The construction of silicate minerals

The SiO4 tetrahedron has a net 4- charge, since silicon has a valency of 4+, and each oxygen is divalent (2-). This means that the silicon ion (Si4+) can satisfy only half of the bonding capacity of its four oxygen neighbours. The remaining bonds are used in one of two ways as silicates crystallize from magma:

1 Some magmas are rich in elements which are attracted to the electronegative tetrahedral oxygen (Box 4.2). The bonds between these elements (e.g. magnesium (Mg)) and oxygen have ionic character (Box 4.2) and result in simple crystal structures, for example olivine (the magnesium-rich form is called forsterite) (Section 4.2.3). The cohesion of forsterite relies on the Mg2+-SiO4 ionic bond. Bonding within the SiO4 tetrahedron has a more covalent character. During weathering, water, a polar solvent (Box 4.1), severs the weaker metal-SiO4 tetrahedron ionic bond, rather than bonds within the tetrahedron itself. This releases metals and free SiO^J- as silicic acid (H4SiO4).

2 In some magmas, electropositive elements (opposite behaviour to electronegative elements) like magnesium are scarce. In these magmas each oxygen ion is likely to bond to two silicon ions, forming bonds of covalent character. The formation of extended networks of silicon-oxygen is called polymerization, and is used to classify structural organization in silicate minerals (Section 4.2.3).

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