Which one of the following statements about the zeolites is false :-

Zeolites are three-dimensional aluminosilicates. They are built from $SiO_4$ tetrahedra in which some silicon atoms are replaced by aluminium atoms, i.e. some $SiO_4^{4-}$ tetrahedra are replaced by $AlO_4^{5-}$ tetrahedra. So they are aluminosilicates with a three-dimensional network; this statement is true.
The extra negative charge produced by each Al is balanced by cations such as $Na^+$, $K^+$ or $Ca^{2+}$. These cations can be exchanged (e.g. for $Ca^{2+}$ and $Mg^{2+}$ in hard water), so zeolites are used as cation exchangers; true.
Their framework has open cavities and channels of molecular size, so they can take up small molecules (molecular sieves, shape-selective catalysts such as ZSM-5); true.
Aluminium in zeolites is present only in tetrahedral $AlO_4$ units. There are no octahedral $AlO_6^{9-}$ units in the framework.
So the false statement is the one claiming that $SiO_4^{4-}$ units are replaced by $AlO_4^{5-}$ and $AlO_6^{9-}$ ions.