الملخص

A total number of 22 samples representing the Jurassic Formations (Hussainiyat, Amij and Muhaiwir) from the clastic units were collected and used for mineralogical studies. X-ray diffraction studies have proved that kaolinite is the dominant clay mineral, the mixed layer illite/ smectite is randomly interstratified and rich in K-poor illite. Based on x-ray pattern decomposition of 001 reflection in conjunction with the chemical analysis of the clay fractions, it has been found that there is a difference in K replacement by H2O between illite of the southern parts, having the formula of [H2O1.2K0.3Al4(Al0.3Si7.7)O20(OH)4], and the illite of the northern parts [ H2O0.9K0.6Al4(Al0.6Si7.4)O20(OH)4]. Based on illite/ smectite composition there is a variation in pH of the deposi-tional environments and the intense weathering between northern and southern parts of the Formations. The variations have been deduced from the clay mineral assemblages. ــــــــــــــــــــــــــــــــــــــــــــــــــــــ