Silicate structures
The basic silicate stucture is one of a central silicon atom surrounded by 4 oxygen atoms forming a pyramid with a triangular base and 4 total sides: a tetrahedron.
If one looks at this structure closely, one realizes that while such an arrangement satisfies the octet rule for Si it does not satisfy it for the oxygen atoms and therefore SiO₄ is a covalent "ion" with four unpaired electrons, one associated with each Oxygen atom. (you can see the unpaired electrons in red in thge dot diagram below)

            .
          :O:
            .  .   .  .   .  .
    . O:Si:O .
            .  .   .  .    .  .
           :O:
             .