Any of a group of substances containing negative ions composed of silicon and oxygen. The silicates are a very extensive group and natural silicates form the major component of most rocks (see silicate minerals). The basic structural unit is the tetrahedral SiO₄ group. This may occur as a simple discrete SiO₄⁴⁻ anion as in the orthosilicates, e.g. phenacite (Be₂SiO₄) and willemite (Zn₂SiO₄). Many larger silicate species are also found. These are composed of SiO₄ tetrahedra linked by sharing oxygen atoms as in the pyrosilicates, Si₂O₇⁶⁻, e.g. Sc₂Si₂O₇. The linking can extend to such forms as bentonite, BaTiSi₃O₉, or alternatively infinite chain anions, which are single strand (pyroxenes) or double strand (amphiboles). Spodumene, LiAl(SiO₃)₂, is a pyroxene and the asbestos minerals are amphiboles. Large two-dimensional sheets are also possible, as in the various micas, and the linking can extend to full three-dimensional framework structures, often with substituted trivalent atoms in the lattice. The zeolites are examples of this. Pauling’s rules provide a useful guide for understanding the structures of silicates.