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

Abstract EN

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 sample), Lower Cretaceous (MAH sample), Lower Turonian (TAF sample) and Pleistocene (YAM sample).

The results of X-ray diffraction (XRD) analysis of clay fraction (particle size < 2 μm) show that the amount of smectite decreases towards more ancient mudrock samples, i.e.

towards an increase of burial depth.

Smectite totally disappeared in the Silurian claystone (SIL sample) at a depth of 1.6 km more likely as a function of burial diagenesis.

The change of smectite into illite, due to burial history, was through a mixed layer illite-smectite 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 mechanism of illite particles was similar regardless 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 shapes for kaolinite were somewhat different.

It was asymptotic in YAM and TAF samples, multimodal in MAH sample, and lognormal in UI and SIL samples.

These variations in CTDs distributions reflect the fact that crystal growth mechanism of kaolinite differed with burial depth.

Kaolinite crystal growth accompanied with nucleation was only at shallow-moderate depths.

This changed to crystal growth without continued nucleation at greater depths.