[en] This study deals with the theoretical and experimental aspects of the x-ray diffraction technique (XRD). The XRD technique is used to investigate fine structure of matter, and it is most efficient method for the determination of the mineralogical composition of rocks. The XRD technique is used also to investigate the clay mineralogical of mud-stones of the Nubian sandstones of north western Sudan. The XRD results revealed that the mud-stone samples are composed, in decreasing abundance's of kaolinite, smectite, chlorite and illite. Non-clay minerals reported include quartz, feldspars and geothite. Kaolinite dominates in most of samples with percentages ranging between 78-96%. Smectite comes second in abundance and ranges between 10-24%, followed by chlorite and illite which showed the lowest abundance's. The dominance of kaolinite over smectite indicates that intense chemical weathering and leaching occurred under warm humid climate interrupted by dry periods. Most probably these clay minerals were produced by inheritance and partly by neo formation. The variation of the chemical composition of these mud stones is due basically to differences in clay mineralogy which was controlled by source rock geology, weathering physicochemical behavior of elements, local environment and climatic condition in the past. (Author)