Silica Tetrahedron Defined and Explained

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

Chemical Details of the Silica Tetrahedron

The silica tetrahedron consists of a silicon atom surrounded by four oxygen atoms. The geometric figure drawn around this arrangement has four sides, each side being a triangle—a tetrahedron.

A ball-and-stick model of it has three oxygens holding up the silicon like the three legs of a stool, with the fourth oxygen sticking straight up above it.

Chemically, the silica tetrahedron works like this: Silicon has 14 electrons, of which two orbit 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, in which case it is 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, and it makes a very good match with silicon. 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–.

Silica Tetrahedron and the 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.

Pronunciation: SIL-ica tet-tra-HEED-drun

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Alden, Andrew. "Silica Tetrahedron Defined and Explained." ThoughtCo, Mar. 9, 2017, Alden, Andrew. (2017, March 9). Silica Tetrahedron Defined and Explained. Retrieved from Alden, Andrew. "Silica Tetrahedron Defined and Explained." ThoughtCo. (accessed January 24, 2018).