Silicates

Silicates are salts of silicic acid, $H_4SiO_4$.
$Si(OH)_4 \xrightarrow{-4H^+} SiO_4^{4-}$ (silicate ion, tetrahedral)

The basic unit of all silicates is the $[SiO_4]^{4-}$ tetrahedron; silicates are classified by the number of oxygen atoms shared per tetrahedron.
No oxygen shared: orthosilicates, $SiO_4^{4-}$.
One oxygen shared between two tetrahedra: pyrosilicates, $Si_2O_7^{6-}$.
Two oxygens shared: chain or cyclic silicates, $(SiO_3)_n^{2n-}$.
Three oxygens shared: sheet silicates, $(Si_2O_5)_n^{2n-}$; all four shared: three-dimensional silicates, $SiO_2$.
Hence sharing of one oxygen atom gives a pyrosilicate.