Microstructural and physicochemical characterization of palygorskite clays from Marrakech High Atlas, Morocco

The studied palygorskite clay mineral has been collected from several localities of Marrakech High Atlas Tertiary series. These series consist of alternation of marl, dolomite, limestone, gypsum and phosphatic beds. Palygorskite currently constitutes more than 90% of the clay fraction of these facies. The aim of this study is to characterise the structure and the physicochemical properties of these fibrous clays. Several technics were performed to characterise these clay sediments: X-ray diffraction (XRD) patterns to study its crystal structure. XRF analysis was performed to study the chemical compositions. Differential thermogravimetric analyses (DTA) were used to study the weight loss due to dehydration and dehydroxylation. The microstructure of these fibrous clays is determined by transmission and scanning electron microscop (TEM/SEM). Physical properties were characterised using the plasticity index of Casagrande, the test for shear strength and the oedometer test. The parameter concerning the adsorption capacity of methylene blue is also determined.

The palygorskite clays of the Marrakech High Atlas consist of a silty clay of uniform appearance, greyish in color, and very low organic plastic. The thickness of deposit varies from several meters to several hundred meters. SEM and TEM observations show that palygorskite clay minerals display bundle of long fibres wish occurs as interwoven fibrous mats forming the laminae and as coatings and pore-filling, and pore-bridging cements in dolomite. These fibres are characterized by Mg rich chemical compositions, high crystalline structure, high specific surface area and excellent sorptive capacity. XRD show reflections (110) at 10.5 A, associated with all harmonics. The ATD curves show three endothermic peaks (100-200, 250-300 and 400-500 °C) corresponding to different dehydration process. A fourth exotherm peak often preceded by an endothermic reaction (800-900 °C) reflet the crystallization of new mineral phases including mullite.

Palygorskite rich samples exhibit a high plasticity index of about 70. The swelling index generally increases with increasing content of palygorskite. In addition, Palygorskite rich samples exhibit high cohesion and a low friction angle. However, the density of the samples decreases with the increase of the content of palygorskite.