Silicates/Titanates

The silicates are the largest, the most interesting and the most complicated class of minerals than any other minerals. Approximately 30% of all minerals are silicates and some geologists estimate that 90% of the Earth's crust is made up of silicates, SiO₄^4- based material. Thus, oxygen and silicon are the two most abundant elements in the earth's crust.

Silicates is based on the basic chemical unit SiO₄^4-, tetrahedron shaped anionic group. The central silicon ion has a charge of positive four while each oxygen has a charge of negative two (-2) and thus each silicon-oxygen bond is equal to one half (½ ) the total bond energy of oxygen. This condition leaves the oxygens with the option of bonding to another silicon ion and therefore linking one SiO₄^4- tetrahedron to another.

At very high pressures, silicon may adopt octahedral coordination, in which each silicon atom is directly attached to six oxygen atoms. This structure occurs in the dense stishovite polymorph of silica found in the lower mantle of the Earth, and it is also formed by shock during meteorite impacts.

There are two forms of silicates:Felsic-The fel stands for feldspar while the sic represents silica. They form in granites and are lighter in weight and color than other silicates because they have less iron and magnesium. Quartz, micas, and the K-feldspars are noteable members of this group.Mafic- Ma stands for magnesium and fic is for iron (ferric). This group of silicates usually form in magmas moving up to fill the gap left when tectonic plates are moving away from each other in the sea floor. Basalt and gabbro are of this type. Olivine and pyroxene are also in this group. They are relatively dense and dark.

A modern approach to classifying silicates is by their structure. This class of minerals uses SiO₄ molecules connected as tetrahedrons. A tetrahedron is a triangular based pyramid. The oxygen atoms occupy the corners of the tetrahedron with the silicon atom in the center.The arrangement of this basic shape is the basis for classification. There are six subclasses. They are:NesoSilicates (Single Tetrahedrons), SoroSilicates (Double Tetrahedrons), InoSilicates( Single& Double Chains), CycloSilicates ( Rings), PhylloSilicates ( Sheets), TectoSilicates (Frameworks).