even at ambient temperatures. The outermost surface is composed of amorphous silicon dioxide, and the interface of silicon sub-oxides with varying oxidation stages. Similar surface characteristics were detected in the surface analysis of silicon particles and particles of amorphous silicon dioxide. The solubility of silicon in water is very low and strongly dependent on the pH of the solution. In addition to silicon, trace amounts of metal ions present as impurities may be dissolved from silicon. Dissolution of silicon in the form of monosilicic acid from silicon particles in synthetic biological fluids is slighly lower than from pyrogenic amorphous silica Aerosil Ox50. This can be at least partly explained by different particle characteristics (e.g. lower surface area).