Abstract. The physical properties of claystones, shales, and slates are highly dependent
on the alignment of phyllosilicate minerals. With increasing overburdening,
the shape and the crystallographic preferred orientation of these minerals are
affected by uniaxial shortening as well as tectonic processes including
recrystallization under elevated pressure and temperature conditions. The
microstructural anisotropy expressed mainly by the alignment of
phyllosilicates significantly predetermines the orientation of fractures,
hence the shear strength and stability of clay-rich sediments and rocks. A
quantitative analysis of phyllosilicate alignment is therefore essential to
evaluate the properties and the mechanical behavior of these rocks. This can
be carried out by analyzing the crystallographic preferred orientation
(texture). Although texture analysis is a common tool in geosciences, it becomes more
difficult in fine-grained rocks owing to for example particle size,
heterogeneity, the polyphase composition, and difficulties in sample
preparation. Methods such as electron backscatter diffraction, neutron
diffraction, or laboratory X-ray diffraction are restricted with respect to
preparation artifacts, sampling size and statistics, water content, etc. To
overcome these issues, we successfully apply high-energy X-ray diffraction as
available at synchrotron research facilities, e.g., at the German Electron
Synchrotron Facility (DESY) in Hamburg, Germany, or the European Synchrotron
Research Facility (ESRF) in Grenoble, France. In combination with Rietveld
refinement we analyze the bulk texture of phyllosilicate-rich rocks. Here we present the results of texture analysis from a wide range of these
rocks: Pleistocene poorly consolidated mud (rocks), affected only by
sedimentation and burial; more highly consolidated but tectonically largely
unaffected Jurassic claystone from the Opalinus Formation of the Swabian Alb;
Carboniferous shales from the Harz mountains representing low-grade
metamorphic and deformed rocks. Our methodical approach to quantifying the
microstructural anisotropy using texture analysis in fine-grained rocks allows
for the quantification of physical properties resulting from the alignment of
phyllosilicates. Furthermore, it enables the prediction of direction-dependent
mechanical strength, which is crucial for the establishment of long-term
repositories for radioactive waste in shales and claystones.
Characterizing the microstructural anisotropy of fine-grained phyllosilicate-rich rocks
