Incorrect statement among the following about zeolites is:

Zeolites are three-dimensional microporous aluminosilicates. Their framework is formed by replacing some of the silicon atoms in the silica ($SiO_2$) lattice with aluminium atoms. The structure consists exclusively of corner-sharing $SiO_4$ and $AlO_4$ tetrahedra. The statement that they contain $AlO_6^{9-}$ (octahedral) units is incorrect. The other statements are correct: they have an open honeycomb-like structure (molecular sieves) allowing them to take up small molecules like water or alcohol; they are aluminosilicates with a 3D network; and they are widely used as cation exchangers (e.g., in softening hard water) due to the presence of exchangeable metal ions balancing the negative charge of the aluminosilicate framework.