Number of oxygen atoms shared per SiO_4^ 4- tetrahedron in I. two...

In two dimensional sheet silicates, three oxygens of each $\mathrm{SiO}_4^{4-}$ units are shared. Hence, it contain $\left(\mathrm{Si}_2 \mathrm{O}_5\right)_n^{2-}$ type anions. Cyclic silicates are obtained by sharing of two oxygenatoms of each $\mathrm{SiO}_4^{4-}$ tetrahedron. Chain silicates are also formed by the sharing of two oxygen atoms of each $\mathrm{SiO}_4^{4-}$ units.