The properties of a zeolite are dependent on the topology of its framework, the size, shape, and accessibility of its free channels, the location charge and size of the cations within the framework, the presence of faults and occluded material, the ordering of T-atoms, and the local environment of T-atoms. The silica and alumina tetrahedra are combined into more complicated secondary units, which form the building blocks of the framework zeolite crystal structures. The tetrahedra are arranged so that the zeolites have an open framework structure, which defines a pore structure with a high surface area. This surface area is different from that of amorphous solids such as silica-alumina in that it is a true part of the crystalline solid. Probably the simplest way of illustrating zeolite structure is by means of skeletal models or diagrams showing the four-connected nets formed by the tetrahedral atoms in the various aluminosilicate frameworks.