Silicate Tetrahedron - Oxygen and silicon together form a strong complex ion, the silicate anion (SiO4)4-. In the silicate anion, the oxygen ions pack into the smallest space possible for four large spheres. The smallest space is taken if the oxygens sit at the corners of a tetrahedron and the small silicon cation sits in the space between the oxygens at the center of the tetrahedron. Because the silicate anion has a negative charge (each oxygen in the anion needs an electron to become stable) the oxygens must accept electrons from cations or share electrons with other silicate anions. Most silicate minerals contain a large number of silicate anions.
Silicate Mineral Structure - The silicate structure is formed by the joining of silicate tetrahedrons in a regular pattern. This regular pattern makes the silicate a crystal with regular interrnal structure. There are five basic structures for the silicate minerals, listed below.
Isolated tetrahedra - in this case none of the oxygens are shared by tetrahedrons, so that individual silica tetrahedrons are bonded together by positively charged ions. Minerals with this structure include olivine and garnet.
Single Chain structure - these form when two oxygen atoms of each tetrahedron are shared with adjacent tetrahedrons. The ratio of oxygen to silicon is 3 to 1. The most common examples are minerals in the pyroxene group.
Double Chain structure - these are characterized by two parallel chains in which every-other tetrahedron along the chain shares an oxygen ion with a adjacent chain. The most common examples are minerals in the amphibole group.
Sheet silicates - When each tetrahedron shares three oxygen ions, the result is a sheet structure characteristic of the mica and clay mineral groups; also, serpentine
Framework silicates - When all four oxygen ions are shared by adjacent tetrahedrons, a three dimensional framework is formed. Quartz and feldspar are the most common examples.
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