It should be noted that this ion, which occurs in the minerals thortveitite , Sc 2 Si 2 O 7 , and hemimorphite ,
Zn 4 (OH) 2 Si 2 O 7 , is isoelectronic with P 2 O 7 4 , S 2 O 7 2 , and Cl 2 O 7 . Except for the fact that the SiO 4 unit
has a 4 charge whereas the PO 4 unit has a 3 charge, there is a great deal of similarity between the
“ polyphosphates ” and the “ polysilicates. ” If three SiO 3 2 (metasilicate) ions are linked in a ring, thcyclic Si 3 O 9 6 ion is formed, in which each Si is surrounded by four oxygen atoms forming a tetrahedron
(see Figure 14.1d ).