Computerized qualitative and quantitative clay mineralogy: introduction and application to known geological cases
Zeelmaekers, E. (2011). Computerized qualitative and quantitative clay mineralogy: introduction and application to known geological cases. PhD Thesis. Katholieke Universiteit Leuven. Groep Wetenschap en Technologie: Heverlee. ISBN 978-90-8649-414-9. XII, 397 pp.
Clay minerals are some of the most abundant minerals in sedimentary rocks and have a unique set of properties, such as a large surface area and the capability to exchange water and cations with their environment from these surfaces. In addition clay minerals serve as raw materials for a wide variety of products and can indicate specific formation or diagenetic conditions. Despite their abundance and importance, clay minerals have historically been characterized with a low qualitative resolution and using only semiquantitative methods because their small crystal size and highly variable composition make them very difficult to study in detail. Fundamental analysis problems were present from preparation through identification to quantification, both in the bulk rock as in the extracted clay fraction. It was recognized that a set of both existing and fundamentally new techniques existed which permits to overcome these problems and provide a highly accurate and reproducible qualitative and quantitative characterization of the clay mineralogy and the bulk rock mineralogy in general. The introduction of these techniques - which are mainly based on X-ray Diffraction (XRD) - in the Laboratory for Applied Geology & Mineralogy was a first major goal of this work. A bulk rock analysis technique was introduced that relies on homogeneous grinding and side-loading of random powders to overcome preparation issues. Data analysis is done using the QUANTA software which is a combination of whole pattern fitting of diffraction patterns of pure mineral standards and single peak quantification through the addition of a zincite internal standard. A critical element is the quantification of clay minerals off their 060-reflections instead of their highly variable 001-reflections. Tests on artificial samples have shown the obtained results to be highly accurate. They are also in close agreement with bulk rock chemistry. Secondly also a detailed clay analysis technique was introduced. It relies on extracting the clays after first removing the cementing agents by a thorough pretreatment of the samples. This permits to produce high quality diffraction patterns of oriented glass slide preparations. These diffraction patterns can then be subjected to detailed qualitative and quantitative modeling using the SYBILLA software. It was demonstrated that the modeling results can be independently validated by chemical and Cation Exchange Cation (CEC) analyses. A second goal of this work was to demonstrate the value of the newly introduced techniques on a number of geological cases for which the traditional analysis methods could not deliver sufficient resolution. In a first study the provenance of muds and suspensions on the Belgian Continental Shelf was studied. After a detailed characterization of the muds themselves and sediments from the potential source areas it was concluded that the muds likely have a local origin, in specific the reworking of older clay-rich sediments, probably supplied by the PaleoScheldt river. A second study focused on a well known clay mineralogical transition near the Eocene-Oligocene boundary in the Southern North Sea Basin which is characterized by a relatively rapid decrease in smectite content. Convincing evidence was found linking the occurrence of the large quantities of smectite in the Eocene deposits to a volcanogenic origin. The transition itself was found to be less rapid than previously assumed and was reinterpreted as having a tectonic rather than climatological origin. In a third study earlier work (limited in resolution) was confirmed, demonstrating a Milankovitch controlled cyclicity of kaolinite in the Boom Clay. In addition to these three geological cases, also the mineralogy of the Boom Clay was characterized in detail.
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