Other Silicates
While feldspars and quartz comprise the bulk of the silicates, and thus the bulk of the earth's crust, other less-common silicates are important. For example, micas, such as muscovite, are characterized by an arrangement of the tetrahedra in sheets. Therefore, micas can be cleaved into thin sheets of material, as the bonds holding the groups of tetrahedra together are not as strong as the bonds holding the silicon and the oxygen together within a tetrahedron. Amphiboles, such as hornblende found in granite, are formed from chains of tetrahedra. Now the olivine group, as previously mentioned, is composed of millions of independent tetrahedra.