Determination of crystallite thickness and crystal growth mechanisms of Jordanian clays by X-ray diffraction method

Five mudrock samples were collected at different stratigraphical positions within the rock column of Jordan. They represent the ages of the late Ordovician-lower Silurian (SIL sample), late Permian (UI samples), Lower Cretaceous (MAH samples), Lower Turonian (TAF samples) and Pleistocene (YAM samples). The results of X-ray diffraction (XRD) analysis of clay fraction ( particle size <2 μ) show that the amount of smectite decrease towards more ancient mudrock samples, i.e. towards an increase of burial depth. Smectite totally disapeared in the Silurian claystone (SIL samples) at a depth of 1.6 km more likely as a fucction of burial diagenesis The change of smectite into illite, due to burial history, was through a mixed layer illite-samectite intermediate stage of alteration. The shape of crystallite thickness distributions (CTDs) for illite was asymptotic in all study samples. This means that the crystal growth meschanism of illite particles was similar regaradless of the burial depth of sediments and was concurrent with nucleation. The XRD patterns showed that the degree of Kaolinite crystallinity increased generally with depth as the 001 reflection becomes sharper and narrower. The CTDs sapes for kaolinite were somewhat different. It was asymptotic in YAM and TAF samples, multimodal in MAH samples, and lognormal in UI and SIL samples. These variations in CTDs distributions reflect the fact crystal growth mechanism of kaolinite differed with burial depth. kaolinite crystal growth accompaniced with nucleation was only shallow-moderate depths. This changed to crystall growth without containued nucleation at greater depths . (authors).