Species	Number of oxygen atoms shared by ${\mathrm{SiO}}_4^{-4}$tetrahedra
orthosilicate(${\mathrm{SiO}}_4^{-4}$)	P
pyrosilicate(${\mathrm{Si}}_2{\mathrm{O}}_7^{-6}$)	Q
Chain silicate(${\mathrm{SiO}}_3^{-2}{)}_{\mathrm{n}}$	R
2 -dimensional sheet structure${\left({\mathrm{Si}}_2{\mathrm{O}}_5^{-2}\right)}_{\mathrm{n}}$	S

Sum of P + Q + R + S is -------------------------------------------