In the present work the validity of the finite cluster approximation is investigated for two zeolite types, faujasite (FAU) and zeolite A (LTA), by comparison of crystal calculations, with cluster calculations, both performed at the same level of accuracy within the EEM frame. The quantities under consideration are the electronegativity, the chemical hardness, the electrostatic potential, and the electrostatic field. A criterion for the determination of the valid region of a cluster is proposed. Also, a method is offered to approximate the structure specific, long-range Madelung potential used for embedding of clusters.