The basic building block of all the silicate minerals: SiO4. One Silicon (Si) ion bonds with four Oxygen(O) ions to form a tetrahedra with the Si at the centre. This bond is very strong, thus silicate minerals are usually hard and have high melting points.
Overall, the silicon tetrahedra has a negative charge: this makes it readily combine with positively charged ions such as Iron(Fe) to form minerals, in this case Olivine.

The tetrahedra can also link together to form chain, sheet and framework minerals. The nature of the linking is used to classify silicates and accounts for their physical and chemical properties. For details on the different linkings, see:

Island Silicates
Chain Silicates
Sheet Silicates
Framework Silicates

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