All silicates are built of a fundamental structural unit, the so-called SiO₄ tetrahedron. The crystal structure may be based on isolated SiO₄ groups or, since each of the four oxygen ions can bond to either one or two silicon (Si) ions, on SiO₄ groups shared to form complex isolated groups or indefinitely extending chains, sheets, or three-dimensional networks. Mixed structures in which more than one type of shared tetrahedra are present are also known.