Question: what exactly is a zeolite?

Alexander Munnoch answered on 25 Nov 2021:

By definition it is a crystalline (it has a repeating unit cell structure) aluminium silicon oxide with pores < 2.0 nm (2 x 10^-9 m) or < 20 angstroms in diameter. They are sometime called molecular sieves because organic molecules are often on the same length scale as the zeolite pores. It's quite advanced but the following website is a brilliant source of structural information – and gives a bit of an idea of the diversity of structure in zeolites. https://asia.iza-structure.org/IZA-SC/framework.php?STC=MFI