Clay minerals and sheet silicates are the main constituents of lutites and clays. These materials are relevant in earth science research as well as in economic geology because of the great variety of applications, based on their particular features at different levels of aggregation in mineral assemblages and on the microstructural and structural characteristics of the mineral constituents frequently characterized by micro- and nanocrystalline appearance. Thus, X-ray diffraction is a main tool for fundamental and applied research of these materials. The present review concerns their microstructural research from powder X-ray diffraction data.