silicate: Any of a group of substancescontaining negative ionscomposed of silicon and oxygen. Thesilicates are a very extensive groupand natural silicates form the majorcomponent of most rocks (see silicateminerals). 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 (see illustration). 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 innite 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 (see illustration), and the linking can extend to full threedimensional framework structures, often with substituted trivalent atoms in the lattice. The zeolites are examples of this.
