This model depicts the basic building block of most silicate minerals - the SiO₄ tetrahedron. It is represented by a ball-and-stick portrayal of the molecule with a single cation of silicon (Si⁴⁺) in the centre (in red) bonded with four surrounding oxygen anions (each O²⁻) in a tetrahedral arrangement (yellow). This is combined with a transparent (purple) depiction of the coordination polyhedron. The molecule cannot exist by itself as there is a residual negative charge of 4-. This charge is accounted for by sharing oxygen anions with adjacent tetrahedra (e.g. in quartz), or by bonding with other cations in silicate minerals (e.g. Mg²⁺ in olivine).