Abstract: In many arid regions of the word lime, gypsum or other cements stabilization of marl soils is used as a convenient and expedient means for the development of foundation base courses and inexpensive wearing courses for transport purposes. The failure of many of these natural and stabilized marl soils to perform their functions have been reported. Mechanical factors generally used to explain the causes for the foundation failures have not been satisfactorily accepted. This study uses X-ray diffraction analysis to explain the general basic causes for deterioration of the support capability of these types of soils. The presence of Polygorskite in marl soils provides it with some very unique features in its natural state, and particularly when it is stabilized with lime, gypsum or other cements. Formation of expansive minerals such as ettringites, as a transformations product of Polygorskite increase the swelling potential of the stabilized soils. The various interactions, reactions and factors contributing to the stability and instability of marl soils, and effects of lime and gypsum on stabilized marl soils are discussed.