The basic building block of the silicates is the silica tetrahedron. Each silicon atom is attached to four oxygen atoms by tetahedral bonds. This results in a 4- charge on the Si04 group.
There are many ways in which the SiO4 tetrahedra can be assembled to build neutral silicate mineral structures. These structures are the major rock-forming minerals.
Olivines and Garnets Isolated tetrahedra balanced by the cations magnesium (Mg), iron (Fe),calcium (Ca)
Pyroxenes Single chains of tetrahedra balanced by similar metal cations and sodium (Na)
Amphiboles Double chains of tetrahedra balanced by similar cations.
Micas and Clay Minerals Sheets of tetrahedra are the building blocks. Aluminum is also involved in these sheet structures which are charge-balanced by
the cations Mg, Na and K.
Feldspars A second group of alumino-silicates, the tetrahedra form three-dimensional frameworks with Ca, Na and K as the balancing cations. The very abundant feldspar are subdivided in the K-Na bearing alkali feldspars and the Ca-Na solid-solution series called the plagioclase feldspars.
Quartz Silica tetrahedra alone can form a neutral three-dimensional framework structure with no need for other cations. This arrangement forms a very stable structure.