Introduction

Clay minerals influence a great variety of chemical, biochemical, and microbiological processes in soils and sediments [1]. These colloids bind cations, act as carriers of radionuclides, and influence the growth and metabolic activity of soil microorganisms, to give but a few examples [2], In the soil, the clay minerals are associated with a great variety of other materials, like different iron oxides, aluminium aquoxides, and humic substances [3]. This association leads to the formation of larger aggregates. Due to the reduced accessibility of the active parts inside the aggregates, the turnover rates of chemicals are reduced [4].

To obtain a measure for the influence of the geometry on the accessibility of the centers, a direct visualization of these aggregates and their internal structure is highly desirable. To avoid samples under investigation being damaged by preparation procedures, these aggregates should be investigated in their natural aqueous environment. Due to the size of the colloidal particles, a direct visualization with the light microscope is not possible. All electron microscopic techniques require that the water be removed from the sample [5], Since these processes can influence the object, a number of different methods has been developed to reduce this risk. On the other hand, x-ray microscopy permits the direct investigation of clay particles in the natural aqueous environment.

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