Abstract: In the sedimentary domain, clays represent about half of the detritic rocks in terms of frequency. Clasts with the diameters smaller than 2 or 4μm, according to the used scale, clays, also called pelitic rocks, have to contain as deposits more than 60% clay minerals of the entire petrographic assembly in order to be categorized as clay rocks. Sediments (the primary material) on the base of which they form after the occurrence of diagenetic processes (porosity reduction, cementation of clasts, dissolving of certain minerals and precipitation of others etc.) are known as muds. The type of clay minerals, their morphology, as well as the morphology of the deposits they belong to, the presence and proportion of different chemical elements characteristic to certain well-established depositing environments, gives us clues about the origin of the primary material and about the transformations, it underwent from the initial depositing stage to the moment the sample is taken.