In silicates, two oxygen atoms per SiO4 tetrahedron are shared giving polymeric anion chains. When each tetetrahedron (SiO44−) shares two O atoms, infinite chains are formed. The formula of these extended anions approximates to (SiO3)n2n− or (SiO32−)n.SiO44− anion is present in orthosilicates. Si2O76− anion is present in pyrosilicates, while (Si2O52−)n anion is present in double-chain silicates as well as in sheet silicates. Structure of silicate: 