Silicon Dioxide, Silica (SiO2)

Formula: SiO2

Melting point: 1713°C

Density: 2.648 g/cc (pure)

Solubility: Only in HF and hot bases. Slightly soluble in water above 350°C

Silicon dioxide is the most abundant mineral in the Earth's crust and possibly in the outer layers of the mantle. As one descends into the planet's interior, silica is gradually replaced by denser oxides, such as iron, until reaching regions of metallic iron.

Its abundance is due to its enormous stability against erosion, deformation, and natural fracture processes. It is the main constituent of sand, the basis of numerous rocks and minerals, and an essential component in complex silicates such as feldspars, which are the most abundant minerals in the crust.

The pure, transparent form of SiO₂ has been known since ancient times as Rock Crystal or Quartz. The Greeks considered it an "eternal ice," impossible to melt, and attributed magical properties to it, such as cooling the skin or soothing burns.

The term "silica" is used in chemistry in the same way as "alumina" for Al₂O₃, referring to the pure oxide, while "quartz" designates the natural crystalline variety.

To date, at least 18 crystalline modifications of quartz have been described, including varieties such as amethyst, agate, chalcedony, smoky quartz, tiger's eye, and jasper. These varieties do not always depend on the chemical composition but rather on the structure and, often, color due to impurities.

The word "crystal" comes from the Greek krýstallos, as quartz was considered the quintessential natural crystal. Before the development of artificial glass, large natural quartz crystals were carved to make cups, engravings, seals, and decorative objects.

The purest quartz, called rock crystal or silica, is hard and brittle, with a hardness of 7 on the Mohs scale. It has excellent compressive strength, but it breaks easily under impact or tensile stress, as it is neither ductile nor malleable. Chemically, it is extremely stable: only hydrofluoric acid and hot concentrated bases attack it, while it is resistant to both oxidizing and reducing agents. This is due to the strength of the silicon-oxygen bonds, which are very difficult to break, although less compact than the aluminum-oxygen bonds in alumina, which explains its lower density and melting point. It is not considered a technical ceramic but rather the base crystal of numerous vitreous materials. Furthermore, quartz has piezoelectric properties: when subjected to mechanical pressure, it generates electricity, making it useful in watches and electronic devices.

Although it is rarely found in its pure form, silicon dioxide is the basis of most glass. Common glass, known as soda-lime glass, contains more than 65% silica, but with additives that improve its moldability and strength. Specialty glasses such as borosilicate, used in laboratories or in thermal shock-resistant utensils, are also made from high-purity silica. In ancient times, quartz crystals were carved to make cups, seals, figurines, and ornaments, sometimes combined with precious metals. Today, although its value has declined thanks to the ease of producing synthetic glass, it is still prized in jewelry and is frequently seen in markets where magical or healing properties are attributed to it without scientific basis. Quartz is also used in electronic and optical devices thanks to its chemical stability and piezoelectric properties, and as a raw material for obtaining high-purity molten glass used in lenses and optical fibers.

Alumina (Al₂O₃) is denser and harder than quartz, with a higher melting point due to its more compact bonds. While alumina is classified as a technical ceramic, quartz is considered the quintessential natural crystal and serves as the basis for the manufacture of numerous glasses and silicates.