Silica (SiO 2 ) is a commonly occurring compound in nature. Various crystalline and noncrystalline silica minerals of inorganic and biogenic origin are known. Depending on temperature, pressure, composition of the precursor phases, etc., several different crystalline modifications are formed. Quartz is the most abundant and well‐known polymorph. Silica occurs as meter‐sized quartz crystals or as amorphous masses made up of submicroscopic crystallites having different microstructures or growth fabrics. In addition, synthetic polymorphs have been produced for which no natural counterparts exist. In industry, quartz sand and silica rocks are widely used as raw materials for production of glass, ceramics, and silicon. Single crystals of quartz are grown artificially for application as resonator crystals in clocks, communication technology, and computer electronics. Both natural and synthetic silica powders are used as fillers to improve the mechanical properties of plastics. The article contains sections titled: 1. Silica Modifications and Products 1.1. Occurrence and Geochemistry 1.2. Si‐O Bond and Crystal Structures 1.3. Crystalline Silica Phases 1.3.1. Crystalline Silica Minerals 1.3.2. Crystalline Nonmineral Silica Phases 1.4. Noncrystalline Silica Minerals 1.5. Colored Silica Minerals 1.6. Silica Rocks 1.7. Crystalline Silica Products 1.7.1. Cultured Quartz Single Crystals 1.7.2. Polycrystalline Silica Products 1.8. Noncrystalline Silica Products 2. Quartz Raw Materials 2.1. Physical Forms and Occurrence 2.2. Processing 2.3. Uses 3. Diatomites 3.1. Introduction 3.2. Formation, Composition, and Quality Criteria 3.3. Occurrence and Mining 3.4. Processing 3.5. Analysis 3.6. Storage and Transport 3.7. Environmental and Health Protection 3.8. Uses 3.9. Recycling 4. Colloidal Silica 4.1. Introduction 4.2. Structure of Colloidal Silica Particles 4.3. Physical and Chemical Properties 4.4. Stability 4.5. Production 4.6. Analysis and Characterization 4.7. Uses 4.8. Storage, Handling, and Transportation 4.9. Economic Aspects 5. Silica Gel 5.1. Introduction 5.2. Structure, Properties, and Characterization 5.3. Production 5.4. Uses 5.5. Economic Aspects 5.6. Legal Aspects 6. Pyrogenic Silica 6.1. Flame Hydrolysis 6.1.1. Production Process 6.1.2. Morphology 6.1.3. Solid‐State Properties 6.1.4. Surface Chemistry 6.1.5. Surface Modification with Silicon Compounds 6.1.6. Characterization 6.1.7. Uses 6.1.8. Industrial Hygiene and Safety 6.2. Electric‐Arc Process 6.3. Plasma Process 7. Precipitated Silicas 7.1. Introduction 7.2. Production 7.3. Properties 7.3.1. Physicochemical Properties 7.3.2. Surface Chemistry and Surface Modification 7.3.3. Chemical Composition and Analysis 7.4. Uses 7.5. Industrial Hygiene and Safety 8. Porosils 8.1. Introduction 8.2. Physical and Chemical Properties 8.2.1. Zeosils 8.2.2. Clathrasils 8.3. Manufacture of Porosils 8.3.1. Synthesis of Porosils 8.3.2. Dealumination of Aluminosilicate Zeolites 8.3.3. Formation of Melanophlogite 9. Toxicology 9.1. Experiences with Humans 9.2. Animal Experiments

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