Silicon dioxide (SiO2) is fairly common inorganic compound. In the nature it occurs in several modifications:
- Quartz – is a trigonal and the most common modification of silica. It is the most abundant mineral found at Earth’s surface .
- Opal – (SiO2·nH2O) is a hydrated amorphous form of silica. Its water content may range from 3 to 21%.
- Silhydrite – (3SiO2·H2O) is a hydrated orthorombic form of silica.
- Tridymite – is an anhydrous form of silica. It crystallizes in triclinic crystal system.
- Cristobalite – tetragonal anhydrous silicon dioxide.
- Mogánite – is a monoclinic silicon dioxide. It often occurs as a minor component of chalcedony, chert or flint.
- Melanophlogite – is a rare form of silica that crystallizes in tetragonal crystal system. It has an interesting zeolite-like structure. Its formula is (46SiO2·6(N2,CO2)·2(CH4,N2) ). The Molecules of the guest compounds are trapped inside a skelet.
- Seifertite – is the densest and the hardest polymorph of silica found in nature so far. It crystallizes in orthorombic crystal system. It was found for the first time in Shergotty meteorite and since then in some other Martian meteorites .
- Keatite – is a tetragonal polymorph. It was found quite recently (2013) in ultrahigh-pressure rocks .
- Stishovite – is a high temperature tetragonal polymorph of silicon dioxide. It is a very rare mineral forming through the metamorphism of Quartz from meteorite impact at extremely high temperatures. Its structure is similar to that of rutile.
Pseudohexagonal crystals of silicon dioxide polymorph tridymite [photo: Michal Hegedus]
Orange quartz, South Africa, Astro gallery, NYC [photo: Michal Hegedus]
Smoky quartz, unknown locality [photo: Michal Hegedus]
Solubilty of silicon dioxide in water strongly depends on its crystalline type. Amorphous silica is about 3.5 times more soluble than its crystalline polymorphs. The peak of water solubility of silicon dioxide is at ~340°C. High quality silica monocrystals are made in a hydrothermal process by dissolving and crystallizing natural quartz in superheated water in a high pressure vessel .
Silicon dioxide is a Lux-Flood acid because it acts as a oxide ion (O2-) acceptor, it react with bases under some conditions:
SiO2 + 2 NaOH → Na2SiO3 + H2O
SiO2 + CaO → CaSiO3
Silicon dioxide is inert with almost all acids, but it reacts with hydrofluoric acid to form hexafluorosilicic acid:
SiO2 + 6 HF → H2SiF6 + 2 H2O
Coloidal form of silica (also called fumed silica) can be obtained by burning SiCl4 in an oxygen rich hydrocarbon flame:
SiCl4 + 2 H2 + O2 → SiO2 + 4 HCl
A simplier procedure, still yielding very fine silica is a reaction between sodium silicate and hydrochloric acid, followed by termal dehydration of formed silicic acid:
Na2SiO3 + 2HCl –> 2NaCl + H2SiO3
H2SiO3 –> SiO2 + H2O
25 mL of water glass was mixed with 25 mL of demineralized water. Hydrochloric acid (6 M, 30 mL) was slowly added with stirring. Silicic acid has precipitated. Water was evaporated and then the mixture was heated to 130 °C for 1h. Disstilled water was added once again in order to dissolve a side product – NaCl. Then it was decanted several times and filtered.
 Hill, T. R., Konishi, H. and Xu, H. (2013): Natural occurrence of keatite precipitates in UHP clinopyroxene from the Kokchetav Massif: A TEM investigation. American Mineralogist, 98, 187-196.
 Holleman, A. F.; Wiberg, E. (2001), Inorganic Chemistry, San Diego: Academic Press, ISBN 0-12-352651-5