The solid phase geometry of a Na- and Ca-montmorillonite has been investigated by using ultra-small- and small-angle X-ray scattering and X-ray diffraction. The scattering domain covered by combining these techniques corresponds to characteristic distances ranging from a few angstroms to a micrometer. The intensity scattered on the whole scattering domain was decomposed into two terms, (i) one assigned to the structure of layers' stacks and (ii) one ascribed to the porous network resulting from the entanglement of individual platelets. The comparison of experimental data with theoretical simulations revealed that the stacks of clay layers are complex and heterogeneous. Ordered stacks of approximately 10 layers separated by 9.5 Angstrom seem to be organized in larger particles comprising around 100 layers. Distances between 25 and 150 Angstrom were observed within those particles. Both samples possess a low pore volume fraction of about a few percent, with pores characterized by a highly anisotropic shape.