Silica Tetrahedron Defined and Explained

Quartz
Colin Gregory/Flickr

The vast majority of minerals in the Earth's rocks, from the crust down to the iron core, are chemically classed as silicates. These silicate minerals are all based on a chemical unit called the silica tetrahedron.

You Say Silicon, I Say Silica

The two are similar, (but neither should be confused with silicone, which is a synthetic material). Silicon, whose atomic number is 14, was discovered by Swedish chemist Jöns Jacob Berzelius in 1824. It is the seventh most abundant element in the universe. Silica is an oxide of silicon—hence its other name, silicon dioxide—and is the primary component of sand.

Tetrahedron Structure

The chemical structure of silica forms a tetrahedron. It consists of a central silicon atom surrounded by four oxygen atoms, with which the central atom bonds. The geometric figure drawn around this arrangement has four sides, each side being an equilateral triangle—a tetrahedron. To envision this, imagine a three-dimensional ball-and-stick model in which three oxygen atoms are holding up their central silicon atom, much like the three legs of a stool, with the fourth oxygen atom sticking straight up above the central atom. 

Oxidation

Chemically, the silica tetrahedron works like this: Silicon has 14 electrons, of which two orbits the nucleus in the innermost shell and eight fill the next shell. The four remaining electrons are in its outermost "valence" shell, leaving it four electrons short, creating, in this case, a  cation with four positive charges. The four outer electrons are easily borrowed by other elements. Oxygen has eight electrons, leaving it two short of a full second shell. Its hunger for electrons is what makes oxygen such a strong oxidizer, an element capable of making substances lose their electrons and, in some cases, degrade. For instance, iron before oxidation is an extremely strong metal until it is exposed to water, in which case it forms rust and degrades.

As such, oxygen is an excellent match with silicon. Only, in this case, they form a very strong bond. Each of the four oxygens in the tetrahedron shares one electron from the silicon atom in a covalent bond, so the resulting oxygen atom is an anion with one negative charge. Therefore the tetrahedron as a whole is a strong anion with four negative charges, SiO44–.

Silicate Minerals

The silica tetrahedron is a very strong and stable combination that easily links up together in minerals, sharing oxygens at their corners. Isolated silica tetrahedra occur in many silicates such as olivine, where the tetrahedra are surrounded by iron and magnesium cations. Pairs of tetrahedra (SiO7) occur in several silicates, the best-known of which is probably hemimorphite. Rings of tetrahedra (Si3O9 or Si6O18) occur in the rare benitoite and the common tourmaline, respectively.

Most silicates, however, are built of long chains and sheets and frameworks of silica tetrahedra. The pyroxenes and amphiboles have single and double chains of silica tetrahedra, respectively. Sheets of linked tetrahedra make up the micas, clays, and other phyllosilicate minerals. Finally, there are frameworks of tetrahedra, in which every corner is shared, resulting in a SiO2 formula. Quartz and the feldspars are the most prominent silicate minerals of this type.

Given the prevalence of the silicate minerals, it is safe to say that they form the basic structure of the planet.